Blast Hardening of Curtain Wall Mullions with Infilled Cementitious Materials

Abstract

Glass façades and curtain walls are frequently employed in the construction industry as part of building envelopes. They offer several functional benefits, such as a pleasing architectural look, energy optimization, acceptable fire resistance, and low maintenance cost. However, they are typically designed for wind pressures and pose a high risk when exposed to blast loads. The glass panes that make up the building envelope are held up by aluminum frames, which consist of vertical elements, referred to as mullions, and horizontal elements, referred to as transoms. Critical infrastructure, such as important government facilities, embassy buildings, office complexes, and industrial facilities that have curtain walls may be vulnerable to blast loads. Experience with such blast loads demonstrates the need for in-depth experimental and analytical studies, especially on mullion hardening as they play the primary role in carrying blast loads to the structural substrate. Literature reviews show that there has been limited prior study on mullion strengthening/hardening. The goal of the current investigation is to develop innovative hardening methods for curtain wall mullions involving infilled cementitious materials to endure impulsive blast loads of high intensity. A combined experimental and analytical study recently conducted at the University of Ottawa revealed that the aluminum mullions of glazed curtain walls sustain severe damage because of their weakness and high deformation demands. Lack of mullion strength and inadequate connections between the vertical and horizontal mullions under blast loads were identified as the major contributors to the unfavorable performance of such walls. The current research project consists of experimental and analytical tasks. The experiments were carried out at the University of Ottawa's Shock Tube Facility. The hardening technology that was researched entails the filling of curtain wall mullions with different types of cementitious materials with and without additional reinforcement. The tests comprise of commercially available hollow aluminum sections as mullions, filled with high-strength mortar (HSM) or engineered cementitious composites (ECC). Some of the mullions were reinforced with steel wires or steel chains inserted as internal reinforcement, others were reinforced externally with carbon fibre reinforced polymer (CFRP) sheets. A total of 16 mullion tests were performed. ii They were exposed to incrementally increasing blast loads to assess the improvements in strength and deformability. Following the tests, analytical research was conducted with the primary goal of validating the experimental findings and the assumption of full composite action between the aluminum mullion and the filling materials. Resistance functions were developed to conduct nonlinear single degree of freedom (SDOF) dynamic analysis. In addition, an analytical parametric investigation was conducted under selected threat scenarios to expand the results. The results indicate that filling aluminum mullions with selected cementitious materials increase strength and deformability of mullions under blast loads. The infill material provides additional compressive resistance while controlling local buckling of the aluminum sections. Cement-based materials with fibres, such as ECC provides additional tensile capacity and deformability adding further to the resistance of the mullions. The additional reinforcement provided internally, in the form of steel cables or steel chains, and externally in the form of CFRP sheets provide further enhancement of the resistance in tension, generating superior performance of curtain wall mullions under high blast loads. The design and construction procedure provided at the end of the current research project provides guidance to practicing engineers and security professionals to mitigate blast risk in modern buildings.