×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Pressure on basement walls

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.

RE: Pressure on basement walls

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

RE: Pressure on basement walls

(OP)
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

RE: Pressure on basement walls

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.

RE: Pressure on basement walls

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

RE: Pressure on basement walls

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

RE: Pressure on basement walls

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.   

RE: Pressure on basement walls

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

RE: Pressure on basement walls

(OP)
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.

RE: Pressure on basement walls

(OP)
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/8a42973df1a...

RE: Pressure on basement walls

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

BigH - Good call on the applied unit weights.

RE: Pressure on basement walls

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 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

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

RE: Pressure on basement walls

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.

RE: Pressure on basement walls

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

RE: Pressure on basement walls

(OP)
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

RE: Pressure on basement walls

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    

RE: Pressure on basement walls

...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*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

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 )

RE: Pressure on basement walls

Just a thought on the "apparent" discepancy.  Geotechnical reports normally provide equivalent fluid pressue for retaining walls and so use an active pressure coefficient in calculating the values.  Here the author might be indicating that for basement walls a "near at-rest" pressure coefficient is more appropriate and hence might have used a factor to increase the pressures.

RE: Pressure on basement walls

I would design this wall in the "at rest" state. The wall should not be deflecting, therefore, active pressure will not occur. As for the Peck 1969 apparent pressure diagram, this pressure is emperical, and is usually reserved for "top-down" construction, such as a braced excavation. This would be where you may drive a sheet pile in flat ground and then excavate behind the wall placing struts as you go down. It is specifically for this top-down construction. If you are placing your backfill against the wall, this would be bottom-up construction, and therefore this pressure diagram would not apply. I would suggest determining an "at rest" lateral earth pressure coefficient Ko for the engineered backfill that you are working with, preferably a clean sand. And then the pressure is p=(Ko)(gamma')(H)+(gamma of water)(H of water) in units of psf/ft of wall run. Please dont confuse pressure with the force calculation, which would be f=1/2pH, in units of lb/ft of wall run. Hope this helps.

RE: Pressure on basement walls

geonerd's right, of course - up to a point.  But using an "at rest" approach will result in an unrealistic distribution of loads on the back of the wall.  What happens to those "locked in" stresses due to compaction against the wall, particularly in clays?

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources