A Novel Sub-Structuring Method for Efficient FE Analysis of Tall Slender Buildings

Jovana Veletic
University of London, London

There is growing interest in designing ever-slenderer buildings. Five of the world’s 24 buildings of 400 meters’ or greater height have a slenderness ratio of 10:1 (height: base width) or more. These structures can form key elements in the new, emerging polycentric cities. This presentation demonstrates a series of novel methods that can be used to save computer run-time in the structural analysis/design stages enabling engineers to explore alternative solutions efficiently. Detailed FE analyses (predicting displacements, internal forces and stresses) of structures such as these can be computationally expensive. For example, a regular slender building with a slenderness ratio of 20:1 (20-meter-square floor plan) will have over 8,000 beam and column elements, requiring more than 20,000 unknowns (degrees of freedom) to be solved by matrix algebra.

Building on an existing FE library (Matlab-CALFEM), the presenters have have introduced (i) a rapid Master-Slave approach -- tying degrees of freedom (DoF) together, for example, to simulate rigid membrane action in certain panels or floors; (ii) advanced shell elements (for the deck and core) and (iii) most importantly, a sub-structuring super-element approach, whereby only part of the building is modeled in detail. This note describes the zoom-in strategy, whereby the engineer can rapidly analyze and examine different portions of the structure (typically blocks of 5-10 floors) while capturing the overall behavior of the building. Such a sub-structuring technique (specifically tuned for tall buildings) can dramatically reduce the size of the stiffness matrix. The consequent memory and run-time savings are reported in the presentation. The software produced also allows substructures (for example outriggers or truss-stiffened floors) to be introduced easily. The analysis approach rapidly reveals optimal and sub-optimal structural solutions when undertaking both preliminary and detailed designs.