PANEL DESIGN, FABRICATION AND LOAD TESTING. R.J. Roy1, A.S. Debaiky2 ... Intended application: short span timber/steel bridge deck. The timber slabs in ...
33rd Annual General Conference of the Canadian Society for Civil Engineering 33e Congrès général annuel de la Société canadienne de génie civil
Toronto, Ontario, Canada June 2-4, 2005 / 2-4 juin 2005
GLASS FIBRE REINFORCED POLYPROPYLENE BRIDGE DECK PANEL DESIGN, FABRICATION AND LOAD TESTING 1
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R.J. Roy , A.S. Debaiky , H. Borazghi and B. Benmokrane 1. Département de génie civil, Université de Sherbrooke, Sherbrooke, Qc, Canada 2. AS composites Inc., Kirkland, Qc, Canada ABSTRACT: The purpose of this work is the design, fabrication and load testing of a pre-fabricated bridge deck slab made of fibre reinforced polymer composite materials. The intended application and design parameters concern the replacement of timber/steel bridges. A prototype deck covering two lateral spans has been designed and fabricated. The materials used were glass fibber fabric reinforced polypropylene (GFRPP) and polyurethane foam. The deck structural profile was developed using the software SAP2000® and has a double wave geometry in the web section enclosed by two flat panels. This prototype is 3.1m long, 1.2m wide and 0.215m thick. The prototype deck was static load tested in a laboratory using hydraulic jacks. The deck supported a total of 515kN without failing (2X257.5kN load patches). Deflections recorded where slightly higher than predicted and may be due to imperfection in the fabrication or lower achieved modulus of elasticity for the GFRPP material. 1. INTRODUCTION This paper summarizes steps taken to develop a new innovative all-fibre reinforced polymer (FRP) composite bridge deck slab. The design process, numerical modeling, fabrication, and testing of the slab as well as proposed improved new design for future field demonstration and application will be covered. This work has been initiated in 2002 and design developments started in Sep. 2003 with the first prototype fabricated in the summer of 2004 and tested in Dec. 2004 at the laboratory of the civil engineering department, Université de Sherbrooke. The target product is aimed to replace the traditional timber deck slabs widely used on Quebec short bridges. These timber deck have a lifespan of 15 years and use creosote treated wood which is a source of polution to the underlying rivers. The target bridges are typically single span, ranging between 6 to 12 m. There are two standard cross sections used in these bridges: i. Narrow section: 4.268 m total width (3.458 m clear traffic width) supported on three (3) parallel steel girders 1.450 m apart. ii. Wide section: 7.518 m total width (6.705 m clear traffic width) supported on five (5) parallel steel girders 1.450 m apart.
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Figure 1. Intended application: short span timber/steel bridge deck. The timber slabs in these bridges are made of standard E-P-S lugs (197x203x4268 mm WxHxL). The lugs are typically arranged in a staggered formation every 406 mm (clear spacing of 209 mm). A wearing surface made of strips of the same wood (203x96x1800 mm WxHxL) and placed atop of the cross lugs is used. This makes the total depth of the deck slab equals to 299 mm. The strips of the wearing surface are connected to the transverse lugs using Ø10 bolts every 812 mm on the longitudinal direction. The Ministère des Transports du Québec uses an in-house design guideline for these deck slabs. It is assumed that the longitudinal wearing surface will bond the transverse lugs so that every wheel load will be shared between three successive transverse lugs in a 25%-50%-25% distribution, where the middle lug of the three carries the 50% ratio. The three successive transverse lugs cover 1218 mm width (in the longitudinal direction). The prototype was designed to be 1218 mm wide to simulate the resistance of the three lugs that share the wheel load. The new slab was designed to have an overall depth of 203 mm, equivalent to that of the timber slab. However, future modification to the initial design can be performed since a thinner wearing surface than the 96 mm wood one could be used. It is foreseen to increase the overall depth of the composite slab to gain higher stiffness and less deflection. The length of the prototype was aimed at 3.2 m, which covers two spans of 1.45 m each and an extra 0.3 m for supports as will be detailed in later sections. For field demonstration and complete applications, the length of the slab strips will be either 7.518 m (for 5-girders bridges) or 4.268 m (for 3-girders bridges), covering the entire width of the bridge in each case. The base materials used to fabricate the prototype deck is TwinTex® fabric (commingled glass fibre reinforced polypropylene, GFRPP) consolidated and formed to the desired shape by vacuum bag o moulding with heating in an oven at 180 C. The slab consisted of two 4.0 mm thick GFRPP corrugated sheets web between two 5.0 mm thick flat GFRPP plates. The four parts were connected together by high tensile bolts and epoxy adhesive. Expanding polyurethane foam is used to fill the inner space. The slab was tested under monotonic load with one point load on each span representing a truck axle. The test results were very encouraging and the slab was able to resist a combined load of 515 kN, which was the capacity of the testing machine. The slab recovered all the deformation upon load removal. The proposed FRP composite deck slab system has several advantages over the traditional wood timber slabs such as: 1. Impermeability to moisture/water, thus minimizing the effect of freeze-thaw cycling. 2. Improved steel girders corrosion protection by preventing the water/salt-contaminated water from moving downwards. 3. Environmental friendly by eliminating the timber deck wood treatment chemicals. 4. High fatigue and durability resistances. 5. Ease of installation and replacement, and in less time. 6. Five times the service life (estimated at 75 years compared to 15 for the timber decks). 7. Higher quality of ride, less noise, and more uniform surface. 8. Integrated guardrail, which is stronger and more comforting to drivers.
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2. LITERATURE REVIEW Several FRP bridge deck demonstration projects and commercials applications have been completed and running in several countries, albeit still in limited volume market wise. The research in North America is still in its early stage, and no field application or demonstration has been made in Canada to date. Luke et al. (Luke et al. 2002) tested a modular bridge deck made of E-glass and polyester, the deck was made of several cells 500mm wide each connected together using epoxy resin. The cross section of the cell was 225mm deep and had triangular openings with 7.75mm thick webs. The authors acknowledged that the triangular configuration used was not the optimum profile, which was the arched web profile, the manufacturing capabilities and cost-efficiency prohibited the fabrication of the arched profile. The deck was tested approved for only 40 t load as specified by the UK’s BS-5400 code. Hayes et al. (Hayes et al. 2000) tested 1224mm wide bridge deck made of glass fibre and polyester. The deck was made up of 12 pultruded square tubes 102x102x6.36mm thick sandwiched between two pultruded 9.53mm thick plates. The section was thus 121mm deep. The elements were all connected using epoxy adhesive, all mated surfaces were abraded before applying the epoxy and pressure was applied to the assembled deck until curing was complete. Additional lateral restraint was provided by through fibre bolts 25.4 mm in diameter at 300mm spacing on the longitudinal direction of the deck. Shekar et al. (Shekar et al. 2003) reported using E-glass and vinylester resin in construction of four highway bridge decks in the US. The cross-section of the decks used was 203mm (8 in) deep using double trapezoidal and hexagonal connectors. The units were assembled at the production plant using polyurethane adhesive and mechanical pressure. Assembled sections were 2.43 m in width (in direction of the traffic) and their length were equal to the width of the bridge in each case except for one bridge where the width was covered with two sections connected together longitudinally over the central beam. The decks had different spans in the four bridges (distance between beams) of 762, 889, 1829, and 2591 mm. The construction of the largest deck (17x54 m) was completed in 6 days, about 10% of the time needed for conventional concrete deck, and using only five workers. The first FRP bridge deck in New York State was placed in late 1999 to replace a deteriorated concrete deck and allow for higher live load on the bridge (Chiewanichakorn et al. 2003). The deck panels were made of E-glass stitched fabric wrapped around foam blocks. The deck was designed using finite element analysis and stresses in the composite materials were limited to only 20% of their ultimate strength, deflection was also limited to Span/800. This extremely conservative design was due to the lack of data and experience on composite bridge decks. Composite action between the FRP deck and the steel girders was deliberately eliminated during design. Field tests after installation showed as-designed stresses in the composites at about 2.9 MPa (fufrp = 221 MPa). An important numerical study was reported by Gan et al. (Gan et al. 1999) on the assessment of different cross-sectional configurations for pultruded FRP bridge deck panels. The authors used commercial finite element package ABAQUS to compare the behaviour of seven shapes most used in research and application, hexagonal (honeycomb), triangular, rectangular, square, thick-top square, enhanced triangular, and enhanced channel were analyzed. The analysis was based on equal cross-sectional area for all seven shapes to reach the optimum cost-performance one since the area of the cross section is the main parameter governing the cost of the bridge deck. The overall depth of the shapes was also kept constant. The authors assumed orthotropic properties for the material and used 3-D block and shell elements for the static and buckling analyses, respectively. It was concluded that the enhanced triangular configuration is the optimum section amongst the seven evaluated. It provided the highest global stiffness and significant improvements in local stiffness and buckling strength. 3. DESIGN PHILOSOPHY The design of the cross section shape and dimension in this project is based on basic criteria to satisfy strength and serviceability requirements of the Canadian Highway Bridge Design Code (CHBDC, 2000) as follows: the overall cross-sectional stiffness should be equal to or greater to that of the existing timber decks. Stiffness taken as the product of inertia times the elasticity modulus was used hereby. The modulus of elasticity of the timber was taken equal to 8 GPa, which is the E50 upper limit for the type of
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wood used as specified by M.T.Q (I.S.P No. 1) (CHBDC, 2000). The modulus of elasticity of the GFRPP o was taken equal to 13 GPa as specified by the manufacturer for the material properties at 20 C. Deflection limit set by CHBDC at span/400 at the service load level was targeted. The span L in this application is 1.45 m giving a maximum allowable deflection of 3.63 mm. It was set that the unsupported length along the upper flange of the section between the joints with the web elements should not exceed 125 mm, which is the half of the smaller dimension of the truck wheel loaded area. This was meant to reduce the probability of local punching failure of the top flange or the local buckling of the webs by ensuring that there are at least two webs supports beneath the wheel contact area. The shape of the cross section was meant to be as simple as possible to facilitate fabrication process. Since the web and flanges will be produced separately then joined together by heating at contact point, any complexity of the shape would require much longer assembly time and therefore cost. The cross-section profile of the web was optimized from four proposed profiles: 1- Adjacent rectangular boxes forming only vertical supports 125 mm apart. 2- Curved double sine waves shaped panels with contact points 125 mm apart. 3- Double diamond shaped panels with 25 mm flat contact points (for less stress concentration at the connection between webs and flanges) 150 mm apart. 4- V-shaped panel covering the whole depth of the web with 25 mm flat contact points (for less stress concentration at the connection between webs and flanges) 150 mm apart. The optimization process was based on the least material needed to satisfy the deflection limitations under service load. Numerical simulation using Finite Element analysis (FEA) was performed to predict the ® behaviour of each proposed design. Commercial software package SAP 2000 was used in this step (SAP2000, 2005). The analyses were made on ¼ models to make use of the symmetry of the bridge deck and loading. The models were 725 mm in the longitudinal direction (1/2 span of 1450 mm between bridge girder), 625 mm wide (approximately half of the 1218 mm width covered by three successive transverse lugs) and 204 mm deep (equal to the depth of timber deck currently used). The four profiles were adjusted during FEA with respect to the flange and web thicknesses for optimum design. The 3.63 mm maximum deflection, including any possible local buckling of the top flange or webs was the governing parameter in the analysis. Stresses and strains in the materials at the service load level were much less than the material ultimate capacity. Figure 2 shows the finite element model, applied load, and stress in the longitudinal direction in model adopted. The third proposed “diamond” profile, which was found to be most efficient, is shown in Figure 3.
Figure 2. Sample ¼ deck slab FEA model with stress results visualization.
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Fibre reinforced polypropylene (GFRPP), E=13 GPa 1.6mm thick sheet
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Structural foam, E=14.7 MPa Figure 3. As-built prototype section dimensions details and material properties.
4. FABRICATION OF THE PROTOTYPE The prototype was fabricated and assembled by three persons. The corrugated webs and the two flat plates (top and bottom sides) were fabricated independently then assembled together using epoxy and high strength bolts. All the parts were fabricated by vacuum bag moulding with heating in an oven at o 180 C, however, the plan for future mass product is to fabricate all parts using a continuous roll-forming machine already in service. The flat plates were fabricated in one piece (1200 x 3300 mm) each, the web sections were fabricated in quarter-sizes (600 x 1650 mm) and joint together using longitudinal splices to form full webs in the longitudinal direction (600 x 3300 mm). High strength bolts were designed to resist the longitudinal shear stresses without the help of the epoxy adhesive. The calculation was made manually and using an ultimate unfactored load of 330 kN on each span, which is equivalent to 3 times the factored service wheel load. Spacing of the blots on the longitudinal direction was 200 mm for bolts connecting the top and bottom plates to the web, and 100 mm for bolts connecting the webs together at mid-height. Figure 4 shows the assembly of the entire section with the different components and stain gage wires visible. After the assembly of the entire section, filler foam was injected into the voids of the web to increase rigidity and help prevent web buckling under loading. For reasons of reduced cost and development time, the polyurethane foam was installed in a free-rising fashion (the mixture just poured in) and this resulted in a softer 14.7 MPa elasticity modulus for the foam (200 MPa was used for design). The full assembly and foaming was completed in four working days by two untrained persons.
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Figure 4. Steps of assembly of the composite deck slab.
5. STATIC LOAD TESTING The static load configuration consisted of two 250×600mm loading areas separated by 1.8m center in center, which represents the width of truck axle (Figure 5). A hydraulic actuator mounted on the loading steel frame was used to apply the loading to the deck slab (Figure 6). The deck slab was placed on three parallel I-beams 1.5m wide, with 83 mm wide flanges. For the purpose of this test, some 20 mm Plywood board peaces were placed between the supporting I-beams and the slab, and between the loading steel plates and the slab to insure no compressive force will be exerted on the heads of deck bolts. The deck was simply deposited on the supports and was retained laterally by steel clamps on each side (Figure 6). The deck was instrumented with 34 electrical strain gauges on the FRP on the top and bottom surfaces, and on the webs. In addition, nine LVDT were used to measure the deflection at different locations.
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Figure 5. Schematic view of the test layout with the assumed service load.
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Figure 6. The test setup with two load cells one at each loading point and the LVDT deflection measuring devices. Notice the presence of the foam in the structural section. 6. TEST RESULTS AND DISCUSSION The deck was loaded four times with increasing load in each test at 220, 380, 450 and 515 kN. It was observed that the slab exhibited a linear deflection behaviour in all the four tests. The four elastic lines were almost identical in slope, indicating no loss of stiffness due to the repeated loading and unloading. During the test of 380kN, it was observed that the loading steel plates (25 mm thick) started to bend upward. Some 2.5" thick plates were used for the remaining tests. The deck withstood the four tests and showed typical flexure behaviour with no buckling or failure in bolts. Figure 7 shows the load-deflection curves for the last test. It should be noted that the three lines to left represent the deformation at the supports, which include the compression of the plywood board strips above the I-beams. Taking this into consideration, the net maximum deflection should be around 28-6=22 mm, as indicated by the dashed lines in Figure 7. At the service load level, the net maximum deflection was approximately 9 mm or span/160 (the target span/400 is 3.6 mm). The difference can be easily justified due to the use of very soft foam instead of the stiff one considered in the original design. In addition, the absence of rubber pads under the steel loading plate caused the deformation to be almost uniform under the plate, which in return extended the deflection line much farther outside the plate area. With respect to stress levels during the tests, the stresses recorded in the composite parts were generally low. For the 515 kN test, stress measurements are presented in Figure 8 with a maximum of 80 MPa intension on the bottom surface under the loading area. This level of stress accounts for only 27% of the materials ultimate strength in tension (300MPa). The maximum compressive stress was 35MPa at the side of the loading area, or 25% of the materials ultimate strength in compression (140MPa). Some cracking noises were heard when high load levels were reached (above 400kN) and few sharp snaps occurred, probably due to plywood breaking, but without any visible damage to the deck. In conclusion, the deck slab resisted and was able to withstand more than twice the service load. The recorded deflections were higher than the allowable limits, but that was acceptable when considering the very soft foam used, and
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the absence of rubber pads under the rigid loading steel plates. The stress levels in the deck were relatively low. This provides enough room for fatigue and creep effects that can take place under service condition, without jeopardizing the integrity of the deck.
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Figure 7. Load-displacement curves during the 515 kN load test at different locations on the deck.
Figure 8. Load-Stress curves during the 515 kN load test at different locations on the deck.
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7. CONCLUSION This paper summarizes the efforts by AS Composites inc. and the NSERC Research Chair in FRP Reinforcement for Concrete Structures at the Department of Civil Engineering of Université de Sherbrooke to develop an innovative composites slab for bridge decks. The composite slab developed is meant to replace the timber deck slabs widely used in Québec. It has the potential to provide 5 times the service life of timber and is environmentally friendly since it does not require any chemical treatment. A first prototype deck slab (1.2x 3.2 m) was designed, fabricated and load tested in a laboratory. This slab was able to carry a total load of 515 kN (twice the design service load) without any fracture, cracking, or damage to the deck elements. A new improved slab design is in development and the fabrication of a second prototype is planned. The new design aims to reduce the required manufacturing and assembly time and make use of A.S. composites continuous roll-forming machine. 8. REFERENCES Chiewanichakorn, M., Aref, A.J. and Alampalli, S. 2003 Failure Analysis of Fiber-Reinforced Polymer Bridge Deck System, Journal of Composites, Technology & Research, 25: 121-129. CHBDC - Canadian Highway Bridge Design Code 2000 CAN/CSA-S6-00 standard, Standards Council of Canada, Ottawa, Canada. Gan, L.H., Ye, L. and Mai, Y. 1999 Design and evaluation of various section profiles for pultruded deck panels, Composite Structures, 47: 719-725. Hayes, M.D., Lesko, J., Haramis, J., Cousins, T.E., Gomez, J. and Massarelli, P. 2000 Laboratory and Field Testing of Composite Bridge Superstructure, Journal of Composites for Construction, ASCE, 4: 120-128. Hayes, M.D., Ohanehi, D., Lesko, J., Cousins, T. and Witcher, D. 2000 Performance of Tube and Plate Fiberglass Composite Bridge Deck, Journal of Composites for Construction, ASCE, 4: 48-55. Luke, S. 2003 The design, installation, and monitoring of an FRP bridge at West Mill, Oxford, Lightweight Bridge Decks, European Bridge Engineering Conference, Rotterdam, the Netherlands. SAP2000 2005 Computers and Structures, Inc. (CSI), Berkeley, California, USA. Shekar, V., Petro, S.H. and GangaRao, H. V. S. 2003 Fiber Reinforced Polymer Composite Bridges In West Virginia, 8th International Conference on Low-Volume Roads, Reno, Nevada.
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