bar support members 20 a and 20 b form an upper cradle 20 for receiving an upper reinforcement bar such as the bar b 2 in fig5 to prevent the reinforcement bars from lifting out of the cradles 20 and 22 on the inner surfaces of the opposing sidewalls 24 a 26 a 24 b 26 b 28 a 30 a and 28 b 30 b are retaining members 32 as shown most clearly in fig2 and 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.