Pile cap reinforcing steel

[BENDOG]

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.

 

[youngstructural]

Radial placement is a very, VERY old idea... It's not very hard to place, so long as the contractor gives the how of his job some forethought.

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...

 

[SlideRuleEra]

Radial bars in a CIRCULAR foundation make a certain amount of sense - they all can be the same length. The way that I have seen radial bars used (in a circular foundation) is to half the number of bars (think of them as spokes in a wheel) as they get progressively closer to the center. This can be done as many times as necessary to keep radial bar spacing above the required minimum. Of course, the bars in each hoop will have a different curvature, but this is manageable.

IMHO, radial bars in an OCTAGONAL foundation do not make any sense at all.

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[youngstructural]

SlideRuleEra: First time I have ever diagreed with you...

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...

 

[SlideRuleEra]

YS - It's a matter of scale, for a smaller foundation I agree that an octagon is a close approximation of a circle. I can see where radial bars in a small octagon would be efficient.

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.

http://www.SlideRuleEra.net

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[youngstructural]

Ah, fair enough... However I did not mean to imply that the detailing of the "Arc Segment" edge be left to the contractor. My apologies for any lack of clarity, however I am advocating providing this detail, and otherwise using a circular layout. I did say that I have seen this detailed well previously.

Glad to know we're on the same page...

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[BENDOG]

To simplify things for the contractor all of the radial bars are the same length, although some will extend closer to the center due to the octagonal shape of the foundation. The spacing is about 12" at the edge and tapers to about 5". I think that I need to do a steel take-off to see if I am saving by using radial steel versus a grid layout.

 

[Hanif08]

Interesting situation.

Can I ask how the piles are arranged? In groups or in rows? And their spacing?

How is the load coming from superstructure?

 

[BENDOG]

There are 128 piles arranged in various circular patterns. The absorber is a flat bottom vessel 70' diameter x 130 ft tall, operating weight about 20,000 kips. the jobsite is in W.VA. so there is low seismic activity.

 

[SlideRuleEra]

BENDOG - Normally, I don't get into issues that were not asked about, but... be sure to check the rigidity of the foundation. Even if the foundation is satisfactory for all loading conditions (from a strength point of view), it may be too flexible. Certain piles may be overstressed during wind loading since the forces are not being distributed properly to the piling. Of course, the easiest way to make the foundation more rigid (higher moment of intertia) is to make it thicker. Six to eight feet thick for a 130' tall absorber vessel on an 80' wide foundation would not be a surprise.

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[BENDOG]

SlideRuleEra - the pile cap is 6 ft thick with #10 bars top and bottom. I should comment that I am reviewing the drawings not the calcs. Calcs were done by others. Thanks for your comment.

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.

 

[Hanif08]

Based on information you have provided, I think the designer has made some simplifications. I envision something like this:

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.

 

[Hanif08]

Oops I did not read your earlier post that pile cap is 6ft thick! If the pile spacing is also relatively small then I anticipate the plate to act like rigid body pushing on the piles with only serious consideration to punching shear. I don't think there will be bending as we know from beam theory.