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RE: Pole Embedment Formula
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Pole Embedment Formula
- From: "Cain, William" <bcain(--nospam--at)ebmud.com>
- Date: Fri, 17 Sep 1999 09:20:31 -0700
Oshin- Be very aware of the basis for the pole embedment formulas in the UBC. The embedded pole formulas in the UBC are based on specific conditions: 1. Not restrained: is assumed to be free to translate and rotate at the point of load application (i.e., it is a cantilever from the point of fixity). 2. Restrained: Some means of translation support is provided at the surface (such as a slab, etc.). It is not based on the pier not being fixed by the ground (for rotation) at this point. The pier will rotate about this point as if it has a simple support at this location. If you are framing into a grade beam that provides fixity to the pier, neither formula applies directly. You can still use the formulas if you make adjustments to your model. For example, a grade beam providing fixity to the pier forces the pier to bend in an s shape, similar to a column in a frame fixed at the top and bottom. To use the "not restrained" formula in the UBC, you must apply the load at the point of inflection (i.e., zero moment). The easiest way to visualize the solution required is to draw the deflected shapes for the pier. A simple, more direct way to deal with these problems that I have found helpful is the method outlined by E. Czerniak in a paper entitled "Resistance to Overturning of Single, Short Piles", Journal of the Structural Division, American Society of Civil Engineers, Proceedings Paper 1188, vol. 83, No. ST 2, March, 1957. The method treats the pier as rigid for depths to diameter ratios less than 10 (although it also provides a fair approximation of more slender piers, possible to ratios as high as 15). It is a general method in that one determines a moment and shear at the resisting surface (i.e., the surface below which resistance is provided by the pier. If your geotechnical engineer says ignore the top 2 feet, for example, the restraining surface begins 2 feet below the ground surface.). For more slender piers, the flexibility of the pier will affect the shears, moments and deflections in the pier. However, remember that most values given for soil or rock resistance are little better than "order of magnitude" values. So don't get too hung up on precision. I usually bound my solutions to get a feel for the sensitivity of the problem. The charts provided in the Czerniak paper simplify this effort. Regards, Bill Cain, SE Oakland, CA -----Original Message----- From: SDGSE(--nospam--at)aol.com [SMTP:SDGSE(--nospam--at)aol.com] Sent: Thursday, September 16, 1999 18:03 PM To: seaint(--nospam--at)seaint.org Subject: Pole Embedment Formula Would it be appropriate to use the restrained embedded pole formula rather than the unrestrained one for the embedment design of a drilled concrete pile in bedrock. The soil engineer has specified the point of fixity into the bedrock, and I was wondering, if the pile is assumed fixed at that point wouldn't the restrained condition apply? Since, I assume, the pole embedment formula was probably based on embedment into soil rather than bedrock. I always used the unrestrained condition in my designs, but this time I have quite a bit of embedment, so trying to reduce the embedment any way I can. Any thoughts? Thanks Oshin Tosounian, S.E.
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