DIY Circular Concrete Footing

Hello ddraheim,

A load of 7 kN is trifling. It is about 1600#. A moment of 15 kNm is not large, but it is too large for a footing of 24" diameter near the surface. If you drill a hole into the ground with a 24" auger, the soil at the bottom of the hole will be disturbed so you cannot rely on 150 kPa bearing. More likely you will be carrying the load by skin friction on the side of the concrete cylinder (pile). Moment will be resisted by lateral forces against the soil. The force at the top will be opposite in direction to the force at the bottom in order to resist the applied moment.

If you live in a cold climate, the minimum depth of pile should be approximately three times the depth of frost penetration in order to avoid frost jacking. If you live in a frost free area, then the depth of pile needs to be adequate to develop lateral forces capable of resisting the applied moment.

And finally, the pile has to be reinforced with a cage of re-bars sufficient to resist the applied moment. With a moment of only 15 kNm, minimum reinforcement should be adequate, say four 15M bars vertically placed in a 16" diameter circle and tied with 10M circular ties @ 16" centres throughout the height of pile.

Bruce ,

what if I make the piers 600mm dia. & 1500mm deep. Use a reinforcement cage like you suggested, and embed the columns (150UC30.0) into the concrete piers leaving 75mm concrete cover between the botton on the column and the bottom of the pier.

ie0 embed the columns 1425mm into the piers.

thanks for your help, Daniel

Hello ddraheim,

In my neck of the woods, the minimum depth of a pile is 15 feet because the frost penetrates to a depth of 5'. You are proposing a depth of 1500 mm (five feet) so I assume you are not concerned about frost jacking.

Even if there is no frost in your area, the pile should be at least 2400 mm (eight feet) deep (my opinion). You can either embed the column in the pile or anchor the column to the top of pile with a baseplate and anchor bolts, whichever you find easier. The baseplate is normally set up about 25 mm from the top of pile and grouted after the column is plumbed.

52 inches deep, #3 or #4 ties every 10 inches, 4 #4 or #5 verticle bars inside the ties. If using anchor bolts, the bolts should be within the top tie and you could throw in an extra tie around them. Keep 3 inches of cover over all bars.

I would like to see your calculation of the statics of that footing, particularly with respect to the applied moment.

Hi, ddraheim!

Is it absolutely necessary for the footing to take a columnar shape? A larger, shallower footprint would be much more convenient, generate less cost and bother to put in place; and be more efficient than a column on a column for transmitting an axial load on the support bed. Just similar to the plate you'd be using to rest the beams on the upper column.

A 24" footing might prevent the column from sloping over on damp earth but if you use it, you should allow for subsidence over the first two years. And don't count on any real support from the vertical surfaces in earth. Your column's also going to be in trouble later from spalling due to ground seepage through the concrete, etc. causing rust on the rebar. A larger shallower spalled footing would be much easier to repair or replace.

Mark.

Your column's also going to be in trouble later from spalling due to ground seepage through the concrete, etc. causing rust on the rebar.

Add mixture to prevent

Hi, bwire!

Very interesting stuff. Thanks for the link. What kind of track record does it have? Can it be used to parge over old spalling sites, e.g.?

Mark