【正文】 he tension there after the concrete is cracked. When the load q is increased, the width of the cracks, the deflection and the stress of steel bars will increase . When the steel approaches the yielding stress ?y , the deflection and the cracked width are so large offering some warning that the pression zone . The failure load q=, is approximately times that for the plain concrete beam. Concrete and reinforcement can work together because there is a sufficiently strong bond between the two materials, there are no relative movements of the bars and the surrounding concrete cracking. The thermal expansion coefficients of the two materials are 105K1 for steel and 105~105K1 for concrete . Generally speaking, reinforced structure possess following features : Durability .With the reinforcing steel protected by the concrete , reinforced concrete concrete beam and reinforced concrete beam Is perhaps one of the most durable materials for construction .It does not rot rust , and is not vulnerable to efflorescence . (2)Fire resistance .Both concrete an steel are not inflammable materials .They would not be affected by fire below the temperature of 200℃ when there is a moderate amount of concrete cover giving sufficient thermal insulation to the embedded reinforcement bars. (3)High stiffness .Most reinforced concrete structures have paratively large cross sections .As concrete has high modulus of elasticity, reinforced concrete structures are usually stiffer than structures of other materials, thus they are less prone to large deformations, This property also makes the reinforced concrete less adaptable to situations requiring certain flexibility, such as highrise buildings under seismic load, and particular provisions have to be made if reinforced concrete is used. (4)Locally available resources. It is always possible to make use of the local resources of labour and materials such as fine and coarse aggregates. Only cement and reinforcement need to be brought in from outside provinces. (5)Cost effective. Comparing with steel structures, reinforced concrete structures are cheaper. (6)Large dead mass, The density of reinforced concrete may reach 2400~2500kg/ with structures of other materials, reinforced concrete structures generally have a heavy dead mass. However, this may be not always disadvantageous, 6m 200 400 (a)plain concrete beam 6m 200 400 (b)Reinfoced concrete beam 2φ 16 particularly for those structures which rely on heavy dead weight to maintain stability, such as gravity dam and other retaining structure. The development and use of light weight aggregate have to a certain extent make concrete structure lighter. (7)Long curing period.. It normally takes a curing period of 28 day under specified conditions for concrete to acquire its full nominal strength. This makes the progress of reinforced concrete structure construction subject to seasonal climate. The development of factory prefabricated members and investment in metal formwork also reduce the consumption of timber formwork materials. (8)Easily cracked. Concrete is weak in tension and is easily cracked in the tension zone. Reinforcing bars are provided not to prevent the concrete from cracking but to take up the tensile force. So most of the reinforced concrete structure in