02-17-2010, 10:51 AM
Like I said in my previous post, one of the main problems that had to be solved in the design and the realization of the building is that of containing the horizontal loads as induced by such agent as wind, seismic wave etc. Looking at this structure, one cannot afford not to recognize that it is a tall slender structure. This is recognized by the fact that if you should compare the vertical dimension to lateral the dimension, the ratio would be very great. But how great is this great to be as to be classified as slender?
If we should make a rough estimate, following the general guide and the formulae attributable to Euler for the classification of columns (this structure, acting as a whole, is like a column that is fixed at the base but free at the top), then the structural height of the building is twice that actual. So we would be talking of a structural height that is in excess of 2 x 800 = 1,600m (theoretically). If the structure was to have a uniform form (say cylindrical) from the base up to the roof top, then we can take the structure as a whole, calculate the ratio of its structural height to the radius of gyration of the structure, thus the slenderness ratio. This will definitely fall outside the 360 unit range that defined the extreme limit of long slender column. Again, since this structure is a reinforced concrete structure, if we should follow the guide as per BS 8110, the slenderness ratio will be the assumed actual height multiplied by 2 (800 x 2 = 1600 = structural height) then divided by the diameter (1600/D). If this is greater than 15 unit (which I am very sure that it is several time greater than), then the structure is a long slender column. We arrive at the conclusion, that in both cases, the structure could be likened to a very! very long slender column. This implies that (due to constructional error-thus inherent eccentricity) any little change in horizontal load will induce a large eccentricity to the structure which is self propagating-thus large out of balance moment, which will increase with every inch increase in height. This will create apart from other problems, that of instability. This is the point that ingenuity in the design and the realization of the project played a great part.
The structure, looking from the top towards the base, has the snail shell form; but looking from the elevations, it has the pyramidal form. The choice of the form did not make only architectural/aesthetic sense but also structural engineering sense. The structure benefited from two basic geometric configurations-the buttressing or the forting form which I will prefer to refer to as the “tripod or dorsal-born form” (on the plan) and the arching form which I will prefer to refer to as the “pyramidal form” (on the vertical plain). These were mainly employed for the provision of lateral and vertical stability to the structure. To counter the effect of wind loading, the structure employed:-
1) the oblong shapes i.e. the extended cylindrical shapes that are known to be one of the best structural shapes at relieving structures from dynamic hydraulic loading thus good for vortex shading )
2) The tapered helical form (running from the base to the summit). This form was achieved by the side stepping of the setbacks which created a continuous spiral curve from the base of the structure on to the summit. The effect was that it created a long passage through which the wind that impacts on the surface of the structure has to follow (instead of destalking directly on the leeward side of the structure at full strength thus producing the large horizontal forces). This elongated pathway which the wave front is forced to follow has the effect that the wind speed, thus the resultant forces are reduced due to damping/attenuation, natural decay, the resistances that it encounters on its pathway which are due to the friction offered by the pathway, interference of the incoming wind wave front on the wave front which was already traversing the pathway (they are moving at different velocities as such have differing wave fronts). On conclusion, since the wave front takes an inclined direction, it will be resolved into the vertical and the horizontal components, with the net effect that the horizontal component (that results in the horizontal deflection and twist of the structure-thus moment and the torsion on the structure) is grossly reduced in comparism to what it would have been if such a measure was not taken. The vertical component of force has an up-lifting effect on the structure which gravity-thus the weight of the structure could take care of conveniently. If the entrance force of the wave front is say 15KN and it flows at an angle of 35° to the horizontal (inclination of the pathway), depending on the roughness of the surface of the pathway as such the co-efficient of friction, the length of the pathway as such the time that it will take a wave front to traverse the pathway etc, let’s assume that the net force is now reduced to 13KN. If this force is resolved into the vertical and horizontal components, then the horizontal component that will have to be designed for (H) = 13 cos 35° = 0.82 x 13 = 10.65KN. This implies that this load has been reduced by 15 – 10.65 = 4.35 (29%)
One of the conditions set out in the codes is that a structure that is to withstand horizontal load has to be regular both in the lateral and vertical direction (the codes specified the respective limits). Does this structure meet up with that condition?
A casual look at the structure will lead to the illusion that it contravened this basic rule (in that it had sharp and well defined setback, contrary to the specification in the codes that the changes in dimension both in the vertical and the horizontal directions should be gradual). In actuality, this is not the case as each section that is terminated is terminated completely, without and part of it extending beyond that level as such will not introduce any eccentricity to the overall structure. Again, the setbacks fell within the 10% of the total area occupied by the structure at that level and 30% of the total area of the structure as at the base (plan). Looking at the structure which comprised basically of oblong shaped components (3 in number) fused together in torn around a cylindrical centre, there is no doubt that the building met this condition (there are no abrupt changes in dimensions within any of the components that formed the base structure (in the sense that the form is within the limits as defined in the codes), there are no penthouses-cantilevering floors, but each and every component terminated completely at the pre-chosen points-the setbacks).This practice has the structural effect that the flanges (the outflanking oblong wings) have tripod or buttressing effect on the core structure.
If we should make a rough estimate, following the general guide and the formulae attributable to Euler for the classification of columns (this structure, acting as a whole, is like a column that is fixed at the base but free at the top), then the structural height of the building is twice that actual. So we would be talking of a structural height that is in excess of 2 x 800 = 1,600m (theoretically). If the structure was to have a uniform form (say cylindrical) from the base up to the roof top, then we can take the structure as a whole, calculate the ratio of its structural height to the radius of gyration of the structure, thus the slenderness ratio. This will definitely fall outside the 360 unit range that defined the extreme limit of long slender column. Again, since this structure is a reinforced concrete structure, if we should follow the guide as per BS 8110, the slenderness ratio will be the assumed actual height multiplied by 2 (800 x 2 = 1600 = structural height) then divided by the diameter (1600/D). If this is greater than 15 unit (which I am very sure that it is several time greater than), then the structure is a long slender column. We arrive at the conclusion, that in both cases, the structure could be likened to a very! very long slender column. This implies that (due to constructional error-thus inherent eccentricity) any little change in horizontal load will induce a large eccentricity to the structure which is self propagating-thus large out of balance moment, which will increase with every inch increase in height. This will create apart from other problems, that of instability. This is the point that ingenuity in the design and the realization of the project played a great part.
The structure, looking from the top towards the base, has the snail shell form; but looking from the elevations, it has the pyramidal form. The choice of the form did not make only architectural/aesthetic sense but also structural engineering sense. The structure benefited from two basic geometric configurations-the buttressing or the forting form which I will prefer to refer to as the “tripod or dorsal-born form” (on the plan) and the arching form which I will prefer to refer to as the “pyramidal form” (on the vertical plain). These were mainly employed for the provision of lateral and vertical stability to the structure. To counter the effect of wind loading, the structure employed:-
1) the oblong shapes i.e. the extended cylindrical shapes that are known to be one of the best structural shapes at relieving structures from dynamic hydraulic loading thus good for vortex shading )
2) The tapered helical form (running from the base to the summit). This form was achieved by the side stepping of the setbacks which created a continuous spiral curve from the base of the structure on to the summit. The effect was that it created a long passage through which the wind that impacts on the surface of the structure has to follow (instead of destalking directly on the leeward side of the structure at full strength thus producing the large horizontal forces). This elongated pathway which the wave front is forced to follow has the effect that the wind speed, thus the resultant forces are reduced due to damping/attenuation, natural decay, the resistances that it encounters on its pathway which are due to the friction offered by the pathway, interference of the incoming wind wave front on the wave front which was already traversing the pathway (they are moving at different velocities as such have differing wave fronts). On conclusion, since the wave front takes an inclined direction, it will be resolved into the vertical and the horizontal components, with the net effect that the horizontal component (that results in the horizontal deflection and twist of the structure-thus moment and the torsion on the structure) is grossly reduced in comparism to what it would have been if such a measure was not taken. The vertical component of force has an up-lifting effect on the structure which gravity-thus the weight of the structure could take care of conveniently. If the entrance force of the wave front is say 15KN and it flows at an angle of 35° to the horizontal (inclination of the pathway), depending on the roughness of the surface of the pathway as such the co-efficient of friction, the length of the pathway as such the time that it will take a wave front to traverse the pathway etc, let’s assume that the net force is now reduced to 13KN. If this force is resolved into the vertical and horizontal components, then the horizontal component that will have to be designed for (H) = 13 cos 35° = 0.82 x 13 = 10.65KN. This implies that this load has been reduced by 15 – 10.65 = 4.35 (29%)
One of the conditions set out in the codes is that a structure that is to withstand horizontal load has to be regular both in the lateral and vertical direction (the codes specified the respective limits). Does this structure meet up with that condition?
A casual look at the structure will lead to the illusion that it contravened this basic rule (in that it had sharp and well defined setback, contrary to the specification in the codes that the changes in dimension both in the vertical and the horizontal directions should be gradual). In actuality, this is not the case as each section that is terminated is terminated completely, without and part of it extending beyond that level as such will not introduce any eccentricity to the overall structure. Again, the setbacks fell within the 10% of the total area occupied by the structure at that level and 30% of the total area of the structure as at the base (plan). Looking at the structure which comprised basically of oblong shaped components (3 in number) fused together in torn around a cylindrical centre, there is no doubt that the building met this condition (there are no abrupt changes in dimensions within any of the components that formed the base structure (in the sense that the form is within the limits as defined in the codes), there are no penthouses-cantilevering floors, but each and every component terminated completely at the pre-chosen points-the setbacks).This practice has the structural effect that the flanges (the outflanking oblong wings) have tripod or buttressing effect on the core structure.