Frost depth 1

[AlmostPE]

I hate to do this but I get mixed answers on this one thread I posted. The foundation forum is not as active as the structural forum. Can you guys help me?

thread256-200120

 

[MarcbSE]

If frost depth is fairly shallow, I will set the bottom of grade beam elevation below the frost depth so void forms are not needed.

If frost depth is deep though, I size the grade beams as needed for the loads and put void forms below them to handle any frost heave.

 

[AlmostPE]

well, I need the void form due to expansive soil, so it will be installed no matter what. Also, the main level is about 4 ft from finish grade. The frost depth here is 3 ft. Thefore, in order to set it at frost depth we need to make the wall about 7 to 8 ft tall.

OR shall I just make the bottom of the wall slightly lower than the finish grade?

 

[MarcbSE]

If you will be using void form anyway then there really is no benefit in lowering the bottom of the grade beams. I would probably set it around a foot below grade anyway to make sure the bottom does not become exposed for whatever reason.

 

[archeng59]

I always put the bottom of the grade beam, not the bottom of the void form, below frost level. The void form, when used, is below the grade beam wherever that is located. If the frost level is shallow, then I put the bottom of the grade beam at least 24" below finished grade to help avoid the effects of moisture changes in the soil thru-out the year.

The overall depth of the grade beam might be affected by the span between the piers. If you have a wide pier spacing, the grade beam might need to be deeper than 24". Obviously, a grade beam is typically a multi-span member, but beam depth is also based on span and not just frost depth.

 

[csd72]

AlmostPE,

See what depth you need the grade beam to avoid using shear ties and then make your decision from there based on how much extra you need to bring it below frost depth.

csd