nodal mass

[zthaer]

dear seismostruct team;

i need to investigate the lateral stiffness associated with each floor, to examine the lateral load resisting system variation with elevation as per ASCE-7 chapter 12 TABLE 12.3-2.
my question is, by examine the eigenvalue analysis output i see the modal mass in both direction , does those values represent the force in the node in that direction, so what i need to do is converting those values to forces and extracting the nodal displacement for all the node in specific floor to calculate the desired stiffness ?

best;

[huffte]

The best way to get the lateral stiffness of the various floors is to place a load at the top and perform a static, linear analysis. The stiffness of a given floor is then equal to the applied load divided by the story drift (the average difference in displacement between top and bottom of columns in that story). I do not believe the eigenvalue analysis results will aid in determining the story stiffness values, but perhaps I am missing something. Best of luck with your work, zthaer.

[zthaer]

dear Tim;
first;
what do you mean by "at the top"? do you mean any single point at the final level ( i.e in the fourth story in 3d building ) , .so, what i got is a single value of force in 4th story with different drift value associated with each node in specific story ( i.e the third ).so the formula for each node in the third story is:
F/delta(i3) - delta ( i0) ?

second;
or you meant extracting the element shear force and displacement ( i.e column )?
Best;

[huffte]

With a single lateral force, F, applied at the top, the shear in every story is F. For all columns in a given story, the drift is delta(top)-delta(bottom) for that column. If a rigid diaphragm is present at the story, then the drift will not vary greatly from column to column within that story. In the ASCE check, you are needing the lateral story stiffness, so you need to locate F so as to minimize torsion; i.e., locate F at the center of stiffness. I would recommend hand calculations to estimate the center of stiffness if the structure is not symmetric in plan. In checking the story stiffness, you don't really need to know how much shear each column takes. You are only interested in total shear and total drift for each story.