Bad idea - second opinion

[trainguy]

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

 

[BFPartners]

The orignial building probably was constructed without an engineer review utilizing Part 9 of the NBCC. Be cautious with expansions. As you mentioned, you might find out that there are moments generated within the slab (below the strip footing placed above the slab on granular). These moments can cause cracking of the slab and the result would be a slab independant of the footing. What about the lateral soil pressures on the wall? Building code restricts the length of basement walls w/o and engineers review and stamp. Note the basement most likely has the minimum required reinforcing (bar at top, bar at mid height, bar at bottom, verts every 6 ft). As far as I am concerned this is garbage! But NBCC says OK... How will the lateral soil pressures be transmitted back into the slab? How does the contractor guarentee load transmittal at the connections? Remember, most basement walls have classic two way (yield force) failure cracks thus the restriction on wall length. If you do not have a confortable "gut" feeling about this work then that feeling probably means something.

 

[flamby]

Structurally this is bad idea. However for a second opinion, I would have Geotechnicals my choice as the same situation on different soils may mean a significant difference.

 

[Ron]

I agree it's a bad idea structurally as there will be significant shear generated in the slab at the walls as well as moment issues from the structure as well as the potential settlement issues.

Both structural and geotechnical evaluation should be done in this case.

 

[AlohaBob]

Another thing to consider....

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.