Slab Detail at Open End Wall of Unheated Building

[ksellc73]

It looks like the topic of slabs in unheated buildings has been covered pretty extensively but my condition is a little different from the threads I have been able to find. I have an unheated PEMB building with an open endwall. The required frost depth in my area is 36". I am struggling with the best way to handle the edge of the slab on this open wall. My initial thought was to pour a strip footing below frost with a grade wall to support the edge of the slab. I would also call out gravel or non-frost susceptible fill behind the wall and under the slab. The question would be to have the slab run over the wall or butt it against the wall and dowel it together. I am also considering a haunched slab with the same fill requirements. Also, is insulation going to have any benefit when the building is this open?

I welcome any thoughts on this condition. Thanks!

 

[BAretired]

I would treat it as an exterior grade slab which, in fact, is what it is. No point using insulation. A strip footing and stem wall is a good idea in case the client wants to close the building in at some time in the future. If you have an apron slab, you should tie the slabs together to prevent differential heaving or settlement, but don't tie either slab to the stem wall. If the slabs want to heave, let them do so freely.

BA

 

[ksellc73]

Thank you for the reply BA. Based on your reply I take it you would not recommend resisting the thrust of the PEMB columns with hairpins as this would tie the slab to the piers? This was a concern of mine but the foundations where going to get very expensive with moment footings or cross ties. Especially for an equipment storage building. This is why it would be very critical that they use a well draining gravel as the sub-base for the slab. I was hoping to remove water from the equation. I guess in this situation you would not use a vapor barrier under the slab as this would hold rain water that may seep down through the slab?

 

[Klitor]

Maybe using a steel tie at eaves level to take care of the thrust would be cheaper, and you could then separate the slab from stem wall?

 

[BAretired]

I believe it is preferable to use cross ties below the slab rather than hairpins, but if hairpins are used, you might want to place a plastic void former under the slab immediately adjacent to the stem walls in order to allow a little differential movement.

A well draining gravel is a good idea for the sub-base. Vapor barrier could be omitted.


BA

 

[Gopher13]

ASCE 32 covers the design of frost protected shallow foundations. It covers both heated and unheated buildings, and should help you determine the thickness of insulation needed and the length of insulation needed to extend beyond the perimeter of your building to keep the ground below your slab from freezing. It also requires the use of non-frost susceptible fill though which seems redundant to me.....If non-frost susceptible fill is used, insulation then wouldn't be needed to keep the ground from freezing (and heaving) because the non-frost susceptible will not heave. So what is the point of both insulation and non-frost susceptible fill?

 

[dhengr]

Ksellc73:
I agree with BA, on several accounts. There isn’t much use or need for insulation in/around this found., since it is a completely open storage shed along this wall. The ground will freeze insulation or not, so a non frost susceptible base or fill material, properly compacted, will be much more effective. Slope the slab to drain out. But, another important detail is that the slab elevation be high enough so that grade can be held well below top of slab and sloped away from the bldg., so the site can be properly drained. Don’t let water accumulate around the found., and slope the apron slab away from the bldg. too. Rest the apron slab on that stem wall for vert. support at the wall. Round the outer corners of the apron with a 12-16" radius corners, or sure as hell the heavy equip. will bust those corners off at about 45̊. I also prefer the cross bldg. tie beams under the slab. They put the trust reacting loads where they belong, at the piers and columns and allow the slab to act somewhat independently. Maybe put a saw cut in the slab over the tie beams.