The terrorist attacks of 1995 (Murrah Federal Building) and 2001 (World Trade Center) focused attention on structural design requirements to prevent the catastrophic damage caused by these events. However, creating practical design requirements for such severe loading proved to be an extremely difficult task. Because prescriptive and performance requirements for blast and progressive collapse mitigation do not appear in the design codes they are not codified into law. The owner, architect and structural engineer are the key stakeholders who carry the responsibility in deciding if the need for blast and progressive collapse resistance is necessary. Generally prescriptive design procedures for blast and progressive collapse resistance result in inefficient costly construction and limitations on the architectural design of such buildings thus are rarely implemented on a project that doesn’t have GSA, DoD or State Department requirements.

The current blast effects/progressive collapse market is relatively small and is primarily government driven. The desire for this type of protection is significant however high cost, difficulty in structural design and impact on program flexibility pushes it into an impractical requirement. We believe that this market will undergo a paradigm shift in growth when resistance to blast effects and progressive collapse can be attained with practical cost effective designs.

With the ConX™️ System, the key stakeholders now have the option of specifying a structural framing system that will carry gravity, wind and seismic loading and, at no additional cost, also provide resistance to blast and progressive collapse, all in one efficient structural framing system. The innovative and highly robust ConX framing system provides a cost effective and easily configurable biaxial moment frame connection that also provides blast and progressive collapse resistance.

In 2004, Ronald Hamburger of Simpson Gumpertz & Heger wrote an article discussing the high costs and architectural limitations of current prescriptive design methods. He felt there was a much more cost effective performance based design method, but the industry needed “a program of research and development similar to that conducted after the 1994 earthquake to determine the types of connection technologies that can be effective in resisting progressive collapse so that less conservative but more reliable approaches to blast-resistant design can be adopted by the community.”

The ConXtech ConXR™️ framing connection debuted in 2004 and the ConXLT™️ connection in 2008. In 2009 Ronald Hamburger wrote “The ConXR and ConXL systems are ideal for building applications that must be designed to resist progressive collapse resulting from vehicle impact, incendiary or explosive device attack.”

“The ConX Connection: A Bolted Special Moment Frame Connection for Seismic and Structural Integrity Applications” March 2009
Ronald O. Hamburger, Simpson Gumpertz & Heger

“Design of Steel Structures for Blast-Related Progressive Collapse Resistance”
Modern Steel Construction, March 2004
Ronald O. Hamburger, Simpson Gumpertz & Heger Andrew Whitaker, University at Buffalo, NY

There are currently two distinct yet similar sets of criteria established by different branches of the federal government to guide in the design of buildings. The DoD Minimum Anti-Terrorism Standards for Buildings and the ISA Security Design Criteria provide standards establishing enhanced protection against the threat of explosive attack against DoD installations, federal courthouses and office buildings respectively.

Each encourages the development of designs with uniformly distributed fully ductile continuous moment frames as a first approach to robustness, redundancy, and resistance to progressive collapse. The assumption that the premium associated with providing continuous ductile moment connections at every column would be cost prohibitive lead to the development of criteria that require them only on the frames that are exposed to the threat potential.

Buildings without accessible lobbies or secured parking below that are within a specified standoff to public roadways, need only be provided with continuous ductile frames at the exterior.

The DoD progressive collapse design requirements are contained in the United Facilities Criteria 4-023-03 (2009), while the ISA progressive collapse criteria are given in the GSA Progressive Collapse Analysis and Design Guidelines (2003).

Standardizations establish enhanced protection against the threat of explosive attack against DoD installations, federal courthouses and office buildings respectively.

The GSA guidelines provide a process of exemption from further consideration of progressive collapse through a set of flow charts that take the user through a series of questions about the structure. 

Using this indirect design approach, structures with minimum levels of strength, continuity and ductility that are designed according to recognized principles that ensure structural integrity are considered to have a low potential for progressive collapse.

Using this indirect design approach, structures with minimum levels of strength, continuity and ductility that are designed according to recognized principles that ensure structural integrity are considered to have a low potential for progressive collapse.

According to the GSA guidelines, new federal buildings without public areas and/or uncontrolled parking with perimeter framing consisting of continuous ductile moment frames qualified according to current AISC seismic provisions require no further consideration of progressive collapse. For building with secured public areas and/or uncontrolled parking, these exemptions can be applied if the continuous ductile moment frames are provided at all affected interior bays. 

Both sets of design criteria have established means by which structures can be designed with some lesser degree of protection than that provided by uniformly distributed fully ductile continuous moment frames along every column line of the building. In the planning stage, the nature of the threat is limited by designing buildings with adequate standoff that do not have public areas and/or uncontrolled parking. UFC 4-023-03 limits the area of a building to which strengthening is applied based on the buildings Occupancy Category. By any measure the level of security provided to even the most important structures falls short of the ideal conceptual structure envisioned by both the GSA guidelines, and the DoD Minimum Anti-Terrorism Standards for Buildings because of the assumption that the cost outweighs the benefit.