settlement beneath a column footing
settlement beneath a column footing
(OP)
Consider an 8-by-8 column footing designed for 2,000 psf and a clay layer 5 to 20 ft below the footing. Run stress distribution beneath the center, edge and corner. Complete the settlement calculations for each stress distribution. You'll get three different results.
Here's my stress distribution (5, 10, 15 and 20 ft in psf):
below the corner: 440, 292, 184, 120
below the edge: 710, 373, 211, 131
below the center: 1170, 482, 243, 143
Knowing that the foundation can't deform to the calculated settlement profile, what is your approach to evaluating the foundation settlement? It's clearly conservative to design for the center stress distribution only. What if Pp-Po is 900 psf, would you really get virgin consolidation beneath the center only (I would doubt it)?
Here I am with my gray hair thinking of a half-dozen ways I could handle this, but would like to hear from others. . .
f-d
Here's my stress distribution (5, 10, 15 and 20 ft in psf):
below the corner: 440, 292, 184, 120
below the edge: 710, 373, 211, 131
below the center: 1170, 482, 243, 143
Knowing that the foundation can't deform to the calculated settlement profile, what is your approach to evaluating the foundation settlement? It's clearly conservative to design for the center stress distribution only. What if Pp-Po is 900 psf, would you really get virgin consolidation beneath the center only (I would doubt it)?
Here I am with my gray hair thinking of a half-dozen ways I could handle this, but would like to hear from others. . .
f-d
¡papá gordo ain't no madre flaca!





RE: settlement beneath a column footing
RE: settlement beneath a column footing
If we consider the footing to be rigid, the "actual" settlement cannot be what is calculated from the center stresses, as the tendency for this settlement to be realized would result in stress re-distribution toward the edge/corner. At some point the soil-structure interaction would normalize to one amount of settlement.
I would tend to agree that some weighted average would be appropropriate, but would be interested in knowing if someone uses this in practice and if it was a calculated method. I would tend to think that 80/10/10 would be a conservative approach, based on "feel". Is there something more refined?
Here is one thought - just attenuate the stresses at 1/2:1. This simplified approach does accomplish one goal, it makes the entire stress below the footing (i.e., within the 1/2:1 envelope) uniform. As a result you'd avoid some varying computation of settlement below the otherwise ridid footing.
f-d
¡papá gordo ain't no madre flaca!
RE: settlement beneath a column footing
RE: settlement beneath a column footing
I take the width of the footing and add to it the depth to the given layer center as the dimensions of a square or rectangle which area is divided into the column load.
When structural engineers or others need to compute stresses at different depths below footings, I have given them this method and they thrive on it due to simplicity. It's not the 60 degree thing, but close. This would be for a case of a less compressible layer over a weaker one where some limiting maximum stress on the weak layer controls what the footing design pressure might be at the higher elevation.
My comps for the subject case came out at: 757, 395,242 and 163
For a commonly used settlement computation method on a simple computer program, I even covert rectangular footings into an equivalent area of a square footing. It usually is sufficiently in the "ball park" to be useful.
Then comes what do you do for a single pedestal water tower on a doughnut shaped footing. An equivalent square footing as plus and an interior square footing for minus numbers has been used for a rough idea for calculating average total settlement. But that does not help the structural guy wanting to know what happens with strong wind and the resulting increased edge pressures. We've generally been quite conservative with design pressure recommendations due to this factor.
What is done by others for this doughnut case?
RE: settlement beneath a column footing
f-d
¡papá gordo ain't no madre flaca!
RE: settlement beneath a column footing
RE: settlement beneath a column footing
Obviously, if the footing is rigid, there has to be a lot of stress transfer to the corners and edges to make it all settle equally. The bending stress in the footing must be greater than that calculated for a uniform contact stress. I have never understood why this is not recognized in the structural design of footings. I suspect that it may become an issue for large footings when the settlement is large. I once saw an 8-foot wide cantilever at the edge of a mat foundation for a grain elevator that sheared off, and concluded that the soil pressures under the cantilever were two to three times the average.