Footings

[tcarr1077]

I am currently working on a project to come up with a footing design for a prefabricated cloth covered garage structure. The loads from the structure are transfered to the footing through legs of the structure spaced at 16'-8" OC. There are 13 legs per side of structure. Each leg takes 37 kips. The site has poor soil condtions with a 1ksf bearing capacity. This projected is to be constructed by a local town with minimul construction resources and a low budget. My question is how could I design this meeting the above requirements. Can it be designed as a continous strip footing or since it has concentrated loads does it have to be designed as a combined footing? Is it to underconservative to poor a continous footing but design it as isolated footings? What are the problems associated with these options. I am new to the engineering field and dont have much guidance, I could use a little guidance.

Thank you
tcarr1077

 

[ishvaaag]

Knowing how to reinforce plates, it will be better analyzing the thing as a plate foundation on springs.

Also be attent to moments coming from the constraints at the feet of the columns if not pinned, and to any horizontal reaction to the columns.

 

[tcarr1077]

Thank you for your response. I am not familure with analyizing plate foundations on springs is there any suggested literature for this subject?

 

[Ron]

tcarr...I would suggest that you get a more experienced engineer on a local level to help you through this. If you are "new to the engineering field" I presume you are not licensed as an engineer, which for this undertaking, you would certainly need to be.

You have a large structure, poor soil conditions, an overturning consideration, settlement and bearing issues. Further, this is going to be a "public structure"....all of these lead one to a local, licensed structural engineer.

 

[hokie66]

The design of the roof structure has a large impact on the design of the footings. Depending on where you are, wind uplift could control. Agree with Ron, this is not simple enough for a newbie without guidance.

 

[DRC1]

Assuming you have the correct loads, and the loads are fairly constant, I would think you could design as 8x8 footings and then extend as a strip. If load is variable, a little more analysis would be required. Get a copy of Bowles Foundation Analysis & Design. It will explain the basics of geotechnical analysis and structural design for spread and continuous footings.
Echo the strong suggestion that you have a PE supervise any work that will be incorperated into design. However, unless we reach out to learn something new.

 

[msquared48]

I agree totally here with Hokie. Wind uplift will control the volume of the footings, but the downward loads the footing footprint.

Being a cloth covered structure though, how much latitude, and ultimately, structural concern, do you have with differential settlement here? If you have a concrete slab, why not pour it integral with the footings and employ a ribbon beam at the edge of the slab to help distribute any settlement.

I really do not see this as a difficult problem.

Mike McCann
MMC Engineering

 

[BigH]

If the site has poor soil conditions (am assuming clay) - why not just preload the site for a couple of years (if they want to stretch the budget - you stretch the time - bit by bit (save - invest etc). Maybe you can decrease the likely settlement, increase the allowable bearing capapcity and then get an experienced engineer to help you. - or why not drive piles? or, if the poor soils are not that thick, do some large hand dug caissons down to better soil (of course, being safe in doing it) - lots of different toys you can play with - but you, as the others say, will need an experienced engineer to help you.

 

[ishvaaag]

The analysis of plates on springs is by now maybe (other than outright footings) the more usual way to design foundations. Springs are used mainly as a substitute for the actual behaviour of the soil, and are derived from ballast factors taken from geotechnical reports.

Albert Fuentes gave 3 ballast factors

0.5 kgf/cm3 for bad soils like yours
4 kgf/cm3 for middle strength soils (working stress, service level 2 kgf/cm2)
12 kgf/cm3 for strong soils.

These surely were k30 ballast factors based on tests sinking 30x30 cm plates (the text as I had it in spanish didn't precise). These values would need be corrected by some formula to a lower value for practical foundations, depending on soil and dimensions. I have found in practice that just taking such values directly for the analysis produces more consistent correlation with the observed behaviour. This surely a fruit of the general conservative approach of the geotechnical sciences when talking values.

It might also be argued that when doing analysis in FEM and being the elements small, taking a ballast factor on the overall width of the foundation be incorrect, but I do NOT have seen this being stated nor practiced in programs, such CYPECAD. The theoretical correction of this may be ascertained from an exam of the literature that I have not time to do, yet I have above stated what is my experience. Each element with each spring reaction force, stiffness will tie everything together in the plate.

I ordinarily bracket the behaviour by taking more than one ballast factor (classes of soils) and examine how the foundation requirements vary. It uses not to be much.

Anyway, the usual approach is making a model of the foundation divided in elements within the program or in autocad or so and then putting foundation constraints and acting forces, or generate a complete 3D model, foundation AND structure. The spring constants you put are the ballast factor multiplied by the tributary area of elements. Then you analyze the model and have settlements and stresses for the plate. You can also derive (or the program directly gives) pressures in the soil at every node of the foundation by multiplying its settlement by the standing ballast factor.

 

[DRC1]

I disagree. I Do agree that the first order of bussines is to confirm that the column loads are correct and do account for wid and other loadings. However, assumming that the loads are correct, They are relatively light. The bigger concern is soil capacity. Using stanadard factors for poor soils is not a good idea. Soil capacity should be and subgrade modulus should be confirmed by a good field exploration program. Springs basically give settlement response rather than bearing capacity. If the soils are sands, most settlement will probably occur during construction. At any rate, bearing would probably be the controlling factor. This seems to be a fairly straight forward foundation problem which should not need a computer solution.

 

[ishvaaag]

I would agree more with you were not because of the loads stated a true mat foundation even of small thickness will be appearing. Anyway, the plate on springs also will care of soil pressure, and likely will show better the effects of the stiffness. I entirely agree that if you have (you should) good geotechnical parameters it is with these that you would proceed, not any standard value; I only was quoting values for illustration.

 

[bb29510]

drive pile

 

[civilperson]

I suggest a two foot wide grade beam at the perimeter designed as a beam with concentrated loads from above and maximum allowable soil pressure from below. (Try 2 foot deep for first iteration of reinforcing design). Uplift can be resisted by footing weight and soil overburden.

 

[ishvaaag]

The problem is there ara 217 tonnes at each side and working stress is about 0.5 kgf/cm2 . This on the length given is a MUCH wider slab than 2 feet. So a narrow strip foundation is not the solution; maybe going deeper may help but we have not the data.

 

[hokie66]

No, ishvaaag, you have your units mixed up. The pressure, using civilperson's solution and the given load, would be 37kips/2/16.67=1.1ksf

 

[ishvaaag]

Hokie and all others that so said, you are right I was taking 16" on centers, thanks.

 

[BigH]

Tried to post this before - but we had server problems. You might wish to explore an "improved trench" concept where you replace the bad material with good granular fill in a trench (or excavation) so that the stresses are maintained within in the improved material - see Das and Hannah, "Model Tests for Shallow Stip Foundations on Granular Soil", ASCE Geotechnical Special Publication No. 16, 1988 - for concept in a general sense.

 

[dicksewerrat]

This sounds more like a PEMB type foundation. I don't think you got all the reactions at the bottom of each "leg".

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com