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ETABS Pier Wall Forces: Why does Shear 22 vary over story height?

ETABS Pier Wall Forces: Why does Shear 22 vary over story height?

ETABS Pier Wall Forces: Why does Shear 22 vary over story height?

I have a 3 storey building with ground floor RC walls and concrete first floor, above first, second floor and roof is and steel and timber frame. I am designing the ground floor concrete shear walls using the demands outputted from ETABS. I have used manual seismic loads applied to the centre of mass of the first floor diaphragm. Each wall has been drawn separately with no spandrel panels over windows for simplicity. I have assigned each individual wall as it's own pier label.

When I view the pier force diagrams, I get the expected moment 33 diagram (triangle shape) starting from zero at the top to a base moment (linearly increasing) therefore I would expect the Shear 22 diagram to be constant. However I get linear varying shear force diagram sometimes starting negative at the top and changing to positive at the bottom.

I have uploaded a pdf with M33 and S22 diagrams for the one of the seismic cases. This is not a max/min case so loads should match up.

At this stage, I am considering outputting the base moments (M33) to excel and manually calculating the shear (S22) for each wall. Shear = Base Moment / Height (3.4m)

Any idea why this is happening?

RE: ETABS Pier Wall Forces: Why does Shear 22 vary over story height?

What type of analysis are you doing? Modal/response spectrum? Using the standard solver are there any instabilities noted in the analysis log?

RE: ETABS Pier Wall Forces: Why does Shear 22 vary over story height?

Yes, seems to be an issue with stability of the model. I ran the standard solver which gave joint error: diag = 0, a problem with zero stiffness. It was occurring to the all the joints at the top of the walls. The problem was I had tried edge releasing the walls out of plane which was causing the issue. I was running a linear static, using user inputted seismic loads.

Attached are correct results. I have showed the original results to show what difference it makes and why instability results needs to be resolved.

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