Footings
Footings
(OP)
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
Thank you
tcarr1077





RE: Footings
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.
RE: Footings
RE: Footings
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.
RE: Footings
RE: 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.
RE: Footings
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
RE: Footings
RE: Footings
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.
RE: Footings
RE: Footings
RE: Footings
RE: Footings
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RE: Footings
RE: Footings
RE: Footings
Richard A. Cornelius, P.E.
WWW.amlinereast.com