Residential found. wall design with extreme lateral load

[cobadger]

I am engineering a house with a 10' tall, poured, basement walls. The soils engineer has indicated a equivalent lateral fluid pressure load of 81psf. The bearing pressure is given as 3100psf. Expansive clay has not been found...according to the soils report. It seems odd to have such a strong soil vertically (3100psf) with such a high lateral load...but I am not a geo. engineer, merely structural.

My question is simply that my math indicates vertical reinforcing of #5 bars 6" o.c. (I have assumed a pin-pin connection for the wall and the main floor as a diaphragm.) The contrator is very unhappy to say the least, as he has never done a foundation with rebar of this magnitude. I'm not asking for an exact answer, but does #5 bars, 6" on center seem reasonable or overkill from anyone else's experience?

 

[HouseBoy]

COE
That's why I suggested that it might be a factored load. Seems like a possible reason that it is so high.
I know the horizntal forces get high out near the nid'length of the wall but he said he wanted only conventional footings and I know, wide footings and the needed steel is asking alot.

I think the most effective solution will involve:
1. Use better analysis tools (either plate charts or FEM)
2. Get at least 10 inch thickness (12 if possible)
3. Detail steel placement in the wall to maximize the effectiveness.

I guess to answer the initial question: does bars at 6" oc in an 8 inch thick wall seem reasonable, I'd say NO. It might be what is required from the simple analysis but I think that analysis is not adequate and the wall thickness is not adequate (he hasn't said yet that he can not go thicker, only that he doesn't want a wide footing).

 

[geodan]

Before you go through all that work designing the wall, I would seriously look at the recommended equivalent fluid pressure of 81 psf!
This.. IS an awfully high equivalent fluid pressure! I would ask what soil values he used to determine this? Because you are dealing with a basement wall and thus it is free from rotation, he is likely using at rest earth pressures.

In order to get a efp of 81, the soil in question would need to have a internal angle of friction of 13 degrees?? (assuming a moist unit weight of 105pcf) This would be a TERRIBLY soft clay! I would really ask for clarification from the geotechnical engineer. You may be overdesigning. Or do you have shallow ground water?

Also, equivalent fluid pressure is commonly used for simple retaining walls for ease of calculation, but it has some limitations. Because simple active earth pressure calculations (without surcharge, slopes, loading) yields a trigular pressure distribution there is a similarity to a fluid pressure analysis with a given equivalent weight. But you cannot properly design surcharge loads, wall geometry, or live/dead loads with the efp approach.

Yeah, its high. I'd recommend backfilling with compacted, free draining sand, (internal angle of friction = 32-36 degrees) Then you will be using (at rest) efp's around 30 to 35pcf range.

Or you could always just use Rankine and calculate the actual pressure envelope.

 

[jhoulette]

In Colorado I have seen several soil reports with similiar soil pressure. They have been increasing the values due to expansive clay. I feel that if it is that bad of expansive material then it shouldn't be used as backfill. I know that is a huge cost, but it is worth it in the long run.

 

[gman11]

I've practiced in Colorado as well. First of all your vertical bearing pressure has nothing to do with the lateral earth pressure. The bearing pressure is going to based on in-situ strength unless the house is placed on fill. The lateral earth pressure is based on dumping uncompacted backfill against the wall.

 

[COEngineeer]

81 is pretty much swamp land.