Very short period Structure

[CreedRD]

Hello All. Its my first time posting in this forum so i apologise if this subject has been talked about before.
When we have very short structures, 1 storey, masonry wall structure with concrete slab (very rigid structure), the periods of the modes are often way lower than 0.20s.
When this happens structural softwares wont show any displacements or ver very low displacement when performing a seismic modal analysis.
Is there any literatura that talks specifically about this kind of structure? Sometimes this rigid structures modal analysis result in 15 even 20 modes to achieve 90% of modal mass participation deforming only small parts of the walls in each mode. Have any of you seen this in any of your analysis? (Sorry for the writing...english is not my main language)

 

[hardbutmild]

In Eurocodes this usually happens with a system of "large lightly reinforced walls" - a system that has a lot of walls (typical for low masonry structures).
There are provisions for how many modes you should consider, I think it is something like 3*number_of_storeys. I believe most codes provide some guidance how to address this issue of not activating enough mass.

Personally, I would do an equivalent lateral force method if possible in this case with a small reduction of seismic forces (only due to overstrength, not due to ductility) - this is a reduction (behaviour) factor of 1,5 in Eurocode. Even if I did modal spectral analysis I would check that the base shear obtained that way is at least similar to the one obtained by ELF method.
The detailing of such walls, diaphragms and footings is not as critical in these cases - that is why they call them "lightly reinforced".
In these types of structures seismic energy is usually dissipated by rocking of large walls, rather than their bending.

 

[TLHS]

Yeah, this basically just becomes a static ELF analysis based on peak spectral. There are probably types of irregularities where a modal analysis might be helpful, but if it's so rigid that you're having issues getting modes to even activate in the dynamic analysis then you're probably not in that kind of situation.

Some codes may require at least a modal analysis, but if it's obvious by inspection that all major modes are well below 0.2 sec (or the equivalent peak spectral cutoff in your code) then there's not really a point. It's just a bunch of overlapping modes with peak spectral acceleration happening.