CONTINUED FROM THE LAST POST
The world’s tallest building, the BURJ (Dubai)
On the other hand, the core structure which extended beyond the setbacks acted as restraint to the setbacks (i.e. the wings that have been terminated. In so doing, these setbacks could be analyzed as columns restrained at both ends-thus reducing their effective height (respective effective height = 0.75 x actual height) and as a result their slenderness ratio. The core structure that extended beyond the point of termination of the outflanking wings could be viewed as the critical lift for the lateral loading. This is due to the fact that the top end is free as such none restrained or supported. The structural layout went a long way to solving this problem in that this lift could be likened as having effective height that is the overall height minus the height of the terminated wings. This drastically reducing the structural effective height. So if the outflanking wings terminated at the height of 600m above the ground level, then the effective height of the extended part of the structure (the ultimate lift) will be 2(800-600) = 400m instead of 1600m!! Again all the changes in dimension were concentric-thus making the geometric center to more or less coincide with the center of mass/stiffness (so doing avoiding conscious introduction of eccentricity). These are only the preliminary conditions. The dynamic loading due to wind loading will definitely suffice considering the exposure condition (wind loadings become grievous with increase in heights, the shape factor etc). To meliorate on that problem (apart from the measures cited earlier), ingenious design had to be adopted as to shade the wind load before they could be applied to the structure. This is achieved by incorporating structures such as fenders, walls, fittings on the external surfaces of the building as to make the wind fronts to intercept themselves, forming interfering waves instead of constructive waves. This pattern of wave, sort of, (may I say), is self-destructive instead of constructive. This is analogous to fighting fire with fire. This process will not eliminate the load but will only reduce it. Given the complexity involved in these interactions and reactions, in actuality, the horizontal forces that would have to be confronted eventually are not easy to estimate as such one will have to resort to the help of computation analysis such as that done with the computational fluid dynamics (the CFD). In most cases, particularly those involving structure of primary importance (as this structure), the wind tunnel study of the behavior of the structure under likely wind loadings have to be carried out. This has the advantage that one could observe physically the behavior of the structure rather than relying wholly on computer simulation (as given by the CFD analysis).
We have to recall that this load is not the only horizontal load to be contended with. The seismic load may be the one that may pose the major problem, thus the critical load (if the structure is located in a seismic zone (and if yes), the characteristics of the site and seismic intensity). In that case, the structures that were incorporated into the building as to reduce the wind loading will not have any effect on the design horizontal loading due to earthquake.
Regards
Teddy