Question

Which of the following is NOT the correct assumption for the analysis and design of a reinforced concrete member?
1. Concrete is assumed to be homogenous
2. The stress-strain relationship for concrete is a curved line, under working loads.
3. The steel area is assumed to be concentrated at the centroid of the steel.
4. At any cross-section, plane sections before bending remain plane after bending.

Answer 1

Correct Answer - Option 2 : The stress-strain relationship for concrete is a curved line, under working loads.

Concepts:

The design of reinforced concrete sections is based on the following assumptions:

1. Plane sections normal to the axis remain plane after bending. It means that the strain at any point in the cross-section is proportional to the distance from the neutral axis. This is valid for both the limit state of collapse and the working stress method.

2. The maximum strain in concrete at the outermost compression fiber is taken as 0.0035 in bending in case of Limit State of Collapse.

3.  The relationship between the stress-strain distributions in concrete is assumed to be a parabolic while, in the case of working stress it is assumed linear.

4. The steel area is assumed to be concentrated at the centroid of steel. This is valid for both the limit state of collapse and the working stress method.

5. For design purposes, the compressive strength of concrete is assumed to be 0.67 times the characteristic strength of concrete. 

6. The tensile strength of concrete is neglected.