Passive Earth Pressures - Building Foundations

[cpdonahue]

I am a geotech engineer being asked to provide passive earth pressure parameters for the design of building foundations. Metal building supported by column footers. Is it conventional to use passive earth pressures for lateral loads on foundations, or is the lateral load typically addressed with shear? Is it appropriate to provide these values to the structural engineer for this application?

 

[HTURKAK]

Sometimes it would be necessary to transfer lateral loads to the foundation soil with passive thrust . ( I did prefer ties rather than trusting passive trust )
However , you are expected to provide passive earth pressure parameters.







Use it up, wear it out;
Make it do, or do without.

NEW ENGLAND MAXIM

 

[civeng80]

Are we talking about a portal frame here ?

 

[cpdonahue]

Not sure what a portal frame is. It's a metal building with masonry walls I believe.

 

[wth]

Portal frame is a steel frame with rigid joints. It supports itself for lateral loads.

I don't have any references, but I've always been told not to use passive pressure for foundation stability, as it requires the structure to move to mobilize it
For certain structures and loads (ie accidental) that's probably OK, but for regular buildings I would use causion.

As you'll need to give passive pressure as a function of deflection, the designer will hopefully realize any troubles arising from that deflection..

 

[milkshakelake]

I would give the passive pressure, but I don't use passive pressures for foundation sliding. Maybe friction coefficient between foundations and soil? I don't think I've ever seen that on a report, but the designer might appreciate it.

 

[wth]

Maybe friction coefficient between foundations and soil? I don't think I've ever seen that on a report, but the designer might appreciate it.

In my part of the world we limit "friction", or roughness as it's called in the literature, Simplified it's r.b = Vd / (Nd * factor<1), where the geotech gives the formula.
The upper allowable limit is 0.9, and the higher the value, the less bearing capacity you'll get.
For a lightweight building with wind loaded columns, this is often the governing case for foundation sizing.

 

[JStructsteel]

to me no different than resisting a retaining wall sliding with passive pressure. There is some gain to it.

 

[milkshakelake]

@wth Never saw that formula before. The geotechs around here need to step up their game!

 

[milkshakelake]

@JStructsteel I think at-rest pressure would be more applicable, and there are some nuances to it. This thread has some discussion on it:

https://www.eng-tips.com/viewthread.cfm?qid=515632

 

[wth]

@milkshakelake I believe it's from, or derived from, Janbu, N. 1954. Stability analysis of slopes with dimensionless parameters. Harvard Soil Mechanics Series,46, 811 , or works by the same author.