Premium for Height

The primary structural skeleton of a tall building can be visualized as a vertical cantilever beam with its base fixed in the ground. The structure has to carry the vertical gravity loads and the lateral wind and earthquake loads. Gravity loads are caused by dead and live loads. Lateral loads tend to snap the building or topple it. The building must therefore have adequate shear and bending resistance and must not lose its vertical load-carrying capability. Fazlur Khan realized for the first time that as buildings became taller, there is a “premium for height” due to lateral loads and the demand on the structural system dramatically increased, and as a result, the total structural material consumption increases drastically (Ali, 2001). If there would be no lateral forces on the building such as wind or earthquake, any high-rise building could be designed just for gravity loads. The floor framing system usually carries almost the same gravity loads at each floor, although the girders along the column lines need to be progressively heavier towards the base of the building to carry increasing lateral forces and to augment the building’s stiffness. The column sizes increase progressively towards the base of the building due to the accumulated increase in the gravity loads transmitted from the floors above.
If we assume the same bay sizes, the material quantities required for floor framing is almost the same regardless of the number of stories. The material needed for floor framing depends upon the span of the framing elements, that is, column-to-column distance and not on the building height. The quantity of materials required for resisting lateral loads, on the other hand, is even more increased and would begin to exceed other structural costs if a rigid-frame system is used for very tall structures. This calls for a structural system that goes well beyond the simple rigid frame concept. Based on his investigations Khan argued that as the height increases beyond 10 stories, the lateral drift starts controlling the design, the stiffness rather than strength becomes the dominant factor, and the premium for height increases rapidly with the number of stories. Following this line of reasoning, Khan recognized that a hierarchy of structural systems could be categorized with respect to relative effectiveness in resisting lateral loads for buildings beyond the 20- to 30-story range (Khan, 1969).