A. Jesumi, M.G. Rajendran / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 2, March -April 2013, pp.729-732729 | P a g eOptimal Bracing System for Steel TowersA. Jesumi*, M.G. Rajendran***(PG student, School of Civil Engineering, Karunya University, Coimbatore –641 114)** (Professor, School of Civil Engineering, Karunya University, Coimbatore –641 114)ABSTRACTThe major system providing lateral loadresistance in steel lattice towers is bracing system.There are different types of bracing systems forsteel lattice towers. The heights of these towersvary from 20 to 500 meters, based on the practicalrequirements. This study has focused onidentifying the economical bracing system for agiven range of tower heights. Towers of height 40mand 50m have been analyzed with different typesof bracing systems under wind loads. The diagonalwind has been found to be the maximum fortowers. The optimal bracing system has beenidentified and reported.Keywords - bracing system, steel lattice towers,wind analysis1. INTRODUCTIONTowers are tall steel framework constructionused for different purposes such as communication,radio transmission, satellite receptions, air trafficcontrols, television transmission, power transmission,flood light stands, oil drilling masts, meteorologicalmeasurements, etc. The present paper discussesmicrowave transmission towers. Lattice towers act asvertical trusses and resists wind load by cantileveraction. The bracing members are arranged in manyforms, which carry solely tension, or alternativelytension and compression. The bracing is made up ofcrossed diagonals, when it is designed to resist onlytension. Based on the direction of wind, one diagonaltakes all the tension while the other diagonal isassumed to remain inactive. Tensile bracing is smallerin cross-section and is usually made up of a back-to-back channel or angle sections. The bracings behaveas struts, when it is designed to take compression.One of the most common arrangements is the crossbracing. The most significant dimension of a tower isits height. It is normally several times larger than thehorizontal dimensions. The area which is occupied atthe ground level is considerably limited and so,slender structures are commonly used.The tapered part of the tower is advantageous withregard to the bracing, as it reduces the design forces.The greater the height of the tower, greater will be thedistance it can transmit radio signals. Towers areclassified as Self-supporting towers and Guyedtowers. Self-supporting towers are generally preferredsince they require less base area. Towers are subjectedto gravity loads and horizontal loads.Bracings hold the structure stable by transferring theloads sideways (not gravity, but wind or earthquakeloads) down to the ground and are used to resistlateral loads, thereby preventing sway of the structure.Bracing increases the resistance of the structureagainst side sway or drift. The higher the structure,the more it is exposed to lateral loads such as windload, since it has higher tendency to sway. If thebracing is weak, the compression member wouldbuckle which leads to failure of the tower. Diagonalbraces are efficient elements for developing stiffnessand resistance to wind loads. There are different typesof bracing systems in common use such as Singlediagonal bracing, double diagonal (X-X) bracing, X-Bbracing, XBX bracing, arch bracing, subdivided Vbracing, diamond lattice system of bracing, K, Y, W,X bracings, etc.K. Agarwal and K. Garg (1994) have assessed free-standing lattice towers for wind loads. It has beenfound that large variations have occurred in windloads on towers and there are several gaps in thepresent recommendations which are to be answeredby more rigorous wind tunnel investigations. Thebehavior of cross-bracings in latticed towers wasstudied by Alan R. Kemp and Roberto H. Behncke(1998). The cross bracings have shown complexbehavior and the number of bolts in the connection ofthe bracing to the main legs have been apparent in theresults. M.Selvaraj, S.M.Kulkarni and R.RameshBabu (2012) have investigated on the behavior ofbuilt up transmission line tower from FiberReinforced Polymer (FRP) pultruded sections. Theyhave discussed experimental studies carried out on anX-braced panel of transmission line tower made fromFRP pultruded sections. The upgradation oftransmission towers using a diaphragm bracingsystem was experimented by F. Albermani, M.Mahendran and S. Kitipornchai (2004). Their resultsshowed that considerable strength improvements wereachieved with diaphragm bracings. The upgradingsystem using the most efficient diaphragm bracingtype has been successfully implemented on anexisting 105 m-height TV tower. F. Al-Mashary, A.Arafah and G. H. Siddiqi  have investigated on theeffective bracing of trussed towers against secondarymoments. The study showed that improper bracingconfiguration of the main and/or secondary bracesinduced high secondary moments. Thus, it isnecessary to identify the economical bracing systemfor a given range of tower height.
A. Jesumi, M.G. Rajendran / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 2, March -April 2013, pp.729-732730 | P a g eThe main purpose of the paper is to demarcate theeconomical bracing system of steel lattice towers.Investigations carried out to analyze towers withdifferent heights and different bracing configurationshave been presented. The towers have been analyzedfor wind loads with STAAD Pro., to compare themaximum joint displacement of each tower.Optimized design has been carried out to estimate andto compare the weight of each tower. The results havebeen used to identify the economical bracing systemfor a given range of height of towers.2. GENERAL CONSIDERATIONSIn this study, five steel lattice towers withdifferent bracing configurations such as the X-B,single diagonal, X-X, K and Y bracings have beenmodeled for a given range of height. The heights ofthe towers are 40m and 50m with a base width of 2mand 5m respectively. The tower of height 40m has 13panels and the tower of height 50m has 16 panels. 70-72% of the height is provided for the tapered part and28-30% of the height is provided for the straight partof the tower.3. TOWER ANALYSISThe towers have been modeled usinggeometric coordinates. The member property wasassigned to each member of the structure. The leg,diagonal bracings and horizontal bracings areprovided with angle sections. Elastic modulus,Poisson’s ratio, density, alpha and damping are thematerial properties used for the analysis. Ahemispherical dome was assumed to be mounted atthe top panels of the towers. The towers wereanalyzed considering it to act as a space structure withpin joints, for dynamic wind loading and optimizeddesign was carried out. The displacements andweights of the towers obtained were compared toarrive at an optimal solution.Fig. 1: Models of the towers with different bracingconfigurations for 40m heightThe elevations of the towers of height 40m and 50mwith different bracing configurations are shown infig.1 and 2 respectively.Fig. 2: Models of towers with different bracingconfigurations for 50m height4. LOADINGThe loads act in three mutually perpendiculardirections such as vertical, normal to the face andparallel to the face of the tower. The loads applied tothe towers are based on the codal provisions in IS:875-1987 part 3. Since towers are tall and flexible, itis critical under wind load. The wind load isdetermined by dividing the tower into different panelsof equal heights. The calculated wind load istransferred on each joint of the exposed face of thetower. For square steel lattice towers, the maximumload occurs when wind blows diagonally. Therefore,IS: 875-1987 part 3 recommends the diagonal wind tobe 1.2 times the wind blowing normal to the face. Thevertical loads act centrally and are distributed equallyamong the four legs. Bending moments produce anequal compression in the two legs of one side, andequal tension in the two legs of the other side whenthe wind is considered acting in any one direction.The shear forces are resisted by the horizontalcomponent of the leg forces and the brace forces.Thus, the taper has a major influence on the design ofthe bracing.The design wind pressure is calculated at increasingheights of a tower from the mean ground level withthe following equation:PZ = 0.6VZ2(1)Where,PZ is the design wind pressure in N/m2VZ is the design wind speed in m/sThe design wind speed is obtained taking intoaccount, factors such as risk coefficient, terrainroughness, height, size of the structure and localtopography. It is expressed as:VZ = Vbk1k2k3 (2)Where,Vb is the basic wind speed in m/s
A. Jesumi, M.G. Rajendran / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 2, March -April 2013, pp.729-732731 | P a g ek1 is the risk coefficientk2 is the terrain, height and structure sizefactork3 is the local topography factorThe basic wind speed for the proposed location was39m/s. The wind load is the product of the dynamicwind pressure, the overall force coefficient and theeffective exposed area of the tower. The forcecoefficient for the exposed surface depends on thesolidity ratio. It is expressed as:F = CF.A.PZ (3)Where,F is the force acting on the structureCF is the force coefficientA is the exposed surface area of the structurePZ is the design wind pressure5. LOAD COMBINATIONThe towers have been analyzed for the followingload combinations according to IS: 875-1987 part 3.1. DL + WL with wind blowing normal to theface of the tower2. DL + WL with wind blowing diagonal to theface of the tower3.6. ANALYSISThe steel lattice towers have been analyzed,idealizing it as a 3D truss as per IS: 800-2007, withvarious bracing configurations. The dead loads actingon the tower are self weight of the tower and selfweight of antenna. The wind loads are consideredacting both normal and diagonal to the face of thetower. Wind loads have been computed by MS Exceland have been incorporated in the analysis done bySTAAD Pro.V8i. The joint displacement with respectto normal wind and diagonal wind has been obtainedfrom the analysis for each tower and has beentabulated and shown in table 1. The sizes of leg andbracing members have been checked for themaximum forces computed from the analysis andoptimized designed has been done as per IS: 800-2007and the weight of each tower has been noted andshown in table 2.7. RESULTS AND DISCUSSIONTable 1: Maximum joint displacement of the towerTable 2: Results showing the weight of the tower05010015020040 50WeightofthetowerHeight of the towerX-BSingle DiagonalX-XKYFig. 3: Weight of each towerFrom the above results in table 2 and fig. 3, it hasbeen found that Y bracing is the most appropriatearrangement of bracing system that resists lateralloads for the given range of heights of the towers, as itshows comparatively lesser weight than the otherbracing systems. The demarcated economical bracingsystem is shown in fig. 4.Fig. 4: The economical bracing system of towerThe displacement undergone by the tower is shown infig. 5.
A. Jesumi, M.G. Rajendran / International Journal of Engineering Research and Applications(IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 2, March -April 2013, pp.729-732732 | P a g eFig. 5: Displacement of the tower8. CONCLUSIONIn this paper, analytical studies have beenpresented to find the most appropriate arrangementand cost-effective bracing system of steel latticetowers for the effective resistance against lateralforces. The joint displacement and weights are thesignificant parameters obtained from the analysis.However, there is no sufficient data regarding thepermissible displacement for towers. From the resultsobtained, Y bracing has been found to be the mosteconomical bracing system up to a height of 50m.Further, the study will be carried out for towers ofgreater heights.ACKNOWLEDGEMENTThe authors thank the management of KarunyaUniversity for having provided the necessary facilitiesto carry out this work.REFERENCE K. Agarwal and K. Garg, Wind LoadAssessment on Free Standing Lattice Towers,Indian Institute of Engineers Journal, vol 75,November 1994, pp. 171-177. Alan R. Kemp and Roberto H. Behncke,Behavior of Cross-Bracing in Latticed Towers,Journal of Structural Engineering, April 1998,pp. 360-367. M.Selvaraj, S.M.Kulkarni and R.Ramesh Babu,Behavioral Analysis Of Built Up TransmissionLine Tower From FRP Pultruded Sections,International Journal of Emerging Technologyand Advanced Engineering, Volume 2, Issue 9,September 2012, ISSN 2250-2459. F. Albermani, M. Mahendran and S.Kitipornchai, Upgrading of TransmissionTowers Using a Diaphragm Bracing System,Engineering Structures, 26, pp. 735-744. F. Al-Mashary, A. Arafah and G. H. Siddiqi,Effective Bracing of Trussed Towers againstSecondary Moments,faculty.ksu.edu.sa/2639/Publications%20PDF/TOWER-BR.DOC. Indian Standard Code of Practice for designloads (other than earthquake) for buildings andstructures IS: 875-1987, part 3 Reaffirmed 1997Second Revision), 1989. Bureau of IndianStandards, New Delhi. Indian Standard Code of Practice for designloads (other than earthquake) for buildings andstructures IS: 875-1987, part 5 Reaffirmed 2008Second Revision), 1988. Bureau of IndianStandards, New Delhi.