Duration of Load Behavior of Lumber Under Dynamic Loading

Abstract

The performance of structural wood members has proven to be generally satisfactory during dynamic load events such as earthquakes or severe wind conditions. The reasons for this, however, are not well understood as few data are available relative to dynamic member performance. Therefore, the true level of safety or conservatism in design is not only unknown, but cannot be accurately estimated with our current level of understanding. Procedures used to adjust member design values for cyclic dynamic loadings are not founded in robust behavior models or experimental test data, but rather experiential inference from past performance and static test data. Duration-of-load factors historically used in design were derived from a model developed from static, pseudo-static, and impact tests performed on small, clear-wood specimens. Other models and design factors have been developed subsequent to this; however, none of the models or design factors have been validated for cyclic dynamic load events such as those attributable to earthquakes and high winds. The present study attempts to partially address this shortcoming through an extensive experimental program. The results of the investigation provide data needed to develop robust behavior models and allow empirical assessment of current design factors. The data indicate the current duration-of-load factor in the NDS and time-effects factor in the LRFD for short duration dynamic loadings appear reasonable.