Pressure on basement walls
Pressure on basement walls
(OP)
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.
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.





RE: Pressure on basement walls
RE: Pressure on basement walls
Thanks
RE: Pressure on basement walls
RE: Pressure on basement walls
RE: Pressure on basement walls
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
RE: Pressure on basement walls
RE: Pressure on basement walls
RE: Pressure on basement walls
RE: Pressure on basement walls
Journal of Light Construction Article ==>
http://www.jlconline.com/cgi-local/view.pdf/8a42973df1a...
RE: Pressure on basement walls
BigH - Good call on the applied unit weights.
RE: Pressure on basement walls
On the whole, though, Brent Anderson's article is pretty good. One might disagree with recommending γequivalent values of 30 pcf; I would not do so for a "non-yielding" wall. But the general approach appears reasonable -
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Pressure on basement walls
RE: Pressure on basement walls
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.
RE: Pressure on basement walls
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/jlconline.storefront/3...
RE: Pressure on basement walls
RE: Pressure on basement walls
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
RE: Pressure on basement walls
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
RE: 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*γ*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"). The factor H is the total height of the wall; γ is the total unit weight ("density") 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!
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.
RE: Pressure on basement walls
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
RE: Pressure on basement walls
RE: Pressure on basement walls
RE: Pressure on basement walls
If you are uncomfortable with the Peck pressure values, then calculate the pressures assuming "at rest" conditions. Calculate the resultant force per foot of wall, then distribute it as a uniform pressure on the wall. Then design the wall using both pressure distributions - and use the more conservative outcome.
Please see FAQ731-376 by VPL for tips on how to make the best use of Eng-Tips Fora.