Line Loads on Unreinforced Slabs 2

[Vinny7]

Can anyone point me in the right direction for information relating to line loads on unreinforced slabs.

I need to check the adequacy of an existing domestic insitu ground bearing slab with a load bearing structural stud built off it. This is lightly loaded but I would like some further guidance on how I could justify the slab is adequate (or not).

 

[Lion06]

Why not just check it as plain concrete? Is it a slab on grade or an elevated slab?

 

[Vinny7]

The existing slab is old. I would expect it is on grade however i doubt it was ever adequately compacted. I suppose if the grade was not compacted and settling the client would have bigger problems than the stud wall?

When you talk about designing it as plain concrete obviously the shear at the face must be less than

0.8 x Root Fcu

what other checks would you do (bending etc.) bearing in mind the slab is relatively long and shallow? Or would you just distribute the load through the slab and get the bearing width at its underside then ensure the pressure in this width is significantly less than the bearing pressure?

 

[concretemasonry]

If this is a basement slab (typically 4" + or - thick), the first thing to do is determine the physical property of the concrete (thickness) and if there is any subgrade support.

In residential construction, it common for the slab thickness to vary from 1" to 6". Also interior slabs frequently get poured (not placed) over uncompacted soils or construction debris because of the construction process.

Have a few holes drilled in the slab and probe with a wire to determine the real slab thickness and the existance of any soil supporting the slab.

If you find a questionable situation, just cut out a strip of the floor and build a proper strip footing on comapcted fill.

You cannot assume anything when it comes to basement constuction.

 

[miecz]

Try to find the army manual online: ARMY TM 5-809-12.

It's titled "Concrete Floor Slabs on Grade Subjected to Heavy Loads." Appendix B has a method for calculating wall loads on a slab.

 

[spats]

If you want the procedure, check out the attached spreadsheet I found on the internet.

 

[spats]

Oops! Goofed up on the attachment.

 

[dik]

spats... do you have a link?

thanks, Dik

 

[steve1]

http://www.usace.army.mil/publications/armytm/tm5-809-12/

 

[msquared48]

If the slab is 3.5" thick (2X4 form height) and only 1500 psi concrete, which is VERY conservative considering old concrete, the capacity of the slab is 4 to 5 klf of wall.

With a 2X4 plate, that load is spread out to a width of only 3 X 3.5" = 10.5". So, the resulting stress to the underlying strata is more than that seen by the slab. You will need a soils report to verify soil bearing values of the range of 4 to 5 ksf.

Therefore, the major problem here will not be the slab, but the capacity of the underlying subgrade to bear the load.

I agree with the above comments to check the underlying strata and slab thickness.

Mike McCann
MMC Engineering