Chapter11 Masonry


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Chapter11 Masonry

  1. 1. 187 11 Masonry Structures INTRODUCTION Load-bearing construction in the affected area is mostly masonry, with some adobe. Masonry buildings in the area are not designed, but merely constructed based on traditional practices that may include some rules of thumb. Masonry construction constitutes over 95 percent of the building stock in the affected area, which by and large did not perform well. Over 1,200,000 masonry buildings either collapsed or were severely damaged. Masonry construction is present in both rural and urban areas. Masonry construction in each region has special characteristics due to the bias towards locally available material, limitations of construction skills, and constraints to construction activity. Past earthquakes have highlighted the inherent weaknesses of this type of construction, and the lessons from the 2001 Bhuj earthquake offer yet another vivid demonstration to the local populace of the vulnerability of their hand-made, unengineered dwellings. This chapter gives an account of the state of the practice of masonry construction in the Kachchh region and a review of its performance during the 2001 Bhuj earthquake. Relevant Indian Standards and other pertinent literature on masonry construction available in the country are also presented. GROWTH OF CONSTRUCTION IN THE KACHCHH REGION: HISTORICAL PERSPECTIVE The Kachchh region has two distinct eras of development. Early construction took place under royal families that lived in the region over the past five centuries. More recent and more significant construction in the region was driven by the India-Pakistan partition in 1947. On the recommendation of Mahatma Gandhi, the Government of India granted 6,070 hectares (15,000 acres) of land near the Port of Kandla for the purpose of developing a township and commissioned the Sindhu Resettlement Corporation (SRC) Limited in 1948 with the main aim of settling and rehabilitating the persons displaced from the Sindh province of West Pakistan, now known as Pakistan. In 1955, after a careful review, the government revised the land available to SRC to 2,600 acres; the Port of Kandla received an adjoining 4,320 acres. By 1958, SRC completed a major part of the construction in two locations, which are today’s cities of Adipur and Gandhidham. However, the growth of the region during 1950-1960 was less than expected. The slow increase in livelihood was due to marginal growth in commerce, trade, industry, and communications. The Anjar earthquake in 1956, whose epicenter was 55 km from Bhuj, and two wars with Pakistan in 1965 and 1971 also discouraged many settlers, and, in fact, there was a significant exodus during 1960-1970. However, the growth of the Kandla Port Trust, the construction of new highways during the 1970s, and the commissioning of the Broad Gauge Railway line in the 1980s, led to an influx of people into the region. Though relatively little
  2. 2. Masonry Structures 188 construction took place during 1960-1995, recent trends show an upsurge in new construction. One interesting feature of this construction is that the use of lime mortar became popular in masonry construction due to the severe shortage of cement experienced countrywide during the early 1980s. Over the past 50 years, the main organizations that have led construction activity in the Kachchh region were the SRC Limited, the Kandla Port Trust, the Indian Railways, the Department of Telecommunications, and the Military Engineering Service. These organizations were responsible for improving infrastructure and introducing new construction technology. The SRC Limited popularized the use of hollow cement block walls, lintel bands, and vertical reinforcements at wall corners in masonry construction, and ties/stirrups with 135-degree hooks in reinforced concrete construction. Such structures performed well during the 2001 Bhuj earthquake. Construction in other parts of Kachchh was only as a consequence of the development in the Gandhidham-Adipur areas. Migrant artisans from Gandhidham area carried the skill and technology to the interiors, and implemented many private projects. Government construction techniques stood as examples for citizen builders to emulate. For instance, lintel bands were common in the construction of the Indian Railways. This may have inspired a few individuals to include these in their own houses. The excessive use of cement-based masonry and the gradual exclusion of lime from masonry was a net outcome of the recent construction. This has resulted in widespread brittle damage in such masonry structures. OVERVIEW OF DAMAGE Collapses of masonry dwellings in the Kachchh region were responsible for the majority of fatalities during the 2001 Bhuj earthquake. The meizoseismal area and adjoining areas that sustained intensities of shaking IX and X include many important towns and villages of the Kachchh region (Figure 11-1), some of which are densely populated. For instance, the town of Bhuj has a population of about 160,000, Anjar of about 55,000 and Bhachau of about 20,000. These towns lie south of the epicenter. There are other villages and towns northwest of the epicenter, such as Manfara and Rapar, which also suffered significant damage. Table 11-1 gives an estimate of the masonry structures and huts damaged during the earthquake. Most of the damage to masonry structures occurred in the Kachchh district, which lies in the most severe seismic zone (V) of the country. Table 11-1. Damage statistics for different types of construction in the Kachchh region. Damage Level* Type of Construction Complete Partial Pukka (well built) houses 187,122 510,419 Kachcha (poorly built) houses 167,205 387,320 Huts 16,266 34,295 Total 370,593 932,034 * From, official site of Government of Gujarat (September 2001)
  3. 3. Masonry Structures 189 �������� ���� ������������������� �� ���������������� ������� ������ ������� ������ ���� ��������� ������� ��� ��� �� ��������� ������� ������� ���� � ��� �� ��������� ����� ������ ������� ������� ������� ������������ �������� �������� ������� �������� �������� ����� ������� ����� ������ ������� �������������� ��� ������� ���������� ����� ������� ���� ����� ����� ����� �������� ������� ������� �� ������� ������ ��������� ������ ����������� ������� ������ ���� ���� �������� ���������� ������ �������� ������ ����� �������� ������� �������� ����� ������� ���������� ������ ���� ����� ������ ������� ����� ���� ������ �� �� ������� ����������� �� ������� ������ � ������ ����������� ������ ���������������� �������� ���������� ���������������� �� � Figure 11-1. Epicentral region of the 2001 Bhuj earthquake showing villages discussed in this report. Aerial photographs of damage are available for Bhachau, Anjar, Ratnal, and adjoining areas of Gandhidham, all in the epicentral region. Figure 11-2 shows the near total destruction of the village of Bhachau, 8 km from epicenter. The town of Ratnal and villages near Gandhidham, 44 km from the epicenter, sustained similar, near complete, devastation. In Anjar, a town where nearly 300 school children died while marching in the Republic Day parade, building damage was widespread. While devastation was nearly complete in some areas, others escaped with only moderate damage. Initially, significantly different soil conditions were suspected for the contrasting damage pattern, but it was the older construction that collapsed, while the newer construction suffered only moderate damage. Damage patterns showed serious lack of earthquake-resistant construction features in the region (Figures 11-3 and 11-4). Villages reduced to rubble showed a more or less uniform choice of construction material and techniques. The overall analysis of these damages reiterates the ills of masonry construction. Random rubble with mud mortar is the most vulnerable type of construction. Wall failure was the most common cause of structural collapse (Figure 11-5). In a few cases, the roof alone collapsed, causing casualties (Figure 11-6). Failure of reinforced concrete slab roof system was present where slab reinforcements lapped at the same location, creating a weak link for fracture (Figure 11-7). Masonry in mud mortar has inherently very poor shear strength. As a consequence, wall thicknesses are necessarily large, sometimes as thick as 750 mm. These thick walls were often not made into a single wythe with interlocking stones that run through the thickness of the wall. In most instances, these walls sustained separation of wythes, thereby losing even the vertical load carrying capacity (Figures 11-8 and 11-9).
  4. 4. Masonry Structures 190 Figure 11-2. Aerial photo of near total Figure 11-3. Damage to masonry construction devastation of village Bhachau. due to inadequate connections between the walls. Figure 11-4. Damage to masonry construction Figure 11-5. Collapse of this masonry structure due to inadequate connections between the walls. was due to wall failure. Figure 11-6. In a few instances, such as this Figure 11-7. Reinforced concrete roof slab masonry structure, the roof alone was responsible with slab reinforcements lapped at the same for structural collapse. location failed.
  5. 5. Masonry Structures 191 Figure 11-8. Lack of through-stones caused Figure 11-9. Separation of wythes caused separation of wythes. collapsed of wall and roof. TYPES OF CONSTRUCTION Masonry dwellings in the Kachchh region include many types of load-bearing construction. In older construction, walls were made from rammed earth and adobe or uncut stone masonry with mud mortar. The roof is generally wooden trusses and clay tile. With the passage of time and the numerous changes that took place over the last 50 years, present day stone masonry uses cut/dressed stones or burnt clay bricks, cement mortar, and reinforced concrete slabs. Dressed-to- size or cut-stones were used in stone masonry for walls in urban areas and in cases where owners could afford higher costs. However, random rubble stone masonry construction predominates in the Kachchh region. Consequently, the building stock in the region has a wide spectrum of masonry construction techniques (Figures 11-10 through 11-16). These include: • Random rubble stone masonry (uncoursed) in mud/cement mortar with clay tile roof.1 • Semi-dressed/dressed stone masonry (coursed) in mud/cement mortar with clay tile roof.1 • Clay brick masonry in mud mortar with tile roof. • Semi-dressed/dressed stone masonry (coursed) in mud/cement mortar with reinforced concrete. • Random rubble stone masonry (uncoursed) in mud/cement mortar with reinforced concrete slab roof. • Burnt clay brick masonry in mud/cement mortar with clay tile1/reinforced concrete slab roof. • Solid/hollow cement block masonry in cement mortar with clay tile1/reinforced concrete slab roof. A large number of masonry structures are hybrid in nature (Figures 11-17 through 11-21). For instance, in a typical two-story construction, the older first story walls may have been constructed in random rubble stone masonry with mud mortar, but the newer extension of the upper story may be in brick masonry laid in cement mortar with reinforced concrete slabs for floors and roof. Such hybrid construction sustained severe damage in the weaker lower story. Sometimes, the opposite has also been observed, where weak thin walls and poor cement mortar in the upper stories led to collapse. 1 Clay tiles are used as covering material and supported on a wooden truss-purlin system or on a framework of wooden joists.
  6. 6. Masonry Structures 192 Figure 11-10. Random rubble stone masonry in Figure 11-11. Stone masonry in cement mortar mud mortar with tile roof. with tile roof. Figure 11-12. Clay brick masonry in mud mortar Figure 11-13. Coursed stone masonry in cement with tile roof. mortar with RC slab roof. Figure 11-14. Random rubble stone masonry in Figure 11-15. Clay brick masonry in cement cement mortar with RC slab roof. mortar with RC slab roof.
  7. 7. Masonry Structures 193 Figure 11-16. Cement block masonry in cement Figure 11-17. Improper connection between mortar with RC slab roof. walls (Samakhyali village). Figure 11-18. Poor shear strength of stone Figure 11-19. Loosely formed roof with masonry in the lower story of structure with lintel Mangalore tiles (Bhuj) led to serious damage/ bands (Bhuj). collapse of structures. Figure 11-20. This 3-story load-bearing Figure 11-21. The first story of this masonry construction (Gandhidham) had exterior walls of structure in Anjar, constructed of random stone masonry and interior walls of hollow cement rubble stone masonry in lime mortar, collapsed blocks. Lintel and plinth bands were provided. completely. The second story, of brick masonry This structure performed well, while its adjoining with cement mortar, experienced minimal 5-story reinforced concrete frame buildings damage. sustained serious damages and collapses.
  8. 8. Masonry Structures 194 LINTEL BANDS Early construction practice in earthquake-prone areas of India did include lintel bands. Indeed, they have been used in the Kachchh region for over 50 years. Early construction spearheaded by governmental agencies like the MES, Indian Railways and Kandla Port Trust, and subsequently joined by the SRC Limited in the early 1950s included the practice of providing lintel bands in their masonry construction. The lintel bands were made of wood, reinforced concrete with cement/lime mortar (Figure 11-22), or even specially shaped hollow cement blocks for placing horizontal reinforcement. However, with time, and as the trauma of a big earthquake in 1956 was forgotten, some of these good practices were slowly lost, and most masonry structures were built with no lintel bands. Over time, an interesting lintel feature was introduced in some masonry construction of the region. Owners of houses used reinforced concrete loft slabs inside the rooms at lintel levels for providing storage space in the house. In many cases, if the house is small, the loft is provided throughout the house. In such instances, these relatively thin reinforced concrete slab strips act as lintel bands (Figure 11-23). In some cases, no lintel band is provided, but the exterior façade is plastered to imitate a lintel band. Figure 11-22. Reinforced concrete band at lintel level. Figure 11-23. Improvisation of loft slabs as lintel band.
  9. 9. Masonry Structures 195 IRREGULARITIES Masonry construction in the region was often built incrementally, and hence the final structural configuration is not always known at the start of the construction. Subsequent additions were driven by functional needs, and the structural consequences of these additions were not understood. Consequently, almost all masonry construction in the affected area is highly irregular in structural configuration. The geometry, size, and shape are often disproportionate and vary abruptly, leading to vertical and horizontal offsets. Loosely formed roofs, like the Mangalore (a south Indian city on the south-west coast of India), tile roofs do not provide good diaphragm action for proper distribution of lateral loads to walls. On the other hand, reinforced concrete slabs achieve good diaphragm action. In buildings with reinforced concrete slabs and configuration irregularities, torsional response was generated, and they performed poorly as they failed to resist the increased torsional shear stresses. FOUNDATIONS Foundations in masonry construction have been constructed fairly consistently throughout the Kachchh region. The topsoil consists of 300-400 mm thick black cotton soil/black silty soil/soft creek soil. Underlying this layer is murrum soil (weathered rock), which may be either hard or soft. Usually, shallow trenches are dug and strip foundations are used. After the trench is dug, a lean cement concrete is laid over the soil. Large granite boulders are hand-broken to a size of 40 mm and less, and used as aggregates in this cement concrete. In the slab concrete, 20 mm, 12 mm and/or 6.25 mm graded aggregates are used. Black granite stone blocks of random shape and of largest dimension up to 450 mm are used in the random rubble plinth masonry. In usual masonry buildings in cities and major towns, a 1.2 m-1.5 m deep 0.9 m-1.2 m wide trench is dug (Figure 11-24a). Then, a 150 mm thick 1:5:10 cement concrete made of 40 mm size hand-broken coarse aggregate is placed on the murrum soil. Next, the plinth masonry is constructed over this cement concrete in two lifts each of 0.7 m height; the first lift is 600 mm wide and the second 450 mm. Uncoursed, randomly cut black basalt/trap stones are used in the masonry with 1:6 cement mortar. A 100-150 mm thick damp-proof course-cum-plinth band is placed on top of the plinth masonry. The band is made in 1:2:4 nominal mix concrete with cement and 20 mm coarse aggregates, and has 4 bars of 12 mm diameter high yield strength deformed bars (fy=415MPa) as longitudinal steel with 6 mm diameter mild steel bars (fy=250MPa) at 150 mm centers as ties. When it is treated only as a damp-proof course, no reinforcements are required by the government specifications. In the rural setting, the above procedure of foundation construction is severely simplified (Figure 11-24b). A 600 mm wide trench is cut in the ground up to a depth equal to the thickness of the black topsoil. The trench is stopped at the first sign of the weathered rock or the soft/hard murrum soil. Only rarely is the 150 mm thick 1:5:10 lean cement concrete layer discussed earlier placed. Even when it is, the base material at the founding level is only random rubble broken stone in mud/lime mortar. The trench is filled with uncoursed trap stones. Dry, hand-mixed 1:6 cement mortar is placed over these random stones and watered to send the mortar into the voids between the stones. The plinth masonry is stopped 150 mm above the ground level. No damp-proof course band is provided in this type of construction. The preparation of the plinth masonry has a few special features. After the trench is dug and the 150 mm thick lean concrete is placed (Figure 11-25), the largest granite stones are picked and their planar faces are made vertical and flush with outside/inside of the wall (Figure 11-26). Then, smaller rubble stones and cement mortar are placed in the voids. The largest stones do not always cross the full width of the foundation and usually a distinct vertical layer of small rubble and cement mortar is present in the vertical mid-thickness of the wall. Since the surface of the
  10. 10. Masonry Structures 196 a b Figure 11-24. Typical foundation specifications for masonry construction in the Kachchh region: a. Formal version adopted in better construction, and b. Basic version adopted in poor quality construction. Figure 11-25. Shallow trenching. Figure 11-26. Large uncoursed granite blocks placed from both sides; a central weak plane is formed. Figure 11-27. Inside foundation surface pointed, Figure 11-28. Masonry in large-block sandstone outside surface left unfinished. under construction. A plinth band is provided.
  11. 11. Masonry Structures 197 granite stone is nonporous and smooth, the cement mortar does not bond effectively with the stone. This, coupled with the vertical mid-layer of the wall mentioned above, invites the masonry to split into two vertical wythes. Often, the vertical surfaces of the plinth masonry are not pointed. In most cases, only one of them is pointed. For example, if a plinth masonry is for a residential unit, only the outside surface is pointed. When the inside surface is required for access and not the outside, as in the case of the foundation of a weighbridge, (where trucks are weighed along the highway), the inside surface is pointed and the outside face is left unfinished (Figure 11-27). In government construction, and in construction by qualified engineers, a damp-proof course is often provided at plinth level (Figure 11-28). Rarely is a plinth band also provided. CONSTRUCTION OF MASONRY WALLS The procedure described above for the foundation masonry is also adopted for walls with random rubble masonry in mud or cement mortar. Unlike in the plinth masonry, these walls are pointed on both faces. In masonry construction using cement blocks, the plinth masonry is done in either granite stone, or in solid cement blocks. The construction materials of the region have characteristics that have led to special construction strategies. For instance, black granite stone is commonly available in the Kachchh region, but is heavy. Lifting large granite blocks for masonry construction at higher elevations is difficult. In addition, a pink variety of sandstone is locally available; it is lighter than granite, but is weaker in compressive strength. A practice has evolved in the region wherein the plinth masonry is made in the heavy granite and the superstructure wall masonry in the light sandstone. Four different types of masonry units are employed for making walls in the Kachchh region, as described below. Random-rubble stone masonry with mud/lime/cement mortar Walls are made with undressed granite stone of different degrees of weathering and up to a maximum dimension of 400 mm in low rise 1-2 story buildings, and up to 600 mm in taller ones. Some of the older construction with mud mortar has wall thickness of about 600 mm even in 1- and 2-story buildings. These thick walls are constructed with one mason working on each side of the wall and placing stones to create even surfaces on the exterior faces. This results in walls that have two predominant vertical layers or wythes. With maximum wall thickness of 600 mm and the largest dimension of stones as 400 mm, no stone covers the full width of the wall and the much-needed interlocking between the two vertical layers is absent. This problem of lack of integrity within the wythes of walls is less severe when wall thickness is about 400 mm or less. Depending on the level of weathering, the surface characteristics, and therefore their bond with the mortar, vary. Small/large block semi-dressed/dressed stones in mud/lime/cement mortar Quarried sandstone or, in some instances, locally available weathered lateritic rock is used. In monumental structures, some private buildings, and many government buildings, semi-dressed (i.e., without smooth polishing of the outer surface) stones are used. The normal practice across India is to use stones with the largest dimension of about 400 mm. However, construction practice in the Kachchh region offers a special construction style. Semi-dressed stones of size about 600 mm × 400 mm × 250 mm are employed in stone masonry (Figure 11-29). Due to this large size of units, the thickness of the mortar usually required between the stones is as large as 80 mm. In such construction, only a few of these large-block stones are required compared to small- block masonry. Under strong seismic shaking, the out-of-plane dislodging of one stone due to either out-of-plumb wall or out-of-plane seismic shaking of the wall can lead to the collapse of a significant portion of the large-block masonry above, jeopardizing the safety and stability of
  12. 12. Masonry Structures 198 Figure 11-29. Stone block units used in masonry construction in the Kachchh region. The tape is held out to 30 cm. the entire building. Small-block stones of about 400 mm × 230 mm × 150 mm are also used. Where provided, pilasters in the long and slender compound walls seemed to have contributed significant out-of-plane stability. Sandstone available in the Kachchh region is of the pink variety. This stone, though lighter than the granite/trap stone, is still heavy for use in wall masonry construction. For this reason, another variety of yellow sandstone is brought from Junagadh in the Saurashtra region. This stone is much lighter and can be cut nearly to brick sizes for ease of handling. The yellow stone is also preferred for the aesthetics of its bright color. Burnt clay bricks in mud/cement mortar In recent times, this type of construction has become increasingly common in the Kachchh region, particularly in urban areas. Consequently, countryside kilns have grown and are producing burnt clay bricks of standard size 230 mm × 115 mm × 75 mm even though the soil is not suitable for making good quality burnt clay bricks. The quality of these bricks is poor and highly variable, even within the same batch. For this reason, bricks are often brought from Ahmedabad and other distant locations. These burnt clay bricks have a frog on one surface, and are used with either mud mortar or cement mortar, depending on the economic considerations of the user. Standard wall thickness of 230 mm is very common in most buildings, even in reinforced concrete frame buildings. However, in two and three story buildings, use of one-and-half brick walls is also observed. There are also instances of the use of 115 mm (half-brick length thick) walls in single-story buildings. This is a matter of serious structural concern, particularly in the severe Seismic Zone V. Solid/hollow cement blocks in cement mortar Both solid (Figure 11-30) and hollow cement blocks with up to three cells (Figure 11-31) of varied sizes and shapes have been in use since 1950, when a Besser Plant was commissioned at Adipur. Over 5,000 buildings of varied sizes and functional utility with lintel bands incorporated into the construction were built with hollow cement blocks by the SRC Limited. Composition and quality control of the manufacture of cement blocks have varied significantly over the years. Around
  13. 13. Masonry Structures 199 Figure 11-30. Solid cement blocks. Figure 11-31 Figure 11-32 Figure 11-33 Figures 11-31, 11-32, and 11-33. Three types of hollow cement blocks. the same time, major government agencies like Indian Railways, the Kandla Port Trust, and the Military Engineering Service also extensively used plinth and/or lintel bands. The hollow blocks permitted vertical reinforcement to be carried through the wall. In some instances, these vertical reinforcements were not anchored into the reinforced concrete slab in order to allow for thermal expansion/contraction of the slab. These buildings, built in the 1950s with hollow cement blocks, performed very well during the 1956 Anjar and 2001 Bhuj earthquakes. However, local engineers now recognize that the vertical reinforcement should be anchored into the slab to provide a positive connection for transfer of forces and ensure against sliding of the reinforced concrete slab. Hollow cement blocks were made of different compositions to give them different finishes. Currently, the hollow blocks construction is waning due to lack of quality in manufacturing. The compressive strength of these blocks largely depends on the level of compaction achieved after placing the concrete mix in the steel molds. The older Besser machine used table vibrations and pressure for compaction, while the locally improvised machines for cement block construction depend on nominal table vibration and hand compaction. Some of the block-making machines recently manufactured in India are small and portable and provide reasonable compaction using table vibration and pressure. In urban areas, the availability of industrial by-products, such as fly ash, has led to the growth
  14. 14. Masonry Structures 200 Figure 11-34. Fly ash bricks have begun to gain popularity as a building material. of cement-based masonry units (Figure 11-34). The fly ash brick masonry blocks are handcast by applying a little pressure with a hand tool. Both solid and hollow units are manufactured at a price competitive to that of the traditional burnt clay bricks. The standard units are 200 mm × 100 mm × 75 mm. Some manufacturers make 230 mm × 100 mm × 75 mm blocks whose lengths match the transverse dimension of the 230 mm columns commonly adopted, which is the thickness of traditional burnt clay brick walls. These cement block units have good thermal characteristics, and in the case of hollow blocks, also lightweight. The hollow bricks also offer the possibility of passing vertical reinforcement through the hollows, as required in severe seismic zones. Table 11-2 presents an overall comparison of the various masonry units in use in the Kachchh region. Masonry walls of hollow block construction follow a relatively simple sequence. The plinth masonry is prepared and the damp-proof course is laid (Figure 11-28). In cases where a plinth band is also intended, special channel unit blocks are placed (Figures 11-35 and 11-36) to act as the formwork for the reinforcement of the plinth band (Figure 11-37). The reinforcement cage is placed and concrete poured in-situ. Vertical reinforcements at wall corners are also introduced in the plinth band and passed through the hollow blocks in wall corners (Figure 11-38). Special blocks are also available for introducing the anchors and fasteners for the window and door frames (Figure 11-39). ROOFING MATERIAL AND ROOF CONSTRUCTION Older roof construction is mostly sloped or pitched, and has wooden truss roofs with purlins supporting the clay tiles (Figure 11-40). Locally available wood logs of rounded cross-section are used, often as main members without any shaping. In most instances, the trusses are not complete with bottom tie members, and one horizontal member (of about 100-150 mm diameter) runs across the length of a room at the ridge level. Two types of roofs are usually used in the Kachchh region. In the first, rafters (of about 100-150 mm diameter) are used. These rafters rest on a horizontal member at the ridge level and directly on the masonry walls at the level of the eaves. Light wood purlins and battens (of size 25 mm × 15 mm) form a grid to place the tiles (Figure 11-40a). In the second type of roof construction, the rafters are done away with, and purlins of significant size (up to 75-100 mm in diameter) are placed directly on the gable wall (Figure 11-40b). One or two intermediate purlins are provided along each of the slopes. The rafter and battens are smaller
  15. 15. Masonry Structures 201 Table 11-2. Comparison of various masonry units used for roof construction in the Kachchh region Building Unit MPa) Cost Comments Sandstone 3.0 1 Good quarries are used up; lately only soft From Junagadh variety is available; becoming expensive; (yellow variety; high water absorption; low mortar strength 380×280×120 at Anjar; and very poor bond is achieved. 400×230×200 at Adipur) Locally available in Bhuj (light pink /brown variety) Hollow cement blocks 2.8-3.5 1.25 Quality is poor (no compaction due to lack of vibration). Solid cement blocks 6.5-7.5 1.50 Higher strength than bricks; no homogeneity, more cracks in walls after seismic shaking. Large size Block is heavy: small movements for (390×200×190) adjusting the position breaks the initial set of the mortar around the blocks in the lower layers due to weak mortars currently in use; cannot be handled in wet condition thus, not soaked before placing. Low bonding due to lesser surface area of blocks in contact with mortar and lesser porosity of blocks, as compared to that of bricks. When such walls collapse, 95% of the blocks are recoverable due to poor bond and higher strength of blocks; in brick walls, only 5% of bricks are recoverable. Small size More joints and hence more homogeneous (230×100×75) than large blocks; higher manufacturing cost. Burnt clay bricks 3.5-4.0 1.50 Bond better with mortar than to solid cement blocks; better insulating property; poor quality. Basalt/trap stone At least 1.75 Fine texture on surface, and hence low bond 50.0-100.0 leading to failure of walls in two wythes; very heavy; used commonly in foundations and in masonry up to plinth level only. Fly ash bricks 6.7-7.5 1.60 Lighter than bricks and blocks; very high water absorption (20-32%), (Permissible Absorption Brick Class - I<15%; II<22%; III<32%); eco- friendly; large sulfur content (since made from lignite-based ash) implies chemical corrosion. Notes: Compressive strength is computed over unit plan area of wall. The cost index noted above includes only the cost of manufacture and supply of the masonry unit, and not of the masonry.
  16. 16. Masonry Structures 202 Figure 11-35. Special channel bricks facilitate Figure 11-36. Cement channel bricks. placement of reinforcement bar. Figure 11-37. Plinth band reinforcement. Figure 11-38. Corner reinforcement. Figure 11-39. Special concrete blocks are used to anchor door and window jambs.
  17. 17. Masonry Structures 203 in size (25 mm × 15 mm) and are simply nailed or tied with coir rope to the main rafters. Thus, there is no positive anchorage of the roof to the walls, other than through bearing of the rafters or purlins along the gable and other walls. Roof tiles are usually of two types: • Mangalore Tiles. Mangalore tile is 400 mm × 230 mm in plan, about 25 mm thick, and weighs about 5 kg (Figure 11-41). Each tile overlaps with the adjacent ones by 30 mm along the width and about 60 mm along the length. The loading from such roofs, including the weight of battens, purlins and rafters, is about 1kN/m2. Pitched roofs with Mangalore tiles are the most common roofing systems employed in the rural areas of Kachchh. No trusses are used in the roof; a grid of rafters and battens are used and the tiles rest on them. A heavy purlin runs across the length of the room at the ridge level. The cross-sections of wooden rafters are much smaller than those of the purlins, and the battens are even smaller. The purlins and rafters merely rest on the top surface of the walls of the room; no positive anchorages are used (Figure 11-42). Mangalore tile roofs can be very heavy and draw large inertial forces. Since they are loosely formed, they are easily displaced (Figure 11-43). • Slit Tube Tiles. Slit-tube tile roofs are also used in the region (Figure 11-44). These are even more loosely formed, and also require an impervious layer of soil/sheeting to prevent rainwater from seeping in. Unfortunately, this layer causes the sliding of the tiles easily. Relative performance of these two roofing systems indicates the former to be more efficient. In semi-rural or urban areas, corrugated galvanized iron/asbestos/tin sheet roofs are also used. Slit-tube clay tile is 150 mm long, 75 mm in diameter and 6-10 mm thick. Each piece weighs about 0.3-0.4 kg. The overlap along the length at the two ends is about 30 mm. Such roofs also impose a dead load of about 1kN/m2. Both Mangalore and slit-tube roofing systems require specially shaped coping element to cover the ridgeline of the pitched roofs. Other roofing materials, such as corrugated sheets made of galvanized iron/asbestos/tin and fastened to wood or steel trusses, are common in better quality construction. Today, masonry dwellings also have reinforced concrete slab roofs. The slab thickness varies from 75 mm to 125 mm. Reinforcement is nominal; usually 6 mm diameter mild steel (smooth) bars or 10 mm/12 a b Figure 11-40. Pitched roofs constructed in the Kachchh region are made of wooden trusses that are often not completed – there is no bottom chord in these trusses. There are two basic forms of the roofs adopted. a.) Roof with heavy rafters and b.) Roof with heavy intermediate purlins. Roof tiles are supported by battens and purlins of small cross-sections laid over these main rafters or purlins.
  18. 18. Masonry Structures 204 Figure 11-41. Mangalore roof tile. Figure 11-42. Roof rests on top of walls. Figure 11-43. Mangalore tiles slid off this roof. Figure 11-44. Slit-tube tile roof. Each tile weighs 0.3-0.4 kg. mm diameter high-strength deformed steel bars at 150 to 250 mm centers along each principal direction. The concrete is hand mixed based on volume batching, and usually of grade M15 (i.e., 28-day characteristic 150 mm cube compressive strength of 15 MPa). Because of poor formwork, the required cover to reinforcement may not be present uniformly throughout the slab. In many instances, the reinforcement bars are seen on the soffit of the slab. The performance of these roofing systems under seismic shaking has been well established. Lighter and stiff-in-plane roofs performed well, while the heavy and loosely formed ones fared poorly. GUIDELINES AND INDIAN STANDARDS The structural design of unreinforced load bearing/non-load bearing masonry walls made of various masonry types is governed by the masonry code (IS:1905-1987). The Indian Standard masonry code specifies materials to be used, maximum permissible stresses, and methods of design. However, selection of materials and special features of design and construction of earthquake- resistant masonry buildings are given in another Indian Standard (IS:4326-1976).
  19. 19. Masonry Structures 205 The International Association for Earthquake Engineering (IAEE) published a manual in English for nonengineered construction (IAEE, 1986). This publication reviews the structural performance during strong earthquake shaking of masonry, earthen, stone and wooden buildings, and nonengineered reinforced concrete construction. It outlines general concepts in earthquake-resistant design of such structures. Further, it gives guidelines for repair, restoration, and strengthening of reinforced concrete structures. This compilation of the basic rules of thumb has been reprinted in India by the Indian Society of Earthquake Technology (ISET), and priced nominally (ISET, 1989). However, even this document did not receive wide audience, as English is not the native language in India, and the IAEE guidelines did not have the formal recognition of enforcing agencies. The Bureau of Indian Standards (New Delhi) has incorporated some contents of the IAEE manual in Indian Standard Guidelines. Two separate publications emerged, one for Earthen Buildings (IS:13827-1993) and another for Low-Strength Masonry Buildings (IS:13828-1993), which covered both brick and stone masonry construction. These publications gave formal recognition to the practice of building earthquake resistance into a greater variety of dwellings. Even though Indian Standards are generally not mandatory in India, they represent documents of authenticated information. In 1999, the Bureau of Indian Standards published the bilingual (English and Hindi) version of these two standards (IS:13827-1999; IS:13828-1999) to increase their usage. The practice of earthquake-resistant construction would be enhanced if these Indian Standards also become available in regional languages. INDIAN STANDARD FOR UNREINFORCED MASONRY (IS:1905-1987) The masonry code gives recommendations for the design of unreinforced load-bearing masonry walls constructed using solid/perforated burnt clay bricks, sand-lime bricks, stones, concrete blocks, lime based blocks, and burnt clay hollow blocks. As per this code, no special provisions are necessary for buildings constructed in Seismic Zones I and II. However, special features are applicable for construction of earthquake-resistant masonry buildings in Seismic Zones III, IV, and V (IS:4326). These specifications were not followed in structures in the affected area, a region in Seismic Zone V. Based on their nominal mix proportions, the masonry code classifies mortars made from cement, lime, Pozzolana and sand into six grades, namely high (H1, H2), medium (M1, M2) and low (L1, L2). These mortars are required to satisfy the minimum compressive strength requirements for use in masonry work. Also, the code requires masonry units to adhere to the strength requirements given in relevant Indian Standards for brick units (burnt clay or sand lime), stones, and concrete blocks (solid and hollow). The use of a wide range of quality of mortars and masonry units noted in the postearthquake reconnaissance surveys indicate that material tests are usually not performed and the above specifications on the mortar and masonry units are not enforced. The code specifies a number of stability-related requirements to be incorporated in the planning of the geometry of the structure and choosing the wall thickness. The code limits the slenderness ratio (h/t or l/t), where h and l are the effective height and effective length of the wall. For walls in cement mortar, the ratio is limited to 27, and for walls in lime mortar up to 2 stories to 20, and for structures taller than 2 stories, the ratio is limited to 13. The standard requires that buildings of typical story heights of 3.2 m, walls in cement mortar are at least 120 mm thick, and those in lime mortar are at least 160 mm (in buildings up to 2 stories) or 250 mm (in buildings with more than 2 stories). These thicknesses are independent of that of plaster. Provisions for estimating effective thickness, length and height of walls are available for various end conditions designed in practice. These provisions seem to be valid for the single story structures built in the Kachchh district.
  20. 20. Masonry Structures 206 In design, the building is required to be analyzed as a whole as per accepted principles of mechanics. The code requires that the components of the structure be made capable of resisting the most adverse combination of loads expected. Provisions for estimating the capacity of individual components are given in the code. The allowable stress method of design is adopted. Permissible stresses are specified and procedures for calculation of those in compression and shear are specified. Thus, a quantitative procedure is available for design of masonry buildings. Unfortunately, quantitative design of masonry buildings is not performed, in general, for any classes of structures. INDIAN STANDARD FOR EARTHQUAKE DESIGN AND CONSTRUCTION OF BUILDINGS (IS:4326-1993) Indian Standard IS:4326-1993 deals with selection of materials, special features of design and construction for earthquake-resistant buildings, including masonry construction using rectangular masonry units, timber construction and buildings with prefabricated flooring/roofing elements. General principles are provided to encourage the construction of buildings with low mass, integral construction including suspended and projecting parts, simple geometric shapes, and ductility. Attention is drawn to special features of building design and construction. For example, when joints are unavoidable in buildings, the code specifies minimum values of separation gaps required between adjoining buildings as given in Table 11-3. IS:4326-1993 classifies buildings into five categories A, B, C, D and E, depending on the value of the design seismic coefficient αh, given by the product βlα0. Here, β is the soil-foundation system factor (1.0-1.5, depending on soil-type and foundation-type), I is the importance factor (1.5 for important structures and 1.0 for all others), and αh is the basic horizontal seismic coefficient (0.04, 0.05 and 0.08 for Seismic Zones III, IV and V, respectively). αh can be interpreted as design seismic base shear per unit seismic weight of the structure. Category E is the most severe with αh ≤ 0.12 and category A, the least severe with 0.04 ≤ αh ≤ 0.05. For buildings in each category, the code specifies the following: mortar mix, the size and position of openings, and the details of earthquake-resistant features like bands, vertical reinforcements, and plan bracings at roof level. INDIAN STANDARD FOR EARTHEN CONSTRUCTION (IS:13827-1993) This Indian Standard covers three types of earthen construction: hand-formed layered construction, block/adobe construction, and rammed earth construction. Each method is presented in detail below. For seismic areas, IS:13827-1993 provides additional recommendations. The Table 11-3. Minimum separation gap to be provided between adjoining buildings with design horizontal seismic coefficient αh of 0.12 as specified in IS:4326-1993. Type of Construction Minimum Separation (mm) Box system or frames with shear walls Min (25; 15n) Moment resisting reinforced concrete frame Min (25; 20n) Moment resisting steel frame 30n Note: n is the number of stories in the building.
  21. 21. Masonry Structures 207 Standard prohibits earthen construction in Seismic Zones IV and V, and restricts the height of such construction to one story in Seismic Zone III. Also, this type of construction is to be avoided in high water table sites, particularly in Seismic Zones IV and V. Roofs are required to be light, well connected within, and adequately tied to the walls. Trussed roofs are preferred over sloping roofs with just rafters or A-type frames. Heavy roofs consisting of wood joists plus earth toppings are prohibited in Seismic Zones IV and V. Tiled/slate roofs are also considered to be vulnerable and are to be avoided in Seismic Zones IV and V. The Standard suggests that the rafters used in the roof be rested on longitudinal wooden elements along the walls to enable a uniform distribution of forces from the roof to the walls. Roof beams or rafters are to be avoided over openings. If these cannot be avoided, lintels over such openings shall be reinforced with additional lumber. The unsupported length of walls between cross walls is required to be less than the smaller of 10t or 64t2/h, where t is the thickness of the wall and h is its height. If walls must be longer than the above limits, they are required to be strengthened by buttresses in between. Further, the height of the wall is required to be less than 8 times its thickness. The upper limit on the size of the openings in walls is specified in absolute dimension as 1.2 m; at least 1.2 m away from the corners. The amount of openings is restricted to be 33 percent of the wall length in Seismic Zone V and 40 percent in Seismic Zones III and IV. Further, the walls are required to be strengthened with the following features: • In dwellings situated in Seismic Zones III, IV and V, the walls are to be tied together with at least two bands, one at the lintel level and another at the roof level. The bands may be made of wood. When pilasters or buttresses are provided, the bands have to pass over them also. • In Seismic Zone V, walls have to be reinforced along the vertical direction with cane or bamboo sticks. These vertical elements have to be tied together with horizontal pieces of bamboo/cane and also anchored in the two bands at the lintel and roof levels. The foundation of such construction is usually strip foundations running along the length of the walls. These strips are required to be founded at least 400 mm below ground, and to have a width of twice the wall thickness. Foundation materials are required to be stronger than those used in the walls. For instance, foundation masonry fired bricks or stones, with lean cement concrete of 1:5:10 or lime concrete of 1:4:8, is suggested by the Standard. It is also recommended that the wall above the foundation up to the plinth be made in the same stronger material recommended for the foundation. The plinth is required to be at least 300 mm above ground. The Standard also suggests that a thick plastic sheet be used to prevent water from seeping upwards through the walls, in addition to requiring a water drain to be built around the outside of the wall to keep the water away from the dwelling. INDIAN STANDARD FOR LOW-STRENGTH CONSTRUCTION (IS:13828-1993) The provisions of this Standard are applicable for brick and random rubble stone masonry construction in Seismic Zones III, IV, and V. For those in Seismic Zones I and II, these provisions are not considered necessary. This Standard refers to low-strength masonry. It clarifies that buildings constructed in accordance with these guidelines are not totally free from collapse under seismic shaking intensities of VIII and IX. However, inclusion of the special design and construction features of the Standard reduces the likelihood of structural collapse. Again, this standard also classifies buildings in Seismic Zones III, IV, and V into five categories – A, B, C, D, and E – as in IS:4326. Buildings in category E and important buildings (with ) are prohibited from using low-strength masonry. However, for buildings in other categories, the Standard makes specific recommendations on the structural configuration and special earthquake-resistant features to be built into them. For example, the limitation on the building height is specified as
  22. 22. Masonry Structures 208 indicated in Table 11-4, with story heights not exceeding 3 m and spans of walls between cross- walls limited to 5 m. Wall thickness is required to be restricted to within 450 mm. The maximum size and preferred location of openings is identified. Brick masonry construction shall be made with bricks of compressive strength not less than 3.5MPa. Strength of bricks and wall thickness are to be chosen depending on the height of the building. Indian Standards suggest special seismic strengthening to increase the earthquake resistance of low-strength masonry buildings, including: 1. Through-stones or bond elements in thick walls. 2. Limitations on size and location of openings in walls. 3. Lintel band over all internal and external walls except partition walls. 4. Roof band (except when the roof is made of reinforced concrete) when roof is flat and gable bands when roof is pitched, resting over the full width of the wall. 5. Vertical reinforcing steel at corners and junctions of walls. 6. In-plan bracing of flexible roof system. 7. Plinth band, where strip footings are made of masonry (other than reinforced concrete or reinforced masonry) and soil is soft or of uneven properties. IS:13828-1993 provides empirical details of each of the above seismic strengthening arrangements for direct implementation in the field during construction. The above seismic strengthening arrangements are required to be built into buildings depending on their category (i.e., A, B, C, or D) and number of stories. OTHER PUBLICATIONS AND BOOKLETS APPLICABLE TO STRUCTURES IN THE BHUJ AREA The publication Vernacular Housing in Seismic Zones of India reviews the seismic vulnerability of masonry construction in India in 1984 (I-UNM 1984). The report is based on a field survey and was published in 1984. It lists the status of various aspects of masonry structures, such as Table 11-4. Limitations on the number of stories to be constructed in low-strength masonry, as specified in IS:13828-1993. Building Category Building Type A B C D Brick Masonry Construction Flat roof 3 3 3 2 Pitched roof (including attic) 2 2 2 1 Stone Masonry Construction Flat roof • Lime-sand or mud mortar 2 2 1 1 • Cement mortar (1:6) 3 3 2 2 Pitched roof (including attic) • Lime-sand or mud mortar 1 1 1 1 • Cement mortar (1:6) 2 2 2 2
  23. 23. Masonry Structures 209 structural integrity and configuration. It identifies deficiencies in masonry structures and suggests measures to overcome those deficiencies. The detailed review of stone masonry houses in the Bhuj region identified the following general characteristics: • Poor roof design: heavy, loosely formed and not properly anchored to the walls • Weak walls: thick random rubble stone walls in mud mortar with weak strength and connections between the adjoining walls • Reasonable foundation design: overall geometry and balance of the structure, quality of workmanship, and size and location of openings. Some academic organizations and nongovernmental organizations in India have also published useful information on earthquake-resistant masonry construction. The Rajiv Gandhi Foundation in New Delhi, in association with the University of Roorkee, Roorkee, published a booklet on “Do’s and Don’ts for Protection” against the earthquake problem. The booklet was published in two languages—in Hindi (RGF 1994a) and English (RGF 1994b). The Rajiv Gandhi Foundation published another booklet on earthquake-resistant house construction (RGF 1994c), which is a good beginning towards taking the subject of earthquake-resistant structures directly to the common man. Similarly, another agency, Lok Vigyan Santhan, Dehra Dun, also published a booklet in Hindi on earthquake-resistant house construction (LVS 1995). This booklet is one in a series of booklets prepared for use by ordinary homeowners, who may not always get an engineer to help them decide how to construct their house. The above are but a few of the examples of efforts to generate awareness and provide information about earthquake-resistant construction to the broadest possible audience. The 2001 Bhuj earthquake provides opportunity to capitalize on and push the mass education of the people of India living in seismically active areas towards building safer and less vulnerable houses. TYPICAL STRUCTURAL DAMAGE The most elementary of masonry construction is random rubble stone (granite) masonry in mud mortar. The wood used in the roofing is not formally cut and shaped. Earthquake-resistant features are not built into this system. There are no connections between the walls, or between the walls and roof. Housing of this type, found primarily in economically weaker sections of the society, performed abysmally during the 2001 Bhuj earthquake. While these low-strength masonry units were high on fulfilling functional needs, they were structurally unsuitable to resist lateral seismic loads because of the building materials used. In this type of construction, little attention is paid to the details that make the roof and walls act together as a single entity. For instance, three months after the earthquake destroyed the original structure, it was being rebuilt using the same rubble in reconstruction. Again, no lintel band is being provided, illustrating that knowledge of earthquake-resistant construction is not being implemented consistently, even after this earthquake (Figure 11-45). The unusually large size (up to 600 mm) pink sandstone masonry units and mud mortar (up to 75 mm thickness) used in making two-story residential buildings resulted in brittle performance (Figure 11-46). Pink sandstone is lighter than granite, readily available, and hence very popular in the Kachchh region. Owing to the coarse shapes of the stones, the thickness of the mud mortar required for leveling is sometimes as large as 8 cm (Figure 11-47). Such large masonry blocks with unusually large mortar thickness of a basically weak mortar material (mud) resulted in very poor performance of a large number of such structures in Bhuj. Heavy purlins carrying the weight of the roof cause stress concentration on the walls at the support points. The stone-mud walls sustained severe cracking at these locations (Figure 11-48). Walls built with large stones and no through-stones separated into wythes impairing the vertical load-carrying capacity. Traces of traditional wisdom were seen in some structures that survived the shaking with little damage,
  24. 24. Masonry Structures 210 where lintel and post system provided lateral resistance (Figure 11-49). This practice may have come from the construction of monumental/heritage construction in the area that used wood frame in a significant way to counter seismic forces. Older construction used a large amount of wood in the post and lintel system, thereby providing some lateral resistance to the inherently weak stone (granite) masonry in mud mortar. Semi-dressed/dressed stone masonry in cement mortar in general sustained lesser damage than random rubble in mud mortar construction. Structures with tall gable walls faced out-of-plane stability problems. Single story construction with semi-dressed sandstone is common. In the majority of cases, lintel bands are not provided in the Kachchh region. These structures sustained only minor damage, such as fine shear cracks in stiff walls. Pink sandstones cut to 450 mm largest dimension were used in the construction of some residential units (Figure 11-50). The plinth in such construction is usually in random rubble granite stone masonry in cement mortar. The strength of these sandstone masonry units can be comparable to that of cement mortar sometimes. In such cases, the shear cracks in walls ran through masonry blocks. Figure 11-45. Random rubble from the original Figure 11-46. Pink sandstone of up to 600 mm (collapsed) structure is being used to rebuild this in size was used in construction of these two- unreinforced masonry house. story housing units. Figure 11-47. Mud mortar is sometimes as thick Figure 11-48. Walls built with large stones as 8 cm. The tape is held out by 30 cm. and no through-stones separated into wythes impairing the vertical load-carrying capacity.
  25. 25. Masonry Structures 211 Figure 11-49. Older construction used a large Figure 11-50. This construction, underway in amount of wood in the post and lintel system, Bhuj at the time of the earthquake, provides a thereby providing some lateral resistance to the plinth band, but not a lintel band. inherently weak stone (granite) masonry in mud mortar. Over the years, the Indian Railways and the Kandla Port Trust have built good engineering practices into their construction. The single-story and two-story random rubble masonry residential units in cement mortar with reinforced concrete slab roofs at the Indian Railways residential colony in Gandhidham have plinth and lintel bands. Pointing is done on the outside and plastering on the inside. The reinforced concrete slab roof is simply rested on the walls. The single-story structures performed very well. The wall-roof interface had nominal sliding and separation, and the short walls between the plinth and lintel bands sustained shear cracks (Figure 11-51). However, the two story residential units suffered damage such as sliding of service water tanks, collapse of parapets, and severe damage to stair towers. Dressed sandstone masonry in cement mortar with plinth and lintel bands was used in single-story row housing at the Kandla Port Trust campus. Only minor cracks were seen in those structures. A good percentage of the recent construction in the Kachchh region is burnt clay brick masonry in cement mortar. These structures showed a full range of performances, depending on the level of earthquake-resistant features built into them. Structures without lintel bands, or with discontinuous lintel bands, performed very poorly. Construction with lintel bands performed well. Unconfined masonry piers sustained damage. The first hollow cement block manufacturing plant was established in the 1950s, and today cement block construction is common throughout the Kachchh region. Cement blocks offer lower lateral resistance than does burnt clay masonry. Offices, academic, and residential buildings in the Kachchh region are built of cement block construction (Figures 11-52 and 11-53). The main features of these buildings include lintel and plinth bands, and vertical reinforcement at wall corners. Hollow cement block construction without plinth and lintel bands performed poorly (Figure 11-54). Collapse of hollow block masonry structures was not observed, though they did sustain shear cracks and sliding along masonry courses. Special hollow cement blocks for construction of columns encouraged construction of multistory buildings (Figures 11-55 through 11-57). A major problem with the hollow cement blocks is their durability. Vertical shearing off of the hollow cement block wall into two wythes and the partial collapse of the wall (Figure 11-58), suggests that excessive weathering of outer layer may have led to the falling off of the outer wythe of the hollow block wall.
  26. 26. Masonry Structures 212 Due to falling quality control in the manufacture of hollow block units, their low compressive strength, and the increasing popularity of fly ash-based cement blocks, solid cement blocks were introduced over the past two decades. However, unlike their hollow counterparts, these units are heavy and cannot be handled comfortably. They have smooth surface characteristics that create a poor bond with the cement mortar. Solid cement blocks tend to have flat surface without the key that is present in standard burnt clay bricks. Their use in tall walls therefore makes them particularly vulnerable in the out-of-plane direction. Consequently, structures with solid cement block structures did not fare too well. Collapse of the roof of a two-story building under construction (Figure 11-59), and of a warehouse building (Figure 11-60) is attributed to the lack of bands to hold the walls together in addition to the tall wall heights. The out-of-plane collapse of the compound wall at the school building in Gandhidham is owing to the large unsupported block masonry panel (Figure 11-61). When the blocks are made as wide as the wall (usually 230 mm), the concept of header and stretcher is not adopted in masonry construction. Long unsupported spans made of such blocks, like compound walls and parapets, did not performed well during this earthquake. In two-story masonry structures, irrespective of whether the masonry is in sandstone, clay brick or cement blocks and in mud/cement mortar, stair towers performed very poorly (Figure 11-62). Where the service water tanks are located over the slab of the stair towers, the problem was even more aggravated. TRADITIONAL CONSTRUCTION IN KACHCHH REGION BHONGAS Traditional houses, or bhongas, in the Kachchh region consist of a single room circular in plan, the diameter varying from 3 m to 6 m (Figure 11-63). The walls are made of sun-dried (adobe) bricks and are about 500 mm thick. The roof is pitched and made of bamboo sticks and thatch. A Figure 11-51. Granite stones up to 350-400 Figure 11-52. Traditional racking in hollow mm are used by the Indian Railways in the cement block shear walls and failure of construction of single and twin residential units Mangalore tile roof was observed at the in Gandhidham. This construction incorporates Commerce College Building in Adipur. There is plinth and lintel bands. The walls are pointed on no lintel band and no plinth band, but a damp- the outside with cement mortar. These single- proof course is provided at the plinth level. story houses performed very well. Nominal cracking was seen at the interface between the roof and the walls, and at the plinth beam level.
  27. 27. Masonry Structures 213 Figure 11-53. Single-story row housing in the Kandla Port Trust residential colony performed well. Figure 11-54. Hollow cement block construction Figure 11-55. Innovation in concrete block without plinth and lintel bands performed poorly technology led to the manufacture of special and sustained severe cracked walls. blocks for making columns and encouraged builders to construct multi-story frame structures using concrete blocks. Figure 11-56. Beams and slabs were of in-situ Figure 11-57. Special elements were also made concrete. for sill and lintel bands so that reinforcement could pass through them.
  28. 28. Masonry Structures 214 Figure 11-58. Hollow cement block wall Figure 11-59. Roof collapse of two-story building separated into two wythes. of solid cement blocks. The building was under construction at the time of the earthquake. Figure 11-60. Warehouse of solid cement blocks Figure 11-61. Out-of-plane collapse of cement had no lintel band. block wall. Figure 11-62. The cantilever projection of masonry work that defines the staircase sustained severe distress. In this case, the tank was resting on the slab (not in the picture), and not on the stair tower.
  29. 29. Masonry Structures 215 central post (100-150 mm diameter) is propped by a wooden log (200-250 mm diameter) running diametrically across the room and resting on the walls, supports the roof (Figure 11-64). Large local stresses are generated in the circular walls at locations where the horizontal post of the roof system rests. The brittle mud walls gave way under these large local stresses, resulting in the collapse of the entire structure. There are no openings built under the point where the wooden log is supported on the wall. No separate foundation is made for this structure. The adobe wall is started about 1.0 m below ground, the plinth is raised about 300 mm above ground, and the wall 2.1 m above that. The major departure from the traditional construction in new construction is that the traditional thatch roof is replaced with a heavy Mangalore tile roof (Figure 11-65). In recent times, some bhongas were also made in burnt clay bricks and cement mortar. Plinth and collar bands are also included in some instances. The bhongas had many earthquake-resistant features such as light roof, walls with small slenderness, few openings, and low height. However, low lateral strength and the heavy log of wood supporting the roof are negative factors. The bhongas suffered varying levels of damage. Most of the older ones and those made in mud mortar suffered collapse; bhongas made with bricks and cement mortar performed better. Figure 11-63. Traditional construction of rural Figure 11-64. Large local stresses are generated houses consisted of circular mud walls and in the circular walls at locations where the thatch roofing. horizontal post of the roof system rests. Figure 11-65. Heavy Mangalore roof tile has recently begun replacing light thatch roof.
  30. 30. Masonry Structures 216 POLS Pols are three-story houses built in Historic times. These pols are examples of the earthquake- resistant construction prevalent in the region. The Kachchh region is known to be subjected to moderate to severe seismic shaking, and earthquake resistance was consciously built into historic housing constructed for centuries. A wood frame with thin clay brick infill masonry in lime mortar formed the basic structural system. Intermediate horizontal bands reduce the panel size of the masonry (Figure 11-66). A pol consists of a highly redundant wooden frame infilled with clay brick masonry in lime mortar (Figure 11-67). This lintel and post method of construction has a flooring system of wooden floor planks finished with heavy stone slabs laid on broken clay brick pieces in mud mortar. It has highly decorative woodwork surrounding the openings and vertical posts (Figures 11-68 and 11-69). In general, these historic structures showed no collapse. In some of these buildings, deterioration due to aging and indiscriminate additions was significant, and some of these structures tilted out-of- plumb after the earthquake. Many of these historic houses performed well during this earthquake. Spalling of plaster and frame infill separation were observed (Figure 11-69). a Figure 11-66. Details of a typical Pol construction. This three-story b house has a long plan. The large number of wood frames used in the transverse direction (shown in black) are highlighted in b.
  31. 31. Masonry Structures 217 Figure 11-67. Pols, historic structures in the region, have highly redundant wood frame infilled with clay brick masonry with lime mortar. Figure 11-68. Decorative bracing on historic Pol. Figure 11-69. Spalled plaster and frame infill separation in this Pol.
  32. 32. Masonry Structures 218 Figure 11-70. Anjar is divided into 12 wards, the most central of which sustained the most damage. DAMAGE IN ANJAR The town of Anjar was shaken not long ago during the 1956 Anjar earthquake (Mw 6.0). Residents of Anjar recall the 1956 event to be of a shorter duration, with relatively lighter shaking intensity than the 2001 event. The population of Anjar was around 30,000 during the 1956 earthquake, and at the time of the January 26, 2001 event it was around 55,000. The town of Anjar has an area of about 12 km2. Older structures that were either undamaged or reconstructed after the 1956 earthquake were spread over a central area of 2.5-3 km2. The town of Anjar is divided into wards (Figure 11-70). The old town lies in the middle of Anjar and consists of wards 3, 4, 5, 9, and 10 (circled by a dashed line). Around this central area lies the new Anjar. Damage was primarily restricted to the five wards of the old Anjar town. Wards 3, 4, and 10 in old Anjar sustained near total collapse of all buildings. These wards were also damaged during the 1956 Anjar earthquake. Ward 10 is apparently reclaimed from a 300-400 year old pond. Wards 5 and 9 sustained lower levels of damages in both this earthquake and the 1956 event. Comprehensive repair and strengthening was not undertaken after the 1956 earthquake. Construction in the old Anjar area is random rubble sandstone masonry in lime mortar with walls up to 750 mm thickness. Use of mud mortar is rare in this type of construction. No earthquake- resistant features like bands were provided. Structures in wards 3, 4, and 10 were rebuilt after they collapsed during the last earthquake from the rubble using the construction methods prevalent at that time. The development of new Anjar took place mostly in the last decade. Construction in new Anjar had similar configurations to those in the old Anjar area. This new masonry construction, however, was different in the following aspects: Structures were made of cement mortar instead of lime/mud mortar, as was common in the old Anjar area. Lintel bands had become a more common feature in residential construction.
  33. 33. Masonry Structures 219 It was expected that construction practice in this old town would take a turn for the better after the 1956 earthquake. However, the country had neither adequate awareness of earthquake-resistant construction at that time, nor any formal guidelines to make masonry construction earthquake- resistant. Post-1956 Anjar earthquake construction in old Anjar was no better than practices in effect before the earthquake. These structures again performed poorly during the 2001 Bhuj earthquake (Figures 11-71 and 11-72). Total or near total collapses of structures in this region suggest the following deficiencies: • Inadequate walls. Structures with stone masonry walls in lime mortar may have weathered over the last four decades, leading to further deterioration of their strength. The re-used sandstone masonry blocks from the rubble of the 1956 event may have poor bond characteristics. The large thickness of up to 750 mm, coupled with the construction of walls in two wythes makes these structures vulnerable under strong seismic shaking (Figure 11- 73). Unsupported masonry panels in tall masonry walls performed poorly (Figure 11-74). Even though there was total devastation in old Anjar, an occasional structure stood upright in this area and showed that construction with lateral force resisting elements (wooden post and lintel systems, or inclined members providing bracing) would have allowed buildings to perform much better under seismic shaking (Figures 11-75 and 11-76). • Inadequate roof-to-wall connection. Wood runners were placed room-wise (i.e., just for the length of the room and not over the full length of the house spanning over the walls) due to lack of adequate lengths (Figure 11-77). This resulted in the floor system of each room behaving independently and pulling apart from the others during strong shaking (Figures 11-78 and 11-79). Also, the failure of the walls into two wythes has contributed to the collapse of the roof systems and the consequent large number of fatalities. Lessons from Anjar highlight the vulnerability of re-used construction material and of structures without earthquake-resistant features. It is important that these features be incorporated into construction, particularly when massive reconstruction work in the area is about to begin. Figure 11-71. The area most affected in Anjar Figure 11-72. Re-use of the rubble from the was the area of town that sustained collapses 1956 earthquake, the extensive use of weak during the 1956 Anjar earthquake and was then lime mortar, and highly irregular dwelling units, rebuilt with the rubble. coupled with the lack of basic earthquake- resistant features, contributed to this total collapse in the older section of Anjar.
  34. 34. Masonry Structures 220 Figure 11-73. The thick walls with small size rubble and no through-stones led to splitting of walls into two wythes and in many cases impaired the gravity load carrying capacity. Figure 11-74. Reducing height of unsupported masonry panels in the tall gable walls may be an important contribution in the reconstruction effort. Figure 11-75. In old Anjar, where almost all structures collapsed, this building with lintel and post system survived.
  35. 35. Masonry Structures 221 Figure 11-76. The inclined reinforced concrete Figure 11-77. Discontinuous runners over slabs of stairs may have provided some lateral interior walls contributed to damage and/or strength to the weak masonry system. structural failure. Figure 11-78. Inadequate floor-to-wall Figure 11-79. Floors pulled apart in strong connection. Flooring system is three layers of ground shaking. closely-spaced cross runners. LESSONS LEARNED The widespread damages experienced by masonry structures can be attributed to inadequacies of the roof and walls. Owing to the difficulties in investigating below the ground, the deficiencies in the foundations, if any, were usually not exposed. Performance of the masonry construction in the quake-affected area suggests that the roofing systems employed in the Kachchh region had the following deficiencies: 1. Heavy roofs with large diameter wooden logs in the roof truss. 2. Roof truss is usually an A-frame with no bottom tie member. This implies that the inverted V-shaped roof system has a greater tendency to open up and flatten out—either because the nailing between the two rafters at the crown is only nominal, or because the purlins are just resting on gable walls, which are themselves tall, slender and vulnerable to out- of-plane collapse.
  36. 36. Masonry Structures 222 3. Rafters rest directly on the masonry walls with no connection between the roof and the walls. This suggests that large inertial forces are not always transferred to the walls, and implies an unstable roof system that can collapse. 4. Roofing elements, namely the tiles, are themselves not integrally connected, and consequently the diaphragm action in the roof is absent. 5. Gable roofs do not have tie bracing in the horizontal plane, particularly at the gable walls, and seismic forces from such roofs are not safely transferred to the walls. Wall systems employed in the Kachchh region also showed numerous deficiencies, namely: 1. Walls are not adequately connected to each other. 2. Walls are not connected to the roof with positive mechanisms. 3. Walls are very thick (up to 400-600 mm), and often fall apart into two distinct wythes. 4. Failure of the large block masonry, long unsupported walls, tall slender walls, and the oversized openings located at undesirable locations are often the cause of wall collapses, and thereby of the buildings. 5. Construction practice is such that one wall is built at a time. Such a construction sequence takes away the basic essence of making an earthquake-resistant house, which is supposedly to act as a single unit. The presence of a clean vertical construction joint at the corners, where the integrity of the wall system is most desired, is a major deficiency in this construction. In limited cases, walls are properly connected through the deployment of proper bonding courses in alternate layers. 6. The walls of buildings built with hollow cement blocks are usually left unplastered and hence suffered extensive weathering. Instances of weathering of the hollow blocks were noted in some places where the outer layer of the hollow blocks has fallen off. This matter may require detailed investigation. Owing to the above structural and constructional features, which clearly violate the requirements of buildings in Seismic Zones IV and V as per the Indian Standard guidelines, the large number of fatalities from building/dwelling collapses during this Mw 7.7 earthquake is no surprise. CONCLUSIONS Masonry construction in the Kachchh region is built by rules of thumb and traditions of construction technology that are handed down from one generation to the next. No engineering calculations are performed to assess their seismic adequacy, nor do experienced engineers always supervise such construction. Thus, there is ample room for this type of construction to deviate from desired construction practice. For these reasons, one cannot guarantee that no collapse will occur in these constructions. Damage to masonry construction in the Kachchh region were due to known ills of masonry and the use of heavy and loosely formed roofs. These damages stand as graphic examples for the people of the region on the vulnerability of their own construction. Similar lessons were learned in the aftermath of two Indian earthquakes of the past decade, namely 1993 Killari earthquake and 1999 Chamoli earthquake. The former primarily demonstrated extensive collapse of random rubble masonry walls due to lack of integrity within them, and the latter primarily showed lack of integrity in the heavy pitched roofs composed of tiles supported on wooden rafters and purlins. Observations made in the aftermath of these two past earthquakes in India have had positive influence on subsequent construction in those areas. It is hoped that the studies on seismic damages sustained by the masonry construction of the Kachchh area will also lead to positive changes in the construction practices of the region.
  37. 37. Masonry Structures 223 Information dissemination on lessons learned from seismic performance of masonry construction during past earthquakes in India is generally absent; there are small efforts in the aftermath of an earthquake, and that to a limited audience. Unfortunately, the only other way of educating local people and artisans en-mass regarding the quality of their own construction is through the test of another real earthquake. Interestingly, in India, very few masonry structures are formally designed, even for gravity loads, despite the existence of a design code for this purpose (IS:1905-1987; IS: 4326-1993) for over three decades. Further, even the Indian Standards dealing with earthquake- resistant masonry, low-strength masonry, and earthen construction published in 1993 (IS:13827- 1993 and IS:13828-1993) are, in general, not implemented owing to lack of knowledge of their existence. Comprehensive long-term planning is urgently required. ACKNOWLEDGMENTS The authors are grateful to the large number of engineers in both government and private sectors of the state of Gujarat, and in particular to all the enthusiastic team of engineers of the SRC Limited, Adipur, who provided information and showed the damaged structures in detail. The authors from IIT Kanpur gratefully acknowledge financial support from the Department of Science and Technology, Government of India for partial support towards the studies on the Bhuj earthquake. REFERENCES IAEE, 1986. Guidelines for Earthquake Resistant Masonry Construction. International Association of Earthquake Engineering, Tokyo, Japan. I-UNM 1984. Vernacular Housing in Seismic Zones of India, A report prepared under Joint Indo-U.S. Program to Improve Low-Strength Masonry Housing. INTERTECT and University of New Mexico for U.S. Foreign Disaster Assistance, Agency for International Development, Washington, D.C. ISET, 1989. A Manual of Earthquake Resistant Masonry Construction. Indian Society of Earthquake Technology University of Roorkee, Roorkee. IS:1905-1987. Indian Standard Structural Use of Unreinforced Masonry—Code of Practice. Bureau of Indian Standards, New Delhi, Third Edition. IS:13827-1993. Indian Standard Improving Earthquake Resistance of Earthen Buildings—Guidelines. Bureau of Indian Standards, New Delhi. IS:13827-1999. Indian Standard Improving Earthquake Resistance of Earthen Buildings—Guidelines. Bilingual Edition, Bureau of Indian Standards, New Delhi. IS:13828-1993. Indian Standard Improving Earthquake Resistance of Low Strength Masonry Buildings— Guidelines. Bureau of Indian Standards, New Delhi. IS:13828-1999. Indian Standard Improving Earthquake Resistance of Low Strength Masonry Buildings— Guidelines. Bilingual Edition, Bureau of Indian Standards, New Delhi. RGF, 1994a. Earthquake Problem: Do’s and Don’ts for Protection. Rajiv Gandhi Foundation, New Delhi, Hindi Edition. RGF, 1994b. Earthquake Problem: Do’s and Don’ts for Protection. Rajiv Gandhi Foundation, New Delhi, English Edition. RGF, 1994c. Earthquake Resistant House. Rajiv Gandhi Foundation, New Delhi, (in Hindi). LVS, 1995. Earthquake Resistant House Construction—Instruction Booklet, Lok Vigyan Sansthan, Dehra Dun, July 1995, (in Hindi).
  38. 38. Return to Table of Contents Next Chapter Masonry Structures 224 CHAPTER CONTRIBUTORS Principal Author C.V.R. Murty, M.EERI, Indian Institute of Technology Kanpur, Kanpur, India Contributing Authors Jaswant N. Arlekar, Indian Institute of Technology Kanpur, Kanpur, India Durgesh C. Rai, M.EERI, Indian Institute of Technology Kanpur, Kanpur, India H. B. Udasi, Sindhu Resettlement Corporation Limited, Adipur, India Debashish Nayak, Ahmedabad Municipal Corporation, Ahmedabad, India Figure Credits C.V.R. Murty and Jaswant N. Arelkar took all the photos in this chapter, except as noted below. Figure 11-2 by J.P. Bardet, M.EERI, University of Southern California, Los Angeles, California, USA Figure 11-21 by Durgesh Rai Figures 11-63 and 11-65 by Robin Choudhurry, M.EERI, University of Woloongong, Australia Figure 11-64 by Kishore Jaiswal, Indian Institute of Technology Bombay, Mumbai, India Figure 11-66 by Debashish Nayak, Ahmedabad Municipal Corporation, Ahmedabad, India