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Rigid Insulation

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SteelPE

SteelPE

Structural
Mar 9, 2006
2,751
I have finished up my first design of a pile foundation. I am now beginning to field questions about alternative details. My design requires the structural slab to be doweled to the perimeter grade beam. This helps distribute lateral forces to the interior grade beams and generally ties the whole building together.

A question has been asked with regards to this detail. Since they are trying for a highly energy efficient structure they are wondering if they can place 1” of rigid insulation in-between the grade beam and the slab. My first thought would be no (due to the fact that we are trying to move forces in and out of the slab). Compression of the insulation would not be a problem because you can get 100 psi compressible insulation (something I didn’t learn about until a few weeks ago).

Is this something to be concerned about or am I making a mountain out of a mole hill?
 
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SteelPE

I will agree with your first thought that the grade beam and the ground floor slab should be joined together.

If insulation is prime concern, why not consider casting the ground floor slab integral with the grade beam, put insulation layer on top of ground floor slab and then construct a (light weight) gound floor on top of insulation.
 
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or add the insulation to the outside of the grade beam and make it part of the building insulation envelope.

Dik
 
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So basically my concerns are valid.

The ideas are great unfortunately, these are ideas that can not be used on this particular project.
 
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Do you need the slab at the ground level to act as a diaphragm or are you just feeling uncomfortable to create a system of ties only using grade beams?

Using grade beams to tie pile caps together (assuming you have followed the requirements of design capacity based on column reaction) is usually sufficient. It creates a "truss" of sorts.

Would it be possible to post a partial plan view of the foundation? Are all the pile caps connected to one another by tie beams? If so, then maybe the slab is redundant?

We are Virginia Tech
Go HOKIES
 
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Can you use HDPE in the joint... it will provide a bit of a thermal break and the floor plate should provide ample heat.

Dik
 
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Slick

In this situation we have 15T wood piles supporting the building. It was decided to use pile caps connected by grade beams for the design of the building. Therefore, all of the piles are connected together (per code requirements).

Running an analysis of a fixed piece of reinforcing 1" high yields a principal stress (bending and shear) of 35 ksi under ASD load combinations. This is a stress of 0.58Fy which is getting pretty high.
 
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Could the exterior grade beam have a curb, say 8" high by whatever wall thickness the architect would like? Pour everything monolithic and provide bars from the top of the grade beam to be bent down into the slab The insulation could then be placed against the curb between the curb and the bars to be bent down to the slab. Follow up with insulation around outside perimeter, assuming there is not a great grade differential from finish floor to the outside grade that should satisfy insulation requirement.

In other words, if your grade beam was 2' high by 2' wide, The formwork would be 2'8" high, by the 2' wide and provide a 8" high by (2'-arch wall thickness) blockout for the slab and insulation thus creatin the curb. Then provide bars to be bent down to slab as mentioned above. Would be easier to just draw it....
 
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