Lateral Stability of Composite Wood I-Joists Under Concentrated-Load Bending

Abstract

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 (EMF), 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 utilizing setups selected to mimic theoretical cases. 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 ratios used in design (RB from 0 to 50). The critical buckling moments (Mcr) recorded from the cantilevered buckling tests were predicted using three lateral stability models. The lateral stability models that 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 geometry and flange material different from the primary study I-joists were tested to provide preliminary insight into the robustness of the recommended model for composite wood I-joists. Cantilevered and simply supported beam test configurations yielded similar Mcr values for composite wood I-joists of the same type and geometry having equal RB 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 Mcr predictions than the current LRFD design model. However, the current design approach in the LRFD manual can be made to satisfactorily predict Mcr 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 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 Mcr. 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.