Abstract: A joist structure having a configuration useful for constructing concrete-forming structures. The joist is comprised of an upwardly opening channel and an oppositely facing downwardly opening keyhole-shaped channel. The joist is an extruded elongate member having the respective channel end walls abutting each other. The upwardly facing channel has a generally rectangular form with the side walls being considerably wider than the end walls to accommodate standard wooden beam sizes, such as a 2 .times. 4. The downwardly facing channel, somewhat narrower and shorter than the upwardly facing channel, has a keyhole shape which considerably increases the compressive and torsion strength of the overall joist. The keyhole shape also accommodates keyhole-shaped inserts for strengthening the joist at high stress points, such as bearing surfaces and the mid-point between spans. The keyhole-shaped inserts can be provided in varying lengths according to the need and can incorporate mounting holes for the joist.