Lateral stability considerations for composite wood i-joists

Three different theoretical beam elastic lateral stability models were combined with the current Load and Resistance Factor Design (LRFD) lateral stability model used for wood I-joist design to identify more accurate predictions of composite wood I-joist buckling behavior. These models were based on Euler, Equivalent Moment Factor, and Nethercot models. Modifications to account for cross sectional geometry, load location relative to the cross section centroid, and lateral bracing were analyzed where applicable within each of the three theoretical models. Material properties of the entire composite wood I-joist cross section for specimens tested in this research were measured and utilized in the studied lateral stability models. The primary study composite wood I-joists were analyzed in both a cantilevered and simply supported configuration. The results from this analysis were compared to determine the adequacy of the current design effective length equations. The cantilevered composite wood I-joists were tested over nearly the entire range of beam slenderness ratio used in design (R_B from 0 to 50). The critical buckling moments (M_cr) recorded from the cantilevered buckling tests were predicted using three lateral stability models. The lateral stability models which provided prediction curves fitting within the 95% confidence interval of the observed cantilevered buckling data were recommended for the lateral stability design of similar composite wood I-joists. Two additional composite wood I-joist types of different geometry and flange material than the primary study composite wood I-joists were tested to provide preliminary insight on the robustness of the recommended model for composite wood I-joists of types and geometries different than the primary study I-joists. Cantilevered and simply supported beam test configurations yielded similar M_cr values for composite wood I-joists of the same type and geometry having equal R_B values. Lateral stability models that included the dimensional, bending stiffness, and torsional rigidity properties of the entire composite wood I-joist cross section yielded far superior M_cr. predictions than the current LRFD design model. However, the current design approach in the LRFD manual can be made to satisfactorily predicted M _cr values of composite wood I-joists provided: (1) the I-joist ultimate moment is used; (2) the elastic buckling moment is calculated using either the Euler Elastic Buckling (EEB) theory or the Equivalent Moment Factor (EMF) theory; and (3) the dimensions, flatwise bending stiffness, and torsional rigidity of the entire composite wood I-joist cross section are used in the calculation of M_cr. When utilized with the current LRFD design approach, the EMF theory appeared to be more adaptable than the EEB theory for predicting the critical buckling moment of composite wood I-joists of varying types and geometries.