Flexural Stiffness and Strength of GFRP-Reinforced Timber Beams

Abstract

An experimental program was conducted at the University of Manitoba to test salvaged timber stringers strengthened with glass fiber-reinforced polymer (GFRP) laminates. A total of 20 creosote-treated Douglas Fir beams with dimensions of 130×330×4,500 mm were tested in three-point bending. Two groups were studied to examine the effect of GFRP reinforcement on the stiffness increase of the beams. The first group of 10 samples were reinforced with GFRP laminates on the tension side only (Group T), whereas the other group of 10 samples was reinforced with GFRP laminates alongside both tension and compression zones (Group TC). This study found that the strengthening with GFRP laminates, on average, increased the strength and the stiffness of the beams, respectively, by 36 and 3% for Group T and by 31 and 3.5% for Group TC. An analysis of a database of fiber-reinforced polymer (FRP)–strengthened timber beams tested by other researchers facilitated further study on the effect of FRP on the behavior of timber. The comprehensive analysis revealed a minimal stiffness increase in timber beams strengthened with FRP. Some evidence exists, however, that the beam span-to-depth ratio is an important factor to consider when strengthening timber beams. Beams with smaller span-to-depth ratios showed some increase in stiffness with increasing the reinforcement ratio; however, beams with larger span-to-depth ratios showed no real enhancement of beam stiffness, unless the reinforcement ratio was around 7 times the minimum code-recommended value. This result supports the current CSA provisions that do not advocate for stiffness increase when this strengthening method is used. The analysis shows that stiffness increase in GFRP-strengthened timber beams, based on results of small-scale samples, is minimal.