Octagon Chimney Foundation
Octagon Chimney Foundation
(OP)
I am designing an octagon foundation for a 550 ft chimney. One of the design references that I have states that reinforcing steel should be placed in two perpendicular directions, lets say N-S and E-W. The reference does not address the reinforcing when the wind is blowing from SW to NE. For this condition you will have two layers of tension steel at 45 degrees to the plane of bending. In the past I have used this approach for small equipment pads with minimal steel, but I do not think it is appropriate for this design. Does anyone have experience or other design references on this type of design.






RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
You should be able to find a reference (octagonal footing) in one of the 5 ACI books (details for all of the code committees) where (I think) they have a design for silos.
I have designed many elevated concrete tanks with concrete stems & concrete footings. I usually design the footing as a circle (I do not have to take in the problems with orthoginal direction of reinf & non orthog direction of overturning moment). I use a combination of both radial & circumferential reinf. using a method (reference is years old & originally developed for chimneys) that calculates circ & radial moments for the inner & outer sections, shears & deflections.
Chimneys have a greater problem of M/Z stresses compared with elevated tanks, because the chimney usually has a relatively small 'footprint' for the footing (smaller radial dimension compared to a tank footing). Hence footing design is more critical (& a larger proportion of the cost of the structure).
The problem with radial reinf (in a circ footing), is that the moment does not progress (change value) at the same rate as the circumf dimension increases (& radial reinf area/metre decreases). You end up with problems of where, how & for how far do you lap the reinf. This can be a problem (inefficient) with very large reinf.
Problems of reinf 'crossover' in the centre of the footing, can be overcome by using the squart root (2) relationship for the centre moment (usually much smaller value than the max moment further out in the footing) for moments in the 45 deg direction.
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
Gaylord and Gaylord show radial reinforcing for a circular foundation and cross reinforcing for octagons, but radial may be the better approach for our design.
Thank you for your response.
RE: Octagon Chimney Foundation
RE: Octagon Chimney Foundation
Chimney Foundations by John Smith & Max Zar.
Journal of the ACI for June 1964.
Title No. 61-39.
There is also design methods for footings on piles.
Lots of graphs & tables (obviously pre computer analysis)
RE: Octagon Chimney Foundation
If you think of your reinforcement as being in the x and y directions. All points of soil pressure have an x coordinate and a y coordinate giving the respective moments from each.
If this doesnt work, then how do flat slabs work under localised load? They only have reinforcement in two directions!
csd
RE: Octagon Chimney Foundation