Collapse of the World Trade Center Towers, 3continued The access and services were carried through the central core, as shown in Fig 5. The floor system comprised 900 deep bar joists spaced at 2.04 m centers and braced by secondary joists. These secondary joists then supported a profiled deck on which was poured a 100 mm thick light-weight concrete slab. The top of the bar joists stood above the soffit of the decking and was cast into the concrete slab to make the bar joists composite in a similar manner to the Speedfloor system. The bar joists spanned between the perimeter frames and the core, as shown in Fig 6. Fig 7 shows an isometric of part of the floor and exterior wall , illustrating some of the details described above. The gravity and lateral load-resisting systems were designed to deliver the strength and stiffness required from a 110 story building with minimum dead load. This was achieved very well, with a steelwork weight of only 44.5 kg/m 2 floor area. The very light and open structure, superbly engineered to meet the design serviceability and ultimate limit state conditions on a building of this height and size, probably made the buildings more vulnerable to collapse from the aircraft impact than would have been the case for a more inefficient and heavier structural system. Passive fire protection was provided to the columns by vermiculite plaster and to the underside of the floor systems by a fire rated suspended ceiling. Each tower had an effective floor area of 319,000 m2 and used 87,000 tons of steelwork. 
Fig. 5 Plan View of Typical Floor ( from [1]). The numbers in the figure are taken from [1] and denote: 11, 12 – Lifts, 10 – Open plan offices. Click to enlarge. 
Fig. 6 Structural System for Typical Floor ( from [1]). The numbers in the figure are taken from [1] and denote: 13 – Perimeter frame, 14 – Bar joists 900 mm deep, 15 – Secondary joists, 16 – Horizontal floor bracing, 17 – Core box columns. Click to enlarge.
Fig. 7 Isometric View of Floor and Exterior Wall System. Click to enlarge. continued... Related Articles | 
