Development of hardwood glulam timber bridge standards

Design standards and specification for 5.5 to 27.4 m(18 to 90 ft.) clear span hardwood glued laminated (glulam) highway bridges have been developed and are available from the Pennsylvania Department of Transportation. Adhesive systems, preservative treatment processes, laminating procedures and key structural properties have been determined and incorporated into the standards for three hardwood species: northern red oak, red maple, and yellow poplar. Important parameters for glulam production from these species include proper surface preparation, assembly time, and clamping pressures. Pressure treatment cycles were developed to attain to 192.2 kg/m³ (12 pcf) retention of creosote to comply with the AWPA C14 Standard. The creosote treatment had no adverse effect upon the flexural strength or stiffness of each specie. Red maple and yellow poplar glulam girders with flexural beam strength of 16.5 MPa (2400 psi) and stiffness of 12.4 GPa (1.8 × 10⁶ psi) were shown to be technically feasible. Beam test results have verified that ASTM D3737 calculation procedures satisfactorily predict the strength and stiffness of hardwood glulam beams and the volume reduction effect for hardwood glulam beams is similar to that for softwood glulam beams.