Octagon Chimney Foundation 1

[BENDOG]

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.

 

[JLNJ]

No matter how you place the reinforcing or how many layeres you have the wind direction will rarely be parallel to it (them). Are you worried about the pier's ability to resolve the forces into the orthogonal directions of the reinforcement?

 

[BENDOG]

Yes. For N-S and E-W wind I can calculate my tensile steel, say "As". For wind SW to NE is it appropriate to consider your tensile steel as 2(0.707)"As"? I was going to consider 4 layers of steel, although they will not over lap in the center because the chimney diameter is very large.

 

[civilperson]

At 45 degree angle to the parallel rebar, the As is as you wrote but the spacing is greater, (1.414 x normal spacing).

 

[BENDOG]

So then the spacing would also be multiplied by 1.414, which will give me the same design steel as the parallel steel. Correct?

 

[shin25]

You should increase the amount of steel, As, by a factor of 1.414 to account for the wind from orthogonal direction.

 

[civilperson]

Same design steel per unit area from 90 degree crossing bars as per 45 degree pattern. The wind load is the same in N-S, E-W and at 45 degree to N-S & E-W (regular octagon has same elevation view from eight different directions).

 

[BarryEng]

Why not reinforce each one of the direction of the 8 faces of the octogon? Normally, wind codes give the max wind force within each 45 deg segment.

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.

 

[BarryEng]

There was a seminar in Bratislavia (years ago) on towers in Europe. There were many examples of towers & footing arrangements. Most of the concrete pencils (communication & TV towers) have the same footing problems (Berlin TV tower etc). Because of these problems, perhaps most have footings in rock?

 

[BENDOG]

We are considering radial and circumferential reinforcing as well as reinforcing each direction of the 8 faces. It appears that radial reinforcing will take less overall steel but the placement may be more difficult. This is a huge octagon, 84 ft wide with 120 piles beneath. Both designs have been sent out for constructability review and and cost analysis and we are waiting for their response.

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.

 

[BigH]

See: thread486-191157 . Fintel's book covers octagon footings, too.

 

[BarryEng]

I finally found the reference I wanted to mention to you, on octagonal footing design (I have a large & unworkable, archiving & filing system).

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)

 

[csd72]

Putting radial reinforcing in is expensive and unnecessary.

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

 

[shin25]

I agree with csd72. The theory behind putting the reinforcing is to arrest the number of cracks as much possible. Reinforcing orthogonal to each other will do the work just fine.