Pressure on basement walls

[VBI]

I saw in a Journal of Light Construction an article that stated the following loads applied on basement walls.

8' wall height Backfill wght of soil = 30 pcf/ft (Class I soil) Pressure calculated = 1250#/ft

8' height with wght of soil = 60 pcf/ft P=2500#/ft

10' hieght with wght of soil = 30 pcf/ft P=2065

These values appear to be off by a factor of 1.3 or so OR am I calculating things wrong. I get for the first example

P=1/2(30)8^2 = 960 #/ft Can anyone shed some light on this. I don't make these calculations often though they appear to be simple enough.

 

[GeoPaveTraffic]

The numbers do appear to be off. Do the total loads include a surcharge load?

 

[VBI]

No surcharge load was indicated. My numbers and approach would be correct then? Simply P=1/2wH^2 would provide total pressure on a basement wall? With w (Wght of soil) ranging from 30pcf/ft to 60 pcf/ft for different soil classes.

Thanks

 

[ashjun]

you perhaps are missing the vital point; what about the active earth pressure from the wall. How did you arrive at the equilibrium equation without that.

 

[GeoPaveTraffic]

ashjun, I'm not getting your point about the active earth pressure from the wall. Could you provide more detail.

 

[ashjun]

Hi,
I think you could look into any soil mechanics on retaining structures.
And please read that as "active earth pressure on the wall and not from the wall"..I am sorry for the mistake.

regards

 

[SirAl]

Looking at the unit weight values, ashjun, it would appear that some form of reduction factor may have been applied (unless the backfill is a very light weight product). Whether this was the active pressure coefficient as you question, who knows. Never the less, 30 pcf is quite low for any backfill type in my opinion. I agree that the pressure values appear flawed (unless I'm missing something). It may be appropriate to notify the author/journal of this apparent discrepancy.

 

[BigH]

Perhaps the low unit weights are due to equivalent fluid pressure convention???

 

[VBI]

I believe it was a equivilant fluid pressure. The author had equivilant fluid pressures for Class I, II,III IV, ETC soil classifications. The bottom line is that when using "equivilant fluid pressures" the pressure will equal 1/2wH^2.

 

[VBI]

Thank you all for your help and interest! I just noticed that the article in question is on line at the following link. It definately states Equivialant FLuid Pressure. But the numbers STILL seem wrong unless I'm doing something wrong.
Journal of Light Construction Article ==>

http://www.jlconline.com/cgi-local/view.pdf/8a42973df1ab3de667e78d501243cb88/

http://www.jlconline.com/cgi-bin/jlconline.storefront/3fad53600014ff49271a401e1d2905e7

 

[SirAl]

VBI - Thanks for providing us with the web site link.

BigH - Good call on the applied unit weights.

 

[Focht3]

It does appear that Brent Anderson, the author of the JLI article, applied a factor of 1.3 to the published values. Personally, I would have spelled that out in my article...

On the whole, though, Brent Anderson's article is pretty good. One might disagree with recommending &[ignore]gamma[/ignore];_equivalent values of 30 pcf; I would not do so for a "non-yielding" wall. But the general approach appears reasonable -



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[BigH]

How did you all access the site? I clicked on it and was told I was not authorized to do so????

 

[Qshake]

What if there is a factor, not mentioned in the article, that addresses a non-yielding wall.

I have seen and heard of cases where the upper support condition for the wall is pinned by the bottom sill plate connection into the wall. Others have argued that the wall should be evaluated as a yielding condition - "active earth pressure".

Whether or not I agree or disagree perhaps that a possible factor that is being overlooked by the author in the explanation.

 

[VBI]

BIGH

Try this ==> It takes you to the preview and then you must click the free button to take you to the article.

http://www.jlconline.com/cgi-bin/jl...02573b0271a401e1d29064b/Product/View/0204desi

 

[BigH]

VBI: danke, merci, terima khasih, muchas gracias, spacibo, etc.

 

[VBI]

I got it at Muchas Gracias...

To all those who responded => Thank you. But I'm still a bit uncertain on how to determine pressure on basement walls.

Can I use an Equivilant Fluid Pressure? AND, treat it like hydrostatic pressure?

Is this an aceptable practice?

Should I be using a Facter as it appears this author did?

I thought figuring earth pressures on basement walls (retaining walls) would be easier than it is proving to be.

Thanks again

 

[VAD]

VBI;

I think that the factor used was a load factor ( factor to cater for uncertainties etc) that is applied in the LRFD design of structures. You will note that in his Table 3 on Steel requirements that he refers that the steel is based on the "Ultimate Strength". In the final structural design the structural engineer or foundation engineer will apply these factors to the earth pressure normally provided in the geotech report for LRFD design.

If still in doubt please contact the author of the paper for clarification.

Regards

 

[Focht3]

...I'm still a bit uncertain on how to determine pressure on basement walls.

It's a gray area of design, and there isn't a lot of good research on the subject. You have to apply a lot of judgment.

Can I use an Equivalent Fluid Pressure? AND, treat it like hydrostatic pressure?

Yes, provided you understand the limitations of the approach. In general, the use of an Equivalent Fluid Pressure works best when a wall is cantilevered i.e. has no bracing or restraint above the bottom of the wall. If the wall is braced or restrained, a trapezoidal pressure distribution (Peck 1969 "State of the Art" paper presented at the ICSMFE in Mexico City) is more appropriate. Peck's pressure distribution starts at zero at the ground surface, increasing to 0.3 to 0.5*&[ignore]gamma[/ignore];*H at the level of the top brace or restraint; the pressure remains constant to the bottom restraint or brace, then decreases to zero at the bottom of the excavation (if the section of wall below the bottom restraint is "free&quot. The factor H is the total height of the wall; &[ignore]gamma[/ignore]; is the total unit weight ("density&quot of the soil; and the 0.3 to 0.5 factor is chosen based on how long the wall will remain in service (with a higher factor for longer service.)

It wasn't intended for permanent walls, but generally gives a somewhat more conservative (and, in my view, realistic) view of the earth pressure magnitude and distribution. When in doubt, analyze the wall both ways, and take the more conservative answer.

Is this an acceptable practice?

The answer depends on what has become locally accepted practice; but generally the answer is 'Yes.'

Should I be using a Factor as it appears this author did?

Yes. The factor will depend on the design method, of course - it may or may not be applied directly to the earth pressure values.

I thought figuring earth pressures on basement walls (retaining walls) would be easier than it is proving to be.

Perhaps you are beginning to understand why many geotechnical engineers consider their sub-discipline as more art than science -

Thanks again

You're welcome!



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[SirAl]

Focht3

Good call (as usual) on the factored load.

In general, the article referenced by VBI was well organized. (The tone of my previous post was not appropriate )