soil pressure with 0.6D+W

[bjb]

Is it required to check soil pressure for a footing using the above load combination? I can see nothing in the IBC or ASCE 7 that would exmpt a check of soil bearing pressure, so my opinion is that by the Code, you have to. On the other hand, this seems overly conservative to me. In the "old days" we only checked to make sure we had the required factor of safety against overturning or uplift. I know that this has been discussed before.

The Code we use is the 2003 IBC. Section 1805.4.1.1 states that footings shall be designed for the most unfavorable effects due to the combinations of loads specified in section 1605.3. Section 1605.3.2 does not require the 0.6 factor on D, and we could use these load combinations to design footings. But if so, how would overturning or uplift be addressed by the Code? Engineering jusgement would say that a minimum factor of safety should be 1.5. Compounding the confusion, ASCE 7-02 does not include the alternate basic load combinations.

 

[WillisV]

thread507-202887

 

[bjb]

I understand that with the 0.6 factor, you don't need an additional safety factor against sliding and overturning. I didn't see in your referenced thread a discussion on soil bearing pressures.

 

[UcfSE]

Both ASCE 7 and IBC 2006 have the load combination 0.6D+W+H. Seems pretty cut and dry to me there. In the alternate section in the IBC, there is the stipulation of not using more than 2/3 of the dead load likely to be in place when dead load effects counteract wind load effects. That doesn't mention soil load though, as I'm sure you found.

I think I would stick with what is more straightforward, which is the basic load combinations. Besides, unless it is interior, how can you have a retaining wall that does not also have wind load on it? That throws you right back into the 2/3 factor for D.

 

[kslee1000]

For most cases, I don't think the reduction in DL will cause problem in soil bearing, consider the transient nature of wind load. Also, I don't think the reduction needs to be considered for the retaining wall with full backfill, except during construction, but at such occasion, the allowable stresses are higher than normal.

However, for the strength design of the under ground structures, the reduction can't be ignored.

 

[ron9876]

This is simply another load case to consider. I don't see any reference to not considering soil pressure with this load case.

 

[kslee1000]

The reference is called "Engineering Judgement", it wouldn't hurt to try some realistic cases.

 

[ron9876]

You have to follow te code. It is an opinion (which I share) that this isn't realistic condition but that is a conversation to have over a beer. We are required to follow the code.

 

[kslee1000]

The 0.6 factor is accounted for the removal of part of structure or overburden, for which, the resulting unbalanced condition deviates from the original assumption, and can be critical.

However, for the case of earth retaining structure, will soil or concrete become lighter with age? Obviously not. So, what is the 0.6 factor accounts for? Ok, let's take out the upper 40% of the soil and concrete, now figure what would result.

 

[ron9876]

Previously there was a required safety factor of 1.5 for resisting/overturning. This new provision is equivalent to a safety factor of 1.67.

What bothers me is that in conditions where there is a material safety factor built in (like the piles in a pile supported shearwall foundation) the code in effect requires a safety factor of 1.67 on top of the approximate safety factor of 2.0 on the pile capacity.

We have heavy winds here and it kills the design. Interior footings in one story buildings have to be so large they look completely out of proportion. When Andrew came thru down here didn't hear anything about footings flying away and most engineers didn't even consider footing uplifts in that era.

Like I said above the code has the load case and we are required to consider it. For a retaining wall if you didn't use this load case what would you use as a safety factor?

The academics have taken control of the codes. Have you seen where the new steel code recommends that you account for the lateral load that comes from leaning columns which are within tolerance. I have been doing this a long time and it gets crazier all the time. Seems like the people that write codes have never been to a construction site.

 

[jechols]

also most allowable soil pressures are selected to reduced settlement due to consolidation from long term loading. When sizing the footing based soil pressure from transient loads we should be able to use a much higher allow. soil pressure. I have had some geotechs allow me to increase the allowable soil brg pressure by 1/3 for wind or seismic but it should be much more imo.

j

 

[VTPE]

Most Geotechs will be giving you NET allowable soil bearing pressure based on SERVICE loads considering depth of footing, overburden, surcharge, and size / shape of footing. That means, the bearing pressure that is allowed at the depth the footings are presumed to be placed.

Consider a footing with no load but its own weight and the weight of the soil over it. Lets say this load is 500 PSF. There is an equal and opposite load from the soil = 500 PSF, therefore there are no internal shears or moments on the footing, it is in equilibrium. As load is added to the column, the pressure on the soil increases to become 500+(additional load). The footing has to be sized so that this load does not exceed the allowable load, qa. The footing concrete has to be designed for the NET increase in the pressure from equilibrium, that is the 500 PSF down is canceled by 500 PSF up so just the additional load causes moments and shears in the footing. If you factor the additional load, it is possible that this load will exceed the allowable soil pressure. However, the factored loads are approximately 1.6* the service loads. The factor of safety applied to the allowable soil pressures is typically 2.5-3. Therefore, the factored net soil pressures will not be more than the pressure that would cause failure in the concrete.

This is a classic case of mixing service resistances with factored loads. If you really want to be consistent, have your geotech provide you with the service loads they used to determine the allowable pressures and keep your factors consitent all the way through. Your problem will probably go away.