Two-Stage Analysis of Podium Structures - Overstrength Factor?

[Shanman_]

Hey All,

Just wanted to get some opinions on whether it is appropriate to use the overstrength factor to amplify forces when designing the a podium slab per ASCE 7-16's section 12.3.3.3 on elements supporting discontinuous walls or frames.

The procedure behind the two-stage analysis - that being the superstructure is first designed independent of the podium structure below - has allowed for an argument to come up against applying it.

Thoughts?

 

[jittles]

WoodWorks has their fairly comprehensive guide on the 5-over-1 system and seems to think that yes, you do need to amplify the forces. They address it from a purely code language perspective.

ASCE 7-10 §12.3.3.3 requires amplifcation of seismic loads in the design
of structural elements supporting discontinuous walls. Previous editions of the IBC and the 1997 Uniform
Building Code exempted concrete slabs supporting light-frame construction from these requirements.
However, ASCE 7-10 does not have this exception thus adding “slabs” to the list of elements needing the
design strength to resist the maximum axial force that can be delivered per the load combinations with
the overstrength factor in ASCE 7-10 §12.4.3.2.

There is the usual option to reduce seismic loads by other methods, such as yielding of a different element limiting the load delivered.

 

[Boof19]

That's a good question - just thinking out loud I would reason the two stage analysis leaves it open to interpret the the upper portion as effectively being a separate structure, eg the design would be "essentially" the same were it on the ground. Now I would ask myself the question - would I need to design my baseplate/anchor bolts/etc. with Overstrength? Whatever your answer, I can see it being rationalized that the same could be applied to the upper portion of the two stage analysis.

Now aside from that you do need to factor up your reactions from the upper story by a ratio of the two systems R-values.

 

[Boof19]

Well there you have it, I detract my logic. Jittles appears to have your answer.

 

[jittles]

I'd still love to hear the logical or fundamental engineering argument if it's out there. That WoodWorks answer really leans on the code, but doesn't offer input on perhaps why the slabs were removed from this code section and now must be designed for overstrength, but previously were exempted.

 

[Shanman_]

Thanks for the reference. I found the section in the 97 UBC too. I was posting with the hopes of finding others that would apply overstrength because that is how I interpret the code as well. I wouldn't necessarily always side myself with Woodworks, as their approaches are typically on the more conservative side (straps & collectors everywhere, etc). The rationale for not applying the overstrength factor as I understand it is because the podium structure being many times more stiff allows itself to behave more like a foundation more so than a supporting beam element. Foundations are typically not designed for overstrength besides when required by ACI anchorage provisions. It is clear in prior editions of ASCE 7 that slabs were included which I don't think can be ignored.