Numerical Blast Response of Pre-Engineered Metal Building Cold-Formed Steel Girts and Purlins

Pre-Engineered Metal Building (PEMB) manufacturers often lap the ends of cold-formed girts and purlins over columns and girders to increase their flexural resistance and stiffness with the goal of minimizing material costs. The 2017 edition of the American Iron and Steel Institute Cold-Formed Steel Design Manual provides assumptions for the static conventional design of cold-formed girts/purlins that are considered good practice assumptions. These assumptions are that lapped girts/purlins achieve full continuity, gross moments of inertia are summed within the lapped portions, and strengths within the lapped portions are the sum of the individual members. These good practice assumptions indicate there should also be an increase in blast capacity for secondary members with lapped ends, which is not discussed in industry blast documents. Blast design methodologies that were established by blast testing indicate that cold-formed beams typically do not maintain ultimate moment capacity at deflections greater than yield because of local compression buckling; however, typical connections and in-plane restraints allow for a limited amount of tension membrane that can develop to offset the loss of strength associated with local buckling. A parametric finite element model is utilized to study the sensitivity of cold-formed girt and purlin blast response to lap splice lengths, boundary conditions, blast loading regime, plastic deformations, and end connection conditions. Observations of member cross-sectional flexural resistance (i.e., local buckling) and the contribution of typical end connections to tension membrane resistance are presented. The results of the parametric finite element models are relied on to propose a revised single-degree-of-freedom (SDOF) blast methodology for cold-formed girts and purlins in PEMBs.