Steel resist tension. Then why we provide steel in compression zone ?
There are several reasons to add compression steel. Keep in mind, supported steel (meaning it can't buckle) resists compression as well. Compression steel helps reduce long term deflections. Concrete creeps under sustained loads. Steel lessens the compression, meaning less sustained compressive stress to cause creep deflection. It makes members more ductile. Since the steel takes some of the compressive stress, the compression block depth is reduced, increasing the strain in the tension steel at failure, resulting in more ductile behavior (the moment at first yield remains largely the same with compression steel added, but the increase in capacity after yield is significant). Compression steel insures that the tension steel yields before the concrete crushes, meaning it helps change the failure mode to tension controlled. It makes beams easier to construct. With bars in the top and bottom, you have longitudinal reinforcement in all 4 corners of the shear stirrups to keep them in place when pouring the concrete. Also, for continuous members, its often easier to run your negative moment steel the full length of the beam rather than trying to cut it off in the positive moment regions. Serviceability concerns. You're going to end up putting steel in that region anyway to for temperature and shrinkage.
