huangyhg

h pile to pile cap connection
i have typically specified the connection of steel h piles to pile caps to be 6" embedment. the question that has always stuck in my mind is: how does one analyze the connection? a quick check of the bearing of the concrete on the area of steel alone typically gives a stress that exceeds the allowable bearing stress of the concrete? thoughts???
a book that i have called steel h piles by bethlehem steel (jan., 1970) states that historically many engineers have specified steel plates as caps, however, it has been shown by tests that they are not normally required for downward gravity load if the pile cap is designed properly and there is adequate confinement of the pile. anchorage may be required for other forces such as uplift.
refer to research report no. 1, "investigation of the strength of the connection between a concrete cap and the embedded end of a steel h-pile", state of ohio department of highways, 1947. more recent investigations have also confirmed the ohio test results.
it is ultimately your choice to conservatively use a cap plate or analyze the research.
a short unreinforced embedment is typical. the connection should be modelled analytically as pinned. as you've noticed the concrete can't come close to developing the bending strength of the pile, and generally doesn't need to.
you can download the .pdf version of "bethlehem steel h piles" from this page of my website
from 1982 aisi publication "the steel pile pile cap connection" (probably out of print):
1) "the elimination of the cap plate facilitates placing concrete around the embedded end of the pile and aids in achieving a better density of concrete in that region of the pile cap".
2) "steel compression piles embedded a minimum of 6 inches and with minimum edge distances recommended in table 2 do not require steel cap plates or any other auxiliary bearing device". (table 2 shows minimum edge distance from center of pile to face of pile cap = 12" for working load less than 110 tons, 16" for 110 to 140 tons, 21" for more than 140 tons).
3) i have an old (1987) copy of nyc building code that said "cap plates will not be required for steel h piles embedded in a reinforced concrete cap".
jec67 - i have also pondered the same thing and believe it is best thought about through my "fat lady with high heels" theorem. it goes like this - say you have a 300 pounder clomping down the hall in an office building with high heels having a 1 square inch bearing area per heel. as one foot comes of the ground leaving her considerable weight teetering on a single heel, doing the math you can calculate the local bearing pressure at an astounding 43,200psf, or 432 times the say 100psf live load of the floor and locally certainly well over the bearing stress of the concrete. but as i am sure you have noticed there do not appear to be many office floors covered with tiny high-heel holes.
so what does that have to do with a pile? - well, the reason we don't have little holes all over are floors is that either 1. not too many 300lb ladies wear high heels or 2. the combined effect of concrete confinement and very localized forces greatly increase the allowable capacity bearing capacity of concrete at the point of load application by allowing it to comfortably redistribute within the surrounding area.
if you want to get a feeling for bearing pressures, confinement, and what is achievable, you might look into the design post-tensioning anchorage zones.
43,000psf local pressure is not too bad for concrete. it is within the uniaxial compression strength. no to mention the material strength with tri-axial compression. like the needle on an old gramophone disk, the load is a pound, presumably, but the bearing area is almost zero. also like a point load applied to a massive bedrock.

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