Bad idea - second opinion
Bad idea - second opinion
(OP)
Hi all.
I am a structural engineer in need of a second opinion. A friend is extending his house, and his contractor is proposing a basement slab at the same level as, and tied into, the perimeter footing, all below frost heave depth, which is 4' - 6" here in Montreal.
This is not a floating slab, it is to be laid directly on compacted soil, without the usual crushed stone etc. The contractor is proposing rebar, and full connection (continuity?) with the foundation wall.
They will then pour the wall footing at the perimeter, on top of the slab.
In my 8 or so years designing building foundations and superstructures, I have never heard of this. I think it's a bad idea, but it's hard for me to verbalize exactly why.
Here are the issues which I perceive:
1) frost heave if the basement is not heated could force the slab up at the center but the fixed perimeter could crack...
2) The slab becomes one big combined footing with positive and negative moment regions, and thus becomes more of a problem requiring engineering analysis versus your traditional home extension project
3) consolidation settlement at the perimeter may affect the slab.
Any other opinions?
Am I way off?
Any comments would be appreciated.
tg
I am a structural engineer in need of a second opinion. A friend is extending his house, and his contractor is proposing a basement slab at the same level as, and tied into, the perimeter footing, all below frost heave depth, which is 4' - 6" here in Montreal.
This is not a floating slab, it is to be laid directly on compacted soil, without the usual crushed stone etc. The contractor is proposing rebar, and full connection (continuity?) with the foundation wall.
They will then pour the wall footing at the perimeter, on top of the slab.
In my 8 or so years designing building foundations and superstructures, I have never heard of this. I think it's a bad idea, but it's hard for me to verbalize exactly why.
Here are the issues which I perceive:
1) frost heave if the basement is not heated could force the slab up at the center but the fixed perimeter could crack...
2) The slab becomes one big combined footing with positive and negative moment regions, and thus becomes more of a problem requiring engineering analysis versus your traditional home extension project
3) consolidation settlement at the perimeter may affect the slab.
Any other opinions?
Am I way off?
Any comments would be appreciated.
tg






RE: Bad idea - second opinion
RE: Bad idea - second opinion
RE: Bad idea - second opinion
Both structural and geotechnical evaluation should be done in this case.
RE: Bad idea - second opinion
Your frost is deeper than here, but....
When we design a structure with the understanding it's heated, that's how the design is predicated. However, we include in our spec the condition that footings are not to be placed on frozen ground. And the temperature of the foundation must be kept above freeaing until construction is complete. We don't need to conservatively provide for the "what if" scenario of an unheated structure during the life cycle. If that were the case, that criteria would be understood and designed at the beginning. The owner assumes the responsibility and liability for poor maintenance if the assumption of minimum temperature isn't provided. Most homes would not only suffer damage to foundations and basements, the plumbing would freeze and break too.
Those type of repairs are common where temperatures fall unexpectedly in areas typically not prone to freezing. Plumbing routed in uninsulated attic spaces is particularly a culprit.