Large Story Shear Forces

[vahidbarzegar]

Hi,
I've been analyzing an 8-Story frame-wall building. Nonlinear dynamic analysis yields large story shears in the first story compared to other stories (about 5 times larger) and almost 90% of this shear is resisted by the shear wall whereas this ratio is 65% in pushover analysis. I tried changing damping ratios, convergence test (NormDispIncr to NormUnbalance), excitation pattern (uniform to multiple support) and element type, but the output is still the same.

Where should i look to find the problem?
P.S. 1:
Shear wall is modeled with fiberBeamColumn Element and lump plasticity model is used for moment frame.
PS 2:
element forces is in equilibrium with nodal forces.

[fmk]

load is going to be taken based on the relative stiffness of the systems (think stiff spring and weak spring in parallel and look at which takes the majority of the force) .. the fact that the dynamic and static pushover are giving different distributions could be due to your choice of the lateral load distribution in the pushover analysis (are you using for example one based on first mode?) .. possible other sources of error are of course your element section definitions.

[vahidbarzegar]

Could this kind of story shear distribution be because of formation of plastic hinge around the mid height of the first story and the uniform drift of upper stories?

[fmk]

yes it is possible .. the load has to get to the base .. in dynamics the resisting forces are inertia, damping and the "elastic" forces from the structure, the fs(u) .. as applied load doe does not change either one of the other resisting loads need to increase or the fs(u) forces increase through the wall resisting forces .. you could verify by doing an elastic analysis and looking at force distribution between wall and frame to ensure not some anomoly in section definition.

[vahidbarzegar]

Do you mean that this shear distribution in the first floor in elastic model should not be in the same order as in nonlinear model if it is due to plastic hinge formation in the first floor?

[fmk]

yes .. the wall and frame should be in same proportion initially as the elastic case .. when the hinge forms at base cannot take any addiional load, then that proportion of the "elastic" forces, the fs(u), must obviously be taken by the wall.