DOB Findings and Recommendations for WTC, Feb 2003

NYC 9/11 Public Portal Document

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to the peripheral ties at each end. The ties may, in whole or in part, be spread evenly in the slabs or may be grouped at or in beams, walls or other appropriate structural .1 elements. The spacing of the ties should not be greater than 1.5Lr where is the spacing of the columns, frames or walls supporting any two adjacent floor spans in the direction of the tie. In walls, the ties should be within 1-foot of the top or bottom of the floor slabs. In each direction, the ties should be capable of resisting a tensile force equal to the greater of F, or (2F0(DL+LL)(La/5) per unit width'*’. 2. At each floor and roof level an effectively continuous peripheral tie should be provided, capable of resisting a tensile force of 3.3F, located within 4-feet of the edge of the building or within the perimeter wall. Each external column and, if the peripheral tie is not located within the wall, the load-bearing external wall should be anchored or tied horizontally into the structure at each floor and roof level with a tie capable of developing ( a force equal to the greater of 6.6Ft (or 0.8hF, if less) artd 3% of the total design ultimate i vertical load carried by the column or wall at that level, where h is the floor to ceiling height in feet. Where the peripheral tie is located within the wall, a positive coimection should be provided between the internal and peripheral ties. .3. Comer columns should be tied into the structure at each floor and roof level in each of two perpendicular directions with ties each capable of developing a force equal to the greater of 6.6F, (or 0.8hF, if less) and 3% of the total design ultimate vertical load carried by the column or wall at that level, where h is the clear height in feet. 4. Each column and each wall carrying vertical load should be tied continuously from the lowest to the highest level. The tie should be capable of resisting a tensile force equal to the maximum design ultimate dead and live load received by the column or wall from any one story. Where a column or a wall at its lowest level is unsupported by an element other than a foundation, a general check for structural integrity should be made to ensure that there is no inherent weakness of structural layout and that adequate means exist to transmit the dead, live and wind loads safely from the highest supported level to the foundations. B) In precast and composite structures, the ties should be effectively continuous. The ties should be effectively anchored, such that the anchorage is capable of carrying the dead weight of the member to that part of the structure which contains the ties. The ties should conform to the requirements of ACI-318 section 16.5 governing 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. 1. The integrity of a bearing is dependent on an overlap of reinforcement in reinforced bearings and a restraint against loss of bearing through movement. The net bearing width should be the greater of 1.75-inches and the design ultimate support reaction per member divided by the product of the effective bearing length and the design ultimate bearing stress. The net bearing width of isolated members should be 0.75-inches greater than for non-isolated members. The effective bearing length should be the least of the bearing length per member, one half the bearing length per member plus 4-inches or 24-mches. The design ultimate bearing stress should be based on the weaker of the bearing surfaces and is calculated as follows: i. 0.4fc for dry bearing on concrete ii. 0.64f c for bedded bearing on concrete iii. 0.8 f c for contact face of a steel bearing plate cast into a member or support with each dimension less than 40% of the corresponding concrete dimensions 2. Ties should satisfy one of the following conditions: i. A bar or tendon in a precast member should be lapped with a bar in cast-in-place connecting concrete bounded on two opposite sides by rough faces of the same ! precast member. ii. A bar or tendon in a precast concrete member lapped with a bar in cast-in-place topping or connecting concrete anchored to the precast member buy enclosing links. The ultimate resistance of the links should not be less than the ultimate tension in the tie.

DL and LL are expressed in Kips/Square Foot (KSF) i Page 32 WTC Building Code Task Force February 2003 Findings and Recommendations

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