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Testing Analysis and Evaluation of a GFRP Deck on Steel Girders William B Stiller P E M ASCE1 Janos Gergely P E M ASCE2 and Rodger Rochelle P E 3 Abstract North Carolina has recently installed a fi ber reinforced polymer FRP deck on steel girders at a site in Union County The bridge was instrumented with foil strain gauges strain transducers and displacement transducers The bridge was then tested with a simulated MS 22 5 design load Experimental data confi rmed full composite interaction between the girders and the FRP deck panels The neutral axis was measured to be 383 mm above the bottom fl ange of the 618 mm deep girder It was found that composite action could be estimated within 3 using a transformed section analysis of the deck panels For two lanes loaded the maximum live load distribution factor was computed to be 0 75 When looking at the overall performance of the structure the deck defl ected 5 mm with the allowable stress at least 10 times over the maximum stress measured in the material The girder defl ection of 7 mm was well within the parameters set forth by AASHTO Simple span defl ection equations were found to conservatively model the anticipated defl ection of the girders when using the transformed section properties DOI 10 1061 ASCE 1084 0702 2006 11 4 394 CE Database subject headings Composite materials Steel beams Bridge decks Load tests North Carolina Fiber reinforced polymers Introduction Most bridges in North Carolina and across the country have been built using traditional materials such as concrete and steel However the space age has introduced new technology and materials to the engineering community such as glass fi ber reinforced polymer GFRP Most recently the state of North Carolina has used GFRP deck panels in a bridge replacement project In this project composite deck panels were used in place of a traditional concrete deck slab The project was funded with a discretionary grant from the Federal Highway Administration s Innovative Bridge Research and Construction Program TheUniversityofNorthCarolinaatCharlotte UNC Charlotte in conjunction with the North Carolina Department of Transportation NCDOT arranged a performance test of North Carolina s fi rst composite deck panel system on November 18 2001 This system was installed for structure 089 022 on October 18 2001 Located in Union County on New Salem Road SR1627 the bridge was originally constructed in 1950 consisting of 11 steel girders with a concrete deck It spanned approximately 12 5 m and carried two lanes of rural traffi c Deemed by NCDOT inspectors as being in need of repair or replacement this proved to be a good test site for the GFRP bridge deck panels The existing deck was removed along with the old girders In their place NCDOT erected seven new W610X140 girders spaced at 1 194 m and topped them with GFRP panels The girders were connected with C310X31 cross frames at the midspan of each girder and at the quarter point of the exterior girders The abutments were cast semiintegral with the girders and GFRP panels Several objectives were established for the testing of the GFRP panels The fi rst objective was to determine if the GFRP panels acted compositely with the steel girders Secondly if composite action was observed could it be predicted using basic transformed section analysis There was also the question of how the live load was distributed across the structure Another objective was to observe the performance of the bridge itself to determine if the American Association of State Highway and Transportation Offi cials AASHTO live load defl ection criteria were being met The fi nal objective was to examine and quantify the amount of stress and strain applied to the GFRP composite material Instrumentation In order to measure the strains and defl ections of the bridge under load there were several instruments selected for the various members being evaluated There were displacement transducers DT strain transducers ST and foil strain gauges SG used for the testing The foil strain gauges were used to evaluate the strain in the steel girders strain transducers were used to evaluate the GFRP deck panels and displacement transducers were used on both materials Fig 1 shows the positions of the instrumentation mentioned in this paper 1Design Engineer Ralph Whitehead Associates Inc 454 S Anderson Rd Suite 3 Rock Hill SC 29730 E mail brad stiller 2Associate Professor Dept of Civil Engineering Univ of North Carolina Charlotte 9201 University City Blvd Charlotte NC 28223 E mail jgergely uncc edu 3State Alternate Delivery Systems Engineer North Carolina Dept of Transportation 1591 Mail Service Ctr Raleigh NC 27699 E mail rdrochelle dot state nc us Note Discussion open until December 1 2006 Separate discussions must be submitted for individual papers To extend the closing date by one month a written request must be fi led with the ASCE Managing Editor 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 the Journal of Bridge Engineering Vol 11 No 4 July 1 2006 ASCE ISSN 1084 0702 2006 4 394 400 25 00 394 JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2006 J Bridge Eng 2006 11 394 400 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved Loading Method The load test of the structure was designed to imitate the moment response of the structure to MS 22 5 loading The MS 22 5 was created to model a fully loaded highway tractor trailer For the structure tested the MS 22 5 truck load with 4 3 m rear axle spacing governed over the lane load for both girder shear and moment In order to test the structure at the service level researchers needed to match as closely as possible the loading condition of an MS 22 5 truck A fully loaded with gravel and water concrete truck was used with a total weight of 305 3 kN Using information about the axle weight and spacing Fig 2 the maximum moment was computed This computation assumed a simply supported structure and yielded a maximum moment of 695 9 kN m which was close to the maximum design moment of 762 4 kN m based on MS 22 5 The structure was tested along fi ve paths The paths were selected to place a wheel load along the centerline of girders G2 and G6 the middle of bays 2 and 5 and on the centerline of the bridge The objective was to select the paths that would place the most stress on both the steel girders and the GFRP panels as well as to provide redundancy in the test for comparative purposes The load paths selected are shown in Fig 3 In all fi ve paths the truck was driven less than 8 km h across the span This was done to negate the effect of impact and to allow the instrumentation to gather as much data as possible The results from load paths 1 and 4 were superimposed to simulate two lanes being simultaneously loaded Composite Action When looking at GFRP composite panels as a replacement for concrete decks it is desirable to have the panels connected to the girders in a manner that will allow the two materials to work together The panels being tested were connected to the girders through prefabricated shear stud pockets Each panel had four pockets per girder see Fig 4 each of which allowed the place ment of three shear studs on the girder Once the studs were welded in place the pockets were fi lled with grout see Fig 5 thus attaching the two materials together Previous testing of a composite deck to steel girder connection in Ohio resulted in the confi rmation of composite action between the two materials Defense Advanced Research Projects Agency 2000 However similar testing of a structure in New York by Alampalli and Kunin 2001 showed no composite action between the panels and the supporting stringers Because the GFRP panels and connection method are slightly different than those previously tested the designers of structure 089 022 conservatively chose to assume no composite action would occur between the girders and the GFRP deck panels This assumption of no composite action was later checked with the experimental data One method to prove composite or noncomposite action in the bridge would be to locate the neutral axis of the girders The midspan of girders 2 and 4 were instrumented with strain gauges on the top and bottom fl ange as indicated in Fig 1 The gauges were placed on the bottom of the bottom fl ange and the bottom of the top fl ange These instruments were used to help locate the position of the neutral axis Assuming the strain in the girder Fig 1 Location and type of selected instrumentation Fig 2 Loading truck axle weight and spacing JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2006 395 J Bridge Eng 2006 11 394 400 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved remains linear which should be the case for girder stresses lower than the yield level the neutral axis as measured from the bottom of the girder y can be calculated as y H t B B T 1 whereH girder depth mm t top flange thickness mm B bottom flange strain mm mm andT top flange strain mm mm The top and bottom fl ange strains at the midpoint of girder 4 were examined from the data collected from all fi ve load paths The data from path 2 indicated a maximum bottom fl ange strain of 130 0 and a maximum top fl ange strain of 78 2 With a girder depth of 618 mm and a top fl ange thickness of 22 mm the neutral axis was computed using Eq 1 to be approximately 372 mm This computation was repeated for the other load paths with an average neutral axis location of 369 mm In order to verify the measured neutral axis location on Girder 4 the same load paths were considered for the top and bottom fl ange gauges at the midspan of Girder 2 When the neutral axis at the midpoint of Girder 2 was computed for all fi ve load paths the position of the neutral axis was computed to be an average of 383 mm above the bottom fl ange of the girder With the neutral axis of the noncomposite girder being located at 309 mm from the bottom fl ange there was composite action taking place The next step in the analysis of the composite action was to determine if it could be proven using material properties and the layout of the girders Current design methods use a transformed section in order to estimate the neutral axis of a composite section Using a similar procedure the modular ratio of the steel and the composite material was used to estimate the neutral axis location Using the manufacturer s information DuraSpan 2002 forthemodulusofelasticityofthedeckpanels EFRP 17 240 MPa andthesteelmodulusofelasticity Es 200 000 MPa the modular ratio was computed as 11 6 Using this ratio and the 1 194 mm girder spacing as the tributary width the transformed width of deck panel for the section was computed 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 at 375 mm above the bottom fl ange of the girder yb As H 2 btran 2 Tpf H h 2 As btran 2 Tpf 2 where yb transformed section neutral axis location from bottom of girder mm As area of the steel girder 17 900 mm2 H height of steel girder 617 mm h height of the GFRP deck panel 195 mm btran transformed width of the section 103 mm and Tpf panel fl ange thickness 17 mm This conventionally transformed section converts the width of the tributary area by using the modular ratio into an equivalent area of steel For a concrete slab of uniform thickness and con sistency it is a good assumption that the entire slab section acts homogenously despite the nominal reinforcing steel However the GFRP deck panels are composed of hollow tubes For the purpose of the transformed section analysis in this paper it was assumed that the material in the web of the tubes did not add signifi cant fl exural resistance to the steel girders The material in the top and bottom plates of the tubes was assumed to handle the majority of the compressive force Fig 3 Truck loading paths Fig 4 Typical deck to girder connection Fig 5 Shear stud pocket layout for typical GFRP panel 396 JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2006 J Bridge Eng 2006 11 394 400 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved The measured neutral axis locations have been averaged for each location The average measured neutral axis location for the midspan of girder 2 is 383 mm When compared to the 375 mm computed neutral axis using the material properties and a transformed section the difference is less than 3 Live Load Distribution Present NCDOT design practice uses distribution factors DF to estimate what percentage of the live load being applied gets transmitted to the girder being designed The American Association of State Highway and Transportation Offi cials specifi cations AASHTO2002 assumethatDF S 1 680 AASHTO Eq 3 23 2 3 1 5 for a concrete slab supported by more than four steel girders where S girder spacing in milli meters and DF is the percentage of a line of wheel loads applied to the girder The designers of structure 089 022 were concerned with how accurate this assumption would be for the composite deck Therefore the designers of the deck panels used a conser vative distribution factor of S 1 525 resulting in a DF 0 783 The test of the bridge determined the distribution factors appli cable to girders of the structure Strain readings were observed to determine the load being taken by the various girders The bottom fl ange of each girder was instrumented with a strain gauge along the midspan of the bridge Using the information gained from these instruments the DF was computed using Eq 3 developed by Stallings and Yoo 1993 DFi n i j 1 k jwj 3 where n number of wheel lines on the bridge i strain at the bottom of the ith girder mm mm j strain at the bottom of the jth girder mm mm wj ratio of section modulus of the jth girder to the typical interior girder and k number of girders According to AASHTO a structure with a road width greater than 6 0 m should be designed for two lanes loaded simul taneously each equal to half the roadway width All fi ve load paths were examined and the distribution factor computed for each girder The superposition of load paths 1 and 4 would best represent this condition The resulting distribution factors for the girders are shown in Table 1 The maximum distribution factor for this 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 This shows a moment distribution that is centered between the values estimated by AASHTO for a concrete deck and the design value used for this bridge Bridge Defl ection Midspan girder defl ections under each load path were recorded for girders 1 4 5 6 and 7 Superimposing load paths 1 and 4 to simulate two lanes loaded yielded a maximum defl ection of girder 4 equal to 7 mm The live load defl ection of the structure is governed by the AASHTO and is established at a limit of L 800 AASHTO 10 6 2 The maximum defl ection of the structure was approximately L 1 680 which was well within the limit established by the AASHTO With the knowledge that the displacements of the structure were well within the limits established a method of estimating the defl ections for future structures was desired For a simple span bridge the maximum displacement is anticipated to occur close to the center of the bridge span Using the simple span assumption a model was generated for an interior girder to validate the Table 1 Live Load Distribution Factors Distribution factor GirderPath 1Path 4Two lanes G10 000 330 33 G20 090 510 59 G30 220 490 70 G40 380 370 75 G50 480 220 71 G60 500 080 58 G70 330 000 33 Total224 Fig 6 Girder 6 defl ection comparison JOURNAL OF BRIDGE ENGINEERING ASCE JULY AUGUST 2006 397 J Bridge Eng 2006 11 394 400 Downloaded from ascelibrary org by Changsha University of Science and Technology on 06 04 13 Copyright ASCE For personal use only all rights reserved defl ection measurements recorded during testing The model used the simple span defl ection equations given by the American Institute of Steel Construction AISC 1998 These equations use the applied load the load location and the composite section properties to compute the defl ection of the girder at any point This defl ection was computed at 1 m increments along the span of the bridge see Fig 6 The composite section moment of inertia based upon the transformed section was used along with the modulus of elastic ity of steel to model the girder section In order to estimate the amount of the axle load applied to the model from the loading truck the distribution factor for one lane loaded was used This 0 50 distribution factor DF was estimated from test data and was used to convert the axle load to a girder load The maximum bridge defl ection recorded was determined to be 5 mm and it occurred during load path 1 at girder 6 The quarter point midspan and three quarter point defl ections were plotted for girder 6 with the path 1 loading Fig 6 Looking at the measured defl ections versus the defl ections computed using the section properties and the simple span beam equations there was a noticeable difference between the com puted and measured values This difference was attributed to the rotational restraint provided by the semiintegral end bents It was assumed that with the restraint at the end of the girders the span would not behave as fully fi xed or simply supported but some where in between In order to verify this the model was adjusted to use the fi xed end beam equations given by AISC 1998 Once all three plots were compared in Fig 6 it was apparent that the girder defl ections measured were between the values estimated using the fi xed support and simple support models Based on this