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approximately 12.5 m and carried two lanes of rural trafc.replacement, this proved to be a good test site for the GFRPbridge deck panels. The existing deck was removed along withthe old girders. In their place, NCDOT erected seven newW610X140 girders spaced at 1.194 m and topped them withGFRP panels. The girders were connected with C310X31 crossframes at the midspan of each girder and at the quarter point ofthe exterior girders. The abutments were cast semiintegral withthe girders and GFRP panels.Several objectives were established for the testing of theGFRP panels. The rst objective was to determine if the GFRPpanels acted compositely with the steel girders. Secondly, ifcomposite action was observed, could it be predicted using basictransformed section analysis? There was also the question ofhow the live load was distributed across the structure. Anotherobjective was to observe the performance of the bridge itself todetermine if the American Association of State Highway andTransportation OfcialsAASHTOlive load deection criteriawere being met. The nal objective was to examine and quantifythe amount of stress and strain applied to the GFRP compositematerial.InstrumentationIn order to measure the strains and deections of the bridge underload, there were several instruments selected for the variousmembers being evaluated. There were displacement transducersDT, strain transducersST, and foil strain gaugesSGused forthe testing. The foil strain gauges were used to evaluate the strainin the steel girders, strain transducers were used to evaluate theGFRP deck panels, and displacement transducers were used onboth materials. Fig. 1 shows the positions of the instrumentationmentioned in this paper.Testing, Analysis, and Evaluation of a GFRP Deckon Steel GirdersWilliam B. Stiller, P.E., M.ASCE1; Janos Gergely, P.E., M.ASCE2; and Rodger Rochelle, P.E.3Abstract:North Carolina has recently installed a ber-reinforced polymerFRPdeck on steel girders at a site in Union County. Thebridge was instrumented with foil strain gauges, strain transducers, and displacement transducers. The bridge was then tested with asimulated MS-22.5 design load. Experimental data conrmed full composite interaction between the girders and the FRP deck panels. Theneutral axis was measured to be 383 mm above the bottom ange of the 618-mm-deep girder. It was found that composite action couldbe estimated within 3% using a transformed section analysis of the deck panels. For two lanes loaded, the maximum live load distributionfactor was computed to be 0.75. When looking at the overall performance of the structure, the deck deected 5 mm, with the allowablestress at least 10 times over the maximum stress measured in the material. The girder deection of 7 mm was well within the parametersset forth by AASHTO. Simple span deection equations were found to conservatively model the anticipated deection of the girders whenusing the transformed section properties.DOI:10.1061/ASCE1084-0702200611:4394CE Database subject headings:Composite materials; Steel beams; Bridge decks; Load tests; North Carolina; Fiber reinforcedpolymers.IntroductionDeemed by NCDOT inspectors as being in need of repair orMost bridges in North Carolina, and across the country, have beenbuilt using traditional materials, such as concrete and steel.However, the space age has introduced new technology andmaterials to the engineering community, such as glass ber-reinforced polymerGFRP. Most recently, the state of NorthCarolina has used GFRP deck panels in a bridge replacementproject. In this project, composite deck panels were used in placeof a traditional concrete deck slab. The project was funded with adiscretionary grant from the Federal Highway AdministrationsInnovative Bridge Research and Construction Program.The University of North Carolina at Charlotte UNC-Charlotte, in conjunction with the North Carolina Departmentof Transportation NCDOT arranged a performance test ofNorth Carolinas rst composite deck panel system on November18, 2001. This system was installed for structure #089-022on October 18, 2001. Located in Union County, on New SalemRoadSR1627, the bridge was originally constructed in 1950consisting of 11 steel girders with a concrete deck. It spanned1Design Engineer, Ralph Whitehead Associates Inc., 454 S. AndersonRd., Suite 3, Rock Hill, SC 29730. E-mail: brad.stiller2Associate Professor, Dept. of Civil Engineering, Univ. of NorthCarolinaCharlotte, 9201 University City Blvd., Charlotte, NC 28223.E-mail: jgergely3State Alternate Delivery Systems Engineer, North Carolina Dept. ofTransportation, 1591 Mail Service Ctr., Raleigh, NC 27699. E-mail:rdrochelledot.state.nc.usNote. Discussion open until December 1, 2006. Separate discussionsmust be submitted for individual papers. To extend the closing date byone month, a written request must be led with the ASCE ManagingEditor. The manuscript for this paper was submitted for review and pos-sible publication on May 27, 2005; approved on September 28, 2005.This paper is part of theJournal of Bridge Engineering, Vol. 11, No. 4,July 1, 2006. ASCE, ISSN 1084-0702/2006/4-394400/$25.00.394/ JOURNAL OF BRIDGE ENGINEERING ASCE / JULY/AUGUST 2006J. Bridge Eng. 2006.11:394-400.together. The panels being tested were connected to the girderspockets per girdersee Fig. 4, each of which allowed the place-ment of three shear studs on the girder. Once the studs werewelded in place, the pockets were lled with groutsee Fig. 5,thus attaching the two materials together.Previous testing of a composite deck to steel girder connectionin Ohio resulted in the conrmation of composite action betweenthe two materialsDefense Advanced Research Projects Agency2000. However, similar testing of a structure in New Yorkby Alampalli and Kunin 2001 showed no composite actionbetween the panels and the supporting stringers. Because theGFRP panels and connection method are slightly different thanthose previously tested, the designers of structure #089-022conservatively chose to assume no composite action would occurbetween the girders and the GFRP deck panels. This assumptionof no composite action was later checked with the experimentaldata.One method to prove composite or noncomposite action in thebridge would be to locate the neutral axis of the girders. Themidspan of girders 2 and 4 were instrumented with strain gaugeson the top and bottom ange as indicated in Fig. 1. The gaugeswere placed on the bottom of the bottom ange and the bottom ofthe top ange. These instruments were used to help locate theposition of the neutral axis. Assuming the strain in the girderFig. 2.Loading truck axle weight and spacingFig. 1.Location and type of selected instrumentationLoading Methodthrough prefabricated shear stud pockets. Each panel had fourThe load test of the structure was designed to imitate the momentresponse of the structure to MS-22.5 loading. The MS-22.5 wascreated to model a fully loaded highway tractor-trailer. For thestructure tested, the MS-22.5 truck load with 4.3 m rear axlespacing governed over the lane load for both girder shear andmoment.In order to test the structure at the service level, researchersneeded to match as closely as possible the loading condition ofan MS-22.5 truck. A fully loaded with gravel and waterconcrete truck was used with a total weight of 305.3 kN. Usinginformation about the axle weight and spacing Fig. 2, themaximum moment was computed. This computation assumed asimply supported structure and yielded a maximum moment of695.9 kN-m, which was close to the maximum design moment of762.4 kN-m based on MS-22.5.The structure was tested along ve paths. The paths wereselected to place a wheel load along the centerline of girders G2and G6, the middle of bays 2 and 5, and on the centerline of thebridge. The objective was to select the paths that would place themost stress on both the steel girders and the GFRP panels, as wellas to provide redundancy in the test for comparative purposes.The load paths selected are shown in Fig. 3. In all ve paths, thetruck was driven less than 8 km/h across the span. This was doneto negate the effect of impact and to allow the instrumentation togather as much data as possible. The results from load paths 1 and4 were superimposed to simulate two lanes being simultaneouslyloaded.Composite ActionWhen looking at GFRP composite panels as a replacement forconcrete decks, it is desirable to have the panels connected to thegirders in a manner that will allow the two materials to workJOURNAL OF BRIDGE ENGINEERING ASCE / JULY/AUGUST 2006 /395J. Bridge Eng. 2006.11:394-400.Ht BBTof girder mm; As=area of the steel girder 17,900 mm2;H=height of steel girder 617 mm; h=height of the GFRPdeck panel 195 mm; btran=transformed width of the section103 mm; andTpf=panel ange thickness17 mm.This conventionally transformed section converts the width ofthe tributary area, by using the modular ratio, into an equivalentarea of steel. For a concrete slab of uniform thickness and con-sistency, it is a good assumption that the entire slab section actshomogenously despite the nominal reinforcing steel. However,the GFRP deck panels are composed of hollow tubes. For thepurpose of the transformed section analysis, in this paper it wasassumed that the material in the web of the tubes did not addsignicant exural resistance to the steel girders. The material inthe top and bottom plates of the tubes was assumed to handle themajority of the compressive force.Fig. 5.Shear stud pocket layout for typical GFRP panelAs + btran2Tpf H+1H hyb=2 2As+btran2 Tpf2Fig. 3.Truck loading pathsFig. 4.Typical deck to girder connectionremains linear, which should be the case for girder stresses lowerthan the yield level, the neutral axis as measured from the bottomof the girderycan be calculated asy=whereyb=transformed section neutral axis location from bottomwhere H=girder depth mm; t=top flange thickness mm;B=bottom flange strain mm/mm; and T=top flange strainmm/mm. The top and bottom ange strains at the midpoint ofgirder 4 were examined from the data collected from all ve loadpaths. The data from path 2 indicated a maximum bottom angestrain of 130.0 and a maximum top ange strain of 78.2.With a girder depth of 618 mm and a top ange thicknessof 22 mm, the neutral axis was computed using Eq.1to beapproximately 372 mm. This computation was repeated for theother load paths with an average neutral axis location of 369 mm.In order to verify the measured neutral axis location on Girder4, the same load paths were considered for the top and bottomange gauges at the midspan of Girder 2. When the neutral axis atthe midpoint of Girder 2 was computed for all ve load paths, theposition of the neutral axis was computed to be an average of383 mm above the bottom ange of the girder. With the neutralaxis of the noncomposite girder being located at 309 mm from thebottom ange, there was composite action taking place.The next step in the analysis of the composite action was todetermine if it could be proven using material properties and thelayout of the girders. Current design methods use a transformedsection in order to estimate the neutral axis of a compositesection. Using a similar procedure, the modular ratio of thesteel and the composite material was used to estimate the neutralaxis location. Using the manufacturers informationDuraSpan2002 for the modulus of elasticity of the deck panelsEFRP=17,240 MPa and the steel modulus of elasticityEs=200,000 MPa, the modular ratio was computed as 11.6.Using this ratio and the 1,194 mm girder spacing as the tributarywidth, the transformed width of deck panel for the section wascomputed to be 103 mm.Using this transformed area and the dimensions of the girder,the neutral axis was computed using Eq.2, to be located at375 mm above the bottom ange of the girder:396/ JOURNAL OF BRIDGE ENGINEERING ASCE / JULY/AUGUST 2006J. Bridge Eng. 2006.11:394-400.Using the information gained from these instruments, the DF wascomputed using Eq.3, developed by Stallings and Yoo1993:DFi=jwjj=1wheren=number of wheel lines on the bridgei=strain at thebottom of theith girdermm/mmj=strain at the bottom of thejth girdermm/mm;wj=ratio of section modulus of thejthgirder to the typical interior girder; andk=number of girders.According to AASHTO, a structure with a road width greatereach girder. The superposition of load paths 1 and 4 would bestrepresent this condition. The resulting distribution factors for thegirders are shown in Table 1. The maximum distribution factor forthis condition was found to be 0.75, which occurs at girder 4.When compared to the spacing of the girders, this DF=S/1,592.design value used for this bridge.Bridge DeectionMidspan girder deections under each load path were recordedfor girders 1, 4, 5, 6, and 7. Superimposing load paths 1 and 4, tothe deections for future structures was desired. For a simple spannik3Table 1.Live Load Distribution FactorsDistribution factorGirder Path 1 Path 4 Two lanesG1 0.00 0.33 0.33G2 0.09 0.51 0.59G3 0.22 0.49 0.70G4 0.38 0.37 0.75G5 0.48 0.22 0.71G6 0.50 0.08 0.58G7 0.33 0.00 0.33Total 2 2 4than 6.0 m should be designed for two lanes loaded simul-taneously, each equal to half the roadway width. All ve loadpaths were examined and the distribution factor computed forThe measured neutral axis locations have been averagedfor each location. The average measured neutral axis location forthe midspan of girder 2 is 383 mm. When compared to the375 mm computed neutral axis using the material properties and atransformed section, the difference is less than 3%.This shows a moment distribution that is centered between thevalues estimated by AASHTOfor a concrete deck and theLive Load DistributionPresent NCDOT design practice uses distribution factorsDFto estimate what percentage of the live load being appliedgets transmitted to the girder being designed. The AmericanAssociation of State Highway and Transportation Ofcialsspecications AASHTO 2002 assume that DF=S/1,680AASHTO Eq. .1.5for a concrete slab supported bysimulate two lanes loaded, yielded a maximum deection ofmore than four steel girders, whereS=girder spacing in milli-girder 4 equal to 7 mm. The live load deection of the structure ismeters, and DF is the percentage of a line of wheel loads appliedgoverned by the AASHTO and is established at a limit ofL/800to the girder. The designers of structure #089-022 were concernedAASHTO 10.6.2. The maximum deection of the structurewith how accurate this assumption would be for the compositewas approximatelyL/1,680, which was well within the limitdeck. Therefore, the designers of the deck panels used a conser-established by the AASHTO.vative distribution factor ofS/1,525, resulting in a DF=0.783. With the knowledge that the displacements of the structureThe test of the bridge determined the distribution factors appli-were well within the limits established, a method of estimatingcable to girders of the structure.Strain readings were observed to determine the load beingbridge, the maximum displacement is anticipated to occur close totaken by the various girders. The bottom ange of each girder wasthe center of the bridge span. Using the simple span assumption,instrumented with a strain gauge along the midspan of the bridge.a model was generated for an interior girder to validate theFig. 6.Girder 6 deection comparisonJOURNAL OF BRIDGE ENGINEERING ASCE / JULY/AUGUST 2006 /397J. Bridge Eng. 2006.11:394-400.Attention was placed on the data gathered when the loadingtruck was following path 2. In path 2, one line of wheels wascentered between girders 5 and 6. This path placed the load onthe side of the bridge where more displacement transducers werepositioned to record the relative deformation of deck panel 3. Thisside of the bridge also contained more strain transducers,mounted to the bottom of the panel. These instruments wereproved to measure accurate strain levels in the composite materialSchiff and Philbrick 1999.The deection of the bays was plotted in Fig. 7 as the trucktraveled along load path 2. From this graph, one can see themaximum deection of the panel to be 5 mm. It was also notedthat the panel seemed to camber up slightly in bay 6. This camberreached a maximum value of 2 mm.The nal area to be investigated was the strain in the panelsthemselves. Being comprised of glass ber-reinforced polymerlaminates with various orientations, the strain in the bers of thepanels was not evaluated, and there was no allowable straingiven by the panel manufacturer for the bers themselves. Themanufacturer provided designers an overall allowable strain onthe bottom of the deck panel for compression2,600 andtension2,600.The strains in bays 5 and 6 were examined for load path 2seeFig. 8. The maximum tensile strain was observed in bay 5. Themaximum strain in the panel was approximately 240. There-fore, the allowable strain in the material is over ten times largerthan the strain experienced in the panel. The maximum negativestrain was recorded in bay 6. The strain at this location reached8. Once again, the strain induced in the material did not evencome close to reaching the strain allowed in the material.ConclusionResearch was conducted to determine whether or not compositeaction could be depended upon for a GFRP composite deckattached to a steel girder. From the strain data gathered fromthe girders, it was determined that there was indeed compositeaction taking place between the girders and the GFRP panels.The neutral axis was measured to be approximately 380 mm fromthe bottom ange. This was consistent at various points along theFig. 7.Panel deection under load path 2deection measurements recorded during testing. The modelused the simple span deection equations given by the AmericanInstitute of Steel ConstructionAISC 1998. These equations usethe applied load, the load location, and the composite sectionproperties to compute the deection of the girder at any point.This deection was computed at 1 m increments along the spanof the bridgesee Fig. 6.The composite section moment of inertia, based upon thetransformed section, was used along with the modulus of elastic-ity of steel to model the girder section. In order to estimate theamount of the axle load applied to the model from the loadingtruck, the distribution factor for one lane loaded was used. This0.50 distribution factorDFwas estimated from test data and wasused to convert the axle load to a girder load.The maximum bridge deection recorded was determined tobe 5 mm, and it occurred during load path 1 at girder 6. Thequarter point, midspan, and three quarter point deections wereplotted for girder 6 with the path 1 loadingFig. 6.Looking at the measured deections versus the deectionscomputed using the section properties and the simple span beamequations, there was a noticeable difference between the com-puted and measured values. This difference was attributed to therotational restraint provided by the semiintegral end bents. It wasassumed that, with the restraint at the end of the girders, the spanwould not behave as fully xed or simply supported, but some-where in between. In order to verify this, the model was adjustedto use the xed end beam equations given by AISC1998.Once all three plots were compared in Fig. 6, it was apparentthat the girder deections measured were between the valuesestimated using the xed support and simple support models.Based on this information, it was determined that the assumptionof a simply supported structure was somewhat conservative.GFRP PerformanceAnother aspect of the testing involved the performance of theGFRP panels themselves. Researchers examined the reaction ofthe deck panels to the applied loading. The two areas of interestwere the GFRP composite deck panel deections and strains.The readings of selected strain transducers and displacementtransducersinstrument positions seen in Fig. 1were used in theevaluation of these two areas.398/ JOURNAL OF BRIDGE ENGINEERING ASCE / JULY/AUGUST 2006J. Bridge Eng. 2006.11:394-400.have found them to stand up to a simulated life of 75 years underMS-22.5 loadingDuraSpan2002. The strain readings taken inthis study lend considerable credence to this previous research.AcknowledgmentsThe writers would like to express their appreciation to theNorth Carolina Department of Transportation and Martin MariettaComposites for their support in providing bridge plans anddata for this research. Our thanks is also extended to Dr. ReidCastrodale of Stalite for his invaluable comments.NotationThe following symbols are used in this paper:As area of steel girder;B steel girder bottom ange strain;btran transformed width;DF distribution factor;DT displacement transducer;EFRP modulus of elasticity of GFRP panel;Es modulus of elasticity of steel girder;H height of steel girder;h height of GFRP panel;k number of girders;n number of wheel lines;S girder spacing;SG strain gauge;ST strain transducer;T steel girder top ange strain;Tpf ange thickness of GFRP panel;t steel girder top ange thickness;wj ratio of section modulus ofjth girder to typicalinterior girder;y measured neutral axis location from bottom ofgirder;yb transformed section neutral axis location frombottom of girder;i strain at bottom ofith girder; andj strain at the bottom ofjth girder.Fig. 8.Panel strain under load path 2girder, as well as under various load conditions. By using themodulus of elasticity of the composite and the girder, a standardtransformed section was computed for the section. With the trans-formed section model, the neutral axis was estimated at 375 mmabove the bottom ange.It was thus concluded that there was composite action takingplace between the deck panel and the girders. This compositeaction could be effectively modeled using a transformed sectionanalysis and should be counted upon in future designs incorpor-ating a similar deck to girder connection.The distribution of the live load was determined from thestrain readings for two lanes loaded. The maximum distributionfactor was approximatelyS/1,592. This is between the values ofS/1,680, recommended by AASHTOfor a concrete deck, andS/1,525, recommended by the panel designers.For bridge #089-022, the maximum girder deection for twolanes loaded was estimated to be 7 mm. This corresponds to aspan ratio ofL/1,680, which is well within the limitL/800established by the AASHTO.Using the transformed section properties along with thestandard beam equations provided by the AISC, a model wasgenerated that provided a conservative estimation of the bridgedeect
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