Integrated Buildings & Structures

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Foundation Depth for Egress Stairwell

  • 1.  Foundation Depth for Egress Stairwell

    Posted 04-21-2020 10:18 PM
    Hello ASCE Community,

    I designed an egress stairwell for a residential client with 8' tall retaining walls on both sides of the stairwell.  I have an image of the foundation plans below.

    The contractor only excavated the wall footings to the depth of the steps.  I'm concerned about frost heave because the footings will only be 18" max from an exposed surface (the steps).

    The reinforcing bars were installed per spec, the footings & wall (dimensions) were also built per spec.  I'd like to get a second opinion as to whether or not I should have the contractor go back out to remove the form work and finish the excavation.  It's more than just the principal of the fact that they didn't follow my design.  Unfortunately, I gave a verbal approval for the excavation as-is but would really like to confirm with some colleagues before moving any further.

    Original Foundation Design
    Cut @ Mid span of stairwell
    Looking up towards the top of the excavation


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    Eric Gilliland Aff.M.ASCE
    Denver CO
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  • 2.  RE: Foundation Depth for Egress Stairwell

    Posted 04-22-2020 02:07 PM
    Since this could conceivably lead to damage that could cause a trip & fall injury in the future, I would insist on the deeper excavation.  I would also carefully explain the reason why to the owner.

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    Mark Myers, P.E., M.ASCE
    Gig Harbor, WA
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  • 3.  RE: Foundation Depth for Egress Stairwell

    Posted 04-22-2020 09:00 PM
    Thanks Mark!  This is what I did.  The homeowner was receptive.

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    Eric Gilliland Aff.M.ASCE
    Gilliland Engineering
    Denver CO
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  • 4.  RE: Foundation Depth for Egress Stairwell

    Posted 04-22-2020 04:44 PM
    Good afternoon Eric,
    You may wish to consider redesigning the foundation system as a frost-protected system using insulation. I do not know what your frost depth is, but you might be able to get away with placing insulation below the stair and as an apron at the top of the stair. NAHB and HUD have published a guide to frost-protected shallow foundations based on ASCE 32, which may be useful to you in performing the design calculations.

    A few other concerns come to mind in looking at your photo and drawing:
    1. Given the wall height (~8ft), it is not apparent that the wall is proportioned for stability when subjected to soil pressure. Now, this assumes that the stair pour is not tied in somehow to fix the base of the wall and create a boat section.
    2. If you are in a location with frost, then presumably you have a period of time in the late winter/early spring when temperatures alternate above and below freezing. During this time moisture infiltrating through the backfill appears to be able to drain through the weep holes onto the steps and freeze. Also it is not clear if weep holes are detailed to prevent migration of soil particles.
    3. The excavation for this stair bulkhead would be greater than the 5ft OSHA allowable for a vertical cut.
    I hope this is of some help to you.

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    Richard J. Driscoll P.E., M.ASCE
    Lebanon NH
    [email protected]
    [Phone]
    www.richardjdriscoll.com
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  • 5.  RE: Foundation Depth for Egress Stairwell

    Posted 04-22-2020 09:00 PM
    Thank you for the information Richard!  I will be redesigning the system and will take your recommendations into consideration.

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    Eric Gilliland Aff.M.ASCE
    Gilliland Engineering
    Denver CO
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