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  • STRUCTURAL INSPECTION - Overstressed basement wall

    I. Descriptions: 2 of 3

    Inspection was carried out on 3/12/01. This inspection is limited to the foundation
    wall being replaced, and the attachment of the structure to the foundation wall. The house is new construction, and is located on a rural lot.

    The structure is a one storey residential house on a poured concrete wall basement,
    and is rectangular in plan with full walkout along one long edge. The opposite wall,
    (north) which must retain earth, is nominally 9' tall and 8" thick. It runs 62 ft
    without any buttressing. The porch foundation wall could have acted as a buttress, but
    it was not tied in with reinforcing bar. The east and west walls are 32 ft long.
    Our best information is that the walls have no vertical reinforcing, and some
    horizontal reinforcing.

    Sometime after backfill the north and east walls showed excessive lateral movement
    at the top, and excessive cracking. The backfill material has been removed and the
    walls have not moved back to original plumb.


    II. Observations:

    The house is anchored to the foundation wall with tie straps embedded into the top
    of the foundation wall at 6 ft spacings. Assuming light soil pressure for sandy
    backfill, 405 lb/ft lateral resistance is needed at the top of the wall. Friction may be
    able to resist 50 or so of the 405 plf.

    This would leave 2130 lb. sideways on each strap. The straps have deformed ex-
    cessively, gouging into the 2x6 base plate. The best installation involves running the
    straps up through centered holes in the plate, then bending them over the top and
    down the sides. The installed straps were bent under the plate, and up the sides. This
    configuration may have less strength, and will exhibit greater deformation under
    lateral load. Cracks in the concrete propagate diagonally from each strap - indicating
    that the straps are (or were) transferring significant lateral force.

    In the best possible case, the straps can transfer about 480 lb. each - as limited by the
    wood plate's crushing resistance. This would indicate that the straps must be no more
    than about 1'-4" o.c.

    The Michigan Building Code calls for a minimum 10" thick wall for this application.
    (Ref: Table 1805.5(1)) Thicker or reinforced walls may be needed depending on
    properties of backfill material. If an 8" wall is used, it must be an engineered,
    reinforced wall. The wall was not built to Code requirements, and would likely have
    had performance problems even with proper top attachment. Calculations show the
    unreinforced concrete will go into tension - unacceptable under the ACI specification.


    III. Conclusions: 3 of 3


    - The wall is not safe and must be rebuilt.

    - The porch foundation must be tied to main wall with appropriate reinforcing steel.

    - The anchors must be installed at a much closer spacing, in order to transfer the lateral
    force into the floor diaphragm. Install anchors per manufacturer directions.
    Consideration should be given to the use of steel rod anchor bolts in the rebuilt wall,
    instead of straps.

    - Rebuilt foundation wall must be in conformance with MBC. Given the trouble so
    far, and the difficulty in knowing actual basement wall pressures in general, It is
    recommended that a reinforced concrete wall be designed, or at least any proposed
    design be reviewed by the Engineer, and submitted to the Building Inspector, prior
    to construction.


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Grand Rapids, MI 49546
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