Creep Effect in Structural Composite Lumber for Bridge Applications

Structural composite lumber (SCL) is a family of newly engineered wood products that are increasing in highway bridge applications. Advantages of SCL are its high strength, flexibility, stiffness, and excellent preservative treatability. A main concern in SCL bridge applications is serviceability performance. The long-term creep behavior of SCL flexural members is not presently determinable. In order to investigate creep effects, deflection monitoring of full-scale SCL T-beam bridge members was performed under ambient conditions and an accelerated aging process. Sixteen beams were monitored under exposed weather conditions with frequent wetting and drying. Variables in the experiment were: lumber type (Douglas fir and southern yellow pine), SCL type (LVL and PSL), preservative type (CCA and penta), and dead load intensity. It was found that creep behavior in SCL bridge members closely follows the Burger theoretical model. LVL, Douglas fir, and CCA treatment causes smaller creep deflections as compared to PSL, Southern Pine, and Penta treatment. NDS type creep multipliers for SCL vary between 2.2 and 2.67 for various subcategories. A single average creep multiplier of 2.3 may be used for treated SCL bridge beams.