huangyhg

formula for cylindrical foundation
i have a background in architectural engineering technology and am currently doing estimating for a site contractor and working on a job for some signage for the naval wepaons station. it's been some time since i have done structural calcs but have a need to run some numbers to determine 2 footings. . i would like to be able to take the square footage of the sign face, apply a 130 mph wind load on it, and determine the size of the 2 footings. a construction management firm got the bid and we were awarded the job. i want to double check the structural firm that the cm is using since we are buying the concrete. i'm planning to use a 4x6x1/4" steel tube for the posts as that is what an existing sign was mounted on. any constructive help would be appreciated.
to determine the footing size an engineer needs to know, at the minimum, the size of the sign and how high it is from ground level. also, the location of the project because if located in a high seismic zone, there is a requirement for all footings to be connected.
bgriddle,
there is a lot of information for cylindrical foundations - we just call them drilled shafts. basically, you have two modes of failure assuming the signage is relatively light: 1) lateral sliding, and 2) overturning.
lateral sliding resistance is applied as a truncated triangular passive pressure distribution against the full width of the drilled shaft. the truncation comes in above the frost penetration depth.
overturning resistance is provided by passive pressure resistance also, but imagine that the passive pressure envelope is twisted around the shaft such that the point of fixity (zero displacement) is between the frost depth and the bottom tip. typically, a factor of safety of 1.5 is applied to the design moment.
jeff
caisson footings depend 100% on the existing soil conditions as outlined in the geotechnical report.
there is no simple formula.
i design a lot of these types of signs and the caissons always end up deeper than the contractor thinks they should be.
i doubt the engineer is going to revise their design just because you question it. i think your only viable option is to consult another engineer to provide an alternate design (if they believe they can save you money).
csd

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