Fatigue Behavior of FRP-Reinforced Douglas-Fir Glued Laminated Bridge Girders

The use of composites in engineered wood products has recently led to the use of fiber-reinforced polymers (FRP) as a reinforcing for glued-laminated (glulam) beams. Bridge girders are among the more common applications of FRP-reinforced glulam beams and therefore the beam is subject to millions of load cycles as well as moisture fluctuations. Significant flexural strength can be gained through the use of such reinforcing, however, the behavior of the wood composite when subject to repeated load cycles and hygrothemal effects is not well understood. In this study, eighteen glulam beams were reinforced in tension with 1.93% E- glass/epoxy reinforcing (defined as the volume of reinforcing fiber divided by the volume of wood) and tested in flexural fatigue at stress levels corresponding to 1 .OFb and 1 .3Fb, where Fb is the allowable flexural capacity. Both full length and partial length reinforced specimens were tested. The FRP sheet was terminated at the theoretical cut off point (or the point at which the reinforcing is no longer needed to sustain the applied loads) with the partial length reinforcing and was explored with and without end restraints on the reinforcing. Unrestrained terminations were beveled to alleviate peeling stresses while restrained terminations were confined by a steel plate and lag screws. Fatigue testing of all specimens cycled the beams in four-point bending for a total of two million cycles with static bending tests performed periodically to track changes in stiffness. Specimens were then broken in static bending to determine residual strength. Loading at 1.OFb fatigued the specimens at a stress ratio of R=0.333 while loading at 1.3Fb produced a stress ratio of R=0.255. Load heads were spaced to produce flexural stress-to-shear stress ratios consistent with those seen by typical in-service timber bridge girders. However, the flexural capacity of the reinforced beams was over-estimated due to lower than expected lamstock properties and the use of a transformed section modulus where the wood section modulus was required. The cumulative effect of this resulted in a conservative testing program where the specimens fatigued at 1.OFb were actually stressed to 1 .52Fb and the specimens fatigued at 1 .3Fb were actually stressed at 1 .98Fb. The results of these tests showed that the full length reinforced beams fatigued at l.OFb were not prone to fatigue failures. At the higher stress level of 1 .3Fb, specimens failed prematurely and exhibited fatigue failures causing bending stiffness losses. The results also showed that with adequate confinement of the FRP terminations, partial length reinforcing may be structurally feasible. Beams with unconfined terminations fared poorly in fatigue. In addition, the effects of hygrothermal stresses in combination with mechanical fatigue are of particular concern. To better understand the effect, both finite difference and finite element modeling was done to quantify the stresses due to hygrothermal fluctuations that are typical over the life span of a timber bridge girder. A kiln schedule was designed to subject beams to extreme high and low moisture contents to reproduce the cumulative damage occurring over a 50 year life span of a timber bridge girder in a New England environment