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Building Frame Shearwall Height Limit on UBC Table 16-N

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When referencing the height limits for structural systems in UBC Table 16-N, what does the height limit mean? For instance for a building frame shearwall system where the typical framing is a coupled shearwall system extending to a "roof" level of 240'-4" above grade, but has a parapet extending an additional 4'-0" and elevator machine room and architectural concrete fin extending to 272' above grade. Would the height limit of 240' for the building frame shearwall system permit such a structure by being interpreted as the height limit for the particular structural system, with the extended machine room walls and architectural fin above the 240' level being considered additional masses to be lumped at the 240' level? Or, should the limit be interpreted to mean the highest point of the structure, or highest roof diaphragm level? 

The elevator machine room and fin are not small items for this particular project. They are both cast in place concrete and connect to the major lateral force resisting elements for the building. It is possible that these "stiff" shearwall type elements extending approximately 30 feet above the building frame shearwall height limit could have a stiffening effect on the coupled walls that comprise the main lateral load resistance for the building. (I.e. the so-called "top-hat" effect where stiff elements at the top of the building control the deflections of the softer coupled walls below.) In such a case, these elements may not be so "minor" in nature.



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