LRFD for Engineered Wood Structures—Connection Behavioral Equations

Abstract

A new design specification for engineered wood structures has been proposed in load and resistance factor design (LRFD) format. This paper provides an overview of the proposed LRFD connections design criteria. The connections design provisions are, in part, calibrated from allowable stress design provisions. Major changes from historic practice, however, result from a change in behavioral equations to a theoretical base for predicting the lateral strength of connections using bolts, screws, and nails. New provisions for axial withdrawal of driven and turned fasteners, as well as combined axial and lateral loading criteria are also proposed. Safety levels were calibrated to historic practice, but some change in design capacity is expected due to format change, conversion to new behavioral equations. and the selection of a calibration point. The LRFD document contains substantial improvement in code clarity. simplification. and structure over the his toric allowable stress specification. A clear mechanism for including design with new wood-based engineering materials is provided.

Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction

“…These estimates have been combined with traditional safety levels and cast into an LRFD format. The development of connection design provisions is described in McLain et al (1993). Note that fasteners are assumed to be installed in clear, relatively flat grain material.…”

Reliability Indices for Bolted and Nailed Connections in Wood Structures

“…Sample sizes were sufficiently large to facilitate statistical distribution fitting techniques to describe test data for wood strength properties. However, LRFD connection design procedures were simply calibrated to existing allowable stress design (ASD) methods ("Standard" 1996;McLain et al 1993;Zahn 1992), primarily due to the fact that sample sizes were insufficient to accurately fit probability distributions to connection strength data. Thus, even though structural load data could be described probabilistically, there was insufficient information to characterize connection strength data probabilistically and calculate structural reliability indices (β).…”

“…However, reliability assessment of connection capacity should be based on the ultimate limit state of the connection. The offset limit state equations specified in current wood design literature are effective at identifying characteristic yield modes for connections subjected to overload conditions (McLain et al 1993). Due to plastic hinge formation in fasteners, connections which exhibit yield modes III (one hinge per shear plane) and IV (two hinges per shear plane) behave in a relatively ductile fashion, resulting in considerable connection displacement and energy dissipation prior to reaching ultimate failure.…”

“…Recent reliability analyses for laterally-loaded connections in timber structures have emphasized calibration to historic proportional limit states (McLain et al 1993;Zahn 1992). Extension of these procedures to ultimate limit states has proven difficult since many historical connection tests were halted at predetermined levels of connection displacement, thus precluding accurate assessments of maximum load capacity, ductility and energy dissipation.…”