Strengthening of Timber Bridges

Transcription

1 Strengthening of Timber Bridges Gentile, C., Svecova, D., Saltzberg, W., and Rizkalla, S. H. ISIS Canada, University of Manitoba, Winnipeg, Manitoba Abstract Most timber bridges in the Province of Manitoba are constructed with sawn Douglas Fir stringers, which are simply-supported with spans ranging from 6 to 10 m. Due to an increase in the allowable gross vehicle weight, most of these bridge stringers may require flexural and/or shear strengthening to increase their load-carrying capacity. This paper presents an innovative approach for the use of fibre reinforced polymers (FRP) for strengthening these bridges. The FRPs have a high strength-to-weight ratio and are non-corrosive. This enables engineers to strengthen old timber bridges without increasing the self-weight of the structure. An experimental program was undertaken at the University of Manitoba to study the behaviour of creosote-treated old Douglas Fir beams strengthened by glass fibre reinforced polymer (GFRP). Halfscale and full-scale timber beams were tested to examine the effectiveness of the technique and to determine the ultimate capacity and ductility of GFRP-strengthened timber bridge girders. The GFRP was bonded to the timber using epoxy. Twenty-two beams were tested using three GFRP reinforcement ratios and plain timber beams as control beams. The proposed technique has proven that the ductility will be dramatically improved and the flexural strength may be increased by 25 to 50 percent. Full-scale tests on three beams of 10 m span confirmed that using GFRP is a feasible technique for strengthening timber bridges. INTRODUCTION Many opportunities exist for the strengthening of timber members, both for new construction and the rehabilitation of existing structures. Techniques for strengthening timber can be used to reduce the size of beams and to increase its strength, thereby creating a more efficient use of the timber supply. Some of the early investigations of reinforcing timber considered available metals as suitable reinforcement. These were aluminum plates bonded to the top and bottom surface of wood cores, or between laminations of glulam (Mark, 1961 and Sliker, 1962). The beams strengthened using this technique failed due to separation and buckling of the aluminum plates. Steel plates and steel cables were used by Bohannan (1962) and Peterson (1965) to prestress laminated timber beams. Both studies reported an increase in strength for the prestressed beams and an increase in stiffness. The beams tested by Bohannan did not show an increase in the stiffness due to unbonded prestressing. GIulam beams were strengthened using steel rebars, as reported by Lantos (1970) and Bulleit et al. (1989). In all of the studies, the variability in strength decreased for the reinforced beams compared to the unreinforced beams. Some recent work has examined the use of carbon and glass FRP sheets to reinforce sawn timber sections (Johns and Lacroix 1999). This work was conducted using small beams with a crosssection of 39-mm by 89-mm, tested on a simplysupported span of 1.4 m. The lack of literature describing the reinforcement of large sawn timbers has motivated the work reported in this paper. This paper describes an experimental program using half-scale beam specimens to investigate the behaviour of timber beams reinforced with GFRP bars. The results from the half-scale specimens and three full-scale timber beams reinforced with GFRP tested to failure are presented. Gentile, Svecova, Saltzberg, and RizkalJa (1999). Strengthening of Timber Bridges Page 1 of6

2 HALF-SCALE BEAM TEST Experimental Program Twenty-two half-scale beams, including plain timber and reinforced specimens, were tested up to failure under monotonic load in four-point bending. The timber was select structural grade, Douglas Fir, with an allowable bending strength, F b, of 11.0 MPa, and an average Young's modulus of 12 GPa (AASHTO 1996). The specimens were 4.3 m long, 100-rom wide and 300-mm high. Figure 1 shows the cross-section of a typical timber specimen reinforced with GFRP bars. T Figure 1 Specimen cross-section The reinforced specimens were made by installing GFRP bars in grooves that were cut 30-mm above the bottom fibres into the sides of the beams. The GFRP bars were 5-mm diameter, with strength of 1800 MPa and a Young's modulus of 56 GPa. An epoxy resin was used to bond the GFRP bars to the timber. Table I summarizes the three different reinforcement ratios, PGP, examined in this experimental program. Table 1 - Reinforcement ratio of beams PGF (%) No. of Specimens o Total 22 All the beams were tested in accordance to ASTM D198 (1992). The beams were tested under fourpoint bending with a shear span of 1.7 m and a simply-supported span of 4.0 m. The load was applied at a constant stroke-controlled rate using a servohydraulic testing machine. Lateral bracing was provided in the middle of each shear span to prevent lateral-torsional instability of the beams during loading. A schematic of the test set-up is shown in Figure 2. lzoomm P/2 P/ mm Figure 2 Test set-up lzoomm Deflections were measured at five locations along the length of the beam using linear variable differential transducers (L VDTs). Strains were monitored in the timber and GFRP reinforcement at the mid-span of each beam. Readings from all L VDTs, strain gauges and load cells were recorded for the duration of the test using a computerized data acquisition system. Load-Deflection Behaviour The load-deflection curves for typical plain and GFRP-reinforced timber specimens are shown in Figure 3. Test results show a large variation in the measured stiffness and strength for both the plain and reinforced specimens. The curves shown in Figure 3 represent the lower and upper bound behaviour of the stiffness range of the tested beams !80 ~ 60! :1 o~----~ b:~====~ ~ ~ o ~_(mm) Figure 3 Typical load-deflection curves Failure of the plain timber specimens usually occurred at point A' for both weak and strong timber, shortly after initiation of cracks in the 2

3 tension zone. For the stiffest and strongest plain timber specimens, some non-linearity of the loaddeflection behaviour was observed, however, failure was always governed by the bending strength of the timber. For the reinforced specimens, the initial loaddeflection O-A was linear until the timber cracked in the tension zone at point A. The stiffness then gradually decreased until maximum load was reached at point B and crushing started in the compression zone. Deflection continued to increase with a minor decrease in load up to point C, at which point a significant crack occurred in the tension zone, causing a rapid drop in load to point D. The beam was capable of additional deflection and load carrying capacity up to point E, where the testing was terminated due to limitations of the testing equipment. Failure Mode All beams were loaded monotonically up to failure. Figure 4 shows a typical beam during loading. Out of the 22 beams tested, 19 beams failed in flexure, while 3 beams failed in a combination of flexure and shear. This paper is focused on flexural strengthening and, therefore, the flexural-shear failures will not be discussed. Two flexural failure modes were observed: compression and tension failure. If the beam exhibited crushing in the compression zone at maximum load, the failure was classified as a compression failure. If crushing did not occur at the maximum measured load, the failure was classified as a tension failure. Figure 4 Typical beam during test For the compression failure, cracking is also present in the tension zone. All of the beams initially cracked in the tension zone, but for the compression failures, crushing in the compression zone controlled the ultimate load. Table 2 summarizes the failure modes observed for the different reinforcement ratios, PGF, in the experimental program. Table 2 - Observed failure modes PGF No. of Failure Modes (%) Beams o 7 Tension (7) Tension (1) Compression (2) Tension (2) Compression (2) Flexure-Shear (2) Tension (2) Compression (3) Flexure-Shear (1) All of the plain timber specimens failed in tension with no sign of crushing in the compression zone. For the reinforced specimens that failed in flexure, 60 percent of the specimens failed in compression failure mode. The addition of the GFRP bars caused a change in the failure mode from brittle tension in the plain timber specimens to a more ductile compression-initiated failure. Due to the high strength of the GFRP reinforcement, the GFRP bars did not rupture in any of the specimens. There was some localized debonding of the bars adjacent to tension cracks in the timber, but the bond between the bars, epoxy and timber remained intact outside the failure region. None of the failures were due to debonding or delamination of the reinforcement, as has been observed in applications using FRP strips or sheets for external reinforcement (Hernandez et al. 1997). Calculated Strength of Beams There was a wide range of strength and stiffness observed for the plain and reinforced timber specimens in this experimental study. An analytical model for the bending strength of lumber developed by Buchanan (1990) was 3

4 modified to include enhancement due to the use of GFRP reinforcement. The model was calibrated using the data collected from this experimental program. The details of this analytical model are described elsewhere (Gentile 2000). Table 3 compares the experimental strengths and model predictions for the half-scale specimens. The letter in parentheses next to the experimental modulus of rupture, MOR, indicates the observed failure mode: (T) for tension and (C) for compression. The model was capable of predicting the strength of the tested beams with reasonable accuracy. Figure 5 shows the calculated strength enhancement for the PGF values used in this experimental study. Table 3 - Experimental and predicted strength Beam ID Fl J1 Bl HI C1 Al G1 MOE (GPa) Exp. MOR (MPa) Model PGF=O 18.8 (T) (T) (T) (n (T) en (T) 54.8 MO~ MOR mod ~~----~----~----~----~--~ l~~--~~----~----~----~--~ cit.. ~ I I~~----t~;-~--_- ~ ~~_~i~ ~ ~f.-~----~--~ ~~~----r-----~~-~-~-=~_~':~'~:~~~.~.~..-.-.~ w ~'=...::::., So """. 0_27% t-i t t ~! PGF-0.41% iii ~... """.0_82% t-i------t t \ ol-~==~==~-----l----~----~ Plain TImber MOR (MPo) Figure 5 Strength enhancement It can be seen in Figure 5 that the strength enhancement varied with the strength of the plain timber. For low strength timber, the strength increase was between 35 and 50 percent, while for high strength timber, the strength increase was between 25 and 30 percent. The strength enhancement was not proportional to the amount of reinforcement. Increasing the ratio PGF of 0.27 to 0.82 percent increased the strength enhancement only by approximately 30 percent while the amount of reinforcement was three times in magnitude. It should be mentioned that this finding cannot be conclusive due to the limited number of specimens tested. There is a need for future research to identify the upper and lower bounds for the reinforcement ratio of timber beams. FULL SCALE BEAM TEST F2 D2 D1 L1 K1 H2 11 PGF= 0.27% (C) (C) (T) 50.9 PGF= 0.41% (T) (T) (C) (C) ExperbnentalPrograrn Based on the results of the half-scale specimens, a set of full-scale beams was tested to investigate the size effect on the performance of the strengthening technique. Four full-scale beams, including one plain timber and three reinforced specimens, were tested up to failure under monotonic load in four-point bending. Figure 6 shows the test set-up for the full-scale beam test. L2 K2 A2 G (T) (C) (T) (C) (C) The specimens were 10.4 m long, 200~mm wide and 600-mm deep. The beams were tested in accordance to ASTM D198 (1992) with a simplysupported span of 10.0 m and a load span of 1.2 m. Figure 7 shows the cross-section of the three GFRP-reinforced specimens tested in this program. The GFRP used in the full-scale 4

5 specimens had a strength of 700 MPa and a Young's modulus of 42 GPa. Figure 6 Test set-up for full-scale beams SOOmm 200mm I< )1 4-13mm GFRP bars Beam 1 200mm I< )I 4-10mm GFRP bars Beam 2 200nvn IE )I 4-13mm GFRPbars Beam 3 Figure 7 Full-scale specimen cross-sections Load-Deflection Behaviour The load-deflection curves for the four full-scale specimens are shown in Figure 8. beam with the highest stiffness was strengthened with the lowest reinforcement ratio, and the two beams with the lowest stiffness were strengthened with the highest reinforcement ratio of 0.42 percent. Note that all reinforced beams behaved linearly up to 95 percent of the ultimate load. Beam 1 was linear-elastic up to failure and failed due to initiation and propagation of tension cracks along a large defect in the timber before crushing in the compression zone. The non-linear behaviour of Beams 2 and 3 was very similar to the behaviour of the reinforced specimens in the half-scale experimental program. Beam 2 was strengthened with a reinforcement ratio of 0.26 percent, but achieved a higher strength than Beam 3 that had 70 percent more reinforcement. The full-scale tests conflrmed that beams from the high end of the strength distribution require less reinforcement than beams from the low end of the strength distribution. Failure Mode Two modes of failure were observed in this experimental program. The plain timber specimen, Beam 4, failed in tension at a deflection of 94-mm. Beam 1 failed in tension due to a weak plain in the timber. Beams 2 and 3 both failed in compression and underwent substantial deflection before failure. Figure 9 shows Beam 2 shortly before failure with a mid-span deflection of 200- mm. The failure modes observed for the fullscale beams were the same as those observed for the half-scale specimens. 250,! ~ 200~ ~ 150,! I.J 100 ~ i 50, i I 250 Figure 8 Load-deflection of full-scale specimen The three reinforced beams used reinforcement ratios from 0.26 percent to 0.42 percenl The Figure 9 Beam 2 at 200-mm deflection 5

6 CONCLUSIONS The behaviour of sawn timber beams reinforced with GFRP bars has been investigated. The failure mode changed from brittle tensile-flexural failure in the plain timber specimens to more ductile compression-flexural failure in the reinforced specimens. A strength increase of 25 to 50 percent was obtained, depending on the reinforcement ratio and strength of the original timber. Results from the half-scale experimental study were reproduced in the tests of full-scale specimens. GFRP bars can be used as a feasible method for the flexural strengthening of sawn timber beams. Further research is needed to determine the long-term performance of the proposed strengthening technique. ACKNOWLEDGEMENTS Chris Gentile wishes to acknowledge the funds received from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the ISIS Canada Network of Centres of Excellence. Financial support and materials provided by the Manitoba Department of Highways and Transportation and Concrete Restoration Services Ltd. is greatly appreciated. Department of Civil and Geological Engineering, University of Manitoba, Winnipeg, Manitoba. Hernandez, R, Davalos, J.F., Sonti, S.S., Kim, Y. and Moody, RC. (1997). "Strength and stiffness of reinforced yellow-poplar glued laminated beams." Res. Pap. FPL-RP-554. Madison, WI: US Dept. of Agriculture, Forest Service, Forest Products Laboratory. Johns, K.c. and Lacroix, S. (1999). "Composite reinforcement of timber in bending.", Submitted to the Canadian Journal for Civil Engineering. Lantos, G. (1970). "The flexural behaviour of steel reinforced laminated timber beams." Wood Science., 2(3), pp Mark, R (1961). "Wood-aluminum beams within and beyond the elastic range." Forest Products Journal, 11(10), pp Peterson, J. (1965). ''Wood beams prestressed with bonded tension elements." Journal of Structural Engineering., ASCE, 91(1), pp Sliker, A. (1962). "Reinforced wood laminated beams." Forest Products Journal, 12(1), pp REFERENCES AASHTO (1996). Standard Specifications for Highway Bridges, American Association of State Highway and Transportation Officials, Washington, DC, ASTM (1992). Standard methods for static tests of timber in structural sizes. ASTM DI Philadelphia, P A: American Society for Testing and Materials. Bohannan, B. (1962). "Prestressed wood members." Forest Products Journal, 12(12), pp Buchanan, A.H. (1990). "Bending strength of lumber." Journal of Structural Engineering, ASCE, 116(5), pp Bulleit, W.M., Sandberg, L.B., and Woods, G.J. (1989). "Steel-reinforced glued laminated timber." Journal of Structural Engineering., ASCE, 115(2), pp Gentile, C. (2000). "Flexural strengthening of timber bridge beams using FRP", M.Sc. Thesis, 6