DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

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largest span between columns in tie direction or 5h, and h is the clear story height expressed in feet. 3. Exterior horizontal ties should be provided from perimeter columns and walls to floor slabs. Tie comer columns both ways. Tie walls uniformly, or not farther than 18- feet apart and within 9-feet of wall ends. Design strength should be at least the lesser of 6.6F, (or 0.8hF, if less) in units of Kips at columns and 2F, (or 0.12hF, if less) Kips/foot at walls, where h is the story height expressed in feet. 4. Vertical ties should be provided floor to floor at load bearing walls 6-inch minimum thickness, masonry strength 725-psi and maximum slenderness (clear height/thickness) of 20. Tie every 16-feet maximum and 8-feet from unrestrained end of wall. Design tie capacity for the larger of (12.7A)(ha/t)' Kips or 6.85-Kip/foot of wall or per column, where A is the horizontal cross-sectional area in square inches of the column or load-bearing wall including piers, ha is the clear wall height and t is the thickness of column or wall. (2) The Alternate Path Method: Wherever the design cannot accommodate the prescriptive design and detailing requirements of the indirect method, each such key member should be notionally removed, one at a time, to determine that its removal would allow the rest of the structure to bridge over the missing member. These criteria should be satisfied while the building is subjected to its own weight D plus a superposed load of 0.25L and a wind load of 0.2W assumed to be acting in combination. D is computed according to Article 2 of I Subchapter 9 of Chapter 1 of Title 27 of the Administrative Code and according to Reference ! Standard RS 9-1 of the same Code. L is computed according to Article 3 of Subchapter 9 of Chapter 1 of Title 27 of the Administrative Code and according to Reference Standard RS 9-2 of the same Code without allowance for the live load reduction permitted in Article 4 of ! Subchapter 9 of Chapter 1 of Title 27 of the same Code. W is computed according to Article 5 of Subchapter 9 of Chapter 1 of Title 27 of the Administrative Code and according to Reference Standard RS 9-5 of the same Code. Proof that the alternate path method is satisfied while the building is subjected to the loads stipulated in the criteria can be provided by the analysis and/or physical simulation. These criteria should be satisfied in accordance with structural analysis based on the Plastic Design or Ultimate Strength method, representing conditions at incipient failure and considered as an independent check of a building designed in accordance with the usual procedures for L Working Stress, Plastic Design or Ultimate Strength Design pursuant to Subchapters 9, 10 and .11 of Chapter 1 of Title 27 of the Administrative Code and all applicable Reference Standards thereto. (3) The Specific Local Resistance Method: Any single element essential to the stability of the structure, together with its structural connections, should not fail under the loads stipulated in this criterion after being subjected to a project specified abnormal or extreme local loading condition. The structure should be detailed to permit load reversals and the connections should be designed to develop the capacity of the members. If an extreme local loading condition is not specified for the project, the key elements should be detailed to develop the ultimate capacity of the materials in shear, flexure and axial load by means of confinement and continuity of reinforcement for reinforced concrete construction and encasement or stiffeners for rolled steel construction. For a column or other compression element, the interaction between flexure and axial load must be considered when establishing the capacities. In those cases where the stability of the element depend upon the lateral support provided by the attached space dividers, these space dividers, or a portion of these space dividers which can provide adequate lateral support, must also satisfy requirements of this paragraph. These essential elements may respond inelastically to the abnormal or extreme local loading conditions; however it must be demonstrated through appropriate analytical methods that these elements can continue to carry the gravity and lateral loads they support.

Page 34 WTC Building Code Task Force February 2003 Findings and Recommendations

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