negative heel pressure at footings? 2

[mbullism]

Is it acceptable to let the heel pressure under a footing subjected to overturning forces to fall below zero for short term (wind/seismic) loading combinations? All the literature I have says this condition is to be avoided, but is that just for long term dead and live loads? I’m looking at the reaction report for a pre-engineered metal building, and satisfying all of the combinations while keeping the resultant within the kern results in substantial isolated footings… (columns on top of a 4’-0” kneewall, then 4’-0” below grade for frost- combined uplift with lateral forces). Can I proportion these footings so that the long term loads result in only positive pressures below the footing, and so that no load combination results in a pressure above the max allowable?

I don’t want our client to think I’m nuts…

 

[archeng59]

when designing retaining walls to resist the permanent soil loads and dead loads (no wind or seismic) I design the footing with no negative pressure at the heel. When considering wind and seismic, I will allow some negative pressure because it is a temporary load condition but I keep the resultant within the kern.

 

[DaveAtkins]

For those of you who "allow negative soil pressure at the heel,"
please explain how this can be justified--adhesion of the soil to the bottom of the footing? What if the soil is granular?
Negative soil pressure is not acceptable, in my opinion.

DaveAtkins

 

[civilperson]

Only the positive soil pressure is used to react for the vertical loading. This leaves a triangular pressure distribution under just a fraction of the footing. The resultant of the soil pressure acts at the third point measured from the toe to the zero pressure location.

 

[mbullism]

DaveAtkins- as is often the case, typing in a thread is different than talking in person. I can't speak for anyone else, but I meant calculated negative/outside the kern. I know it's actually zero. I simply meant that the point of zero bearing pressure is not at the edge of the heel but is back under the footing... part of the heel subject to lifting off the soil/footing interface.

What I was looking for is 1)if this was o.k. in the short term, 2) under what circumstances, and 3)to what extent. I hear a lot of "yes, we do that for wind/seismic", and a couple of folks that either gave me their judgement as to how much, or pointed me to published info on it...

My apologies for any confusion.

 

[DaveAtkins]

Well then,
I am okay with a triangular bearing pressure under the footing for ANY load case, as long as the allowable bearing pressure is not exceeded.

DaveAtkins

 

[structuralaggie]

apsix - my point is that a different expression would be needed for the bearing if you get an uplift at the heel. One can be derived, but the expression q=P/B(1+6e/B) will not yield the correct bearing pressure based on the assumpitons it was derived under. Granted, a small amout of uplift will not affect the pressue that much.

 

[JEmH]

Just in case this horse isn't dead yet....
If I'm looking at a service limit state I try to keep bearing pressure on the entire footing, or at least be close. At a strength limit state I'm less worried and generally like the resultant in the middle half or so. Also, if I'm on rock that is stiffer and much harder than my footing, I usually don't mind having a relatively small "triangle."

 

[mitchelon]

Recommend you discuss this at the Geotech forum. Ten Structural Eng can't all be right when there opinions are different.

 

[miecz]

For transportation projects, AASHTO Standard Specification (5.5.5)requires the resultant of the bearing pressure for static loading to be within B/6 of the center of the foundation, for retaining walls on soil. For seismic loading, the resulting must be within B/3 of the center of the footing.

 

[JLNJ]

csd72

It seems to me that the overturning on a compressible material like dirt is not completely analogous to the cup on the table (and that was my point). We don't know the real stress distribution in the soil and the spiked pressure we assume when the resultant falls outside the kern may not be conservative.

I certainly use the same outside-the-kern methodology as others but I often wonder about the actual stress distribution.

 

[structuralaggie]

jmiec - keepint the resultant within the middle third of the footing is analogous to not having negative pressure.

 

[csd72]

JLNJ,

Try it with a compressible table cloth on the table and it is exactly analogous.

You, like many on this forum are getting two modes of failure mixed up.

Overturning is one mode of failure

Soil bearing capacity is a completely different mode of failure.

As long as the soil is strong enough, it will not yield enough to make any significant difference to the overturning behaviour.

 

[ron9876]

I am in Florida and when the code was changed to include the 0.6D+Wind load case footings starting getting out of control. In this condition I am comfortable with uplift at one edge of the foundation as long as the soil pressure is ok. (This is like a SF of 1.67)

I am currently designing footings for interior columns for a one story manufacturing building. Man the numbers work but these sizes/depths are crazy. Didn't see any footings pull out of the ground when Andrew came thru here.

 

[Wollmann]

For spread footings for bridges, the AASHTO LRFD specifications have rules for maximum eccentricity under strength load combinations: For spread footings on soil e < B/4, where B is footing width, for rock e < 3/8 B. Therefore, assuming triangular distribution, on soil you will need compression over 75% of the footing width, on rock over only 37.5%.

(Ref. AASHTO LRFD Specifications, 3rd Edition, Articles 10.6.3.1.5 and 10.6.3.2.5)