I've done a lot of Octagonal foundations for monopoles or vertical vessels or such. But, never really done circular. They're pretty similar, except that octagons are easier and cheaper to form and build.
I've worked on clarifiers at wastewater plants which are very large round foundations. If you choose a round foundation, use a radial reinforcing pattern instead of a grid pattern if possible. The grid pattern will generate ALOT of different bar lengths which can slow construction and lead to misplaced bars.
Personally, I have never done any circular foundation before but the closest is an octagon. You can refer to PIP STE 03350 Vertical vessel foundation design guide for octagonal foundations.
In past i have designed circular foundation . Typically chosen for vessel and circular tank foundations and circular water tanks. If bending moment governs , the affective area could be find from foundation books ( one of them Principles of
Foundation Engineering by BRAJA. M DAS ).
The reinforcement would be radial and tangential bars.
If you write WIND TURBINE FOUND. and search the web, one of the outcomes ;
Sometimes you just need a circular foundation particularly if you have a center drain but agree octagon is very common especially for aboveground storage tanks
You need a circular foundation .. when you have a circular tank. Vertical vessel or wind turbine, probably not.
Using octagons you don't have to waste so much time bending all those bars into circles as well. Just cut off straight bars. Easy.
Speaking of which, why not do that in circular foundations. No reason that mat steel needs to be circles. Kind of funny if you think about it. Looks like a big waste of time to me.
1503: If you are sloping to a center drain then you are better off with circular rolled bars. Imagine doing a unique V bend for every bar then placing those unique bars in the field. Rodbusters would lose it.
Steveh49:
"GC_Hopi, don't the tangential bars all need a different radius? "
Yes, they do, and the spacing for the radial bars is also variable, since bending moment diagram is variable.
There is no reason to reinforce in the radial and tangential pattern from the structural mechanics perspective. The equivalent reinforcement (accounting for slab twisting Mxy) in orthogonal directions x and y is easily found by the Wood and Armer method, or by the method by Baumann, or by a third method which I currently do not remember the name of. If you google "wind turbine foundation", you will find many such examples.
V bends. That's crazy. Why? To make them all have exactly 2" clearence or something? Talk about barbusters losing it??? No wonder.
I would put all mat bars in horizontally on bottom, and or top too as needed. If the mat surface is not level, on top or bottom, just keep the 2" minimum and let the clearence vary in other locations as necessary to keep all bars horizontal. No different than bottom mat steel when you have to overexcavate a little.
If you over excavate, do you bend the bars down in that spot? Hope not. You just run them straight and flat across.
I'd only curve the bars to fit inside a tank ringwall.
An octagon is two squares, so a lazy guy can still work comfortably.
And oh BTW please do not use radial bars, or bent into octagons. The drawing on page 30 in the reference I supplied shows criss cross bars in the mat. Do not confuse the finite element meshes shown in other diagrams as reinforcement placement directions.
The reinforcement in the image posted by GC_Hopi looks quite manageable if the tangential bars can be ordered straight and curved while tying the mat, ie if they're flexible enough compared the the radius. Then transition to XY reo at the centre when the radius is too tight and too many radial bar need to be terminated.
Octagon has straight forms but a little wasted reo in the variable lap lengths compared with radial-tangent reo.
What's the benefit from the octagon = two squares? The squares are truncated.
If the foundation is too large, one square wastes a lot of concrete, plus diagonal wind generates a high soil bearing out on the corners, due to the L^2, which really aren't necessary. An octagon has more uniform wind load from all directions. Not as good as a circle, but a good compromise.
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