Allowable Bearing Capacity Calculation? 4

[tonyiggy]

I am a structural engineer, Usually at the beginning of a project we get a geotech report from the owner with the allowable soil bearing capacity we should use to design our footings. However I remember form school that one of the parameters for determining bearing capacity is the size of the footer (terzaghi, vesic methods). How can the geotechnical engineer give us a blanket allowable bearing capacity if it is based upon the size of the footer? 2nd question Is the blanket bearing capacity given to us say 3000 PSF overly conservative? Typically we design all footer for the allowable BC that is in the geotech report. However would I expect an 8x8 square footer to have a higher bearing capacity verses a 3x3 footer or would there be some other reason a larger footer would have the same BC as a smaller footer?
Thanks

 

[phamENG]

tonyiggy - yes, the size does have an impact on bearing capacity. But you have to start somewhere. They have enough experience to know that the structural engineers in their area specify a continuous wall footing of x' wide 90% of the time. So they can use that as a starting point. If you read close enough, they'll probably stick in there somewhere that the wall footings have to be a minimum of 18" wide and column footings a minimum of 3' square. Those are usually based on a shear failure in the underlying soils. Conversely, larger footings will result in more settlement over time as they have a wider and deeper influence zone.

So, while from a strength perspective your 8x8 may be able to carry more load prior to failure, long term settlement will drive down the bearing pressure you want to design for. They don't necessarily equal out entirely, but they're close enough.

The level of accuracy you're looking for isn't really practical in most designs. You have to consider that the geotechs are making recommendations based on 2 or 3 samples of soil from a site of several acres in most cases. They are providing generalizations specific to the site.

 

[MTNClimber]

Chicken before the egg or egg before the chicken? If I asked the structural for the size of the footings (not footer), they would have to ask for the allowable bearing capacity. We have to make assumptions for shallow spread footings. If it's a mat foundation or we expect very large loads we would have to do more coordination with the structural. Luckily most building loads are easily estimated and sometimes we include that assumption in our reports.

Just as important is the estimated settlement which actually controls the allowable bearing capacity. Typically structures can only tolerate X amount of settlement which always happens before you hit the ultimate bearing capacity of the soil.

 

[oldestguy]

In my experience practically all of the jobs had reports based on the building proposed as well as approximate range of column or wall loads. If your report was a general thing, I'd go back to who wrote it and describe your job with approximate loads and ask that the report be revised if necessary for your situation.

 

[tonyiggy]

Thanks for sharing your knowledge with me. I appreciate it.

 

[emmgjld]

oldestguy nailed it.
The structural, at the BEGINNING of the project, must determine if any provided reports are even applicable.
Do any existing reports need re-evaluation based upon the proposed use?
Do the existing reports even provide what kind of construction is anticipated, at the time of writing?

 

[BigH]

Another item to consider, and we did this for many projects with desicated crust overlying less stiff clays - is that we would give a chart of "allowable bearing pressure" (not capacity) for different footing widths - and if necesary another chart depending on adjacent footings. Remember that seldom does the bearing capacity rule (i.e., a strength derivation) as settlement is typically of the most concern, namely allowable bearing pressure. A case in point where the permissible settlement of a structure is 10 mm. This would demand an allowable bearing pressure far less than the allowable bearing capacity and even the more often used allowable bearing pressure that is based on 25 mm or 40 mm (depending on country and practice).

 

[Settingsun]

BigH, do you express allowable bearing capacity in units of pressure? How clear is it in your reports that allowable bearing pressure and capacity are not the same? I just searched and the first few websites didn't seem to draw this distinction. Allowable bearing capacity included restriction of settlement on those sites.

 

[EireChch]

The values presented in any report should be the lesser of shear (capacity) and settlement (bearing pressure).

I have always been thought and understood it as capacity (in kPa, psf or whatever) is shear and allowable bearing pressure (units as before) is settlement.

Both should be in units of pressure

 

[BUGGAR]

The geotech should give some type of foundation on which the recommended soil bearing is based. Such as "shallow strip footing" or...

 

[fattdad]

yes, the report should stipulate the type of building, the extent to which there is below-grade construction and anticipated wall and column loads. The bearing capacity and settlement assessments should wrap these stipulations into the preliminary engineering for the project. The structural engineer should note these stipulations when reviewing the report and alert the geotechnical engineer if any of the geotechnical report claims are no longer valid and address these as the design is finalized.

f-d

ípapß gordo ainÆt no madre flaca!

 

[EireChch]

We typically provide bearing pressures (based on shear and settlement) for footings from 1,2,3,4,5m wide and 1,2,3,4,5m deep. For rafts we provide 5,8,10,20m wide. I think this is undoubtedly the best way to do it.

I use a macro in my spreadsheet so once you input the soil profile and settlement tolerance it it give bearing pressures for each condition.

It also give modulus of subgrade reaction which you structurals love.

Also, footing not footer!!!

 

[CT9904]

hello, if I may be allowed to add to OP's question, from what i remember bearing capacity in addition to being dependent on soil and foundation characteristics also depends on characteristics of loading such as the base overturning moment and resultant load inclination from lateral loads. so my question is, how do you account for them when providing allowable bearing capacity recommendations (blanket single allowable bearing capacity recommendation or allowable bearing capacity recommendations for expected footing sizes and depth)?

 

[EireChch]

You dont account for them, recommendations would come with a disclaimer of no eccentricity of load has been considered etc.

 

[oldestguy]

For a situation where the borings, etc. are done for a structure with those sort of loads, yes you account for them then.

 

[r13]

mattend,

You should let the consultant know the type of structure under consideration - building, retaining wall, pole, tower...The geotechnical engineer then will perform investigation, sampling, testing in according to the type of the structure, and anticipated type of loads, and provide relevant soil strength accounting those concerns.