Experimental program and simplified nonlinear design expression for glass curtain walls with low-level blast resistance

Abstract

A series of five full-scale, nearly conventional, curtain wall specimens was tested in the UNC Charlotte Structures Laboratory. Specimens were subjected to quasi-static, uniform, out-of-plane loading to failure under displacement control. The tests were performed to obtain complete resistance curves, including the nonlinear behavior of the specimens up to 'ultimate failure'. Ultimate failure was defined as mullion fracture or significant breach of the curtain wall system when viewed as the protective barrier between building occupants and the external blast load. Representative load-deflection and load-strain resistance curves are presented. The energy absorbed by the curtain wall system up to three different limit states - first cracking of glass, first yield of mullions, and fracture/breach of the system (ultimate failure) - and maximum mullion end rotations are computed from the experimental results. Ultimate energy absorption capacity - the recoverable linear strain energy plus the nonlinear energy due to formation of damage mechanisms - and maximum mullion end rotations are essential for reliable and economical design of blast resistant curtain walls. To this end, a simplified methodology is presented for analytically approximating curtain wall resistance functions that can be input to an energy expression that models nonlinear structural dynamic behavior due to an 'impulsive' loading. The blast resistance of a curtain wall can then be approximated using this procedure. It is shown that a nearly conventional curtain wall, a conventional system with two modifications - use of laminated glass lites that are structurally glazed (wet-glazed) to a conventional framing system with structural silicone sealant - had nearly 14 times the ultimate energy absorption capacity and nearly four times the blast resistance as the fully conventional system.