Deflection Limits for Steel Framed Podium Construction

[Pers Eng]

We are designing a 4 story residential building with the first level being composite steel framed with concrete filled metal deck and then 3 level of wood construction. On similar past projects we have used CIP concrete for the podium level where deflections due to gravity loads are minimum. Wondering if based on others experience we should be limiting DL and LL deflections in the composite steel beams to a more stringent requirement that normal floor limits since it will be supporting wood framed load-bearing walls for the upper levels and roof. Google searches and other references have been silent on this.

 

[Nor Cal SE]

I haven't had a design specifically like this, and the issue is not straightforward, but I would suggest you would have reason to consider more stringent deflection criteria if one you have brittle finishes (for interior, more brittle than sheetrock, and for exterior, more brittle than stucco or plaster).

Other than that, I presume you would be okay, especially since your building is residential and should have fairly stable live loads unlike a public assembly hall, dance floor, gym, or other type of structure where your live load could have widely varying magnitude and vibration potential.

 

[Agent666]

If you have a lot of load transfers (walls located on joists, columns on beams, etc) with potential for accumulated creep/shrinkage in the concrete podium and creep in the upper timber levels then it would be prudent to use a stricter level.

For example if there is a wall that is located mid span of the concrete slab spans, then there will be some permanent long term deflection effects that reflect in the superstructure relative to say a timber wall located directly over a podium level wall.

Work out the worst long term deflection at the worst location in the upper levels from all contributing members and work back from there to something that seems acceptable in terms of final floor levelness in the upper levels, vibrations from supporting walls that might be not over podium walls and the finishes being adopted (internal and external). Ensure the concrete podium level is fully de-propped before lining the upper timber levels, this builds in at least some of the initial short term deflection from the podium.