ASCE 7-16 12.2.3.2 Two-Stage Analysis Question - Base Reactions

[justanormalguy]

I have a question about how exactly to analyze the lower portion when using the Two-stage seismic analysis procedure per ASCE 7-16 (section 12.2.3.2).

I have a 5 story wood structure sitting on a two story concrete structure with the second floor being the concrete podium. When we apply the upper structure reactions at the podium slab - Do you usually check the slab for the stresses from the T/C forces, or do you assume that the slab just transfer the loads to the podium slab. I think you just applies the forces and redistribute them to the concrete structure? Any input is appreciated it.

 

[DaveAtkins]

I do not have experience with this, but I think I understand the concept (others can chime in to correct me...I won't mind).

The reason for a two stage analysis is that it is overly conservative to take the weight of the seven story concrete plus wood structure and distribute it to the various levels as you would a building which is all one material and lateral load resisting system. You would be loading the wood portion of the structure with seismic force caused by the concrete portion of the structure.

Therefore, you analyze the upper five stories, then analyze the lower two stories separately. You use the appropriate "R" for each system independently.

But to answer your question, yes, any forces generated at the base of the upper five story portion must be resisted by the lower two story portion. Since the upper portion in this case will have a lower "R" than the lower portion, it is conservative to directly apply the forces from the upper portion (with the lower "R") to the lower portion.

I am not sure what you mean by "T/C" forces. Are you referring to uplift and downward force due to overturning? If so, then yes, these forces need to be applied to the lower portion.

Sorry for the longwinded response...I am trying to verify my understanding along with you!

DaveAtkins

 

[jerseyshore]

There was a really good discussion on this over the summer, maybe someone can link that for you.

 

[Eng16080]

There seems to be a question like this that comes up every few weeks. You might want to search for past threads, as jerseyshore mentioned, as there's a lot of good content. In any case, here is some quick advise which hopefully helps:
[ol 1]
[li]First of all, to answer your question: "Do you usually check the slab for the stresses from the T/C forces, or do you assume that the slab just transfer the loads to the podium slab?" Yes, design the slab for the T/C forces. It's like any other part of a structure. You always need to check that there's a valid load path. Nothing magical happens when those forces hit the podium. They don't just go away.[/li]
[li]Be sure that the loads from the upper structure to the podium have been amplified by the ratio of Rupper divided by Rlower, per Section 12.2.3.2(d). That includes rho as well for the higher seismic categories.[/li]
[li]Be sure that an overstrength factor (omega) is being used if necessary. This factor is often applicable where you have Type 4 horizontal and vertical irregularities, which is common at podium slabs. An example is if you have a wood shear wall above the podium and a braced frame below with the two not being perfectly aligned (see Section 12.3.3.3). The overstrength factor (omega) is defined in Table 12.2-1 and is often quite severe (probably 3 or 2.5 in your case). The application of this factor is somewhat confusing, though. In cases where the connection of the structure above the irregularity has sufficient ductility, then the overstrength factor isn't always necessary. Again, there are some good past discussions on here about this from people more knowledgeable than myself.[/li]
[li]Finally, there's a publication by Woodworks titled "Five-Story Wood-Frame Structure over Podium Slab" which is very helpful.[/li]
[/ol]

 

[driftLimiter]

We talk about this quite often.

Here was my post regarding this. Some designers use a 'lumped' overturning approach to start out. Then dial it in more.

Link

Concentrated overturning forces can lead to increase in slab demand for flexure primarily when they are near middle of span.
The forces can also be quite demanding from an anchorage perspective.

Our approach is to model the loads where the are, then follow the load path into the walls etc.