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Basement wall analysis

Basement wall analysis

Basement wall analysis

(OP)
The stresses created in basement walls by earth pressures against their exterior faces are resisted by bending of the walls in horizontal and vertical spans. The distribution of the total lateral load between the vertical and horizontal span depends on the height and length of the wall, and its stiffness in the spans. The lateral load on a basement wall will be carried entirely in the vertical span when the length of the wall between supports approaches three and a half to four times the height.

Question: can someone explain where the 3.5 to 4.0 times the height comes from? Either explain the theory or please point me to a code reference (explanation of theory preferred).

Thank you.

RE: Basement wall analysis

Well, I would argue very strongly against the 3.5t to 4 times "rule" you quote here for the following reason. I don't care where or who it comes from.

About 25 years ago, I was called out to a site where the contractor had backfilled a 14 high concrete wall while it was still b=very green causing the wall to crack. The crack pattern was a 45 degree line from the top of the wall at the corner, to the foundation. That tells me that the load to the wall as you move away from the corner increases linearly to the full lateral load of the full height of the wall at a distance of "H" from each corner. I would reinforce your wall accordingly.

Mike McCann
MMC Engineering

RE: Basement wall analysis

(OP)
Mike,
Thanks for the reply. Do you remember the length of the wall?

RE: Basement wall analysis

It was probably 60 feet or more and designed as a cantilever retaining wall initially, later to have a roof framed to it later after being backfilled. Not the same as a true basement wall with the reinforcing and deflection patterns, but similar as far as the anticipated failure pattern.

The extra steel is cheap compared to the cost of fixing the problem after the fact.

Mike McCann
MMC Engineering

RE: Basement wall analysis

not exactly the same, but a two way slab ratio of long span to short is 2.0

RE: Basement wall analysis

(OP)
@Mike:

quoted directly from Handbook of Construction Contracting VOL I, by Jack P. Jones:....

"The total lateral load is divided equally between horizontal and vertical spans, if the wall’s length between supports is no greater than its height. The lateral load is carried entirely in the vertical span when the wall’s length between supports is 3.5 to 4 times the height. The distribution of the lateral load falls somewhere between these limits for other ratios of wall length to height."

@Group:

I am still not sure where Jack is getting the 3.5 to 4. I am just looking for some clarity on this.

RE: Basement wall analysis

I would agree with the first sentence as it is a two way slab.

No problem, so he must be applying ratio factors from a two way slab analysis, similar to those found in the 1963 ACI code.

But those ratios do not bear out his statement of 3.5 to 4 times, more like to between 1 to 2, to 1, and those ratios are based on a uniform load, not a triangular load from soil.

I, too, would like to see his justification here. I consider it a stretch.

Mike McCann
MMC Engineering

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