Progressive collapse design of buildings continues to be an active area of research and development. Not only is the topic important in terms of protecting building occupants but it is also an intellectually-engaging issue, with a wide spectrum of views, ranging from those who feel progressive collapse design is not needed to those who have an obligation to protect the public. The range of technical issues is quite broad also, as there are many types of structural systems and materials, the structural response is typically complex and nonlinear, and there are a number of analysis and design approaches that can be taken. Finally, there is also passionate discussion generated by those who mistake progressive collapse design for hardened structure design, those who propose risk-based approaches when it is known that there is insufficient data to assess risk and those who feel progressive collapse design should not be employed until more research is done and all answers are known.
Research into progressive collapse (PC) is ongoing in the United States (US) and Canada. As part of ongoing efforts by the US Department of Defense (DoD) to better protect its personnel, Unified Facilities Criteria (UFC) 4-023-03 Design of Buildings to Resist Progressive Collapse has recently been improved. Test programs are ongoing in North America, including tests of full-scale reinforced concrete and steel structures as well as additional test and analysis efforts at universities and government agencies. Finally, commercially-available structural design software, such as SAP2000 and ETABS, are oftenused to design for progressive collapse, following the applicable design requirements such as UFC 4-023-03. SAP2000 can also be used as an analysis tool and can provide similar or better results than those determined with high-fidelity physics based (HFPB) tools. A brief summary of these efforts are provided in this paper.
The development of UFC 4-023-03 Design of Buildings to Resist Progressive Collapse was initiated by the US DoD in 2003. The DoD had developed earlier PC design guidance in response to the Oklahoma City bombing  but desired a more comprehensive approach after the World Trade Center attacks. There have been two major versions of UFC 4-023-03, with the first release in 2005  and the second in 2009 . The 2009 version of UFC 4-023-03 has recently been modified as discussed later in this paper.
The motivation for the development of UFC 4-023-03 was the lack of progressive collapse design guidance within the US civilian design community. The design of US government facilities typically follows consensus civilian building codes and standards of practice, which were not available for progressive collapse design. As a result, the DoD, was forced to develop criteria to reduce the vulnerability of structures to progressive collapse.