REINFORCEMENT REQUIREMENTS FOR DOUBLY REINFOCED BEAMS.

A doubly reinfocement concrete beam is reinforced in both compression and tension regions .The section of the beam may be rectangle,Tor L section .The necessity of using steel in the compression region arises due to two main reasons (1) When depth of beam is restricted ,the strength available from a singly reinforced beam is inadequate.(2) At a support of a continues beam where the bending moment changes the sign ,Such situation may also arise in the design of a beam circular in the plan .TYPES OF THE PROBLEMS :-- There are two types of problems in doubly reinfoced beams just as in the singly reinforced beams (1) Analysis of the section and (2) Design of the section .The analysis of the doubly reinfoced section involves the determination of the amount of resistance knowing breadth ,depth ,area of tension and compression steels and their covers .The analysis is similar to that for the singly reinforced beam except that the compressive force consists of two parts , in steel and in concrete .The stress in compression steel at the collapse limit state may be the yield stress or some what less depending upon the position of the nuetral axis.The stress in compression steel must be compitable  with the strain in the compression steel. When a section or a given dimension is required to carry a given factored bending moment in excess of what it can carry as singly reinforced section ,compression reinforcement becomes necessary.Certain additional tension reinforcement will also be required .However the total tension reinforcement can not exceed the maximum permissible tension reinforcement. The design of a doubly reinforced section involves the determination of cross sectional dimensions and area of tension and compression reinforcements.The first step is to determine whether compression steel is required .This may be done by comparing the factored bending moment with the limiting moment of resistance of a singly reinforced beam.The amount of compression steel may be obtained by writting the equations of the equilibrium.MINIMUM AND MAXIMUM REINFORCEMENT :-- The minimum and maximum amounts of tension reinforcement in a doubly reinforced beam are the same as those in a singly reinforced beam. The minimum compression reinforcement can,of course be zero ,in that case ,it will be a singly reinforced beam .The maximum compression reinforcement can not exceed 4 percent of the gross cross -sectional area of the beam .It is too required to generate the design charts and design tables for the doubly reinforced beams as per the design &scope of the ratio and percentage for different equations of the concrete mix , type of steel and mix of grade .