Design innovations
A typical floor layout and elevator arrangement of the WTC towers. To solve the problem of wind sway or vibration in the construction of the towers, chief engineer Leslie Robertson took a then unusual approach — instead of bracing the buildings corner-to-corner or using internal walls, the towers were essentially hollow steel tubes surrounding a strong central core. The 208 feet (63.4 m) wide facade was, in effect, a prefabricated steel lattice, with columns on 39 inch (100 cm) centers acting as wind bracing to resist all overturning forces; the central core took the majority of the gravity loads of the building. A very light, economical structure was built by keeping the wind bracing in the most efficient area, the outside surface of the building, thus not transferring the forces through the floor membrane to the core, as in most curtain-wall structures. The core supported the weight of the entire building and the outer shell containing 240 vertical steel columns called Vierendeel trusses around the outside of the building, which were bound to each other using ordinary steel trusses. In addition, 10,000 dampers were included in the structure. With a strong shell and core such as this, the exterior walls could be simply light steel and concrete. With the massive core and lightweight shell for structural integrity, Robertson created a tower that was extremely light for its size. This method of construction also meant that the twin towers had the world's highest load-bearing walls.
