Nonlinear modeling of concentrically braced frames

Abstract

Concentrically braced frames (CBFs) comprise a large proportion of lateral-force-resisting systems used in steel building infrastructure around the world. Many categories of CBFs exist, including special, ordinary, and non-seismically detailed (i.e., current “R = 3” or pre-1988 construction) CBFs. Experimental testing of these different types of CBFs has shown that they have widely varying nonlinear behavior depending on the relative strengths of their yielding mechanisms and failure modes and level of ductile detailing. Numerical modeling of this range of behavior types is necessary to evaluate the seismic performance, including to quantify potential damage to special CBFs and the vulnerability of lower-ductility CBFs. Special CBFs have been the focus of many previous nonlinear modeling recommendations, including simulation of brace fracture, gusset-plate flexural strength and stiffness, and gusset-plate contribution to frame stiffness. However, recommendations for lower-ductility CBFs have not been well established. To provide important guidance for modeling these common systems, new recommendations based on the large quantity of available experimental data are proposed for simulating fracture of rectangular HSS braces with varying local slenderness, asymmetric load histories, and concrete in-fill; axial yielding of gusset plates; fracture of brace-to-gusset-plate welds; fracture of gusset-plate interface welds; post-fracture, secondary yielding mechanisms in gusset-plate interface connections; yielding beams in the chevron configuration; and buckling columns. These recommendations are validated using experiments of two-story chevron CBFs with yielding beams which simulated an existing, pre-1988 CBF (i.e., a nonductile CBF) and a subsequent repair where the braces and gusset plates were replaced.