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RE: Void Forms under Grade Beams

[Subject Prev][Subject Next][Thread Prev][Thread Next] If you've been designing foundations in the San Francisco Bay Area, particularly down on the peninsula, you would know that there's all kinds of problems with foundation materials.  I'd go along with whatever the foundation engineer says, even though you might (in 8 or 9 years, possibly just under the statue of limitations) still be called in a suit.   Many of the geotechs try to cover themselves as much as possible - many can't get insurance and live with no property!

Uplift on grade beams, piers, pier friction uplift, lateral creep, difference in expansiveness in the same plan envelop, even global movement of a whole hill (such as an area in Ladera) are pretty much the common problems as well as being near one of the faults.  I know that many of the foundations we designed in the 70's and 80's cost more than the residence.  

Also, have your geotech review your final design and write you a letter saying that it's ok.

Neil Moore, S.E.
neil moore and associates

At 12:28 PM 5/24/2005, Gerard Madden, SE wrote:
Thanks to all. What he is now saying is that I need to design the piers for an uplift skin friction of 1500 psf on the upper four feet of the piers only. This seems way too high to me, since the downward skin friction is only 450 psf?.
He backed down for the dual requirements, but initially told me I had to design the piers for the uplift on the grade beams even if I used the void forms. Now he wants me to design for a 25 kip uplift from 4 feet of skin friction???? I questioned this too and he?s researching?.
*head a shakin*
-----Original Message-----
From: Arvel L. Williams, P.E. [mailto:awilliams(--nospam--at)]
Sent: Tuesday, May 24, 2005 12:18 PM
To: seaint(--nospam--at)
Subject: RE: Void Forms under Grade Beams
Grade beams on void boxes - No Uplift unless tied to floor.
Uplift on piers - yes through skin friction, need to account for this in the design and steel in pier.  Geotech should give you active zone (depth) and uplift force per unit area.
Note:  If you bell the piers then the tensile stresses are likely to control the steel percentage.
-----Original Message-----
From: Gerard Madden, SE [mailto:gmadden(--nospam--at)]
Sent: Tuesday, May 24, 2005 1:34 PM
To: seaint(--nospam--at)
Subject: Void Forms under Grade Beams
I have a soils engineer requiring me to design grade beams for a pier and grade beam residential foundation for an uplift of 1500 psf. He is also requiring void forms underneath the grade beams?. I thought this was an either or type of thing, not both. It?s a 1 story house and this will easily govern the design.
Any input appreciated.