3 the retaining members 32 extend slightly over the channels c 1 and c 2 to prevent the reinforcement bars from moving upward and out of the channels c 1 and c 2 as the figures indicate the upper surfaces of the retaining members 32 are beveled sloped or curved slightly downward so that when the reinforcement bars are pressed downward toward the channels c 1 and c 2 force is transferred outward to cause the sidewalls 24 a b 26 a b 28 a b and 30 a b to flex outward and allow the reinforcement bars to snap into the channels c 1 and c 2 the lower surfaces of the retaining members 32 are preferably not beveled but rather have a square or barbed corners for effectively capturing the reinforcement bars within the channels c 1 and c 2 compared to prior chair designs that have used opposing tapered slots in a conical or cylindrical wall the opposing sidewalls and retaining members of the present invention provide a significantly improved retention mechanism in the preferred embodiment of the invention all of the components of the chair 10 are formed from one continuous piece of thermoplastic such as polypropylene which though rigid enough to support the weight of the reinforcement bars is flexible enough to allow the sidewalls 24 a b 26 a b 28 a b and 30 a b to flex outward to receive the reinforcement bars as described above thus when a reinforcement bar is laid across the cradle 20 on top of the retaining members 32 and is pressed downward.