August 9, 2012

How to Model a Column within a Wall in RISA-3D

While RISA-3D does not currently have pilaster design, there are still plenty of situations where you would want to have a column embedded within a wall. The idea behind this is that the column handles the gravity loads for any beams which frame into it, while the wall handles lateral loads. No load sharing is intended between them, although they do occupy the same space.

The problem with simply modeling a column within a wall is that the column is (by default) a Physical Member and it therefore is fully merged with the wall along its height. In the example below, a beam frames into two wide flange columns which are embedded within masonry walls.

Iso View1

The green arrows in the image below represent the reactions at the base of the wall. As you can see, of the 20k of applied load, only about half of that load is carried through the columns. The other half has bled out into the walls, distributing by arching action into a distributed load at the base of the wall. This is not a realistic distribution unless the column has shear studs embedded into the masonry.

Wall Base Reactions

You can get around this by changing the columns to be non-physical members ( click on the column and uncheck the "Physical" box in the Additional Properties section). Then, move the nodes at the beam/column intersection to be 1 inch above the top of the wall. That way the column will not connect to the wall, and the load will never touch the wall.

Corrected Wall Base Reactions1

Tags: RISA-3D Walls

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