Can somebody give me a clue on how to determine the quantity of reinforcement to be put in a bored pile subjected to shear force and large bending moments ?
Classical approach would be to count a reduced concrete section and concrete strength, then determine rebar, or just the reduced strength. Stay within safe cover for rebar or else you are in a steel encased pile where again, its capacity will be diminished at some rate of corrossion along the life of the structure.
If shear and bending moment control, it is more like a beam than a pile, in which case you would design it as a beam. But if the axial load is also high, you would design it as a column. So use column interaction for axial load and bending, and beam shear analysis for the shear.
hokie - but if you design it as a beam (your first case) - are you designing it as a fully supported beam or partially or .... Just wondering. Would you design for full depth reinforcement - or just that affected by the moment??
BigH,
I assume by "fully supported" you mean braced by the soil. I wouldn't worry about bracing for a circular pile acting as a beam. As for the reinforcement, I would just provide reinforcement where the pile is bending. Below that, I would treat it like any other bored pile.
After the shear forces/bending moments and axial forces have been calculated, design the pile as a column. What codes are you designing too? The Australia piling code allows the minimum reinforcement to be reduced from 1% (min reinforcement when designing as a column to the Australian concrete code) to 0.5%. Also, depending on the supervision/workmanship of the pile, the Australian piling code also requires and additional material reduction factor to be applied, that is an additional 0.75 applied to the 0.6 to 0.8 that is already applied to a pile. I have always assumed that the soil provides full restraint to the pile and therefore the pile is designed as a short column. Shear forces should be checked but I have never known shear forces to be critical in the design of 4-5' diameter piles.
If shear reinforcement is critical make sure that the reinforcement is fully developed by providing an additional loop of the helix.
Asixth,
You can also have an unreinforced pile/pier:
"An unreinforced pile shall be permitted where the design
action effect does not exceed the design strength calculated for plain concrete members in
accordance with AS 3600"
Thus as long as your pier meets the requriements of your concrete structures code you all good, ie, beam or columnas fully restrainted per Hokie66, unless you have quick sand or an abnormal situtation.
I design pile reinforcement everyday- determine moment and shear distribution and design like a beam-column. I use CSIcol or PCAcol w/ factored loads to determine the amount of reinforcement. Newer codes address the minimum cage length and shear reinforcement, and the need for fixity at the pile top. All designs should be fixed head unless the pile cap will rotate.
The fixity of the pile to the pile cap I believe comes from earthquake requirements. thus if you not in a earthquake zone, Like myself, fixing the pile head is not required.
You are correct - based on IBC 2006, cap fixity is not required unless you have a SDC=C or worse. However, if the pile cap & pile head do not rotate, fixity will be implied and the maximum moment will occur at the pile top. I never understood how you can design for free head conditions if the pile cap or pile top does not rotate. Either they both rotate or they don't unless you have a true hinge at the connection. An example of a true free head condition is a single pile supporting a cell tower.
kleo,
It is very hard to get a fixed end connection with timber piles, here you have to assume pin type connection, or have a nice deep pile cap. For concrete piles that are assumed to be pinned at the top, you generally only supply 50mm embedment. Some people provide enough reinforcement to ensure ductility, thus that the connection can go into plastic deformations but be assumed as pinned. Some don’t.
