huangyhg

foundation design for settlement
i am not very familiar with design of reinforced concrete yet, i really have only taken the one class in college. now, i am given the task to design a foundation for a house based on the total settlement of up to 5" and differential settlement of approximately 3". i can't find a good example anywhere. i am thinking of a just designing a continuous footing for the loading and just see what the midspan deflection will be. is this the normal approach? i should mention that this settlement is due to liquefaction of the soils, so it would be a sudden, dynamic settlement that is governing my design.
thanks for your help.
you're kidding aren't you?
we did something to that order, for a local tract homebuilder who had unwittingly located some lots on really bad soil conditions (where was the geotech?).
we ended up designing post-tensioned foundation with compressible soil criteria, and the resulting foundation had a good share of internal post-tension beams (ribbed foundation). when the homebuilder priced the construction cost, he went ballistic...i guess they provided a park area in lieu of a few houses.
at any rate, the post-tension institute has literature on this subject. there is software available (geotechnical tool kit).
thank you sacrebleu, but actually, i have contacted the post-tensioning institute with regard to liquefaction and the design of a post tension slab foundation, but i have not gotten a response from them. i did get a response from a couple of the designers of gtk software, and they do not recommend that we use the program for this soil condition. basically, i was told that the program (compressible mode) is designed for slow gradual settlement resulting from the water being squeezed out of the soil under the vertical loading. they haven't offered me a good solution as to how i should design a pt foundation for liquefaction condition.
dbuzz, i am sorry to sound so ignorant, but to be quite frank, i have not been able to find a good source to fully understand and resolve this question. like i said, i am really not experienced in design of reinforced concrete. do you think you can help me? will you?
i think dbuzz's point was that if you don't know any more about it than that, you shouldn't be doing it on your own in the first place. ideally, you learn from working with someone that knows it all.
have you tried looking for a local structural engineer? might be a lot better avenue that anonymous internet help.
i'd be curious too, about how you were "given the task". are you working for an engineer or engineering firm? who will have the professional responsibility for the completed job?
i think some data is missing to arrive at design of footing of house you are going to take up.in order to arrive at foundation design ,you need to know type of soil,depth of water table, load on walls and these are then corelated to terzaghi's formula for bearing capacity.have you got tested the bearing capacity of soil before going into the type of foundation and actual calculation of footing design?
mubash is right. before approaching such a task you should request a soil report. merely designing footings based on limiting the total and differential settlements without knowing the soil properties is not enough. once you have the neccessary information such as bearing capacity, foundation levels, modulus of subgrade reaction,...,etc. only then you can addequately design footings.
university of washington
then use grade beams, piles, and elevated first floor (wood-farmed)
if you aren't that familiar with r.c. engineering you have a lot of work to do, but you should also have a senior engineer watching over you as well. to answer your other questions, you cannot just design the footing as a simply supported beam and check the midspan deflection. the whole thing settles, it does not simply deflect in the middle due to superimposed loading. it is more of a beam on an elastic foundation problem. you should be asking your seniors these questions also.
thanks all,
i do have a soils report, soil bearing info, modulus of subgrade, etc. the project is also tract homes, but it wouldn't be a few lots, it would be more like 100 lots. i don't think that our client would have a problem with using a pt foundation, i'm just not sure how i should go about designing it. i would love to use the gtk program, but and a little leary about doing so because i was told i am not supposed to. i know that other pt designers have, and i know that some soils engineers will provide values of center and edge lift that will "come close" to what i need.
i was hoping that i could find an approach to design a pt or conventional foundation "to withstand the specified settlements", as is the verbage on the soils report, since i know there is absolutely no way the client will go with pile foundation or a mat foundation.
(by the way, my boss in on vacation-i can't put this off until he gets back).
thanks for you help.

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