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Special Concentric Braced Frames (SCBFs) can provide an efficient design solution for tall buildings in zones of moderate to high seismicity. Such a solution provides a direct axial load path and offers the moderate amount of ductility typically sought for tall buildings. The scale of these buildings is such that braces are large enough to provide reasonably symmetrical behaviour in tension and compression. However, on a typical perimeter column grid of 8-12m the brace angle resulting from conventional single storey braces is inefficient.
A more efficient solution is achieved with multi-storey braces spanning across 2-3 floors between columns, taking restraint at intermediate floors to preserve global compactness of the braces and reasonably symmetrical behaviour. However, the design code framework (e.g. AISC 341-10) does not extend to such a configuration, and there is little or no test data available.
This paper reports on the findings from an analytical case study carried out for the design of Reforma 509, a 238m tall mixed-use tower currently under construction in Mexico City, where 3-storey special concentric braces are employed to optimise the efficiency of the structural performance. The study concludes that a multi-storey brace configuration with intermediate restraints can exhibit distributed yielding and provide a similar amount of ductility to conventional single-storey braces. However, to ensure this behaviour the local compactness criteria should be increased to a higher level than the minimum requirements of AISC 341-10. The study also concludes that such an increase in the criteria is beneficial also for single-storey braces.