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24/10/2023 3:16 pm
Hi All
Assume I have a hypothetical case of a 2 story building with steel portal frame on the upper floor supported on a reinforced concrete frame lower floor. Would you design the upper steel portal floor as a Part?
My understanding is that it will ultimately depend on the base fixity of the portal, i.e with a fixed base, the analysis should consider the upper floor as a primary structure whereas with a pinned base, the steel portal will essentially be a cantilever off the first floor and hence a part. In reality, the base connection will likely be semi-rigid; how do we then deal with such situations?
I note that the commentary clause C6.2.1.3 gives us options to use either ESM or Section 8. Keen to know your thoughts.
25/10/2023 10:06 pm
Hi Rajeev,
I think the steel portal is still considered primary structure regardless of whether it is fixed base or pinned base – it is the primary lateral load resisting system for the upper floor. (Note the definition of a part from 1170.5 is “an element that is not intended to participate in the overall resistance of the structure to horizontal displacement under earthquake conditions, for the direction being considered”)
If the upper level will respond to earthquake loading significantly differently to the lower level, e.g. if it is much more flexible (say a portal frame on top of a concrete wall structure), or if it is significantly lighter, then there is probably a reasonable case to consider whether parts loading is appropriate.
In the case of the top level being significantly lighter, you may want to investigate how parts loading compares to equivalent static loading, because in some instances the 8% mass lumped at roof from the equivalent static method can result in a higher lateral load than using the parts method.
I hope that helps.
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