For inspectors especially those that perform new construction phase inspections it’s somewhat confusing in the building codes if the tables for concrete wall reinforcement apply or if engineering design is required. In the first graphic the full height 8’ basement wall, if it has more than 4 foot of unbalanced fill has to be restrained. To be restrained the sub-floor (joists and plywood) have to be in place prior to back-fill to resist inward movement at the top of the wall. The 6’ and 4’ step down wall sections are not restrained at the top, which requires special design if they retain more that 4’ of unbalanced fill, have a surcharge or retains hydrostatic water pressure. See the second graphic for showing surcharges like sloped soil or a live load surcharge like a vehicle and hydrostatic water pressure. In the first graphic the 6’ basement wall section retains more than 4’ of unbalanced fill, which requires an engineered design as a cantilevered retaining wall. The 4’ basement wall section would not require and engineered design unless there was a surcharge or hydrostatic pressure.
NOTE: Only the surcharge within the 45 degree dashed line is considered in the wall design.
I have attached actual photos of a step-down foundation with a soil surcharge that pushed the top of the foundation wall in cracking the basement floor along the edge of the footing. This wall was retaining some soil surcharge and hydrostatic pressure. The water drains along the foundation saturating the soil. The backfill soil was inorganic clay that will swell when wet, which is not recommended for any basement wall.
Never seen enough pressure on a wall to push and crack a floor like that before Randy.
Do you think that there might not have been any dowels in the footing for that to happen or the footing moved along with the wall?
The footing was doweled into the wall so the footing rotated with the wall. The basement floor was poured on top of the footing. I didn’t show the photo, but I placed a level on the floor and you can see the floor rotated with the footing. I have seen this a few times over the past 26 years.
To be considered restrained a full height basement wall the subfloor system resists the horizontal pressure at the top of the wall and the basement floor slab resists the horizontal pressure at the bottom. The concrete basement wall is designed as a simple beam pinned at the top and bottom. The step down foundation wall has a cripple wall on top. The joint between the bottom of the cripple wall and the top of the step down foundation wall is considered pinned. A pinned joint is considered a hinge and unable to resist any moment. The building code limits the soil height to 4 feet, which limits the amount of horizontal earth pressure. If there is hydrostatic pressure, surcharge or high shrink-swell soils then the step down foundation wall has to be designed regardless of the soil height.
Nice graphic Randy and my bad, I completely forgot this was a step down wall.
Now I know exactly what happened and why.
I guess one could say that the existing ground must have been saturated bad and without the proper spread footing to prevent the overturn due to the unbalance pressures, it is easy to see why it happened. Must of been a clay site or unstable soil with an under designed footing.
Marcel,
Your assessment is correct. Saturated clay soil (heavy) plus some soil surcharge combined with some hydrostatic pressure. If they had used the proper backfill and surface grading they would have likely avoided this problem.
