pile support flat slab bottom steel elevation 2

[structSU10]

Having a debate - flat slab with piles embeded 4". Do you run the rebar like a pile cap, 0-3" above the top of pile, or provide adequate cover above the bottom of slab so you are below the top of pile.

 

[SlideRuleEra]

In general, for slabs with many piling place the rebar 3" above top of piles (i.e. 7" above underlying soil). Here are a few reasons:

1) For piles with significant loading, concrete above top of piles needs to be relatively thick to resist punching shear. Two rebar mats (upper an lower) used since loading the slab will produce both positive and negative moments in the slab. Placing the lower mat even lower (so that it surrounds each pile) is not needed since the effective spacing between upper and lower mat is already adequate.

2) Both the upper and lower mats are essentially identical, making rebar detailing / fabrication and especially placement easier, faster and lower cost.

3) The "extra" concrete that is seemingly "wasted" (by having 7" cover below the bottom mat) is a bargain. A slab on piles is going to be on "poor" soil (otherwise, piles would not have been used). Or, in the case of a pile supported mat foundation, down in an excavation subject to bad weather (and groundwater intrusion where I worked). Being able to place simple bar mats and pour concrete quickly is priceless.

If seismic, uplift (e.g hydrostatic) or high lateral loads are concerns, reevaluate all of the above. Pile embedment greater than 4" is probably warranted along with more complex rebar.

http://www.SlideRuleEra.net

 

[structSU10]

I agree with that concept - it is the way I am showing it but others in the office say they have never done it in the past. I appreciate your feedback / validation.

 

[hokie66]

What kind of piles? Why are they embedded so far? How thick is your slab above the pile?

 

[dik]

CIP piles go to the U/S of the slab, with a tolerance of 1" before they have to start chipping. In my book, anyway.

Dik

 

[r13]

3" clear cover is only required for footing area directly cast over the soil. For 4" pile embedment, you can set the steel mat 4.5" above footing base, or 3", if you are allowed and willing to chip out the stick out pile head. I would vote against the latter though.