Pushover analysis of concrete column

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jayram

Pushover analysis of concrete column

Post by jayram » Thu Aug 05, 2004 11:23 am

Hi,

I want to do a pushover analysis of an RC column under a constant axial load (127 kN). So I first do an axial load analysis, then set loadConstant and then do the pushover analysis. However, when I look at the axial force (during pushover) at the gauss points, it becomes much greater than 127 kN. Consequently, the section has much greater moment capacity than that obtained by sectional analysis. On the other hand, the reaction force in the axial direction remains at 127 kN. I have defined linear geometry. Can anyone please explain what is going on.

Thank you
Jayram

fmk
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Post by fmk » Mon Aug 09, 2004 11:20 am

you are probably using the dispBeamColumn .. use the (forceBeamColumn) nonlinearBeamColumn instead .. given nodal disp the force based element iterates to get a displacement field which exatcly satisfies force equi of element (force field known exactly for force based element) .. the displacement based element just assumes a displacement field based on nodal displacements and passes these to the sections .. if you look at the section forces for each section along member for the column of yours and the dispBeamColumn some will be neg, some positive and you can use integration (gauss-lobotto) to see what the overall force in the elemnt is .. if you use the forceBeamColumn all axial force in sections should be the same .. the force beam column is the 'better' element assuming it can converge!

NOTE: the displacement based beam column element is doing what most all FE elements actually do to determine displacements at the gauss points. so it is not a 'bad' element, just a typical one, with typical limitations.

shisha
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Post by shisha » Tue Aug 10, 2004 1:52 am

To add to Frank's reply to Jayram's question about the column pushover:

..the dispBeamColumn element can only model a linear curvature distribution by vitue of the cubic shape functions (assuming this to be the usual choice) it uses to approximate the displacement field within the element, cannot capture the concentration of the plastification (due to flexure) near base of the column unless you use a small element whose length should be approximately the length of the plastic hinge that you expect.

Only if you use this kind of a element geometry with a small element at the base of he column can yield accurate behavior of the column.

The forceBeamColumn is much better in this regard as one element can be made to represent the entire lenght of the column.

shisha.
shisha.

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