Beams and columns are the two most important structural units with which contractors deal. Slabs and walls are, from a design standpoint, merely modifications or adaptions of these basic sections. A review of some design considerations and related construction problems follows. The simplest concrete beam is of rectangular cross section with steel in the bottom to resist tension; it is supported at both ends and not fixed in any way. But this simple design becomes impractical and uneconomical as loads increase because more and more concrete is required to resist the increase in compressive forces. Restrictions in headroom and the high cost in time and materials of forming massive beams led designers to more sophisticated solutions. By designing a beam that takes its shape from strengths and properties of concrete and steel, a weight and space saving section is achieved. Thus there is the T-beam, with a wide upper flange furnishing the mass of concrete required to resist compressive forces and a narrow, deep leg with steel in the bottom to resist tensile forces. Such ribbed slab sections often are used efficiently for floor and roof construction. The need for accurate placement of the steel at its correct position in these narrow legs is obvious. The nature of the supports for beams has an influence on the amount, shape and location of reinforcement. The simply supported beam, with ends free to move, always is in compression in the top portions and under tension at the bottom, no matter what the load. The reinforcement is, therefore, placed in the bottom. The column is normally the principal member to provide the reactive force required to support the beams and slabs discussed earlier. Provided that walls brace the building against wind and beams of slabs do not transmit moment to the column, a simple column can be subject only to direct compressive load. The design and reinforcement of columns should then be relatively simple. That this is far from true is laid to flexibility, or lack of it. The load-carrying ability of a reinforced concrete column is highly dependent upon the stiffness of the two materials acting in combination. A column also undergoing bending due to wind or earthquake becomes vastly more complex.