Limiting Foundation Width
Limiting Foundation Width
(OP)
Hi all
I am a structural engineer with I think satisfactory understanding of soil mechanics , I got soil report for geotechnical engineer for design of a foundation , there was a note in the report stating that Isolated footings or strip footings shall be used with maximum width 3 Ms.
Just I wonder why the geotechnical engineer limited the foundation type and its width?!!
The foundation type is shallow with Allowable bearing capacity 110kN/m2 and recommended foundation level is (-1.50Ms) at this level and under Soil was giving results around 25 and increasing for SPT. Borehole depth is 10Ms.
I am a structural engineer with I think satisfactory understanding of soil mechanics , I got soil report for geotechnical engineer for design of a foundation , there was a note in the report stating that Isolated footings or strip footings shall be used with maximum width 3 Ms.
Just I wonder why the geotechnical engineer limited the foundation type and its width?!!
The foundation type is shallow with Allowable bearing capacity 110kN/m2 and recommended foundation level is (-1.50Ms) at this level and under Soil was giving results around 25 and increasing for SPT. Borehole depth is 10Ms.





RE: Limiting Foundation Width
He may not be limiting the foundation type. This may just be a poor choice of words. He may be alluding to "If" footings are used, limit the width to 3m.
Ask the geotech for some clarification. He probably has read that line so many times it makes sense to him, and he doesn't realize it could be misinterpreted.
RE: Limiting Foundation Width
RE: Limiting Foundation Width
B.c.=110kN/m2
Max. Allowable settlement =25mm
RE: Limiting Foundation Width
Not sure where this sort of a statement comes from. It would be assumed that the load does not change as you vary footing width.
Per UcFSE
'as you increase footing size you decrease bearing pressure but increase settlement." THIS IS NOT TRUE. For the same loading and with a wider footing, Bearing pressure is less and the setlement is less. That's the controlling factor (settlement).
If the statement were true, one would use very small footings. We widen footings to decrease settlement potential. For closely spaced load points not much is gained when overlaps of stress bulbs occur however.
The best advice is to get the report writer to explain the statement.
RE: Limiting Foundation Width
RE: Limiting Foundation Width
Also, I agree with McCoy. If there are lower lying loose or soft layers, then the depth of influence of wider foundations may cause additional settlement.
It is also possible that they want to limit the influence of the foundation to the depth of the boring.
Did they recommend deeper borings for larger loads?
RE: Limiting Foundation Width
Depth of borehole was 10 Ms in the three reports, I wonder if he is limiting foundation width to borehole depth!!
Is not 10 Ms enough as a borehole for shallow foundation?!
About loads I have is that max. Load is 2100kN where 4.5x4.5 square footing is enough. The building is only 2 stories. other problem is if I have a concrete core for lift which is not heavy loaded at all but the foundation must be sized for each dimension so that it is 5.00Msx5.00Ms this was considered unaccepted by other firm which is checking our work because they said the statement I mentioned about footing width, there refusal was not because of technical reasons but just soil say this (just people dealing with papers)
RE: Limiting Foundation Width
RE: Limiting Foundation Width
Let's not get all mixed up with zones of influence until you compute the ratio of added pressure to existing pressure as far down as you think is important, not just to bore hole depth. That is what is important and causes the settlement A high strength layer over a soft one may tend to distribute the stresses better below, but I doubt that that was the reason. See next para here.
I strongly suspect the report writer uses a boiler plate report that he or she just edits for each new job report. When in a hurry a lot of standard language comes along. I've made the same mistake a few times.
RE: Limiting Foundation Width
Oldestguy is absolutely correct in that this must be a boiler-plate recommendation. We all have them and use them; but it shows the absolute necessity (to avoid embarassment) of thoroughly confirming that the boiler-plate recommedations make sense for a specific project.
RE: Limiting Foundation Width
RE: Limiting Foundation Width
here is another point what if i used isolated footing following his regulations and i have maximum width of it to be 3 Ms but the distance between them 20 cms?!
RE: Limiting Foundation Width