Simplify Seismic Calculations

[Logan82]

Hi,

I am looking for a simple and conservative calculation method for earthquake loads on low rise steel and reinforced concrete buildings. The projects I work on are typically in a low seismic area, so earthquakes are not really a big concern.

The reasons are this:
-Calculations in codes are quite time consuming, and they change significantly at each code version. I do not need that level of optimization for the type of buildings I design.
-Even using Advance Design America, if the participating mass is under 90 %, the seismic load results from the software can't be used according to their documentation, which happens often.

 

[STrctPono]

I thought that the Equivalent Lateral Force Procedure was simple?

 

[271828]

Have you tried the Equivalent Lateral Force Procedure mentioned by STrctPono? It is pretty simple. ASCE 7 Section 12.8.

With low seismic, why are you jumping to a procedure that is computing participating mass?

 

[Agent666]

271828, the fact that the load might be low is irrelevant if a code requires a response spectrum analysis. The requirement for undertaking a response spectrum analysis is not dependant on the level of load in any standard I've ever used. If the structure is irregular (fails the 'irregularity' checks in a code) then it usually triggers the need to do a 3D RSA instead of being able to use a simpler equivalent lateral force procedure.

To OP, include more modes until you have less than 90% mass participation.

https://engineervsheep.com

 

[271828]

Agreed. I sloppily used the term "low seismic." I don't think irregular buildings requiring RSA is the typical application mentioned in the OP.

 

[Logan82]

Thank you for your answers regarding the equivalent lateral force procedure!

Agent666, I tried to increase the number of modes several times in the cases when my participating mass was under 90 %. Sometimes it did work, but not always. Are there other tips to use to reach the 90 % participating mass?

 

[STrctPono]

Logan82,

Are you running CQC or SRSS? Are you getting most of your mass participation from 2-4 distinct modes or is it spread out very thinly over lots of modes? If your building is regular and has very distinct low frequency mode shapes then you should be able to achieve your 90% modal mass participation pretty easily. If you have areas of your structure with lots of mass that vibrates only at a higher frequency then that would be a culprit. I may be wrong (as I don't design buildings) but wouldn't heavy lower levels and lighter upper levels cause this issue? I imagine that this is common and would think that running more modes would solve it. At some point though, you perhaps need to look at the way it was modeled and see if there is a more efficient way of doing it. Running 40 mode shapes for a 3 story building and still not achieving target mass participation would be a red flag. I usually can tell right away from looking at my mode shapes if something is modeled incorrectly. Your mode shapes can clue you in to whether you have bad boundary conditions or perhaps lumped your mass incorrectly. If you are including any foundation elements in your model or have a basement this may be a culprit also.

 

[Logan82]

Thank you very much for the tips. My question regarded the design of 1 story, 10 m high, sometimes irregular building. Braced in 1 direction, but in the other they are not braced (by using moment connections). There are almost always 2 predominant modes (X and Z directions). There are no foundations included. I will read further regarding CQC vs SRSS. Boundary conditions may be a problem, but I am wondering how they can represent a problem, since the combination loads are converging (even though the earthquake load does not reach 90 %).