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STEEL CAISSON PILE

STEEL CAISSON PILE

STEEL CAISSON PILE

(OP)
I am designing a concrete filled caisson pile, with a steel core using 150 ksi threaded rod as the core, and steel pipe having a yield stress of 100 ksi

ICC (I am looking at the NYC edition) tells you to use 0.33 Fy for the concrete, 0.35 Fy for the steel pipe and 0.50 Fy for the steel core.

My question is:
To determine total capacity of the pile,
if the steel pipe and steel core have different yield stress, do you have use the lower yield stress (which in this case is 100 ksi) for both the pipe and the steel core, or use the values for the two steel materials as they are.

RE: STEEL CAISSON PILE

You want to check your strains in the material. In order to carry a partucular strees level, the material needs to acheive a certian level of strain. This is function of both the elacticity and the geometry of each component.
Say the load in the cassion is 200 tons and you assume the steel, based onmaximum allowable stresses can carry 150 tons and the concrete can carry 5o. (I know the mathe doesn't jibe with ther OP but work with me here) If your anticipated deflection in the steel is an 1/8 inch for the 150 tons, but at a 1/16, the concrete has reached its allowable stress, by the time it deflects the next 1/16, it will be overstressed.
Since the steel components and the concrete must deflect equally, all the components must be below their respective allowables at the anticipated strain.
This is a problem on small cassions and minipiles where load sharing is anticipated, but smaller or high strength members require significant deflection to acheive stress levels anticipated.  

RE: STEEL CAISSON PILE

DRC1 is correct, the steel and concrete have to shorten the same amount, thus the load sharing has to consider strain compatibility.  One question, where did you get the fy values for the steel components?  They are very high and the material is probably not very ductile.

To answer your specific question, as the core and shell will shorten the same amount, the maximum steel stress will be limited by the material with the lower yield.

The code you are using probably allows a greater contribution from the internal reinforcement because it is so well confined.

Widespread use of large concrete filled steel columns (I know yours is a pile, but that is a type of column) is a relatively new thing, say 20 years or so, and there has been a lot of recent research into this area.  The code provisions you quoted are probably very conservative as to the load carried by the concrete portion, as the concrete benefits greatly from confinement by the shell.

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