EFFECT OF SPAN-LENGTH ON THE RELIABILITY OF TRUSS BOTTOM CHORDS

As input to the development of an LRFD specification for wood construction, seven truss patterns were studied to determine the impact of lower chord length on (conditional) probability of failure. The truss spans ranged from 5.59 m (18.33 ft) to 20.2 m (66.33 ft). The tensile properties of the lower chord were simulated using a lumber model that characterizes the serial correlation of tensile strength and modulus of elasticity (MOE) along 0.76 m (30 in.) segments of lumber. A companion value of bending strength for each piece of lower chord lumber was simulated based on the average MOE of the segments. Combined stress analysis was used to calculate the theoretical probability of failure of all the segments composing the lower chord. Based on the results and assumptions of the study, truss span length had no significant effect on the reliability of truss bottom chords. Also, truss pattern and lumber quality appear to have strong impacts on the reliability of bottom chords.