numerate the situations in which doubly reinforced concrete

 Enumerate the situations in which doubly reinforced concrete beams become necessary. What is the role of compression steel?

Solution :-

When the bending moment required to be resisted is more than the moment of resistance of a balanced section of singly reinforced beam of given size, there are two alternatives:

(1) To use an over-reinforced section

(ii) To use doubly reinforced section. An over reinforced section is always uneconomical and also undesirable because of sudden failure probability. Also the increase in the moment of resistance is not in proportion to the increase in the area of tensile reinforcement. The reason behind this is that the concrete, having reached maximum allowable stress, cannot take more additional load without adding compression steel. The other alternative is to provide reinforcement in the compression side of the beam and thus to increase the moment of resistance of the beam beyond that of a balanced section.

Doubly reinforced sections are also useful in following situations :-

(1) Where the members are subjected to probable reversal of external loads and thereby the bending moment in the section reverses, such as in concrete piles etc. 

(2)When the members are subjected to loading, eccentric to either side of the axis, such as in columns subjected to wind loads.

(3) When the members are subjected to accidental lateral loads, shock or impact.

The steel reinforcement provided in the compression zone is subjected to compressive stress. However, concrete undergoes creep strains due to continued compressive stress, with the result that the strain in concrete goes on increasing with time. This increases compressive strain in steel in addition to creep strain in compressive steel. Thus the total compressive strain in compressive steel will be much greater than the strain in surrounding concrete due to flexure alone. Thus, compressive steel takes up all the additional compressive stresses beyond the permissible compressive stress for concrete making the section safe against failure in flexure.