Quote:
Originally Posted by LouisVanDerWright
Turns out the frustrums have virtually no significant effect on the need for the blow through, sheer walls, or tuned dampers:
http://www.structuremag.org/?p=13221
The article goes on to talk about how the frustrums [sic] ended up having a negligible effect on the engineering of this structure. The shapes are gradual enough that it doesn't really function much different than a normal collum [sic] on the corners. The sheer walls are only where they are because the building is too narrow for a core and outrigger design like Trump tower. The outrigger collums [sic] would have to have 15'x15' to have the same effect as the sheer walls in the end product. |
I'm not so sure about that... after all, the whole point of the article was to address the many challenges the frustums presented to the structural engineers. Although it is certainly true that the article didn't state that the frustums didn't necessarily have a documented direct relationship to the need for the dampers or the blow-through, it was only after very intensive wind-tunnel testing with this
specific massing did they determine what the exact figures were to achieve the 'comfort' needs at the top of this tower... From the same article:
"[a] … solution was explored where column locations follow the tapering building exterior, shifting at every floor and remaining tight to the façade. With column axial loads as great as 15,000 kips at the base, the implication of shifting columns presented real challenges to global stability and building balance.
This geometry ultimately proved viable, incorporating column horizontal offsets of approximately 5 inches at every level… at the top and bottom of each frustum, where the direction of column stepping reverses. This required a significant lateral restoring force at that elevation, supplied by the floor slab.
At frustum wide points, the induced force is an outward pull on the slab, felt in two directions at corner columns… At frustum narrow points, the equilibrating force compresses the slab… The developer’s goal of maximizing clear height and having clean slab soffits meant this P-delta problem needed to be solved without adding beams or other thickened elements that would conventionally carry compression. Analyzing these slabs as compression elements, while considering the gravity deflections and slab post-tensioning as initial conditions, revealed an arrangement of supplemental reinforcement necessary to keep these elements stable.
Another critical design decision, stemming from having 100 unique floor plans [defined almost wholly by the structural, not the architectural plan], was determining how much granularity to apply to the detailing of the slab reinforcement (rebar, post-tensioning tendons, and studrails). The most optimized design suggested every level entailed a unique design, perfectly matched to the spans. However, the resulting volume of details would create a massive burden on the contractor to manage shop drawings, fabrication, storage, placement, and special inspection unique to each level.
The other extreme would be to apply the design for the worst-case slab, the widest level and longest spans, to all levels in that frustum. This solution would be both overly wasteful of material and would create an overload of load-balancing from the post-tensioning tendons when applied to the short-span levels.
The chosen design approach hit the mid-point between these competing interests of optimization and constructability. Each 13-story block of slabs is broken into three designs, considering small, medium, and large span levels separately. Other than the 20 levels of hotel, parking, and mechanical use, the remaining 80 levels are covered by just 18 unique slab designs."
The article also states the exterior columns anchoring the 'outrigger' shear walls would actually need to be 10' square... still too big of course.
Anyway, it is a very interesting read and certainly goes into some detail that the frustum massing had on the engineering not to mention the unit planning... as we have already discovered with many of these floor plans, we can all see the effect the structure has on the function of this building. I would still love to see a plan comparison of the same unit tier at the narrowest frustum floor, one with the removal of 5'(!!) around its perimeter with the same layout.
I remember seeing earlier bKL schemes (where this 3 tower scheme originated, I believe) where the two end towers had a structural exoskeleton that resolved all of the shear forces... hmmm. It was kind of an elegant solution for a Chicago supertall (champion of structural expressionism)...
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