Since the terrorist attacks of September 11, 2001 and the collapse of three very tall buildings at New York's World Trade Center, some members of the public, building officials, legislators and engineers have stated a belief that present design standards do not provide sufficient safeguards to ensure that buildings will be able to withstand collapse in the event of reasonably foreseeable attacks and incidents. The issue at question, however, is what constitutes a reasonably foreseeable event. In fact our present building codes and standards do require design to provide collapse prevention for reasonably foreseeable events including snow storms, wind storms, fires and earthquakes. Indeed, this is one of the primary purposes of these codes and standards. However, the magnitude of the event for which collapse protection is provided is not intended to cover the worst occurrence of such events that could ever occur. Rather, the intensity of design events are generally selected for these hazards in a manner that balances the cost of providing protection against collapse and the probability of experiencing the initiating event. Thus, in typical buildings, we design for wind loads with a 50-year mean recurrence interval in hurricane-prone regions we design for 100-year storms and for most of the nation, we design to withstand collapse 2,500-year earthquakes. While we can reasonably foresee that a tornado may strike a building in the mid-west|or that a meteor may land on a building at any location in the U.S., we design for neither of these events, as well as many other foreseeable events because for any one building, the likelihood is so low that the cost of providing the protection would far outweigh the benefits of having such protection. Recognizing the difficulty of identifying specific design scenarios beyond those already covered by the building codes and standards that should be included in design, in their report on the World Trade Center Collapse, the National Institute of Standards and Technology (NIST) recommended a series of improvements to current design practice to reduce the potential for disproportionate collapse of structures. The concept behind designing to prevent disproportionate collapse is that it does not matter what the initiating event is, an airplane impact, a meteorite, or terrorist bomb, relatively small initiating events should not result in large-scale failure and collapse. NIST and other organization introduced proposals to the International Code Council that required specific disproportionate collapse analyses of many types of structures. In response, the National Council of Structural Engineering Associations, in cooperation with industry associations representing the concrete, masonry, steel and timber construction industries developed a series of alternative proposals to place supplemental structural integrity criteria into the International Building Code. This paper provides an overview of these proposed requirements.
Steel connection design for structural integrity
April 29, 2008
Publication: Crossing Borders: Proceedings of the 2008 Structures Congress. April 24-26, 2008, Vancouver, BC, Canada, ASCE
Services: Structural Design