International Journal of Constructive Research in Civil Engineering (IJCRCE)Volume 2, Issue 2, 2016, PP 1-10ISSN 2454-8693 (Online)www.arcjournals.orgAnalysis and Design of Prestressed Box Girder Bridge byIRC: 112-2011Phani Kumar.ChStudent of final year M.Tech Structural Engineering,Mandava Institute of Engineering and Technology, Andhra PradeshS.V.V.K.Babu, M.TechAssistant Profeessor, Civil Engg. Department,Sri Vasavi Institute of Engineering and Technology, Andhra PradeshD.Aditya Sai Ram, M.TechAssistant Professor, Civil Engg. Department,Mandava Institute of Engineering and Technology, Andhra PradeshAbstract: Bridge construction today has achieved a worldwide level of importance. Bridges are the keyelements in any road network and use of prestress girder type bridges gaining popularity in bridge engineeringfraternity because of its better stability, serviceability, economy, aesthetic appearance and structural efficiency.In this thesis analysis and design of prestressed concrete bridges (Deck Slab, T-Girder and Box Girder) arecarried out using IRC:112-2011. The unified concrete code (IRC:112) published by the Indian Road Congressin November 2011 combining the code for reinforced concrete and prestressed concrete structures represents anew generation code, which is significantly different as compared to previous codes (i.e. IRC:21 for RCCstructures and IRC:18 for PSC structures). IRC:21 and IRC:18 stands withdrawn, with the publication ofIRC:112. The fundamental difference between IRC:112 and old codes is that IRC:112 based on limit statetheory while the previous codes were based on working stress design philosophy.Keywords: prestress, Deck slab, T-slab, Box Girder, IRC:112.1. INTRODUCTIONBridges are defined as structures which are provided a passage over a gap without closing waybeneath. They may be needed for a passage of railway, roadway, footpath and even for carriage offluid, bridge site should be so chosen that it gives maximum commercial and social benefits,efficiency, effectiveness and equality. Bridges are nation‟s lifelines and backbones in the event ofwar. Bridges symbolize ideals and aspirations of humanity. They span barriers that divide, bringpeople, communities and nations into closer proximity. They shorten distances, speed transportationand facilitate commerce. Bridges are symbols of humanity‟s heroic struggle towards mastery of forcesof nature and these are silent monuments of mankind‟s indomitable will to attain it. Bridgeconstruction constitutes an importance element in communication and is an important factor inprogress of civilization, bridges stand as tributes to the work of civil engineers.Classification of Bridges:Bridges are classified based on different criteria as followsAccording to function as aqueduct (canal over a river), viaduct (road or railways valley, pedestrian,highway, railway, road-cum-rail or pipe line bridge.According to material of construction of super structure as timber, masonry, iron, steel, reinforcedconcrete, prestress concrete, composite or aluminum bridge.According to form or type of super structure as slab, beam, truss, arch, suspension bridge.According to inter span relation as simple, continuous and cantilever bridge.According to position of bridge floor relative to superstructure as a deck, trough, half-trough orsuspension bridge.According to span length as culvert (less than 8m), minor bridge (8m to 30m) or long span bridge. ARCPage 1

Phani Kumar.Ch et al.2. BASIC CONCEPT OF PRESTRESSINGPrestressing is the application of an initial load on a structure, to enable it to counteract the stressesarising from subsequent loads during its service period. prestressing has been practiced from ancienttimes the behaviour of spokes of the bicycle when it is loaded, is also example of prestressing.Prestressed concrete is basically a concrete in which internal stresses of a suitable magnitude anddistribution are introduced so that the stresses resulting from external loads are counteracted todesired degree. In reinforced concrete members, the prestress is commonly the steel reinforcement.The minimum grade of concrete in prestressing technique is M40 for pre tensioning and M35 for posttensioning. The tensile strength of concrete is only 8-14% of its compressive strength of concrete.Fig1.1. Behavior of RC member with and without prestressingTypes of Prestress Girders and its Purposes:One of the most commonly used forms of superstructure in concrete bridges is precast girders withcast-in-situ slab. This type of superstructure is generally used for spans between 20 to 40 m. Majorityof prestress concrete bridges, constructed in India are post tension type. The span to depth ratio isusually kept as 20 for simply supported spans and 25 for continuous spans. The girder spacing of 2 to3 m. The deck slab overhang should be provided as required to provide the desirable aesthetic effectand to reduce transfer moments. Different types of girder bridges as shown in Figure 2.1a. Cross section of T-Girder with cast in situ deckb. I-Girder with cast in situ deckc. Box girder with cast in situ deckFig2.1. Different types of girder sections3. FINITE ELEMENT METHODThe finite element method is a technique for analyzing complicated structure by nationality cutting upthe continuum of the prototype into a number of small elements which are connected at discrete jointscalled nodes. For each element, approximate stiffness equation are derived relating the displacementsInternational Journal of Constructive Research in Civil Engineering (IJCRCE)Page 2

Analysis and Design of Prestressed Box Girder Bridge by IRC: 112-2011of the nodes to the node forces between elements and, in the same way that slope defection equationscan be solved for joints in a continuous beam, an electronic computer is used to solved the very largenumber of simultaneous equations that relate node forces and displacements.a) 3 Noded Triangle, 4 Noded Quadrilateral, 6 Noded Triangle, 8 Noded Quadrilateralb) 9 Noded Hexahedron, 10 Noded tetrahedron, 20 Noded Curved solid4. IRC RECOMMENDATIONS ON DESIGN OF BRIDGESThe first and major step in any bridge analysis is selection of type of loading, they are dead load, liveload, impact effect, wind load, longitudinal force due to tractive effort of vehicles, longitudinal forcedue to braking of vehicle, seismic effects, earth pressure, vehicle collision forces etc. Out of theseloads live load plays a major role.Vehicle Live Loads:Vehicle live loads are categorized based on their configuration and intensity as IRC Class 70R, IRCClass AA (tracked and wheeled type), IRC Class A and IRC Class B loading.Load Combinations:All critical loading stages shall be investigated. The stages stated belowAt the stage of prestressingconstruction stages including temporary loading, transport, handling and erection or any occasionalloads that may occur during launching of girdersdesign loads according to IRC:6 that includes service dead load, prestress with full losses andservice dead load, live load and prestress with full lossesFor the combination of loads with differential temperature gradient effects, maximum 50 per centlive load shall be consideredUltimate strength: A prestressed concrete members checked for failure conditions at an ultimateload ofa.1.25 G 2 SG 2.5 Q---under moderate conditionsb.1.5 G 2 SG 2.5 Q--- under severe exposure conditions.For sections, where the dead load causes effects opposite to those of live loads shall be checkedfor G SG 2.5 Q.Calculation of Ultimate Strength:There are two conditions of failure at which strength should be calculated and minimum of these shallbe considered for design. They area) Failure by yield of steelMult 0.9 db As fp(2.1)b) Failure by crushing of concreteMult 0.176 b db2 fckInternational Journal of Constructive Research in Civil Engineering (IJCRCE)(2.2)Page 3

Phani Kumar.Ch et al.5. ANALYSIS OF BOX GIRDER BRIDGESThe methods for the analysis of box girder bridges are as followsSimple line analysis or beam analysisGrillage analysisBEF Analysis (Beams on elastic foundation)Space frame analysisFinite element methodFor study of box girder bridges finite element method is more accurate method.5.1. Description of ModelLoading on Box Girder Bridge:The various type of loads, forces and stresses to be considered in the analysis and design of thevarious components of the bridges are given in IRC 6.Thickness of Web:The thickness of the web shall not be less than d/36 plus twice the clear cover to the reinforcementplus diameter of the duct hole where„d‟ is the overall depth of the box girder measured from the top ofthe deck slab to the bottom of the soffit or 200 mm plus the diameter of duct holes, whichever isgreater.Thickness of Bottom Flange:The thickness of the bottom flange of box girder shall be not less than 1/20 th of the clear web spacingat the junction with bottom flange or 200 mm whichever is more.Thickness of Top Flange:The minimum thickness of the deck slab including that at cantilever tips be 200 mm. For top andbottom flange having prestressing cables, the thickness of such flange shall not be less than 150 mmplus diameter of duct hole.Losses in Prestress:While assessing the stresses in concrete and steel during tensioning operations and later in service,due regard shall be paid to all losses and variations in stress resulting from creep of concrete,shrinkage of concrete, relaxation of steel, the shortening (elastic deformation) of concrete at transfer,and friction and slip of anchorage.Calculation of Ultimate Strength:Ultimate moment resistance of sections, under these two alternative conditions of failure shall becalculated by the following formulae and the smaller of the two values shall be taken as the ultimatemoment of resistance for design.Failure by Yield of SteelMult 0.9 db As FpFailure by Crushing of ConcreteMult 0.176 b db2 fck(5.1)6. ANALYSIS AND DESIGN OF POST TENSIONED DECK TYPE BOX GIRDER BRIDGEA post tensioned deck type box girder bridge of clear span 30 m and width of roadway is 7.5 m isconsidered for the analysis. Live loads are taken as per IRC:6. Cross section of box girder is shown inFigure 6.1 and mathematical modeling is done using SAP2000 and is shown in Figure 5.3. Theoverhang face of the girder is 1.2 m and deck slab thickness is 0.25 m. Bottom slab thickness is 0.25m and girder thickness is 0.35 m. Material properties used are M50 grade of concrete and Fe415 gradesteel. The tendon profile considered is parabolic in nature. The Bridge analysis for different span todepth ratios (L/d) ratio starting from 15 to 19 and different span to depth ratios (L/d) are considered asfollows.International Journal of Constructive Research in Civil Engineering (IJCRCE)Page 4

Analysis and Design of Prestressed Box Girder Bridge by IRC: 112-2011Case1 L/d 15, d 2.0Case2 L/d 16, d 1.9Case3 L/d 17, d 1.8Case4 L/d 18, d 1.7Case5 L/d 19, d 1.6(5.2)6.1Cross Section of Box-Girder6.1. Mathematical ModelingThe mathematical model of a box girder bridge having a span of 30 m is shown in Figure6.2Fig6.2. Modeling of box girder bridgeThe tendon profile considered for the design of post tensioned box girder bridge is parabolic andmathematical model is shown in Figure 6.3Fig6.3. Tendon profile6.2. Validation of ResultsThe bending moment, shear force and deflection results are obtained by using SAP2000. The bendingmoment and shear force are obtained by considering different loading conditions consisting of deadload, super imposed dead load and live load. The results are shown below for case 1. The variation ofbending moment and shear force along the length up to mid span is shown in Tables 6.1 and 6.2.International Journal of Constructive Research in Civil Engineering (IJCRCE)Page 5

Phani Kumar.Ch et al.Table 6.1. Bending moment variation along span (tm)Span 817222190.4L39235670.5L0000Table 6.2. Shear variation along span (kN)Span 7141880.3L78419128Moment due to DL SIDL (Mg) 1631 tmTotal Maximum moment (Mt) 2219 tmInitial Stresses:fck 50 MPafci 40 MPafct 20 MPafcw 16.5 MPaftt 2 MPaftw 0 MPafbr 16 MPaLoss ratio 0.8The variation of prestress force, eccentricity and number of cables with respect to span to depth ratiosare summarized in Table6.3Table 6.3. Calculation of prestress force and eccentricityL/d1516171819fsup (MPa)1.571.591.61.621.64finf (MPa)0.810.780.760.730.7Prestressing Force P (kN)54285370530952515194e (mm)850800750700650using the freyssinet system, anchorage type 7K-15 in 65mm cable ducts (IS:6006-1983)No of Cables44444The following checks are performed for the above mentioned case1.Check for Section Modulus:Required section modulus Zreq 571 x 106 mm3Provided section modulus Zpro 341 x 108 mm3Zpro Zreq, Hence the section provided is adequate.Check for Stresses:At transfer stageStress at top 1.52 MPa fctStress at bottom 0.87 MPa fttAt working stageStress at top 1.4 MPa fcwStress at bottom 0.48 MPa (As per IS1343:1980 there is no tensile stress)International Journal of Constructive Research in Civil Engineering (IJCRCE)Page 6

Analysis and Design of Prestressed Box Girder Bridge by IRC: 112-2011All the stresses at top and bottom fibers at transfer and service loads are well within the permissiblelimits.Check for Flexural Strength:For the centre of span sectionAccording to IRC: 18-2000,Mu 1.5 M(G) 2 M(SG) 2.5 M(Q)Mu 40025 kNmThe ultimate flexural strength is calculated as followsFailure by Yielding of SteelMu 0.9 db Ap fpMu 118244 kNmFailure by Crushing of ConcreteMu 0.176 bw d2fck (2/3) 0.8 (b-bw) (d-0.5Df) Df fckMu 234556 kNmThe ultimate flexural strength Mu 118244 40025 kNm, Hence safe.Deflection Check:Table 6.4. Check for deflectionSpan/Depth Prestressing Force (kN)155428Eccentricity (mm)850Deflection (mm