DOUBLY REINFORCE SECTIONS FOR RCC STRUCTURES

DOUBLY REINFORCE SECTIONS

INTRODUCTION

A doubly reinforced concrete section is reinforced in both compression and tension regions. The section of the beam or slab may be a rectangle, T and L section. The necessity of using steel in the compression region arises due to two main reasons:

(I) When depth of the section is restricted, the strength available from a singly reinforced section is inadequate.

(II) At a support of a continuous beam or slab where bending moment changes sign. Such a situation may also arise in the design of a beam circular in plan.

TYPES OF PROBLEM

There are two of problem in doubly reinforced beams just as in the singly reinforced beams:

(i) Analysis of section, and

(ii) Design of section.

The analysis of doubly reinforced section involves the determination of the amount of moment of resistance knowing breadth, depth, area of tension and compression steels and their covers. The analysis is similar to that for the singly reinforced sections 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 on the position of the neutral axis. The stress in compression steel must be compatible with the strain in the compression steel.

When a section. of a given dimension is required to carry a given favored bending moment in excess of what it can carry as a singly reinforced section, compression reinforcement becomes necessary. Certain additional tension reinforcement will also be required. However, the total tension reinforcement cannot exceed the maximum permissible tension reinforcement.

The design of a doubly reinforced sector involves the determination of cross sectional dimensions and areas 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 section. The amount of compression steel may be obtained by writing the equations of equilibrium.

Working Stress Method of Rc Structures

Working Stress method has been the traditional method used for reinforced concrete design where it is assumed that concrete is elastic, steel and concrete sct together elastically, and the relationship between loads and stresses linear right upto the collapse of the structure. The basis of the method is that the permissible stress for concrete and steel are not exceded any where in the structure when it is subjected to the worst combination of bending assuming that both materials obey the Hooke’s law. The elastic theory assumes a linear variation of strain and stress from zero at the neutral axis to a maximum at the extreme fibre.

The permissible stresses are prescribed by a building code to provide suitable factors of safety to allow for uncertainties in the estimation of working loads and variation in properties of materials IS:456-2000 user a factor of safety equal to 3 on the 28 days cube strength to obtain the permissible compressive stress in bending in concrete; and equal to 1.78 on the yield strength of steel in tension to obtain the permissible tensile stress in reinforcement.