Understand that concrete is strong in compression, very weak in tension.
In a cantilever bending load, like an overhang, the top surface is in tension and the bottom
surface is in compression. That is why Ian said (exactly correclty) to put the 1/2"
rebar near the top. At the top, the steel will take the tension loads.
IF THE PRIMARY
LOAD IS AN OVER HANG.
Our range has these benches and they are about 36" wide and about 4-5 ft long, with about
a 8-10"Wx 24" rounded cutout at the back on each side. They are set on top of a 8" diam
sonotube for each of three legs. One at the front on each side, maybe 6" overhang
and one a the back centered. So in this design, the primary load is NOT the overhang.
In that design, the overhangs mean nearly nothing, the primary load will be the
weight of the concrete and the gear on top. That means bending load between pillars, which
puts the tension side on the BOTTOM, compression on top. So, in that situation, I'd put one (or
two 3-4" apart,) 1/2" rebars with the centerline of them forming a V with the bottom
over the rear pillar and the tops of the two arms centered on the top of the front pillars.
Then maybe one rebar across between the front pillars. And THESE should be near
the bottom, because of the bending load between the pillars.
Sonotubes are inexpensive cardboard tubes, designed specifically as forms to make
concrete pillars. Make the pillar mix a bit wetter than normal and work a rebar up and
down a LOT to work out air. Too dry will make a bad fill. "Too wet" won't be much of
an issue unless you go really overboard.
The SAKRETE 8 in. x 48 in. Form Tube is designed for one-time applications to reduce the cost of pouring footings, round columns and piers. Pour concrete directly into the tube for a neat and easy form.
www.homedepot.com
Dig in about 2 ft or more, depending on frost line, deeper for farther north and set the
tube in hole, jig it vertical and pour full of sakrete. A single 1/2" rebar in the middle would be
good. Cut off the tubes very carefully with a level laid across them, jig them plumb
and pour full, flush tops level before pouring. Make your bench top and somehow
(about 6 men, some straps or ropes and a couple of 6 ft 2x4s should do it) lift into place
on top of the three pillars.
Makes a hell of a solid bench which will last about forever in the weather. Letting
the rebar in the pillars stick out an inch and a half and putting in PVC pipes about
2" long with duct tape over the top at exactly the right location to make sockets. Will probably need a
6 mil poly lined plywood floor on your mold with locating holes for these pipes, made from the
already located rebar pieces sticking out of the pillars so they will guaranteed all line
up right. The PVC pipe makes sockets in the bottom of the bench top to sit on the pins
sticking out from the pillars.
Under a $150 in materials, my guess. 12-15 bags of sack crete at around $6
a bag. maybe 20 ft of rebar. Three $10 sonotubes. And a bunch of work.
If you just put blocks on the ground the whole thing will move around over time.
Free engineering.
If you make a T-shaped concrete block support, with a foundation to stabilize it,
say 18-24" concrete footer. Then the overhang can be shortish and the long
way will be on the blocks so no bending load where it would be un supported
between pillars. Probably want to pour a couple of the block cores as pillars
to stabilize them. With that design, minimal rebar near the top would be fine.
If you want, I can post a photo of our benches. Nice not to have anything under
to interfere with chairs and feet.
Bill