Punching shear on rat slab.

[tmoe]

Hi all,

Im designing a shoring system for a contractor who is cutting floor joists for the work he is doing. There are shoring jacks supporting a second story load (with sleepers to distribute load) which rest just inboard of where he will be making his cut. As a result, I had intended to put 4x4 posts (which he has a bunch of lying around)under the joists to support them. The post would span about 18" to a 2" rat slab below.

Im using equation 55.17 shown here for punching shear stress and Im showing failure.

I thought maybe putting a sleeper under the post, such as a 2X6 or 2x10 would help spread out the load, but I have tried a few options and it appears that changing the area the post is distributed over has little effect on eq. 55.18. The minimum value seems to always end up as 2, and thus gives the same result when plugged into 55.17.

Any ideas on how I can solve this such that punching is ok?

 

[XR250]

How much force per post are we talking?

 

[BAretired]

I'm from Alberta. We're rat free. Could we call it a mouse slab?

BA

 

[dhengr]

BA:
What’s a moose slab? What’s the average area of a moose paw, so I can figure the bearing area?

How thick is the slab, conc. strength, allowable underlying soil bearing pressure, what post size and loading, etc. You certainly should get a higher capacity out of a 4x4 post when it had a 2' long 4x4 under it and atop the slab, than if the post bears directly on the slab. That’s dependent on the bending strength of that horiz. bending member, but it does increase the bearing area. If you have to depend on a cookbook and a specific formula to do this problem, that might be an indication that you should talk this over with your boss and get some local help on this design, for a better understanding of the whole concept.

 

[msquared48]

http://news.nationalgeographic.com/news/2003/03/0331_030331_rats_2.html

Mike McCann
MMC Engineering

 

[hokie66]

All joking aside, and I did enjoy them, forget trying to get a 2" concrete slab to work structurally, for punching shear or anything else. You will have to provide spreaders/cribbing/temporary footings for the shoring loads on the soil. Neglect the "rat" slab.