DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

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(2) Two adjacent “wall panels or nominal lengths thereof ’ forming an exterior comer of the building. (3) One or more elements forming a “nominal extent of flooring”. (4) One column ! (5) Any other one element of a stmctural subsystem that is judged to be vital to the building’s I stability. Nominal Length of Wall Panel is the smaller of the following lengths as appropriate to the design in question: (1) The length between adjacent lateral supports. (2) The length between a free edge and the nearest lateral support. (3) A length equal to 2.25 times the clear height of the wall panel. Lateral Support is considered to occur at; I i (1) A substantial partition perpendicular to the wall. (2) A strengthened vertical portion of the wall (not exceeding 1/3 story height in the horizontal direction). Substantial Partition is defined as having an average weight of not less than 30 pounds per square foot, tied with connections that are capable of resisting a force of 2.1 or 0.7+0.14Ns in units of Kips per foot height of wall (which ever is less), and Ns is the number of stories in the structure. Nominal Extent of Floor denotes the following: (1) for a floor spanning in one direction, the extent is the clear span. In the perpendicular direction, the extent is to be taken as the smaller of the following: i. the distance between adjacent “substantial” partitions arranged in the direction of the floor span. ii. The distance between a free edge and the nearest “substantial” partition arranged in the direction of the floor span. iii. In the case where partitions are not “substantial” the extent is to be taken as 2.25 times the clear span. (2) for a floor spanning in two directions, the extent should be taken as the area bounded by the clear spans in both directions. (c) Methods of Evaluation. (1) The Indirect Method: If all members are structurally connected by joints capable of transferring the specified capacity in tension, shear, or compression (as appropriate) without reliance on friction due to gravity loads or when additional tie members are provided as specified below, then the layout and configuration of the building and the interaction between (or strength of) its members should be deemed to provide adequate protection against progressive collapse under abnormal load. The structure must be able to withstand at a minimum, a horizontal load of 1.5% of each floor’s weight applied simultaneously on all floors, checked separately from the effects of seismic or wind. The structure must also include effective ties for the key structural elements. These consist of peripheral ties, internal ties, horizontal ties to colunms and walls and vertical ties. The tie force, F„ should be the lesser of 4.1 or 1.4+0.27Ns in units of Kips per foot width and Ns is the number of stories in the structure. The specified capacity of such ties should be no less than the capacities indicated for the different types of construction, as described below: A) In reinforced concrete structures, the reinforcement provided for other purposes may be regarded as forming part of, or the whole of, these ties and should conform to the requirements of RS9___ (ACI-318 Section 7.13 Requirements for Structural Integrity). In addition, the building should be checked for the presence of the following ties. The same reinforcing may be used to satisfy both requirements. Bars may be considered anchored to another tie at right angles if the bars extend beyond all the bars of the other tie for an effective anchorage length (based on the force of the bars) beyond the center-line of the bars. The ties must be adequately anchored where substantial changes in construction or reentrant comers interrupt the continuity of the ties.

1. Internal ties should be at each floor and roof level in two perpendicular directions. The ties should be effectively continuous throughout their length and anchored February 2003 WTC Building Code Task Force Page 31 Findings and Recommendations

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