settlement from vertical and moment loads on mat foundation 3

[danyul]

hello
can someone help with this problem?

16'x16' "mat" foundation on clay very soft to 18 feet then stiff. several cases: vert load of 210kips, then vert load with seismic moment of 722ft-kips.

vertical settlement is not a worry but rather "tilting" is more of a worry.

i need to calc the differential settlement with both the moment and vert loads to see if it is allowable.

do i just find the qmax and calc settlement under that end with that pressure? and assume the other side is zero?

thanks

 

[BigH]

Remember that unless dynamic (live loading) is more or less continuously applied, it will not really lead to significant consolidation settlements (e.g., seismic) - now seismic loading may have other effects due to liquefaction, etc. - but these would be addressed separately.

 

[danyul]

i see...

is there a way to figure out the settlement due to a moment that is applied continuously?

 

[civilperson]

Sure, find the soil pressure at the base of the mat. This will either be trapezoidal or triangular in shape from low pressure to high pressure. Calculate the settlement for various strips of equal pressure and compare.

 

[fattdad]

If you have a mat with eccentric loading you can calculate the equivalent footing size from the resultant.

example: You have a 16 ft square footing that supports a dead load of 210 kips. That's 820 psf. If you were to consider 722 ft-kips that's a moment arm of 3.4 ft, which would reduce your footing size from 16 ft square to something of about 11 or 12 ft square (you'd have to use Pythagorus (sp)). Let's say 11 ft. That would then result on a bearing stress of 1,736 psf. You could then consider the settlement from 722 psf and then from 1,736 psf and look at the contrast.

Just a first thought. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[danyul]

ok sorry i meant to say differential settlement. so if i find the settlement at 722 psf and at approx 1736 psf that will be the settlements on each end of the slab?

so say the 722 psf end has 1 inch and the 1736psf end has 2 inches then the differential settlement would be 1 inch over the 16 feet?

i was thinking it would be zero inches on the zero qmin side and therefor 2 inches over the 16 feet...

 

[danyul]

i just noticed it would be the settlement of 820psf not 722psf

 

[BigH]

A point Danyul - just the way I am - but don't see how anyone "reports" loads to nearest 1 psf. Gives impression of far too much accuracy than reality. Also, don't forget that if you have the "E" value for your immediate settlement or your consolidation parameters - that if you are within 25% of actual on your estimate - you've had a good day.

 

[SEAGROUP]

HOW WE CAN CALCULATE THE SUTTELLMENT FOR MAT FOUNDATION IF THE BOR LOG IS AS FOLLOWING
1-2m, Depth Silty sand with gravel
2-4m, Sitly Clay
4-5m, well graded gravel
5-10m, Silty sand with clay

 

[BigH]

seagroup - would be better to have started a new thread - however, you need to give much more information than what you did.
1. What is founding detph?
2. What is characteristic N values
3. What is the undrained shear strength estimate for layer 2?
4. What lies below 10 m - a mat foundation is usually "large" - say 10 to 15 m or more in least dimension - and the depth investigated (as given) do not extend deep enough.

Just a start -

 

[fattdad]

Just to agree with BigH's last post.

I'd also add, what is your soil classification method? There are pros and cons to ASTM soil classification, but here's an instance where it would be helpful. In the ASTM there would be no classification, "silty sand with clay" and the term, "silty clay" is pretty rare. For me, knowing atterberg limits and natural moisture would be good for your layers 2 and 4. Also, is this alluvial, glacial or residual terrain?

On the matter of whether you are deep enough with your site evaluation, I'd likely agree there too. If you are considering a 100 ft by 100 ft mat foundation, I'd want data to greater depths, or I'd like to know from local experience that the "seat of settlement" is unlikely to extend below some depth (i.e., there is a regional till layer typically below 50 ft or the depth to bedrock is about 70 ft, etc.).

Good luck and if you have more information, just post it.

f-d

¡papá gordo ain’t no madre flaca!

 

[Riggly]

danyul,
How many soil borings do you have at that site? My approach would be to take a close look at the soil profile, and look at the variation of the thickness of the clay layer, STP etc, within the influence zone of the foundation. Calculate both the immediate and consolidation settlement depending on the soil profile. The variation between your results my indicate your possible differential settlement.

Ensure a good layer of granualar material beneath the foundation to distribute the load on the foundation, to alleviate some of the potential such settlement problem.

Why don't you consider deep foundation? For the depth of soft layer you mentioned, sound like a good candidate for deep foundation, such as auger cast pile.

Good luck