DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

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5.2.2 Commentary General Considerations These guidelines are intended to enhance the probability that if localized damage occurs as the result of an abnormal loading event, the structure will not progressively collapse or be damaged to an extent disproportionate to the original cause of the damage. The initiating damage may be the result of an accidental impact or overload, misuse or structural alteration, fire or explosive event. Each of these unintended loading conditions or reductions in structural capacity, that may originally be localized, may leave the structure vulnerable to a much more extensive structural collapse unless measures are taken to mitigate that possibility. The guidelines for general structural integrity, robustness and resistance to disproportionate collapse are based on a preliminary understanding of recent events and existing guidance from other jurisdictions, such as the U.K. Standards and Regulations, where structures designed to these standards demonstrated desirable performance when subjected to abnormal loading. The intent of these guidelines is to alert the engineer to conditions that may initiate progressive collapse. Recent events suggest that extended fire exposure, a relatively common hazard, can impact a building’s ability to resist its load. These experiences indicate that building designs should consider resistance to collapse resulting from extended exposure to fire just as they are designed to resist collapse when exposed to other loading conditions (i.e. gravity, earthquakes, wind, etc.). By considering resistance to fire induced collapse these guidelines may increase the safety of occupants and emergency personnel, decrease the damage to neighboring buildings and people, and reduce the financial losses associated with collapse. The behavior of the structural system under fire should be considered an integral part of the structural design process. Registered Fire Protection Engineers can assess anticipated fuel loads and identify the potential fire hazards. This will determine a range of thermal changes and temperature sensitive parameters to which the structural members may be subjected. A registered Structural Engineer can assess the impact of these calculated thermal changes to the structural members and determine the ability of the structure to resist progressive collapse. These conditions may include buildings that have an unusual fire load that may be associated with unusual occupant characteristics, building characteristics, content/fuel load characteristics, and other specific risks. For high-risk conditions, the engineer should consider the behavior of the building and its ability to resist progressive collapse. Fire can expose multiple elements throughout a space to high temperatures, thus the potential thermal impact on all the affected elements should be determined. The possibility of the loss of multiple structural components should be assessed in relation to the multiple related events, such as an earthquake or explosion, which may cause a fire to start. Fire produces a differential thermal expansion, reduces the modulus of elasticity, and reduces the yield strength of steel. In a fire the expansion of the different elements of the structure is dissimilar due to its heat capacity, conductivity, mass, insulation and proximity to the fire. The full scale fire testing on an 8- story building at the Building Research Establishment (BRE) conducted at Cardington U.K., demonstrated the adequacy of specific steel buildings to a given set of fire scenarios. The successful performance of these structures was in part due to their connections having a sufficient amount of tension capacity to absorb the load transfers. The performance of concrete buildings under fire is dependent in part on the cover/protection of the reinforcing steel, the thermal insulation provided by the concrete, and its ability to resist spalling. Hence, when undertaking analyses for concrete buildings, the effect of heat on spalling and the reduction in protection of reinforcing steel should be assessed. At a minimum, the analysis should verify that the structure remains standing; however, additional performance criteria regarding the level of tolerated damage and the extent of its ability to continue to be serviceable should be established by the stakeholders. A simplified linear analysis may at times be possible by assuming a temperature, an elastic modulus and yield strength for the structural elements exposed to the fire. There is a significant body of published information about structures under fire that should be reviewed when undertaking these types of analyses. In the case of overload, accidental impact or blast, the protection of structures to resist progressive collapse may be achieved by either allowing for multiple load paths or by hardening the vulnerable structural elements to be resistant to a postulated abnormal loading condition. The desired detailing may either be

’ Kirby B.R. British Steel data on the Cardington fire tests. Technical Report, British Steel, 2000. February 2003 WTC Building Code Task Force Page 35 Findings and Recommendations

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