Pile cap reinforcing steel
Pile cap reinforcing steel
(OP)
I am reviewing designs of a piled foundation for a large absorber vessel at a power plant. The pile cap is an 80 ft diameter octagon foundation, and the reinforcing steel is spaced radially about the center. The bars are terminated about 16 ft from the center to avoid congestion. At the center portion bars are then shown in a N-S /E-W grid, overlapping the radial bars.
I have not designed an octagon of this size, but I am used to running bars in a NS/EW grid only. Has anyone used radial reinforcing before? Is it a cost savings over using a grid pattern? It appears that it would be a nightmare for the field to place these bars radially.
I have not designed an octagon of this size, but I am used to running bars in a NS/EW grid only. Has anyone used radial reinforcing before? Is it a cost savings over using a grid pattern? It appears that it would be a nightmare for the field to place these bars radially.






RE: Pile cap reinforcing steel
I would expect the radial placement to reduce the total required amount of steel, but increase the fabrication costs. The long circular loops needed to provide steel in the circumferential direction are not cheap or easy to produce for typical bar sizes and cannot be shop fabricated and transported (unless you use smaller curved bars with laps, which negates the material savings).
Good luck,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Pile cap reinforcing steel
IMHO, radial bars in an OCTAGONAL foundation do not make any sense at all.
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RE: Pile cap reinforcing steel
An octagonal foundation, given the eight sides, is quite close to a circular shape. The same circular reinforcing simplifies the detailing by "pretending" the closest edge is the outside circumference of a circle, and detailing appropriately. The remaining steel is trimmed and lapped as for an outside return corner. This actually works quite well, and although I have not personally done this, I have seen it detailed.
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Pile cap reinforcing steel
But the octagonal foundation in this question has an inscribed radius of 40 ft (I assume that is what the thread poster means). The circumscribed radius is 43.3 ft. That 3.3 ft difference is quite a bit to leave it up to a Contractor to figure out how to reinforce in the field.
Of course what you propose can be done, and done well. But I would rather have a better documented design.
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RE: Pile cap reinforcing steel
Glad to know we're on the same page...
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: Pile cap reinforcing steel
RE: Pile cap reinforcing steel
Can I ask how the piles are arranged? In groups or in rows? And their spacing?
How is the load coming from superstructure?
RE: Pile cap reinforcing steel
RE: Pile cap reinforcing steel
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RE: Pile cap reinforcing steel
I am designing a piled octagon foundation for a 75 ft diameter x 53 ft high raw water tank. It appears that there will be less reinforcing steel using a radial pattern versus a grid pattern.
RE: Pile cap reinforcing steel
Total weight Q = 20000kips
Diameter of vessel = 70'
Base area A = 0,25*pi*D2 =3848ft2
Total uniform base pressure w = Q/A ~ 5200psf
I understand the piles are arranged in a circular fashion like rings and probably equally spaced (within a ring).
Then one can take a segment long the radius and assume it to be a continous beam with piles as supports and uniform load equal to w*b where b is the tributary area. If we do like this then we estimate top/bottom reinforcement radially. Otherwise I cannot imagine how reinforcement is calculated in radial lines.
There are few problems with this type of analysis:
1) Considering that the cap is atleast 2ft thick or more, we may not be able to calculate this as a beam unless the pile spacing in radial direction is atleast 10ft.
2) The behaviour of the cap will depend on its stiffness and pile spacing. There may be some two action in the hoop direction.
It would be nice to see the behaviour in a FEM model with piles as linear spring and loaded with uniform pressure.
RE: Pile cap reinforcing steel