Sp20 masonry design_and_construction


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Sp20 masonry design_and_construction

  3. 3. .COMPOSITION dF THE SPECIAL COMMITTEE FOR IMPLEMENTATIONOF SCIENCE AND TECHNOLOGY PROJECTS (SCIP)ChairmanDR H. C. VISVESVARAYAVice-ChancellarUniversity of RoorkeeRoorkeeMembersDr M. RamaiahDr R. K. BhandariShri V. Rao AiyagariShri Harish ChandraShri M. L. MehtaShri S. K. Datta (Alfernare)Shri J. D. ChaturvediShri J. Venkataraman(Member Secretary~RepresentingStructural Engineering Research Centre(CSIR), MadrasCentral Building Research Institute,RoorkeeDepartment of Science & Technology,New DelhiCentral PublicNew DelhiWorks Department,Metallurgical and Engineering Consultants(India) Ltd, RanchiPlanning Commission, New DelhiBureau of Indian Standards, New Delhi
  4. 4. As in the Original Standard, this Page is Intentionally Left Blank
  5. 5. FOREWORDUsers of various civil en ineering codes have been feeling the need forexplanatory handbooks and iot er compilations based on Indian Standards. Theneed has been further emphasized in view of the first publication of the NationalBuilding Code of India in 1970 (which has since been revised in 1983) and itsimplementation. The Expert Group set up in 1972 by the Department of Scienceand Technology, Government of India carried out in-depth studies in various areasof civil engineering and construction practices. During the preparation of the FifthFive-Year Plan in 1975, the Group was assigned the task of producing a Scienceand Technology Plan for research, development and extension work in the sector ofhousing and construction technology. One of the items of this plan was theformulation of design handbooks,‘explanatory handbooks and design aids based onthe National Building Code and various lndian Standards and other activities in thepromotion of the National Building Code. The Expert Group gave high priority tothis item and on the recommendation of the Department of Science andTechnology, the Planning Commission approved the following two projects whichwere assigned to the Bureau of Indian Standards (erstwhile Indian StandardsInstitution):a) Development programme on code implementation for building and civilengineering construction, andb) Typification for industrial buildings.A special committee for Implementation of Science and Technology Projects(SClP) consisting of experts connected with different aspects was set up in 1974 to‘advise the BIS Directorate General in identifying and for guiding the developmentof the work. Under the first project, the Committee has identified several subjectsfor preparing explanatory hadbooks/compilations covering appropriate .IndianStandards/codes specifications which include the following:*Handbooks Published:I. Aids for Reinforced Concrete to IS 456 : 1978 (SP 16 : 1980)Explanatory Handbook on Codes of Earthquake Engineering (1s 1893 : 1975and IS 4326 : 1976) (SP 22 : 1982)Handbook on Concrete Mixes (SP 23 : 1982)Explanatory Handbook on Indian Standard Code of Practice for Plain andReinforced Concrete (IS 456 : 1978) (SP 24 : 1983)Handbook on Causes and Prevention of Cracks in Buildings (SP 25 : 1984)Summaries of Indian Standards for Building Materials (SP 21 : 1983)Handbook on Concrete Reinforcement and Detailing (SP 34 : 1987)Handbook on Functional Requirements of lndustrial Buildings (Lightingand Ventilation) (SP 32 : 1986)Handbook on Timber Engineering (SP 33 : 1986)Handbook on Water Supply and Drainage with Special Emphasis onPlumbing (SP 35 : 1987)I I. Handbook on Functional Requirements of Buildings (other thanIndustrial Buildings) (SP 41 : 1987)S&iects Under Programme:--Foundation of Buildings--Construction Safety PracticesBuilding Construction Practices*Handbooks published are available for sale from BIS Headquarters, and from all Branchesand Regianal Offices of BE.
  6. 6. -Formwork-Fire Safety-Tall Buildings-Loading CodeThe Explanatory Handbook on Masonry Code SP 20 (S&T) was firstpublished in 1981 to provide commentary on various clauses of Part Vl, Section 4of National Building Code of India 1970 (which was based on IS 1905 : 1969version) with the object of promoting and facilitating the use of the masonry code.The handbook has been found to be very helpful to professional engineers. Thishandbook while providing commentary on various clauses highlighted certainimprovements modifications that were needed in the basic code (IS 1905 : 1969).Subsequent to publication of the handbook, the code (IS 1905) was revised in 1980(IS 1905 : 1980) taking into consideration the recommendations contained in thehandbook (see also Introduction).The code was further revised in 1987 as a result of the experience gained withthe use of 1980 version of the code and also other developments in other parts ofthe world in the design and construction refinements of masonry structures. Therevision of the handbook was also taken up simultaneously along with the revisionof the code to make it up-to-date. At the same time it was felt that it would behelpful and handy to professionals if information relating to construction practicesbased on various Indian Standards on masonry was also included along with thecommentary. Therefore masonry construction practices have now been included inthe revised handbook and the title of the handbook has been changed accordingly.The revised handbook is in two parts. Part I gives commentary on ‘IS1905 : 1987 Code of practice for structural use of unreinforced masonry (secondrevision)’ along with design examples (IS 1905 : 1987 has also been included in theNational Building Code Part VI, Section 4 Masonry through amendment No. 2)and Part 2 gives all construction aspects relating to masonry construction based onrelevant Indian Standards and other literature available on the subject.The following points are to be kept in view while using the handbook:4b)c)/4e)f)g)Wherever the expression ‘The Code’ has been used in the handbook, it refers toIS 1905 : 1987.Part 1 of the handbook is to be read along with IS 1905 : 1987.The clause numbers in Part I of handbook refer to the corresponding clausenumbers in IS 1905 : 1987. The clauses are explained in the same sequence asthey occur in IS 1905 : 1987. When there are no comments to a particularclause, the same has been omitted.For convenience figures and tables appearing in Part 1 of the handbook areidentified with Prefix ‘E’ to distinguish them from those used in the code. Forexample, Figure E-3 refers to the figure in the handbook whereas Figure 3refers to that given in the Code. Where a clause is pre-fixed by letter ‘E’, itrefers to comments on that clause in the handbook.Notations as per IS 1905 : 1987 are maintained with additional notationswherever necessary.The handbook does not form part of any Indian Standard on the subject anddoes not have the status of an Indian Standard. Wherever there is any disputeabout the interpretation or opinion expressed in this handbook, the provisionsof the Code(s) only shall apply; the provisions of this handbook should beconsidered as only supplementary and informative.References cited have been listed at the end of each part of the handbook.The handbook, it is hoped would be useful to practising engineers and fieldengineers in the design and construction of masonry work. It would also be helpfulto students of civil engineering to acquaint themselves with the various provistonsof the basic Code on masonry and construction practices.The handbook is based on first draft revision prepared by Shri M. S. Bhatia,retired Engineer-in-Chief, Central Public Works Department (Government of(vi)
  7. 7. India). The draft handbook was circulated for review to Central Public WorksDepartment, New Delhi; Engineer-in-Chiefs Branch, Army Headquarters, NewDelhi; Sr. Civil Engineer (Design), Northern Railway, New Delhi; NationalBuildings Organization, New Delhi; Central Building Research Institute, Roorkee;Central Road Research Institute, New Delhi; Shri M.G. Virmani, New Delhi;Public Works Deptt., Govt of Tamilnadu, Madras: Maulana Azad College ofTechnology, Bhopal: Public Works Deptt. Maharashtra, Bombay; Tirath RamAhuja Pvt Ltd, New Delhi; National Buildings Construction Corporation Ltd, NewDelhi; National Council for Cement and Building Materials, New Delhi; StructuralEngineering Research Centre, Madras; M/s C. R. Narayana Rao, Madras; TataConsulting Engineers, Bombay; Indian Institute of Technology, Kanpur; CivilEngineering Department, University of Roorkee, Roorkee; Punjab Public WorksDepartment, Patiala; Structural Designers and Consultants Pvt Ltd, Bombay;Indian Institute of Technology, Kharagpur; Indian Institute of Architects, NewDelhi; School of Planning & Architecture, New Delhi; Shri D. N. Chopra, NewDelhi; Shri T. S. Khareghate, Bombay; Department of Earthquake Engineering,University of Roorkee, Roorkee; Housing and Urban Development CorporationLtd, New Delhi; Tamilnadu Housing Board, Madras; Delhi DevelopmentAuthority, New Delhi; Andhra Pradesh Housing Board, Hyderabad; RajasthanHousing Board, Jaipur; Shri Thomas Mathew, Cochin; Central VigilanceCommission, New Delhi; Chief Municipal Architect, Calcutta and the viewsexpressed were taken into consideration while finalizing the handbook.
  8. 8. As in the Original Standard, this Page is Intentionally Left Blank
  9. 9. INTRODUCTIONUntil 1950’s there were no engineering methods of designing masonry forbuildings and thickness of walls was being based on ‘Rule-of-Thumb’ Tables givenin Building Codes and Regulations. As a result walls used to be very thick andmasonry structures were found to be very uneconomical beyond 3 or 4 storeys.Buildings exceeding 3 or 4 storeys had thus to be constructed with steel or RCCframes. Since 1950’s intensive theocritical and experimental research has beenconducted on various aspects of masonry in advanced countries. As a resultdifferent factors which effect strength, stability and performance of masonrystructures have been identified and methods of design based on engineeringprinciples evolved. Most of the countries have therefore switched over to use of so-called “calculated or engineering masonry” of structures. Simultaneously methods.of manufacture of bricks and construction techniques have been considerablyimproved upon.The basic advantage of masonry construction lies in the fact that in loadbearing structures, it performs a variety of functions, namely, supporting loads,subdividing space, providing thermal and acoustic insulation, affording fire andweather protection, etc, which in a framed building, have to be provided forseparately. No doubt, use of masonry in load bearing structures has certainlimitations, but it is suited for a building in which floor area is subdivided into alarge number of rooms of small or medium size and in which the floor plan isrepeated in each storey throughout the height of the building. These conditions aremet with in residential buildings, hostels, nursing homes, hospitals, schools andcertain types of administrative buildings. Extensive research, including large scaletesting, has been carried out in regard to the behaviour of masonry which hasenabled engineers and architects to design tall masonry structures on soundengineering principles with greater exactitude, economy and confidence. There aremany recent examples in other countries of well designed 12 to 20 storeyed loadbearing masonry buildings which have only 25 to 40 cm thick walls. This is incontrast to the 16 storey ‘Monadnock Building’ in Chicago designed by John Rortin 1891 with 180 cm thick brick walls at the base.In India there has not been much progress in the construction of tall loadbearing masonry structures, mainly because quality of bricks generallymanufactured in the country is poor, their normal strength being of the order ofonly 7 to IO N/mmZ. In many Western countries, bricks of even medium qualityhave crushing strength of 30 to 50 N/mm?. However, recently mechanized brickplants have been set up at a few piaces in the country which are producing bricks ofstrength 17.5 to 25 N/mm’. Thus, it should now be possible in some parts of thecountry to go in for 5 to 6 storeyed load bearing structures at costs less than thoseof RCC framed structures. With this development, structural design of load bearingmasonry buildings has assumed additional importance in India as well. In factunder the Experimental Projects Scheme of the National Buildings Organization,50 residential units in 5 storeyed blocks, having one brick, that is 25.4 cm thick loadbearing brick masonry walls in all the storeys were constructed at Manicktola,Calcutta in 1975 and construction of 20 residential units in’ 5 storeyed blocks,having one brick, that is, 22.9 cm thick walls, have been constructed in New Delhi.Buildings are presently designed in western countries mostly by allowable stressmethod of design. Walls are designed as vertical cantilevers with no momenttransfer at wall to floor connection. Lateral loads are distributed to cross wallsaccording to their stiffness and locations by the diaphragm action of floor and roofslabs acting as horizontal beams. It has been found that eccentricity of load from aslab at the top of a masonry element gets reduced at the bottom support of the wall.In some countries limit state design method is now coming into vogue because ofbetter reliability and economy obtained through the adoption of this method. Forlarge and important projects strength of masonry is based on ‘for-the-job’ prismtests instead of lacing reliance on standard tables. In seismic zones masonryconsisting of ho low blocks is reinforced vertically to take tension. In somePcountries structural advantage is taken of the fact that use of through-wall unitsresults in stronger masonry. In tall single storey long-span buildings such as
  10. 10. churches, sports stadia, large halls, etc, use of Diaphragm type masonry wails isproving to be an economical innovation.Before concluding, a brief mention may be made of some special features andtrends of load bearing masonry in other countries for information of designers.These are as follows:a) Manufacture and use of high strength burnt day units (70 to 100 N/mm?) withperforations for passing vertical reinforcement where necessary.b) Use of high-bond organic modified mortars (polymers) to obtain masonry withvery high compressive as well as bond strength.c) Basing design calculations for structural masonry on prism/cube strength ofmasonry with units and mortar actually proposed to be used in the job.d) Use of ‘Through-Wall-Units’ in order to achieve higher efficiency of masonry(ratio of masonry strength to unit strength). With the use of these units, verticalwall-joints in masonry are eliminated.e) Use of floors/roofs of high stiffness in order to reduce eccentricity of loadingon walls.f) Use of prefabricated brick panels in masonry.g) Use of facing bricks in conjunction with normal bricks fbr external walls, forarchitectural effects.h) Prestressing of masonry elements.There is need and considerable scope in this country of intensifyingexperimental, research and study in the field of load bearing masonry in order to‘make better and more economical use of this wonderful and versatiie buildingmaterial-the brick.In India we have been trying to keep pace to some extent with thedevelopments taking place in other countries in ragard to masonry and in 1961, ISI(now BIS) published its first Code on Masonry which made provisions for design ofmasonry based on working stress method. This code. was revised in 1969. Certainprovisions were upgraded based on improvements brought about in Codes of someother countries. Unfortunately there has not been much of research on the subjectin our country. In 1976, BIS undertook the task of publishing a handbook onmasonry and during the preparatory work certain deficiencies in the Code came tolight. The Code was therefore further revised in 1980 and a comprehensivehandbook on masonry with clause-wise commentary and analysis, Illustrativeexamples and design tables was published in 1981. In view of the growing interestsof the users of Code and handbook and practical importance of this subject todesigners and builders,, the Code as well as the handbook and practical importanceOf this subject to designers and builders, the Code as well as the ‘handbgok havebeen further revised and updated. Now Building Regulations incorporate theconcept of engineered masonry contained in the Code, where as earlier in ourcountry one brickwall could only be a single storey building. Presently one brickwalls are built-up to 4-5 storeys in many parts of the country.The revised Code along with the handbook it is hoped would be ofconsiderable help to engineers in the design and construction of masonry buildingsespecially tall buildings.
  11. 11. CONTENTSFOREWORDINTRODUCTIONPART 1SCOPETERMlNOLOGYMATERIALS3.1 Masonry Units3.2 MortarDESIGN CONSIDERATIONS4.14.24.34”.:4:64.7GeneralLateral Supports and Stability4.2.2 StabilityEffective Height4.3.1 Wall4.3.2 Column4.3.3 Openings in WallsEffective LengthEffective ThicknessSlenderness RatioEccentricitySTRUCTURAL DESIGN5.1 General5.3 Load Dispersion5.3. I General5.3.2 Arching Action5.3.3 Lintels5.4 Permissible Stresses5.4. I Permissible Compressive Stress5.4. I. 1 Stress reduction factor5.4. I .2 Area reduction factor5.4.1.3 Shape reduction factor5.4.1.4 Increase in permissible compressive stresses._ . . .allowed fbr eccentric vertical and/or lateralloads under certain conditions5.4:2 Permissible Tensile Stress5.4.3 Permissible Shear Stress5.5 Design Thickness/Cross Section5.5. I5. and Columns Subjected to Vertical Loads5.5.1.1 Solid wallsWalls and Columns Mainly Subjected to LateralLoads5.5.2.1 Free standing walls5.5.2.2 Retaining wahsWalls and Columns Subjected to Vertical as well._as Lateral LoadsWalls Subjected to In-Plan Bending and VerticalLoads (Shear Walls)Non-Load Bearing Walls6. GENERAL REQUIREMENTS6.1 Methods of Construction6.2 Minimum Thickness of Walls from. . . . . .PageVix3344688810IOIO10ItI:2425::::2252626282828
  12. 12. 6.3 Workmanship6.4 Joints to Control Deformation and Cracking6.6 CorbellingANNEXESANNEX H-l WORKED EXAMPLES ON DESIGN OFSTRUCTURAL MASONRY31ANNEX H-2 DESIGN OF BRICK MASONRY FOR 53RESIDENTIAL BUILDINGS UP TO 3 STOREYSANNEX H-3 DESIGN OF BRICK MASONRY FOR 73OFFICE BUILDINGS UP TO 3 STOREYSANNEX H-4 NOTATIONS, SYMBOLS AND ABBREVlATldNS 104PART 2SECTION 1 GENERAL 107I. I IntroductionI .2 Materials1.3 Masonry UnitsI .4 Mortar1.5 Scaffolding1.6 Curing107107108III122124SECTlON 2 BRICK MASONRY 1242. I General2.2 Bonds2.3 Laying of Brick Masonry2.4 Fixing Door and Window Frames2.5 Honey-Combed Brick Masonry2.6 Brick Masonry Curved on Plan2.7 Extension of Old Brick Masonry2.8 Corbelling2.9 Efflorescence124124126127127127127I28128SECTlON 3 STONE MASONRY 1293. I General3.2 Random Rubble Masonry3.3 Coursed Rubble Masonry3.4 Ashlar Masonry3.5 Laterite Stone Masonry3.6 Stone Veneering3.7 Miscellaneous items129129131132133133134SECTION 4 CONCRETE BLOCK MASONRY 1384. I General4.2 Handling and Storage of Blocks4.3 Laying of Blocks4.4 Rendering and Other Finishes4.5 Treatment at Openings and Fixing of Door and Window Frames4.6 Provision of Lintels4.7 Intersecting Walls4.8 Provision of Floor/ RoofSECTION 5 MASONRY ELEMENTS1381391391401401401401401405. I General5.2 Cavity Wall5.3 Retaining Wall5.4 Masonry Arches1401401411425.5 Masonry Domes 143
  13. 13. SECTION 6 SOME MISCELLANEOUS MATTERS RELA.TlNGTO MASONRY6. I General6.2 Chases, Recesses and Holes6.3 Brick Nogging and Dhajji Walling6.4 Window Sills6.5 Copings on Compound Walls and Parapets6.6 Use of Fire Bricks6.7 Flues and Chimneys in Residential Buildings6.8 Protection of Masonry During Construction6.9 Use of Reinforcement in Masonry6.10 Prevention of Cracks in Masonry6.1 I Walling with Materials Other than MasonryLIST OF REFERENCES AND BIBLIOGRAPHY143143l-43144145145145145146146147147149(xiii)
  15. 15. As in the Original Standard, this Page is Intentionally Left Blank
  16. 16. SP 20(S&T) : 19911 SCOPE1.1 BIS has not yet formulated any Code ofpractice for design and construction of reinforcedmasonry since quality of bricks generally availablein the country at present is not suitable for use inreinl’orced mansonry. Bricks for this purposesl~o~~lcl necessarily be of high strength and shouldalso be dense. so that moisture absorption is less.If bricks have high moisture absorption,reinforcement gets corroded in course of time,thereby lowering its life expectancy.1.2 Mud mortar for masonry as bonding materialis normally not used in the present dayconstruction because of its poor bonding quality.Mud mortar does attain some strength on drying,but it readily absorbs moisture on coming incontact with moisture or rain and loses itsstrength when wet. For temporary and low costsingli: storeyed houses, however, its use issometimes made particularly in rural areas, wheneconomy in cost is the main consideration. (Someinformation on use of mud mortar construction isgiven in i.4.3.4 and of Part 2.)2 TERMINOLOGYSome of the terms defined in this clause areillustrated in Fig. E-i to E- Need for making_ a distinction betweencolumn and wall arises because a column can takelesser unit load than a wall. This behaviour ofmasonry is based on experimental research and,in this context, it will be relevant to quote fromthe Proceedings of the Conference on Planningand Design of Tall Buildings’ as follows:“Walls and Columns-Plain Masonry--Modeof failure. The characteristic failure of wallBED BLOCKI fwELEVATIONunder compressive loading takes the form ofvertical tensile cracks at mid-height and in linewith the vertical mortar joints. The cracks candevelop at such frequency as to becomeprogressively slender columns side by side. Thelower elasticity of mortar causes verticalcompressive load to impart. lateral strainmovements to the mortar, which producestensile stresses in the brick by inter-face bondwhilst maintaining the bed-joint mortar incompression. The mortar is then in conditionof triaxial compressive stress and the brickcarries vertical compression in combinationwith.biaxial lateral tension. Greater the heightto length ratio of the wall, higher the value ofhorizontal tensile stresses ‘at the vertical jointsand, therefore, weaker the wall against verticalsplitting under load.”Since a column has greater height to lengthratio in comparison to a wall, it has a lowerpermmisible stress under a vertiFal load.A masonry column has been defined as avertical member the width of which does notexceed 4 times the thickness. This provision isbased on British Standard CP Ill : Part 2 : 19702.However, in the National Building Code ofCanada and also Recommended Practice forEngineered Bricks Masonry4, 1969, a column hasbeen defined as a member whose width does notexceed 3 times the thickness.2.9 Hollow UnitsShellback’ found that in perforated bricks, typeand distribution of voids influence the strength ofbricks but for perforation areas up to 35 percentof the cross-section, the bricks hav’e been found tobehave as if solid. That explains the backgroundFIG. E-l BED BLOCKHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1SECTION XX
  17. 17. SP 20(S&T) : 1991FIG. E-2 BUTTRESSfm t11l----b--+bf?LtFIG. E-3 COLUMNt------b_Cross Area = a bNet Area = ab -3x when x is thearea of one coreNOTE -LOSS of areadue to yoova is ignoredFIG. E-4 CROS~ECTIONAL AREA OF MASONRYUNITCROSS JOINTWALL JOINTFIG. E-5 JOINTS IN MASONRYIfor the definition of ‘hollow units’.3 MATERIALS3.1 Masonry Units9ii)Choice of masonry units is generally madefrom the consideration of: (a) localavailability, (b) compressive strength, (c)durability, (d) cost, and (e) ease ofconstruction. Brick has the advantage overstone that it lends itself to easy constructionand requires less labour for laying. Stonemasonry, because of practical limitations ofdressing to shape and size, usually has to bethicker and results in unnecessary extracost. Thus, the first choice for a building atany place, would be brick, if it is availableat reasonable cost with requisite strengthand good quality. In hills as well as incertain plains where soil suitable formaking bricks is not available or cost offuel for burning bricks is very high andstone is locally available, the choice wouldbe stone. If type and quality of stoneavailable is such that it cannot be easilydressed to shape and size, or if the cost ofdressing is too high, use of concrete blocksmay prove to be more economical,particularly when construction is to bemore than two storeys, since thickness ofwalls can be kept within economical limitsby using concrete blocks. In areas wherebricks and stone of suitable quality are notavailable and concrete blocks cannot bemanufactured at reasonable cost, and limeand sand of good quality are available,masonry units could be of sand-lime bricks.However, for manufacture of sand-limebricks, special equipment is required, andthus use of sand-lime bricks is not commonin India as yet.Strength of bricks in lndia varies fromregion to region depending on the nature ofavailable soil and technique adopted formoulding and burning. Some research hasbeen done for manufacture of bricks ofimproved quality from soils such as blackcotton and moorum, which ordinarily givebricks of very low strength. The followingstatement based on information collectedby BIS some time back, will give a generalidea of the average strength of bricks inN/mm? available in various parts of India,employing commonly known methods formoulding and burning of bricks:Delhi and Pm$abUttar PradeshMadhya PradeshMaharashtraGujaratRajasthanWest BengalAnd hra PradeshAssam7 to IOIO to 203.5 to 553 to IO3IO to 2033.54 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1
  18. 18. SP. 20 (S&T) : 1991iii)E-8A RCC SLAB GIVINGLATERAL SUPPORT TOA WALL AT TOPE-6B CROSS WALLS GIVINGLATERAL SUPPORT TO A WALLE-SC PIERS GIVING LATERALSUPPORT TO A WALLE-SD R CC BEAM GIVING LATERALSUPPORT TO A COLUMN IN THEDIRECTION OF ITS THICKNESS ‘t’E-6E Rcc BEAMS GIVING LATERALSUPPORT TO A COLUMN IN THEDIRECTION OF THlCKNESS’t’AS WELL AS WIDTH ‘b’FIG. E-6 LATERALSUPPORTSIn certain cities like Calcutta and Madras,machine-made bricks are now beingproduced, which give compressive strengthsvarying between 17.5 and 25 N/mm*.The following relation generally holds goodbetween strength of bricks and maximumnumber of storeys in case of simpleresidential buildings having one brick thickwalls and rooms of medium size:Ni mm2 Storeys3 to 3.5 I7 210 3I.5 4It is, however, possible tothese levels by optimizationand structural designs,to 2to 3to 4to 5go higher thanof architecturalfor example,HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART I
  19. 19. SP 20(5&T) : 1991,-SHEAR WALLSHEAR WALLPLANHorizontal force P acting on wall A is resistedby cross walls B which act as shear wall.FIG. E-7 SHEAR WALLPLASTERLOADlEARINGMASONRYiv)v)6-FACING UNITSl~OCD TOffESULTIN CONMONACTIONU*Ot!RLOADFIG. E-8 FACED WALLadopting cellular type of plan, reducingstorey heights, keeping openings away fromintersections of walls, limiting spans ofrooms and size of openings, designing floorand roof slabs so as to distribute loadsevenly on various walls, and using selectedbricks in the lower one or more storeys, etc.When building with stone as masonry unitin coursesl minimum thickness of wallsfrom practical considerations has to be 30to 40 cm depending upon type and qualityof stone used. However, CBRI haveinnovated a technique of making precaststone blocks for use as masonry units, sothat it has become feasible to build stonemasonry walls of 20 to 25 cm thickness,resulting in economy in cost (see 13.7 ofPart 2).As a general rule, apart from strength ofmasonry units and grade of mortar,vi)3.1.1STONEVENEERFIG. E-9 VENEERED WALLstrength of masonry depends on surfacecharacteristics and uniformity of size andshape of units as well as certain propertiesof mortar. Units which are true in shapeand size, can be laid with comparativelythinner joints, thereby resulting in higherstrength. For this reason, use of A gradebricks gives masonry of higher strength ascompared to that with B grade bricks, eventhough crushing strength of bricks of thetwo grades may be same. For similarreasons ashlar stone masonry which usesaccurately dressed and shaped stones ismuch stronger than ordinary coursed stonemasonry.For detailed information on variousmasonry units reference may be made to 1.3of Part 2.Bond between mortar and masonry unitsdepends on suction rate of masonry units.Masonry units, which have been previously usedin masonry would not possess adequate suctionrate and may not develop normal bond andcompressive strengths when reused. It is thereforenot advisable to reuse such units in locationswhere stress in m’asonry is critical.3.2. Mortari) Particulars of mortars for masonry arecontained in IS 2250 : 19815. lmportantrequirements, characteristics and propertiesof commonly used mortars are summarisedbelow for ready information. For moredetailed information on mortars referencemay be made to 1.4 of Part 2 of thisHandbook.ii) Requirements of a good mortar formasonry are strength, workability, waterretentivity and low drying shrinkage. Astrong mortar will have adequate crushingstrength as well as adequate tensile andshear strength. It is necessary that mortarshould attain initial set early enough toHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1
  20. 20. iii)enable work to proceed at a reasonablepace. At the same time it should gainstrength within reasonable period so thatmasonry is in a position to take load early.A workable mortar will hang from thetrowel and will spread easily. A mortar withgood water retentivity will not readily losewater and stiffen on coming in contact withmasonry units, and will remain plastic longenough to be easily adjusted in line andlevel. This property of good waterretentivity will enable the mortar to developgood bond with masonry units and fill thevoids, so that masonry has adequateresistance against rain-penetration.Mortars are intimate mixtures of somecementing materials, such as cement, limeand fine aggregate (such as sand, burntclay/surkhi, cinder, etc). When only fatlime is used, which sets very slowly throughthe process of carbonation, it becomesnecessary, for the sake of better strength, touse some pouolanic material, such as burntclay/surkhi or cinder. Plasticizers are usedin plain cement-sand mortars to improveworkability. Mortars could be broadlyclassified as cement mortars, lime mortarsand cement-lime mortars. Maincharacteristics and properties of these threecategories of mortars are as under.a) Cement mortars- These consist ofcement and sand, varying in proportionfrom 1 : 8 to 1 : 3, -strength andworkability improvinin the proportion ofwith the increasecement. Mortarsricher than 1 : 3 are not used in masonrybecause these cause high shrinkage anddo not increase in strength of masonry.Mortars leaner than 1 : 5 tend tobecome harsh and unworkable and areprone to segregation. Cement mortarsset early and gain strength quickly.Setting action of mortar is on account ofchemical changes in cement incombination with water, and thus thesemortars can set and harden in wetlocations. In case of lean mortars, voidsin sand are not fully filled, andtherefore, these are not impervious. Richmortars though having good strengthhave high shrinkage and are thus moreliable to cracking.b) Lime mortars - These consist ofintimate mixtures of lime as binder andsand, burnt clay/surkhi, cinder as fineaggregate in the proportion 1 : 2 to1 : 3. As a general rule, lime mortarsgain strength slowly and have lowultimate strength. Mortars usinghydraulic lime attain somewhat betterstrength than those using fat lime. Infact, lime mortars using fat lime do notharden at all’in wet locations. Propertiesiv)clSP 20(S&T) : 1991of mortar using semi-hydraulic lime areintermediate between those of hydraulicand fat lime mortars. When using fatlime, it is necessary to use somepozzolanic material such as burntclay/surkhi or cinder to improvestrength of the mortar. The mainadvantage of lime mortar lies in its goodworkability, good water retentivity andlow shrinkage. Masonry in lime mortarhas, thus, better resistance againstrainpenetration and is less liable tocracking, though strength is much lessthan that of masonry in cement mortar.Cement-lime mortars - These mortarshave the good qualities of cement as wellas lime mortars, that is, mediumstrength along with good workability,good water retentivity, freedom fromcracks and good resistance against rain-penetration. Commonly adoptedproportions of the mortar (cement : lime: sand) are 1 : 1 : 6, 1 : 2 : 9 and1 : 3 : 12. When mix proportion ofbinder (cement and lime) to sand is keptas I : 3, it gives a very dense mortarsince voids of sand are fully filled.Mortar for masonry should be selected withcare keeping the following in view. Itshould be noted that cement-lime mortarsare much better than cement mortars formasonry work in most of the structures.4b)If binder contains more of cement andless of lime, it develops strength early,and is strong when matured. A richcement mortar is needed, firstly, whenmasonry units of high strength are usedso as to get strong masonry; secondly,when early strength is necessary forworking .under frosty conditions; andthirdly, when masonry is in wet locationas in foundation below plinth, where adense mortar being less pervious canbetter resist the effect of soluble salts.An unnecessarily strong mortarconcentrates the effect of any differentialmovement of masonry in fewer andwider cracks while a weak mortar(mortar having more of lime and less ofcement) will accommodate movements,and cracking will be distributed as thinhair cracks which are less noticeable.Also stresses due to expansion ofmasonry units are reduced, if a weakmortar is used. Lean mortars of cementalone _,are harsh, pervious and lessworkable. Thus when strong mortars arenot required from considerations ofstrength or for working under frostyconditions or for work in wet locations,it is preferable to use composite mortarsof cement, lime and sand, in appropriateproportions. Figure E-10 baaed on BRSHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 1
  21. 21. SP 20(S&T) : 1991r/r/d Strength of brickworkII Stcngth of mortar0CEMENT 1 1 1 1 1LIME 0 ‘4s 1 2 3SAND 3 3 6 9 12MORTAR RATIO BY VOLUMEEffects of mortar mix proportions on the crush-idg strengths of mortar and brickwork built withmedium strength bricks.Strengths are shown relative to the strength ofa I : 3 cement-sand mortar and the brickwork builtwith it.FIG. E-IO RELATION BETWEEN STRENGTH OFBPICK WORK AND STRENGTH OFMORTARDigest 6th illustrates the relationbetween strength of mortar andbrickwork for a number of mortar mixeswhen bricks of medium strength (20 to35 N/mm* according 10 BritishStandards) are used. As the proportionof lime in mortar is increased, thoughmortar loses strength, reduction instrength of brickwork is not much.c) It has been observed from experimentalresults that lime-based mortars givehigher ratio of strength of brickwork tomortar as compared to non-limemortars. This can be explained asfollows: Normally brickwork fails undera compressive load on account ofvertical tensile splitting, for which bondstrength of mortar is more importantthan its compressive strength. Sincelime-based mortars have much higherbond strength, as compared to cementmortars, the former produce brickworkof higher strength. Table E-l giving testresults abstracted from SIBMACproceedings7 illustrates this point veryclearly.Table E-l Effect of Mortar Mix on Strengthof Brickwork[Using Clay Brick of Strength 32.7 N/mm*(4 750 ibf/in*)]Mortar Mix Mortar Brickwork Ratio(Cement: Compressive CompressiveLime: Strength StrengthSand) (28 Days) (28 Days)x Y Yx(1) (2) (3) (4)n/ mm2 (Ibf/ in*) N/mm2 (Ibf/ inz)I : ‘A : 3 17.8 (2 590) 8.9 (I 290) 0.501:%:41/2 10.8 (I 570) 9.3 (1 345) 0.86I:1:6 4.7 (680) 8.5 (I 235) 1.82I:2:Y 1.7 (245) 4.6 (660) 2.69NOTE~ Lime used was in the form of well matured putty.v) Optimum mortar mixes from considerationof maximum strength of brickwork forvarious brick strengths based on Digest No.616 (Second Series) and Table I of theCode, are given in Table E-2 for generalguidance.Table 82 Optimum Mortar Mixes for MaximumMasonry Strength with Bricks ofVarious StrengthsBrickStrengthMortar Mtx(By Volume)MortarType(1)(N mm?)Below 5(Cement : Lime : Sand)(2)1 :0:6I : 2c : 90: IA:2-3(3)M25-14.9 I :0:5 MII:ic:615-24.9 I : 0.: 4 H2I : 1/c: 4%25.0 or above I : o-I/,c : 3 HINo IF.- Lime of grade B can be used as an alternative tolime C.4 DESIGN CONSIDERATIONS4.1 GeneralIn order to ensure uniformity of loading, openingsin walls should not be too large and these shouldbe of ‘hole in wall’ type as far as possible; bearingsfor lintels and bed blocks under beams should beliberal in sizes; heavy concentration of loadsshould be avoided by judicious planning and8 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1
  22. 22. SP 20(S&T) : 1991sections of load bearing members should bevaried where feasible with the loadings so as toobtain more or less uniform stress in adjoiningparts of members. One of the commonly occuringcauses of cracks in masonry is wide variation instress in masonry in adjoining parts.NOTE-- A ‘hole in wall’ type opening is defined as anopening where total width or height of solid masonryaround the opening is equal to or greater than the corres-ponding window dimension.4.2 Lateral Supports and Stability4.2.2 Stabilityi) In a masonry structure, there are generallyinbuilt, out of balance vertical forces dueto imperfection in workmanship andverticality of walls which tend to make thestructure unstable. Thus for stabilitycalculations of a lateral support, horizontalforce equal to 2.5 percent of all verticalloads acting above that lateral support isassumed for checking the adequacy of thatsupport. This ,horizontal force is inaddition to any other lateral force, namelywind or seismic that the structure may besubjected to.ii) It ‘should be borne in mind that assumedhorizontal force of 2.5 percent is the totalout of balance force due to vertical loads atthe particular support and it does notinclude out of balance forces acting atother supports. Further it should be keptin view that horizontal force of 2.5 percentof vertical loads need not be considered forelements of construction that providelateral stability to the structure as a whole. Provision ,in sub-clause (a) is as per 1964version of IS : 875.b) A cross wall acting as a stiffening wallprovides stability to the wall at its junctionwith the cross wall thereby resistingmovement of wall at horizontal intervalsand sharing a part of the lateral load.Further in conjunction with the floorsupported on the wall, it resists horizontalmovement of the top of the wall. For thefirst mode of stiffenmg, it is necessary thatcross wall is built jointly with the loadbearing wall or is adequately anchored to itand there should be no opening in the crosswall close to its junction with the main wall(refer clause of the Code); for the_second mode, the floor should be capable ofacting as a horizontal girder and also thefloor should be so connected to the crosswalls that lateral forces are transmitted tothe cross walls through shear resistancebetween floor and cross walls.c) When bricks of old size that is, 23 X 11.5 X7.7 cm (FPS System) are used, Table E-3may be used in place of Table 2 of the Codefor buildings up to 3 storeys.Table E-3 Thickness* and Spacing of StiffeningWalls (Brick Size 23 X 11.5 X 7.7 cm)SI Thickness Height of Stiffening WallNo. of Load Storey/ABearing Minimum Maxim”2Wall to Thickness Spacingbe Stiffened(1) (2)(cm)I. II.52. 233. 34.5 andabove(3) (4) (5)(m) (cm) (m)3.25 11.5 4.503.25 Il.5 6.005.00 I I.5 Cross walls in conjunction with floors in abuilding provide stability to the structureagainst the effect of lateral loads that is,wind, etc. In case of halls, we have onlyend walls and there are no intermediatecross walls. If hall is longer than 8.0 m, theend walls may not be able to provideadequate stability (depending upon theextent of lateral loads) and therefore, it isnecessary to check stability and stresses bystructural analysis.ii) If roofing over a hall consists of RCCbeams and slab, it will be able to functionas a horizontal girder for transmitting thelateral loads to the end walls. The longwalls will therefore function as proppedcantilevers, and should be designedaccordingly, providing diaphragm walls, iffound necessary. Use of diaphragm wallshas been ex lained in E-5.5.3. Also endwalls will I!e subjected to shear andbending and should be designed forpermissible shear and no-tension. It isnecessary that RCC slab of the roofingsystem must bear on the end walls so thatlateral load is transmitted to these wallsthrough shear resistance. Method ofstructural analysis of a hall is illustrated inSolved Example E-l When a hall or a factory type building isprovided with trussed roofing thelongitudinal walls cannot be deemed to belaterally supported at the top unless trussesare braced at the tie beam level as shown inFig. E-l 1. With braced trusses as lateralsupports, longitudinal walls will functionas propped, cantilevers and should bedesigned accordingly. Even when designedas propped cantilever, ordinary solid wallsmay have. to be fairly thicker and thereforeHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 9
  23. 23. SP 20(5&T) : 1991PURLINSANCHOREDGABLE WALTIE BEAMS OF ROOFTRUSSES FIXED TODIAGONAL BRACINGOF TIE BEAMS OF TRSO THAT ROOF ACTSHORIZONTAL GIRDERTRANSMITS WIND FORTO GABLE WALLSFIG. E-l I DIAGONAL BRACING OF TRUSSESbecome uneconomical. In that situationuse of diaphragm walls may be resorted tosince that can result in considerableeconomy.When bricks of size 23 X 11.5 X 7.7 cm(FPS) are used, Table E-4 may be used inplace of Table 3 of the Code.Table E-4 Minimum Thickness of BasementWalls (Brick Size 23 X 11.5 X 7.7 cm)SI Minimum Height of the Ground AboveNO. Thickness of Basement Floor with WallBasement Wall Loading (Permanent Load) of‘More than Less tha;;M kN/m 50 kN/m(1) (2) (3) (4)(cm) (m) (m)I. 34.5 2.50 2.002. 23 I.35 I .ooNOTE ~- Permanent load means only dead load and it doesnor include live load. )(7)10A free standing wall has no cross walls togive it stability against overturning due tolateral loads that is, wind or seismic loads. Itthus acts like a cantilever fixed at the baseand free at the top. For design of freestanding walls please see comments onE- and E- a wall is intended to retain some drymaterial and there is no likelihood of anyhydrostatic pressure, the design of wallcould be based on permissible tension inmasonry. A retaining wall intended tosupport earth should be designed as agravity structure, placing no reliance onflexural movement of resistance, since watercan get access to the back of the wall andimpose pressure through tensile cracks if anyand endanger the structure.4.3 Effective Height4.3.1 Wall (Table 4-Note I)Referring to Note I of Table 4, strictly speakingactual height of a wall for the purpose of workingout its effective height should be taken to be theclear distance between the supports. However, inthe Code it has been given as the height betweencentres of supports, which is in accordance withthe provisions of British Standard CP-I I I : Part2 : 19702 as well as Australian Standard 1640-19748. Since thickness of floors is generally verysmall as compared to height of floors, this methodof reckoning actual height will not make anyappreciable difference in the end results. Onecould, therefore, take actual height as given in theCode or clear distance between supports as maybe found convenient to use in calculations.Wall (Table 4-Note 5)Implication of this note is that when wailthickness is not less than 2/3 of the thickness ofthe pier, a concentrated load on the pier, will beborne by the pier as well as the wall. In this casewe may design the element just as a wallsupporting a concentrated load, taking advantageof the increase in the supporting area due to thepier projection. In case thickness of wall is lessthan 2/3 of the thickness of pier, we have todesign the pier just like a column, for whichpermissible stress is less because of greatereffective height and further supporting area willbc only that of the pier that is, without gettingany benefit in design of the adjoining walls oneither side. However in case, the wall and piersarc supporting a distributed load, we would getthe advantage of stiffening effect of peirs asin 4.5.2 of the Code.4.3.2 Column- In case of columns actual heightahould be taken as the clear height of a columnbetween supftorts as illustrated in Fig. E- Opening in Wallsi) An RCC slab bearing on a wall is assumedto provide full restraint to the wall while aActual Height H = Clear distance betweensupports.FIG.E-12 ACTUAL HEIGHT OF A COLUMNHANDBOOK ON MASONRY DESlGN AND CONSTRUCTION-PART 1
  24. 24. SP 20(S&T) : 1991calculation is taken as 19 cm, though nominalthickness is 20 cm. Similarly in case of brickmasonry with bricks of old size (FPS System)actual thickness of one-b&k wall would be takenas 22 cm though nominal size of brick is 23 cm.ii)timber floor comprising timber joints andplanking is assumed to provide only partialrestraint. The clause makes stipulations forreckoning effective height of columnsformed by openings in a wall for the twocases:a) when wall has full restraint at top andbottom; andb) when wall has partial restraint at topand bottom. These two cases areillustrated in Fig. E-13.In the case of (b) (see Fig. E-13), if heightof neither opening exceeds 0.5H, wallmasonry would provide some support tothe column formed by openings in thedirection parallel to the wall and for thisreason effective height for the axisperpendicular toethe wall is taken as Handotherwise it is to be taken as 2H. For thedirection perpendicular to the wall, there isa likelihood of a situation when no joistrests on the column formed between theopenings and thus effective height is takenas 2H that is, for a column having notatera support at the top.4.4 Effective LengthWhen a wall has more than-one opening such thatthere is no opening within a distance of H/8froma cross wall and wall length between openings arenot columns by definition, the design of the wallshould be based on the value of SR obtained fromthe consideration of height or length, whichever isless.4.5 Effective Thickness4.5.1 In case of masonry using modular bricks,actual thickness of a one-brick wall for design,.- COLUMN FORMED BY OPENINGSrRCC SLABEffective Height, h,, = 0.75H + 0.25H1h,, = HE-13A Walls Having Full RestraintFIG. E-13 EFFECTIVE HEIGHT4.5.2 (See also comments on Note 5 of Table 4.)When ratio tP/ t, is 1.5 or less and the wall ishaving distributed load, Note 5 of Table 4 wouldbe applicable. It follows from this thatinterpolation of values in Table 6 are valid onlywhen t,/ t, exceeds has been observed from tests that acavity wail is 30 percent weaker than asolid wall of the same thicknes’s as thecombined thickness of two leaves of thecavity wall, because bonding action of tiescannot be as good as that of normal bondin a solid wall. That explains why effectivethickness of a cavity wall is taken as two-thirds of the sum of the act,ual thickness oftwo leaves.In this type of wall either one leaf (inner)or both leaves could be load bearing. Inthe former case, effective thickness will betwo-thirds the sum of the two leaves or theactual thickness of the loaded leafwhichever is more. In the latter caseeffective thickness will be two-thirds of thesum of thickness of both the leaves, or theactual thickness of the stronger leaf,whichever is more.4.6 Slenderness Ratioi) Under a vertical load a wall would buckleeither around a horizontal axis parallel toFor H, d 0.5 Hh,. = H4,,=2 HFor H, > 0.5 Hh., = 2 Hh,:2 HE-13B Walls Having Partial RestraintOF WALLS WITH OPENINGSHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 II
  25. 25. SP 20(S&T) : 1991the length of the wall or around a verticalaxis as illustrated in Fig. E-14. Buckling isresisted by horizontal supports such asfloors and roofs, as well as by verticalsupports such as cross walls, piers andbuttresses. Thus capacity of the walls totake vertical loads depends both onhorizontal supports that is, floor or roof aswell as on vertical supports that is, crosswalls, piers and buttresses. However, forthe sake of simplicity and erring on safeside, lesser of the two slenderness ratios,namely, one derived from height and theother derived from length is taken intoconsideration for determining permissiblestresses in .masonry walls, thus ignoringstrengthening effect of other supports.SECTIONPLANE-14A Section of a Wall E-14B Plan View of a Wallwith Tendency to Buckle with Tendency to BuckleAround Horizontal Axis Around Vertical Axis UnderUnder Vertical Load Vertical LoadFIG. E-14 BUCKLING OF WALLSii) In case of columns, there will be two valuesof SR as illustrated in Fig. 8 of the Code.For the purpose of design, higher of thetwo values is taken into account sincecolumn will buckle around that axis withreference to which the value of SR iscritical, that is, greater.iii)I2Load carrying capacity of a masonrymember depends upon its slendernessratio. As this ratio increases, cripplingstress of the member gets reduced becauseof limitations of workmanship and elasticinstability. A masonry member may fail,either due to excessive stress or due tobuckling (see Fig. E-22). According toSahlin (p. 1003)1, for materials of normalstrength with SR less than 30, the loadcarrying capacity of a member at ultimateload is limited by stress, while for highervalue of SR failure is initiated by buckling.Further, mode of failure of a very shortmember having h/r ratio of less than 4 isiv)v)vi)predominantly through shear action, whilewith h/r = 4 or more, failure is by verticaltensile splitting. From consideration ofstructural soundness and economy ofdesign, most codes control the maximumslenderness ratio of walls and columns soas to ensure failure by excessive stressrather than buckling.Limiting values of SR are less for masonrybuilt in lime mortar, as compared to thatbuilt in cement mortar, because the former,being relatively weaker, is more liable tobuckling. Similarly,, values of maximumSR are less for taller buildings sinceimperfections in workmanship in regard toverticality are likely to be morepronounced in case of taller buildings.Limiting values of SR for column is lessthan that of walls because a column canbuckle around either of the two horizontalaxes, while walls can buckle aroundhorizontal axis only.Since slenderness of a masonry elementincreases its tendency to buckle,permissible compressive stress of anelement is related to its slenderness ratioand is determined by applying Stressreduction factor (ks) as given in Table 9 ofthe Coda Values of Stress reduction factorhave been worked out (see Appendix B ofBS 56289) by taking into considerationaccentricity in loading because ofslenderness. Strictly speaking full value ofstress reduction factor is applicable onlyfor central one-fifth height of the member.In practice however for the sake ofsimplicity in design calculations, stressreduction factor is applied to the masonrythroughout its storey height (Note 3 underTable 9 of the Code is an exception) andfor designing masonry for a particularstorey height, generally stress is workedout at the section just above the bottomsupport assuming it to be maximum at thatsection. Theoretically critical section in .astorey occurs at a height 0.6 H above thebottom support as explained later in E-4.7.Thus provisions of the Code and the designprocedure in question, as commonlyfollowed, is an approximation, that errs onthe safe side.Advantage of Note 3 under Table 9 of theCode is taken when considering bearingstress under a concentrated load from abeam. Bearing stress is worked outimmediately below the beam and thisshould not exceed the Basic compressivestress of masonry (see Table 8 of theCode). Also stress in masonry is workedout at a depth of $ from the bottom of thebeam. This should not exceed thepermissible compressive stress in masonry.HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION--PART 1
  26. 26. vii)If actual stress exceeds allowable stress ineither case, a concrete bed block isprovided below the beam (see Solvedexample E-9 for design of a Bed block).In accordance with of the Code,some increase in permissible compressivestress is allowed for concentrated loadswhich are concentric. For Checking bearingstress under such a load, however, someauthorities on masonry recommend aconservative approach-that is, either totake advantage of Note 3 of Table 9 of theCode or to take advantage of provisions of5.4.1.5 of the Code but do not apply boththe provisions of the code at the same time.In this connection reference may be madeto commentary portion 4.13.6 of theAustralian Standard 1640-1974s which isappended to that standard.4.7 Eccentricity9ii)Eccentricity of vertical loading on amasonry element increases its tendency tobuckling and reduces its load carryingcapacity; its effect is thus similar to that ofslenderness of the member. Thus combinedeffect of slenderness and eccentricity istaken into consideration in designcalculations by the factor known as Stressreduction factor (ks) as given in Table 9 ofthe Code.Eccentricity caused by an ectientric verticalload is maximum at the top of a member,that is, at the point of loading and it isassumed to reduce linearly to zero at thebottom of the member that is, just abovethe bottom lateral support, whileeccentricity on account of slenderness of amember is zero at the two supports and ismaximum at the middle. Taking thecombined effect of eccenrricity of loadingand slenderness critical stress in masonryoccurs at a section 0.6H above the bottomsupport as shown in Fig. E-15.For the sake of simplicity, however, indesign calculations, it is assumed thatMUMTRICITYeX ea ete, = eccentricity due to loading.e. = eccentricity due to slenderness.e, = combined eccentricity which is maximumat 0.6 H from bottom support.FIG. E-15 ECCENTRICITYOF LOADING ON A WALLSP 20(S&T) : 1991critical section in a storey height is at thetop of bottom support and masonry isdesigned accordingly. In other words thedesign method commonly adopted includesextra self weight of 0.6H of the memberand thus errs on the safe side to someextent. In view of the fact that designcalculations for masonry are not veryprecise, the above approximation isjustified.5 STRUCTURAL DESIGN5.1 Generali) Some general guidance on the designconcept of load bearing masonry structuresis given in the following paragraphs.ii) A building is basically subjected to twotypes of loads, namely:a) vertical loads on account of dead loadsof materials used in construction, pluslive loads due to occupancy; andb) lateral loads due to wind and seismicforces. While all walls in general cantake vertical loads, ability of a wall totake lateral loads depends on itsdisposition in relation to the direction oflateral load. This could be bestexplained with the help of anillustration.In Fig. E-16, the wall A has goodresistance against a lateral load, whilewall B offers very little resistance tosuch load. The lateral loads acting onthe face of a building are transmittedthrough floors (which act as horizontalbeams) to cross walls which act ashorizontal beams) to cross walls whichact as shear walls. From cross walls,loads are transmitted to the foundation.This action is illustrated in Fig. E-17.Stress pattern in cross walls due tolateral loads is illustrated in Fig. E-18.WALL UNDERResistance of brick wall to take lateral loadsis greater in case of wall A than that in case ofwall E.FIG.E-16 ABILITY OF A WALL TO TAKE LATERALLOADSHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 13
  27. 27. SP 20(S&T) : 1991BASE OF WALL ‘3’Wind load on the facade wall I is transferred viafloor slabs 2 to the cross walls 3 and thence toground.The strength and stiffness of 2 that is floors ashorizontal girder is vital; floors of lightweightconstruction should be used with care.FIG. E-17 FUNCTION OF LATERAL SUPPORT ToWALLiii) As a result of lateral load, in the crosswalls there will be an increase ofcompressive stress on the leeward side, and14decrease of compressive stress on the wind-ward side. These walls should be designedfor ‘no tension’ and permissiblecompressive stress. It will be of interest tonote that a wall which is carrying-greatervertical loads, will be in a better position toresist lateral loads than the one which islightly loaded in the vertical direction. Thispoint should be kept in view whileplanning the structure so as to achieveeconomy in structural design.iv) A structure should have adequate stabilityin the direction of both the principal axes.The so called ‘cross wall’ construction maynot have much lateral resistance in thelongitudinal direction. In multi-storeyedbuildings, it is desirable to adopt ‘cellular’or ‘box type’ construction fromconsideration of stability and economy asillustrated in Fig. E-19.WIND LOAD ON SHADEDAREA IS RESISTED BYTHE CROSS WALLSTRESSDIAGRAMDEAD LOADWIND LOADCOMBINEDFIG.E-18 STRESS PATTERN IN CROSS WALL ACTING AS SHEAR WALLHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART I
  28. 28. SP 20(5&T) : 1991UNSlSTABLEPLANE-19A CROSS WALL CONSTRUCTION-UNSTABLEIN LONGITUDINAL DIRECTIONSTABLESTABLE-PLANE-19B CELLULAR OR BOX TYPE CONSTRUCTIONSTABLE IN BOTH DIRECTIONSFIG. E-19 STABILITY OF CROSS WALL ANDCELLULAR (Box TYPE) CONSTRUCTIONv)vi)vii)Size, shape and location of openings in theexternal walls have considerable influenceon stability and magnitude of stresses dueto lateral loads. This has been illustrated inFig. E-20.If openings in longitudinal walls are solocated that portions of these walls act asflanges to cross walls, the strength of thecross walls get considerably increased andstructure becomes much more stable, aswill be seen from Fig. E-21.Ordinarily a load-bearing masonrystructure is designed for permissiblecompressive and shear stresses (with notension) as a vertical cantilever by acceptedprinciples of engineering mechanics. Nomoment transfer is allowed for, at floor towall connections and lateral forces areassumed to be resisted by diaphragmaction of floor, roof slabs, which acting ashorizontal beams, transmit lateral forces tocross walls in proportion to their relativestiffness (moment of inertia). Variousmodes of failure of masonry are illustratedin Fig. E-22.This wall will not resist lateral loading assuccessfully as wall 2; it tends to act as threeseparate short lengths rather thanThis wall will tend to act as one long portion ofbrickwork and will be more resistant to lateralloading.FIG. E-20 EFFECT OF OPENINGS ON SHEARSTRENGTH OF WALLSviii)ix)For working out stresses in various walls,it is faster to tabulate stresses floor-wise forsuch walls .carrying greater loads.Computations for vertical loads and lateralloads are made separately in the firstinstance, and the results from the twocomputations are superimposed to arriveat the net value of stresses.In any particular floor, from practicalconsiderations, generally, quality of bricksand mix of mortar IS kept the samethroughout. Also in the vertical directionchange in thickness of walls is made onlyat floor levels.5.3 Load Dispersion53.1 GenerulL Pre-1980 version of the CodePI ovided for dispersion of axial loads applied to aHANDBOOK ON MASONRY DESIGN AND CONSTRtICTION-PART I 1s
  30. 30. -.E-22C SHEAR FAILURE OF A MASONRYCROSS WALL UNDER LATERAL LOADINGEXCESSIVE COMPRESSWESrRESS AT TOE ANDMASONR CONSEPUENT TEt4Sll.EEXCESSI SPUTTINO AND CRUSMNOTENSlON OF MASONRYE-22D EXCESSIVE COMPRESSIVE STRESS IN CROSSWALLS RESULTING IN CRUSHNG OF MASONRYAT THE TOE UNDER LATERAL LOADINGFIG. E-22 VARIOUS MODES OF FAILURE OFMAXINRYmasonry wall at an angle of 4Y to the vertical,distributed uniformaly through a triangularsection of the wall. This was based on provisionsof B. 5. CP-Ill : Part 2 : 19702. According toBrick Institute of America4, though distributionof stress through an angle of 45” is borne out byANGLE OF DISPERSALL50Ww =Tixz5*SP 2O(S&T) : 1991experimental studies carried out at the Universityof Edinburgh, as: umption regarding evendistribution of stress does not seem to have beenfully substantiate. The Institute thereforerecommended that angle of distribution ofconcentrated loads in a masonry wall should notexceed 30 degrees. This recommendation is inconformity with provisions of the correspondingGerman Standard (DIN 1053-1952) SwissStandard (Technical Standard 113-1965) and thepublication: ‘Brick and Tile Engineering 1962 byHarry C. Plummer’. In view of the above, angle ofdispersion had been changed from 45” to 30” in1980 version of the Code (see Fig. E-23).53.2 Arching Actioni) Arching in masonry is a well knownphenomenon by which part of the loadover an opening in the wall gets transferredto the sides of the opening. For goodarching action masonry units should havegood shear strength and these should belaid in proper masonry bond using a goodquality mortar. Further, portions of thewall on both sides of the opening should belong enough [see E-533(i)] to serve aseffective abutments for the arched masonryabove the opening since horizontal thrustfor the arch is to be provided by the shearresistance of the masonry at the springinglevel on both sides of the opening. If anopening is too close to the end of a wall,shear stress in masonry at springing levelof imaginary arch may be excessive andthus no advantage can be taken of archingin masonry for design of lintels.ii) To explain the effect of arching on designof lintels and stress in masonry, let usANGLE OF DISPERSAL 30”Ww=po2h tan30W - Concentrated toadw- Distributed load after dispersal at depth h fromplane of application of concentrated loadFIG. E-23 DISPERSAL OF CONCENTRATED LOAD IN MASONRYHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1
  31. 31. SP 20(S&T) : 1991consider a wall of length AB with anopening of effective span PQ = L as shownin Fig. E-24 PRQ is an equilateral trianglewith PQ as its base.Because of arching action, loads of floorand masonry above the equilateral triangleget transferred to the sides of the wall.Therefore lintel at PQ is designed for loadof masonry contained in the triangle PRQ.To work out approximate stress inmasonry in various stretches, it is assumedthat:a) load from the lintel gets uniformlydistributed over the supports,b) masonry and floor loads above thetriangle PRQ get uniformly distributedover the stretches of masonry CD andEF at the soffit level of the lintel, CDand EF being limited in length to L/2and over the stretches GH and JK at thefloor level, limited in length to L orL-H2whichever is less, H being theheight of top of the opening from thefloor level.In case some other opening occursbetween the lintel and horizontal plane 25cm above the apex R of the triangle,iii)arching action gets interrupted because ofinadequate depth of masonry above thetriangle to function as an effective archingring. Also if there is some other loadbetween the lintel and horizontal plane 25cm above the apex R of the triangle,loading on the lintel gets affected.in case of buildings of conventional designwith openings of moderate size which arereasonably concentric, some authorities onmasonry recommend a simplified approachfor design. In simplified approach, stress inmasonry at plinth level is assumed to beuniformly distributed in different stretchesof masonry, taking loadings in each stretchas indicated in Fig. E-25 without makingany deduction in weight of masonry for theopenings. It is assumed that the extrastresses obtained in masonry by making nodeduction for openings, compensates moreor less for concentrations of stresses due toopenings. This approach is _ of specialsignificance in the design of multistoreyedload-bearing structure where interveningfloor slabs tend to disperse the upperstorey loads more or less uniformly on theinter-opening spaces below the slabs andthus at plinth level stress in masonry, asworked out by the above approach isexpected to be reasonably accurate.EXTRA FLOOR LOAD ON STRETCH CD AND GHMASONRY LOADx = L OR F, whichever is less.FIG. E-24 ARCHING ACTION IN MASONRYHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION--PART I
  32. 32. NOTE - Loads on Sections A to E of the buildingare considered to be acting on wall lengths a to erespectively.FIG. E-255.3.3 Lintelsi) Lintels over openings are designed takinginto consideration arching action inmasonry where feasible as explainedearlier. It is a common practice to assumethat length of walls on both sides of anopening should be at least half the effectivespan of the opening for transfer of load tosides by arch action. In case it is less, lintelshould be designed for full load over theopening regardless of the height of thefloor slab as shown in Fig. E-26A.FLOOR LOAD ON LINTELMASONRY LOAD ON LINTELI I r] FLOORE-26A Effective Load whenLl<$ii)SF 20(S&T) : 1991When location and size of opening is suchthat arching action can take place, lintel isdesigned for the load of masonry includedin the equilateral triangle over the lintel asshown in ,Fig. E-26B. In case floor or roofslab falls within a part of the triangle inquestion or the triangle is within theinfluence of a concentrated load or someother opening occurs within a part of thetriangle, loading on the lintel will getmodified as given in (iii), (iv) and (v).MASONRY LOADON LINTELFLOORE-26B Effective Load when LI andL2 >, L/2 and Floor/ Roof Slab does notintercept the Equilateral friangle Overthe Linteliii)iv)v)vi)When stretches of wall on sides are equalto or greater than L/2 and equilateraltriangle above the lintel is intercepted bythe floor/roof slab, the lintel is designedfor load of masonry contained in theequilateral triangle plus load from the floorfalling within the triangle as shown in Fig.E-26C.When stretches of wall on the sides of theopening are equal to or greater than L/I 2with the equilateral triangle over the lintelintercepted by floor slab and anotheropening comes within the horizontal plane2.5 cm above the apex of the triangle, lintelis to be designed for loads shown in Fig.E-26 D.When any other load is coming betweenthe lintel and horizontal plane 25 cm abovethe apex of the equilateral triangle over thelintel, the latter is designed for the loads asshown in Fig. E-26E.It may be clarified that in fact load comingon a lintel is indeterminate and the abovesuggestions for the design of lintels arebased on empirical rules derived fromHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 19
  33. 33. SP 20(S&T) : 1991Ll ORL2 +E-26C Effective Load when LI and LZ 2L/2, and Equilateral Triangle Over theLintel is Intercepted by Floor Slab Abovewith no Other Opening to Intercept Archvii) Economy in the design of lintels may beeffected by taking advantage of compositeaction between lintel and the masonryabove it. For this purpose centering of thelintel should not be removed till bothmasonry (up to 25 cm above the apex ofequilateral triangle above the lintel) andRCC of the lintel have gained sufficientstrength so as to be able to bear stresses inthe composite beam having masonry incompressive zone and RCC lintel in thetensile zone. Behaviour of composite beamin this case is anologous to that of gradebeam in pile foundation.From experimental research, it has beenobserved that single brickwidth walls forvertical loads are stronger than multiplebrick width wails as can be readily seenfrom the test results reproduced below(Swiss results quoted by Mark’“):(1) (2) (3)Multiple 25.4-38.1 0.68brick-width (10-15)Theoretical explanation for the abovebehaviour of masonry is that presence ofvertical joints, which have a much lowerlateral tensile strength, reduces thecompressive stress of masonry under axialloading. Thus greater is the frequency ofvertical joints, lesser is the complessivestrength of masonry. Thus a 20123 cmthick brick wall (one brick-length) isweaker than a IO/ Il.5 cm brick wall ofsingle brick-width because of presence ofvertical joints in both the directions in theformer. Table 8 of the Code for BasicCompressive Strength of Masonry, whichis based on British Standard*, may bepresumed to hold good for one brick-length or thicker walls and thus in case ofhalf-brick load bearing walls some increasein Basic stress may be permitted at thediscretion of the designer.For similar reasons, concrete blockworkmasonry which has proportionately lesservertical joints is stronger than brickworkmasonry‘J, though the Code at present doesnot make any stipulation about it. In othercountries, for high rise load-bearingstructure advantage of this phenomenon istaken by making use of ‘through-wall units’of burnt clay, thereby attaining higherpermissible strength for brick masonry,and making it feasible to go high withsingle unit thick walls.5.4 Permissible Stresses5.4.1 Permissible Compressive Stress5.4.1 .l Stress reduction .factorWhen a wall or column is subjected to an axialplus an eccentric load (see Fig. E-27) resultanteccentricity of loading (??) may be worked out asfollows:w= w, + w2Taking moments about AB,Wz= W, X 0+ W2eWallConstruction(I)Singlebrick-width-do--do-Wall RelativeThiikness Strengthcm (in)(2) (3)12.7 (5) 1.oo15.2 (6) 0.8917.8-25.4 0.80(7-10).‘. = _-_!%2_WI + w25.4.1.2 Area reduction jhctori) Provision of Area reduction factor in-thisCode was originally similar to that in 1970version of British Standard Code CP 1112.When the Code was revised in 1980;upperlimit of ‘small area’ was reduced from 0.3to 0.2 m* based on the provision in BS5628 Part 1 : 1978’.20 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1Actionexperrence and general principles ofengineering.
  35. 35. SP 20(S&T) : 1991eF-i5.4.1.4 Increase in permissible compressivestresses allowed for eccentric vertical and/ore lateral loads under certain conditions9 Eccentric vertical load (vertical load pluslateral load in case of free standing walls)on masonry causes bending stress inA B addition to axial stress. It has been found-- ---_ ------ that masonry can take 25 percent greatercompressive stress, when it is due tobending than when it is due to pure axialload, because maximum stress in case ofbending occurs at the extreme fibres andthen it gets reduced linearly while in axialcompression, stress is more or less uniformthroughout the section. For similar reasonspermissible compressive stress in concretefor beams is greater than that in columnsI 1 Isubjected to vertical loads. This rule ofL---t----ihigher Permissible compressive stress whendue to bending can also be explained fromthe consideration that beyond elastic limitWI = axial load.redistribution of stresses takes placeW2 = eccentric load at distance ‘8 from centre line.because of plasticity and thus stress blockW = resultant load at distance ‘d’ from centre line.is in practice more or less rectangular inT = resultant eccentricity.shape instead of triangular as is normallyassumed in accordance with the elastic‘FIG. E-27 RESULTANT ECCENTRICITY theory. This enables the member to takegreater load.ii) Area reduction factor due to ‘small area’ ofa member is based on the concept thatthere is statistically greater probability offailure of a small section due to sub-standard units as compared to a .largeelement. However American and theCanadian Codes do not include anyprovision for smallness of area. The reasonfor this seems to be that factor ofsafety/load factors inherent in a Codeshould be enough to cover the contingencymentioned above for this provision. In theAustralian Code (1974)8 and draft 1SOstandard (l987)ii limits for smallness ofarea in this context are taken 0.13 and 0.10m2, respectively. Strictly sfor this provision in the 8eaking necessityode arises whenthere is appreciable variation in strength ofindividual units. In view of the fact thatstrength of masonry units beingmanufactured at present in our countrycan appreciably vary, the necessity for thisprovision is justified in our code.ii) When loading on a masonry element hassome eccentricity, the Code lays down thedesign approach for various ranges ofeccentricity ratios namely (a) eccentricity1ratio of - or less; (b) eccentricity ratio241 Iexceeding - but not exceeding 6 , and (c)24Basis of theeccentricity ratio exceeding kdesign approach is explained below (seea/so Fig. E-28).1a) Eccentricity’ ratio of - or less-24Refering to Fig. E-28B, I#’ is totalvertical load per unit of wall withresultant eccentricity r, t is thickness ofwall,fi and fiare the stresses at the twofaces of the wall. and f;n is Permissiblecompressive stress for axial loading. Shape reduction factorShape modification factor is based on the generalprinciple that lesser the number of horizontaljoints in masonry, greater its strength or loadcarrying capacity. It has, however, been foundfrom experimental studies that for units strongerthan 15 N/ mm2, extent of joints in masonry doesnot have any significant effect on strength ofmasonry because of use of the comparatively highstrength mortar that normally goes with high-strength units..f;=f+”Zf+”ZSubstituting values of A, M and Z22 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1
  36. 36. SP 20(S&T) : 1991W-L -LfC fcT -rc=oW=S,tFIG. E-28Ae= t/24j-1 = 1.25fcf*= 0.75 j-cw=ji tFIG. E-28Bt/24 < e’< t/6f, = 1.25 fct te =- e’= -6 6t6= 1.25 fcw 1.25Xt=-I, 2STRESSFIG. E-28Dfc = 1.25 fc2wz--c-r3 2-( 1zW=2FIG. E-28EFIG. E-28CW= permissible load per unit length of wall.fc = permissible compressive stress of masonry.P = resultant eccentricity of loading,I = thickness of wall.FIG. E-28 VARIATION IN STRESS DISTRIBUTIO.NECCENTRICITY OF LOADINGHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART IWITH CHANGE IN23
  37. 37. SP 20(S&T) : 1991b)for eccentricity ratio 4 = 1since JZ is equal to axiil cc?ipresIiIItstres J;,As we allow 25 percent additionalcompressive stress in case of eccentricloading, it follows that maximumcompressive stress 6) for eccentricityratio up to & does not exceed axialcompressive stress by more than 25percent which is permitted by the code.Therefore for eccentricity ratio of $ orless, it is not necessary to compute andadd bending stress to the axial stress.The designer is expected to work outonly axial compressive stress for thepurpose of design and see that it doesnot exceed Permissible compressivestress for axial load.’ Design load, W =A t..Eccentricity ratio exceeding & but notexceeding i (see Fig. E-28C and E-28D)We X 6Bending stress = t?;for eccentricity ratio l.(substituting inthe above equations), 6f;JY+W-2W _-t t tp$_!LotThus on one face compressive stress getsdoubled and on the other face it is fullynullified by tensile stress and there is notension in the cross section. For loadingwith eccentricity ratio between.--!- and !-24 6’we have to limit the maximum stressfito 1.25 fcj;=F(l+y)=l.25f.. Design Load,..iii)cl Eccentricity ratio exceeding $ (see Fig.E-28E)-We had seen from (b) abovethat when eccentricity ratio reaches theIvalue.-, stress is zero on one face; when6-1this ratio exceeds - there will be tension6on one face rendering ineffective a partof the section of the masonry and stressdistribution in this case would thus beas shown in Fig. E-28E. Averagecompressive stress:f =UO_fl---a2 2Since fi has to be limited to 1.25 fcfi; =1.25 h2The design load Win this case will beequal to average compressive stressmultiplied by length ab of the stresstriangle abc. Since for equlibrium, theload must pass through the centroid ofthe stress triangle abc and the load is atta distance of - - 7from the compressive2face, we getab t---=--3 2TThus design load, W = average stressX ab= 1.25 xfc2x 3 (t - qFrom the above equation we can seethat theoretically design load W is zerowhen Z= t/2. However from practicalconsiderations E should be limited tot/3.In Appendix C of the Code, use ofconcrete bed block has been suggested in3.2 and 3.3. It seems necessary to add thatin case some tension is likely to develop inmasonry beta use of eccentricity of.concentrated loads, the bed blocks shouldbe suitably reinforced and these should belong enough so as to prevent tensile cracksin masonry due to eccentricity of loading.5.1.2 Permissible Tensile StressIn accordance with Note 2 of the clause tensilestress up to 0.1 N/mm2 and 0.07 N/m@ in themasonry of boundry/compound walls ispermitted when mortar used ‘in masonry is of M 1and M2 grade respectively or better. Thisrelaxation has been made to effect economy in the24 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1
  38. 38. design of the boundry/compound walls sincethere is not much risk to life and property in theevent of failure of such walls.5.4.3 Permissible Shear StressIn 1969 version of the Code, provision forPermissible value of ‘shear stress (based on B.S.CP 111 : Part 2 : 19702) was 0.15 N/mm* (1.5kg/cm?) for walls built in mortar not leaner thanI : I : 6 cement : lime : saad mortar. In the 1981version of the Handbook it had been brought outthat experimental research on the subject hadproved that when masonry is preloaded, that is,when it ids having vertical load, it is capable ofr&sting greater amount of shear force. AustralianCode AS : 1640-19748 had also reflected thisalready. Based on that in 1980 version ofIS 1905, value of Permissible shear stress wassuitably modified and was related to amount ofpreloading, subject t6 a maximum of 0.5 N/mm*and minimum of 0.1 N/mm?.5.4.4 If there is tension in any part of a section ofmasonry, that part is likely to be cracked and thuscanndt be depended upon for resisting any shearforce. The clause is based on this consideration.This situation is likely to occur in masonryelements subjected to bending.5.5 Design Thickness/Cross Section5.5.1 Walls and Columns Subjected to VerticalLoad5.5.1.1 Solid wallsBrick work is generally finished by either pointingor plastering and with that in view, it is necessaryto rake the joints while the mortar is green, incase of plaster work raking is intended to providekey for bending the plaster with the background.Strictly speaking thickness of masonry forpurposes of design in these cases is actualthickness less depth of raking. However in case ofdesign of masonry based on permissible tensilestress (as for example design of a free standingwall), if walls are plastered over (plaster of normalthickness i.e. 12 to 15 mm) with mortar of samegrade as used in masonry or M2 grade--whichever is stronger or are flush pointed withmortar of Ml grade or stronger, raking may beignored.5.5.2 Walls and Columns Mainly Subjected toLateral Loads5.5.2.1 Free standing wallsi) 1980 version of the Code provided fordesign of a free-standing wall as a gravitystructure that is, without placing relianceon the flexural moment of resistance of thewall due to tensile strength of masonry. Itwas seen that this approach to designresulted in fairly thick walls and maximumheight of an unplastered 23 cm thick wall(one-brick thick of conventional size) couldbe only about 0.86 m while it has been aii)iii)SP 20(S&T) : 1991common practice since long to build suchwalls to heights much greater than 0.86 m.It was further seen from Table 9 of 1980version of the Code (based on BSCP121 : Part 1 : 197312) that height tothickness ratio of free-standing walls givenin relation to certain wind speeds could notbe sustained unless flexural moment ofresistance of the wall is taken intoconsideration. From a study of practicesbeing followed in some other countries inthis regard, it is evident that, for design offree-standing walls, it is appropriate totake into consideration flexural moment ofresistance of masonry according to thegrade of mortar used for the masonry.Method of working out thickness of free-standing walls by taking advantage offlexural moment of resistance of the wallhas been given in Solved Example E-13. Itwould be seen that self-weight of a freestanding wall reduces tensile stress inmasonry caused by lateral load that is,wind pressure. Thus heavier the masonryunits, lesser is the design thickness of wallfor a particular height. It is, therefore,advantageous to build compound walls instone masonry in place of brick masonrywhen stone is readily available andthickness has to be greater than one brick.Also it should be kept in view that use oflight-weight units such as hollowbricks/ blocks in free-standing walls hasobvious structural disadvantage.As a general rule, a straight compoundwall of uniform thickness is noteconomical except for low heights or inareas of low wind pressure. Therefore,when either height is appreciable or windpressure is high, economy in the cost of thewall could be achieved by staggering, zig-zagging or by providing diaphragm walls.Instances of design of staggered anddiaphragm compound walls are given inSolved Examples E-14 and E-15. It can beseen that for wind pressure of 750 N/m*,maximum height of a 23 cm thick brickwall using grade Ml mortar can be 1.5 mfor a straight wall, 3.2 m, for a staggeredwall and 4.0 m for a diaphragm wall.5 .S.2.2 Retaining wallsThis clause is similar to of the Code andmethod of design of a retaining wall, based on thepermissible amount of tension in masonry, issimilar to that for a free standing wall.5.5.3 Walls and Columns Subjected to Vertical asI+‘t~llas Latera; Loadsi) Longitudinal walls of tall single storeywide span buildings with trussed roofs suchas industrial buildings, godowns, sportshalls, gymnasia, etc, which do not have anyHANDBOOK ON MASONRY DESIGN AND CONSTHIICTION-PART 1 25
  39. 39. SP 20(S&T) : 1991ii)intermediate cross walls other than gablewalls, tend to be very thick anduneconomical if designed as solid walls,since vertical load is not much and thelateral load due to wind predominates.This would be particularly so when thetrusses are not adequately braced at the tiebeam level so as to be able to act ashorizontal girders for transmitting thelateral loads to the gable walls. In this case,the walls act as simple cantilevers andflexural stres; at hti;et;+e will be quitehigh. trusses areadequately braced to provide girder actionand are suitably anchored to the gablewalls, longitudinal walls would function aspropped cantilevers, thus resulting inconsiderable reduction in bendingmoments on the long walls as shown inFig. E-29.In UK, masonry diaphragm walls havebeen adopted in wide-span tall, singlestorey buildings and these have provedvery economical and successful. Principleof a diaphragm wall is similar to that of arolled steel lrjoist that is, placing morematerial at places where stresses are more.As a result f ratio of a diaphragm wall ismuch higher than that of a solid wall,thereby resulting in economy.iii) A typical arrangement for laying bricks ina diaphragm wall is shown in Fig. E-30. Byvarying the depth and spacing of ribs interms of brick units, designer can obtain anarrangement that meets the requirement inany particular case. Placing of ribs isdecided on the consideration thatprojecting flange length on either side ofrib does not exceed 6 times the thickness ofthe flange. Thus rib-spacing is limited to 12tr + t, where tr and t, stand for flange andrib thickness respectively. Brick layout indiaphragm wall is planned such thatproper masonry bond is obtained with theleast number of cut bricks. Designersinterested in getting more detailedrTRUSS NOTBRACED=xX31 I PE-29A Trusses Not BracedFIG. E-29 EFFECT OF BRACING OFinformation regarding use of diaphragmwalls ma refer to ‘Brick Diaphragm Wallsin Tall mgle Storey Buildings’ by W. G.s”Curtin and G. Shawl35.5.4 Walls Subjected to In-Plane Bending andVertical Loads (Shear Walls)A cross wall which functions as a stiffening wallto an external load-bearing wall, is subjected toin-plane bending.’ If it is also supporting afloor/roof load, it is subjected to vertical load inaddition to In-plane bending. The designprocedure in this case is given in Example E-l 1. Itshould be kept in view ,that such a wall whensubjected to vertical load gets strengthened, sincevertical load reduces or nullifies tension due tobending and also increases the value ofpermissible shear stress (sre a/so commentson 5.4.3).5.5.5 Non-Load Bearing Walls9ii)Non-load bearing panel and curtain walls,if not designed on the basis .of guidelinesgiven in Appendix D of the Code, may beapportioned with the help of Table E-5which is extracted from RecommendedPractices for Engineered Brick Masonry4.The table is based on the assumption thatwall is simply supported only’ in onedirection either verticahy or horizontallywithout any opening or otherinterruptions. Where the wall is supportedin both directions, the allowable distancebetween lateral supports may be increasedsuch that the sum of the horizontal andvertical spans between supports does notexceed three times the permissible distancepermitted for supporting in the verticaldirection.Guidelines given in Appendix D of theCode are based on some research in whichmainly rectangular panels withoutopenings were tested. If openings are smallthat is, hole-in-wall type (see E4.1 Note),there would be no appreciable effect onstrength of panels, since timber or metalframes that are built into the openingscompensate to a great extent for the loss of,-TRUSSm BRACEDPROPl%Ki:E-29B Trusses BracedTRUSSED ROOFS ON BUILDINGS26 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART I
  40. 40. SP 20(S&T) : 1991FIG. E-30 TYPICAL BRICK LAYING ARRANGEMENT FOR DIAPHRAGMWALLSTable E-5 Span to Thickness Ratio of Non-Load Bearing Panel/Curtain WallsVertical Span Horizontal SpanDesign Wind ACement-Lime’ Gkment-limeAPressure Gement-Lime Cement-Limdkg/m* Mortar I : I : 6 Mortar I : ‘/2: 4% Mortar I : I : 6 Mortar I : %: 4%(1) (2) (3) (4) (5)25 38 43 54 6150 27 30 38 4315 22 25 31 35100 19 21 27 30125 17 19 24 27150 15 I7 22 25.NOTE- Partition walls which are not subjected to any wind pressure that is, internal partition walls may be apportionedwith the help of the above Table by assuming a minimum design wind pressure of 250 N/ml.strength of the panel due to the openings. In situations where design by formingHowever, when the openings are large or sub-panels is not feasible, panel may bewhen the openings cannot be categorised analysed using theory of flat plates (foras of ‘hole-in-wall’ type, it may often be example, yield line theory or finite elementpossible to design the panel by dividing it method) taking into consideration endinto sub-panels as shown in Fig. E-31. conditions as appropriate.HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 27
  41. 41. SP 20(S&T) : 1991A 0 C DI I I I1 !EP.N*III*E ____ _________I II IJ MK L=A I3 C DJ KK L L MORnIJ Ml2cYYl Denotes freeedge._ Denotes simply supported edge.Arrow, indicate span ring modes of subpmek.FIG. E-31 DESIGN OF PANEL HAVING ALARGE OPENING6 GENERAL REQUIREMENTS6.1 Methods of ConstructionInformation regarding constructional aspects ofmasonry based on IS Codes (relating to materialsof construction, Codes of practice, etc) is given’ina classified form in Part 2 of this Handbook forthe convenience of the designers, architects andbuilders.6.2 Minimum Thickness of Walls FromConsiderations Other than Structurali) Requirements for thickness of walls fromconsiderations other than strength andstability have been discussed below withregard to fire resistance, thermalii)iii)iv)insulation, sound insulation and resistanceto rain penetration.Resistance to Fire-The subject of fireresistance of buildings has been dealt withcomprehensively in appropriate IndianStandards’4 and also in Part IV of theNational Building Code of India 1983which may be referred to in this regard.Tlwrmal Insulation ~ Thickness of. walls incase of non-industrial buildings fromconsideration of thermal insulation shouldbe worked out for the climatic conditionsof the place where a building is to beconstructed on the basis of IS3792 : 197815. Even though no IndianStandard has yet been published on thesubject for industrial buildings, data andinformation given. in the above IndianStandard would be of some assistance indeciding the thickness of walls fromconsideration of thermal insulation.Sound Insulation qf .Value qf’ Walla)b)Indian Standard IS 1950 : 196216 laysdown sound insulation standards ofwalls for non-industrial buildings suchas dwellings, schools, hospitals andoffice buildings. Salient features of thatstandard are summarised below forready information.While deciding thickness/specificationsof walls, it is necessary to consider,firstly the level of ambient noise in thelocality where building is to beconstructed depending upon intensity oftraffic and type of occupancy of thebuilding. Noise level of traffic variesfrom 70 decibels (abbreviated as dB) forlight traffic to 90 dB for heavy traffic.Requirements of sound insulation fordifferent buildings from considerationof ambient noise level and occupancyare given in Table E-6. These values areapplicable to external walls for reducingout-door air-borne noise.Table E-6 Requirements of Sound InsulationValues (dB) of External Walls of BuildingsAgainst Air-borne Noise[Clause 6.2(iv) (b)]SI Type ofNo. BuildingFor Noisy For QuietLocations Locations(90 dB Level) (70 dB Level)(1) (2) (31 (4)I. Dwellings 45 252. Schools 45 253. Hospitals 50 304. Offices 40 2028 HANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART I
  42. 42. c) Sound insulation values of party andinternal wallls are decided onconsiderations of levels of indoor noiseemanating from adjacent buildings oradjacent rooms and these should be asgiven in Tab16 E-7.Table E-7 Sound Insulation Values for Partyand Internal WailssiNo.Situation SoundInsulationValuesdB(1) (2)I. Between living/ bed room in onehouse or flat and living/bedrooms in another(3)502. Elsewhere between houses orflats403. Between one room and another in 30the same house or flat4. Between teaching rooms in aschool405. ‘Betweenone room and anotherin office306. Between one ward and anotherin a hospital:NormalExtra quiet4045d) Sound insulation values of non-poroushomogeneous rigid constructions, suchas a well plastered brick/stone masonryor concrete wall, vary as the logarithmof weight per unit area and thus increasewith the thickness of wall. These valuesare given in Table E-8.Table E-8 Sound Insulation Values of SolidConstructionsWeight Per rnz of Sound InsulationWall Area Valuekg dBS 22.825 33.250 37.6100 42.0I50 44.1200 46.4250 47.9300 49.1350 50.0400 50.9450 51.6500 52.3600 53.6SP 20(54&T) : 1991e) Based on the data given in Table E-8,insulation values of brick walls plasteredon both sides work out as in Table E-9.Table E-9 Sound Insulation Values of MasonryWalls Plastered on Both SidesThickness of Wall (cm) dB7.7 45.1IO 41.311.5 48.020 51.323 52.2NOTE---Thickness of walls given above arenominal and exclusive of thickness of phaster.f) As a general guide, it may be taken thatfor noise insulation a one-brick wall (20or 23 cm thick/plastered on both sidesas external wall’and a ‘/2 brick wall (10or 1I .5 cm thick) plastered on both sidesas internal walls are adequate.v) Resistance to Rain Penetration -Recommendations for thickness of wallsof different types of masonry fromconsideration of resistance to rainpenetration based generally 01, 1s2212 : 196217 are given in Table E-10.6.3 WorkmanshipCommon defects of workmanship in masonry are:a)b)clImproper mixing of mortar;Excessive water cement ratio of mortar;Incorrect adjustment of suction rate ofbricks;d) Unduly thick bed joints;e) Uneven or furrowed bed joints;.f-1Voids in perpend joints; andg) Disturbance of bricks after laying.Improper mixing of mortar and excessive watercement ratio may reduce the strength of mortar tohalf, thereby affecting the strength of masonry.Suction rate of bricks has a very pronouncedeffect on the strength of brick-work and thereforeit should be controlled carefully. Water absorbedfrom mortar by bricks leaves cavities in themortar, which get filled with air and therebyreduce the strength of mortar:. Brick work builtwith. saturated bricks develop poor adherencebetween brick and mortar. Thus flexural strengthas well as shear strength of such brickwork wouldbe low. At the same time such a brickwork will beprone to excessive cracking due to high shrinkageand thus rain-resisting qualities of the brickworkwill be poor. British Ceramic Association havesuggested a suction rate of 2 kg/ mini m2, while inaccordance with Canadian Code7 and AmericanHANDBOOK ON MASONRY DESIGN AND CONSTRUCTION-PART 1 29