An emerging specialty in the design of building structures is the design for blast or impact loading. Once only a concern for government and military structures, private sector building owners are also requesting that blast resistance be a consideration in the design of their structures with increasing frequency. In response to the growing market for blast-resistant design, the American Institute of Steel Construction (AISC) has just released a Design Guide for Blast Resistant Structures [Design Guide 26]. This document, which can be accessed online at www.aisc.org, covers many elements of blast resistant design from loading to member design to progressive collapse mitigation.
One particularly interesting section is Chapter 7 on connection design. Section 7.4 Connection Strength requires that “Connections should be designed to develop the full plastic capacity of the supported members, so that the plastic response of the structure can be mobilized in resisting blast-induced stresses. It should be noted that the dynamic plastic capacity of an element, loaded briefly by impulsive loading, is often greater than the static plastic capacity.” In addition, Section 7.7 Bracing and Moment-Resisting Connections states that “In addition to the modifications required for available strength discussed previously, bracing and moment-resisting connections should be designed in accordance with the AISC Seismic Design Manual.” The implication of both of these clauses is that the lateral force resisting systems of blast-resistant structures should be designed and detailed in the same way as one would design and detail seismic-resisting frames. For braced frames, this means that brace connections must develop the expected yield strength of the brace member which, if you’ve designed seismic bracing connections, you know is AgRyFy.
Connecting a member for a force exceeding its nominal tensile strength is not a simple matter. In fact, the challenges in seismic bracing connection design are exactly why our High-Strength Connectors were developed at the University of Toronto. The High-Strength Connector (HSC) accommodates a complete joint penetration groove weld around the entire circumference of a circular hollow structural section or pipe brace member, thereby ensuring that the connection strength exceeds the member’s expected yield strength. The simplicity and elegance of the HSC, which make it an appealing choice for seismic bracing connections, make the HSC a natural option for bracing connections in blast resistant steel buildings as well. So the next time your client requests that you consider blast loading for their building refer to AISC’s Design Guide 26, and also consider High-Strength Connectors for your bracing connections.
