DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

I 5.2 Draft Progressive Collapse Guidelines t

5.2.1 Guidelines (a) General Considerations Progressive collapse is the propagation of collapse to an extent disproportionate to the initiating zone of damage and is interpreted as structural failure beyond the point of initial damage extending vertically over more than three stories and horizontally over an area greater than one structural bay or 20 percent of the horizontal area of the building, whichever is less. ASCE 7-98*'’ cites three design alternatives for providing structural resistance to progressive collapse, including the indirect design approach, the direct design alternate path and the direct design method of specific local resistance. The indirect approach is a f I prescriptive consideration of resistance to progressive collapse through the provision of minimum levels of strength, continuity and ductility. The direct design alternate path method allows local failure to occur but provides alternate load paths to bridge over the damage and avert collapse; The direct design method of specific local resistance provides sufficient strength to resist failure. If the design cannot accommodate the prescriptive design and detailing requirements of the indirect method, the alternate path approach can be used to quantify the extent ofductile detailing required for new construction. Where neither the indirect nor the alternate path approaches can be accommodated or where project specific threats are identified, the individual members can be locally hardened and detailed to develop the full resistance of each key element against unanticipated load without failing the connections or supporting members framing it. When the building contains materials that pose a high risk including high fiiel loads, combustible/flammable liquids explosive materials, toxic gases, etc. the effects of fire must be considered by separate analysis. This balanced approach develops the full resistance available to the key members and provides comprehensive resistance to progressive collapse. The load path of the structure should be taken in consideration in determining the appropriate method of evaluation. Where the Direct Design Alternate Path Method, or the Direct Design Specific Local Resistance method are utilized, licensed professional engineer(s) should demonstrate by analysis that the design is resistant to progressive collapse when exposed to credible threats including fire. To protect against a reduction in capacity and differential thermal expansion that may result in progressive collapse from extended exposure to fire, the structural members and components should be designed and have a fire resistance rating appropriate to their designed function. These structural members and components should be appropriate for associated hazards/threats and credible fire loads, the predicted fire intensity and duration of these fire loads, the height and the use/occupancy of the specific building or space, established goals, objectives, and level of risk acceptable to the stakeholders, fire and life safety features provided, and proximity to adjacent I structures and properties so as to limit the potential for progressive collapse. ! Transfer structures should be continuous over several supports with substantial structure framing into these members to create a two-way redundancy that provides an alternate load path in the event of a localized failure. The column connections, which support the transfer structures, should provide sustained strength despite inelastic deformations and designed as full moment connections. Transfer structures and the columns that support the transfer members should be hardened to the requirements of the specific local resistance. These guidelines also apply to transfer systems and to all elements that have no redundancy, such as tension hangers and tension ties in trusses. These elements should have connections that develop the full axial strength of the member (AgFy). All elements of a transfer system will be designed for the lowest strength reduction factor used in the design of any supported, supporting or transfer element. The transfer system and non-redundant key members are to be designed to sustain extended exposure to fire corresponding to the bum out of the floor on which it is located. (b) Definitions Kev Members are defined as those structural elements at any one story whose loss results in a collapse of the structure more than one story above or below the element under consideration, or over a horizontal area in excess of that stipulated in the criterion. These critical load-bearing members correspond to: j (1) Any single “wall panel or nominal length thereof.” i MW

' American Society of Civil Engineers Minimum Design Loads for Buildiiigs and Other Structures, Revision of ANSI/ASCE 7-98, Section Cl.4 General Structural Integrity Page 30 WTC Building Code Task Force February 2003 Findings and Recommendations

NYC-WTC_000166516