Concrete resists forces that tend to compress it, but it needs steel reinforcing bar, commonly called rebar, to help with tensile forces, which are those that tend to pull it apart. The purpose of columns as support structures is primarily to resist the compressive forces, but that's not purely the case. There are other forces at work as well, and even the downward forces may be translated from a vertical to a horizontal direction. Without proper reinforcement, concrete columns can buckle or burst outward.
Cut the column's vertical pieces out of No. 4 rebar with a rebar cutter. A typical column has four vertical pieces, depending on the size of the column. Make the length equal to the distance from top of the column to the bottom of the footing.
Use the bending tool on a rebar cutter to bend the last six inches of each vertical bar 90 degrees. The vertical bars should now be L-shaped. The bottom of the L will help to integrate the column with the footing. The height of the vertical pieces should now allow three inches clearance above and below the rebar.
Cut No. 3 rebar for making rectangular stirrups for encircling the vertical bars. Cut enough pieces so that you can space them vertically in the column at one foot on center. Make the length of each piece four inches shorter than the perimeter of the column.
Bend the rectangular stirrups with the bending tool. Make the length and width of each stirrup four inches shorter than the length and width of the column. That will allow 12 inches for overlapping. Use a pair of pliers and tie wire to tie each end of the overlap.
Insert all the upright pieces into the stirrups and tie the top and bottom stirrups first to establish the shape of the rebar "cage." Tie each of the four upright pieces into one of the four corners of the rectangular stirrups. Secure them tightly with tie wire. Make sure the bottom part of each of the L shapes is pointing away from the center of the column.
Slide the stirrups that are in between the top and bottom stirrups into position at one foot on center. Tie each of the corners to one of the upright bars.
Position the finished cage above the bottom of the footing with dobies, which are small concrete blocks used for spacing and positioning rebar. The dobies should maintain the cage three inches off the bottom off the footing, and the top of the cage should be three inches below the top of the column.
Pour the footing with the rebar cage in place. When the footing dries, build the column forms around the rebar.
Have your building project properly engineered. Adhere to all local building codes.
Wear gloves, long sleeves, long pants and steel-toed boots when tying rebar.