Load spread under footings
Load spread under footings
(OP)
Hi ya,
A small question that I have been asked.
In routine consolidation analysis, the load spread under a shallow foundation can be taken as varying from 30 to 45 degrees. This is used to compute additional stresses induced within the clay due to the superstructure.
I have been asked why 30 to 45?..I feel this question might have been addressed before (which unfortunately I have been unable to locate).
In relation to the above, does the angle depend upon the type of soil or just the footing? Loose the soil, less is the angle of dispersion of the load?
Would someone also guide me to the relevant literature?
Thanks very much!
A small question that I have been asked.
In routine consolidation analysis, the load spread under a shallow foundation can be taken as varying from 30 to 45 degrees. This is used to compute additional stresses induced within the clay due to the superstructure.
I have been asked why 30 to 45?..I feel this question might have been addressed before (which unfortunately I have been unable to locate).
In relation to the above, does the angle depend upon the type of soil or just the footing? Loose the soil, less is the angle of dispersion of the load?
Would someone also guide me to the relevant literature?
Thanks very much!





RE: Load spread under footings
on the type of footing and the soils you will get varing distributions. 30 to 45 degrees is a good rule of thumb. Get a hold of any Foundation Engineering Book by either Das, Cernica, or Bowles. You need to look at the pressure distributions isobars that occur under footings. On excavation details I have always seen the following: With crushed stone under the footings the distribution angle is 26.5 degrees + or -. With compacted fill its about 45 degrees. This is just my experience and I cannot explain the technical differences. Not following these basic values may lead to a soil failure, as the bearing pressure will push out the soil sideways. Good Luck.
RE: Load spread under footings
The choice of 30° or 45° depends on what you are trying to solve. If you are looking at stress overlap, then it is common to use the more punitive (i.e. "conservative") angle. If you are looking at settlement, though, the answer is usually 30° or 2:1 (V:H) slope (which is the same distribution.)
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RE: Load spread under footings
RE: Load spread under footings
I am still unclear about one thing:
sands- is it appropriate to use load dispersion of 30 degrees in loose sands and 45 degrees in medium to medium dense sands?
Likewise, in clays how about the affect of void ratio?
Regards
RE: Load spread under footings
Please share your analytical results with the group -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.
RE: Load spread under footings
RE: Load spread under footings
Focht3:
This was not a part of any project, but a question in general. Once I come across any similar works, I shall update on this forum.
Yes, estimating the sensitivity of the results due to the angle of load dispersion is a good idea. Will follow on that.
I conclude that there is no "correct" assumption of this dispersion angle and it is mainly dependent on the engineer's judgement or at his fancy.
Regards
RE: Load spread under footings
My "sensitivity analysis" was intended as much as a teaching tool as anything. And by that I mean "self teaching" - if you get conflicting recommendations, run the analyses in a way that helps you see how sensitive the final answer is to a given assumption. Then you can decide whether the worry was worth all the effort -
Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.