April 22, 2014
How can I check a Slab on Grade for Post Loads?
Need to first know how to model a slab on grade? See our Slabs on Grade article to learn how to model slabs on grade:
One of the most critical aspects of slab on grade design is the consideration of punching shear due to posts/columns/racks which bear directly on the slab. With RISAFoundation it is easy to model these conditions and get complete punching shear checks. In the example below an L-shaped slab has four concentrated loads from an equipment rack which is sitting atop the slab.
The slab will automatically be designed for flexure and one-way shear, but in this case it is the punching shear which will most likely control the design. In order to check for punching shear you must model the profile of the baseplate at the bottom of each post. Click the button highlighted in the image above in order to draw a post. Choose to draw a Post/Column, as opposed to a concrete pedestal, and define the dimensions of the baseplate:
Posts are always centered on a node, and any load which the post is carrying should also be applied directly to that node:
After solving you can review the punching shear results, which include a unity check (UC) showing the ratio of punching shear stress to capacity. As long as this value is less than 1.0 your slab on grade is thick enough to handle the post load.