Applying seismic load to FEA model
Applying seismic load to FEA model
(OP)
Applying seismic load to FEA model
This is probably a question that is specific to STAAD.Pro, but perhaps not.
I am wondering how everyone applies seismic load to their models (of buildings) when using the equivalent static force procedure. As an example, I have a large steel industrial building with a metal deck roof and horizontal roof bracing. I have seen some people use the 'master and slave' function to assign this type of roof as a rigid diaphragm and then apply the base shear as a point load on the center of mass of the building. I do not believe this is an accurate representation of how lateral load is being distributed by the roof diaphragm (metal deck w/ horizontal bracing.) Also, this method would not provide the load demand on any of the roof bracing, as the bracing is not being engaged in the model. The only time I see the 'master and slave' function being useful is when you have a reinforced concrete slab. I believe the most accurate way of applying seismic load to a model when using the equivalent static force procedure would be to apply it to each individual frame in proportion to its mass. However, with accidental eccentricities and 4 different directions, I can have 8 seismic load cases – this can be quite labor intensive. Is this latter approach the correct approach that one should use for a building of this type? Your thoughts please?
Thank you.
This is probably a question that is specific to STAAD.Pro, but perhaps not.
I am wondering how everyone applies seismic load to their models (of buildings) when using the equivalent static force procedure. As an example, I have a large steel industrial building with a metal deck roof and horizontal roof bracing. I have seen some people use the 'master and slave' function to assign this type of roof as a rigid diaphragm and then apply the base shear as a point load on the center of mass of the building. I do not believe this is an accurate representation of how lateral load is being distributed by the roof diaphragm (metal deck w/ horizontal bracing.) Also, this method would not provide the load demand on any of the roof bracing, as the bracing is not being engaged in the model. The only time I see the 'master and slave' function being useful is when you have a reinforced concrete slab. I believe the most accurate way of applying seismic load to a model when using the equivalent static force procedure would be to apply it to each individual frame in proportion to its mass. However, with accidental eccentricities and 4 different directions, I can have 8 seismic load cases – this can be quite labor intensive. Is this latter approach the correct approach that one should use for a building of this type? Your thoughts please?
Thank you.






RE: Applying seismic load to FEA model
Lately I have started to see some texts where when the model does not make use of rigid diaphragms, but actual stiffnesses anywhere, accidental eccentricities are forfeited. Something of the kind with proper fine-tuning will be a much welcome simplification for EQ combinations; in the end 5% of eccentricity must not make much for one good lateral resisting system and almost any theorization covering the issue should be even better than the messing eccentricities. However, the thing seems not to have filtered to the usual codes as yet.
RE: Applying seismic load to FEA model
I still believe that a rigid diaphragm (or master/slave approach) is good for many roof cases where the layout of the frames is well balanced and the rigid diaphragm distribution of the loads to the frames is "good enough". Using a rigid diaphragm simplifies a lot of calculations, prevents interaction of the frame beams and the diaphragm elements, speeds up the analysis, etc.
If you are looking to design the rod braces too, I wager you could check them by hand with the time you just saved.
RE: Applying seismic load to FEA model
Personally I am a response spectrum person myself. It gets the seismic forces down low.
RE: Applying seismic load to FEA model
RE: Applying seismic load to FEA model
You can idealize your diphragm as a flat plate with the proper stiffness to match your metal deck + bracing. If it was only metal deck there is a method to determine the proper plate thickness. With the horizontal bracing, you'd need to run some hand calcs to determine the correct stiffness of the plate. If you go this route, the plate will pick up some of the gravity loads from your roof beams and girders, so watch out that those aren't under-designed (it's worth doing a hand check).
All of the lateral loads on the out of plate walls (in plane walls too) can be captured by line loads on the roof/floor level beams.
Lateral loads on the roof system and on the modeled members can be captured by the selfweight X=Cs or Y=Cs (where Cs is your lateral coefficient) command.
This is the common practice for an ELF analysis. It is very similar for a response spectrum analysis, but you need to capture your masses with reference loads.
RE: Applying seismic load to FEA model
The National Building Code of Canada states that accidental torsion must be applied regardless of whether the diaphragm is flexible or rigid. This makes no sense to me, because a flexible diaphragm can not develop torsion. I wonder what the IBC has to say about accidental torsion and flexible diaphragms?
RE: Applying seismic load to FEA model