 | | Collapse of the World Trade Center Towers, 2continued Details of the Buildings Fig 1 shows the two towers in service. These towers were the principal buildings in a complex of city development. Fig.2 shows the location of these towers on the World Trade Center site, along with the direction of impacts. 
Fig. 2 Site Plan of World Trade Center Development Showing Location of the Two Towers. The direction of arrows adjacent to each tower shows the approximate direction of the planes on impact. (The direction of arrows adjacent to each tower shows the approximate direction of the planes on impact.) This map should be orientated 40 degrees clockwise from the stated directions – the North face is actually closer to North-East.
Each tower was 411 meters high, 63.5 x 63.5 meters square on plan, core 24 x 42 m. There were 110 stories in each tower. Each tower was 411 meters high, 63.5 x 63.5 meters square on plan, core 24 x 42 m. There were 110 stories in each tower. The towers were one of the best examples of “tube tower construction”, a structural form ideally suited to providing the strength and stiffness required for very tall buildings. On each façade a rigid moment-resisting frame was formed comprising 59 box-section columns, spaced at 1.02 meter centers, connected by deep spandrel beams. The frames did not run into the corners, however, there a shear connection between the two adjacent frames was provided so that the frames, together with the floors, formed a torsionally rigid framed tube fixed to the foundations. This framed tube carried all wind loads. The floors spanned without intermediate columns to the core, which was supported on 44 box-section columns designed and detailed to carry vertical loading only. Fig 3 shows this concept in an isometric view, while one of the 450 x 450 mm exterior frame box columns is shown in Fig. 4. 
Fig. 3 Isometric View of Building (from [1]), the numbers in the figure are taken from [1] and denote: 13 – Perimeter frame, 17 – Core box columns 450 mm square, 20 – Floor slab.
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Fig. 4 Cross Section Through Exterior Box Column (from [1]) The numbers in the figure are taken from [1] and denote: 36 – Steel column, 38, 39 – Fire resistant plaster, 40 – Aluminum façade, 42 – Window glass, 43 – Window frame.
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