Staad.PRO Seismic Response Spectrum (CQC)

Hi. I am working on a steel structure and i have some issues with the seismic loads.

That are the flexure MZ diagrams for the pure seismic load case in X direction (longitudinal).

It caught my attention the results for the beam element marked in green.

The masses were defined as follows, for X,Y,Z:

If i delete the masses in Y i don´t observe this problem anymore.

And i don´t understan if this is correct, because the response spectrum is defined for X direction:

In anycase, i don´t undestrand either why i have this problem in the green beam and not in the purple beam, considering that the loads are exactly the same.

Also, the resultant base shear shows significant differences, depending if the Y masses are considered or not.

I appreciate any help.

Thanks!

Parents
  • Seismic masses are always lumped at the nodes of the member. The green beam has a node at the middle, so the mass at the node is considered and that impacts the bending moment diagram. The purple beam does not have a node at the middle, so the masses are at the ends of the beam only, which does not cause any BMD spikes at the middle. If you would insert a middle node on the purple beam, the results will be similar to the green beam.



Reply
  • Seismic masses are always lumped at the nodes of the member. The green beam has a node at the middle, so the mass at the node is considered and that impacts the bending moment diagram. The purple beam does not have a node at the middle, so the masses are at the ends of the beam only, which does not cause any BMD spikes at the middle. If you would insert a middle node on the purple beam, the results will be similar to the green beam.



Children
  • Thanks Modestas. I understand. But if that is a local issue on that beam because of the existance (or not) of a middle node...  what explains the important differences in the reactions values that i observe between the two cases?

  • You mentioned that the base shear differs if the masses in Y direction are removed. That is expected - different mass input will lead to different base shear. You should be providing the mass in all 3 global directions - X, Y and Z.



  • Then i guess there is something i am not getting right.

    In the load case "EOX: Seismic loads in X" i define a response spectrum in X direction. Doesn´t that command work with the masses defined in x direction only? In which case it wolud be the same wheather i define or not masses in Y or Z...

  • Please note that in a response spectrum analysis, the result diagrams (axial force, shear forces, torsion and bending moments) can be accurately computed only at the nodes. In between the nodes, the value is not known exactly, and hence calculated by doing a linear interpolation between the start and end nodes. 

    Result diagrams are plotted by connecting the values at the start, end, and 11 intermediate equidistant points. The start and 11 intermediate points are "actions". The member end value is a "reaction", which is converted to an action by changing the sign.

    In your case, if you add a node on the beam where you can't see MZ diagram, an additional point will be there for RS analysis. You will get to see-

    In Response Spectrum analysis, responses like member forces are obtained form static analysis on each modes considered in the analysis. Design lateral force at each floor in each mode is considered as Ak * (Phi)ik * Pk * Wi

    Ak= design acceleration spectrum of a mode

    (Phi)ik= mode shape co-efficient

    Pk= Mass participation factor , Wi= seismic weight

    (Phi)ik determination requires a modal analysis with lumped mass system where you require to define mass in X, Y an Z direction.

    After getting the modal responses final results are obtained by using modal combination methods like SRSS, CQC etc

    I hope you can understand that for X directional RS analysis masses defined in X by applying loading in X direction would not be sufficient as mode shape co-efficient would be wrong.

    0726.Cruce Av A_Rev B1_EO_con_masa_Y.std