This set of Design of RC Structures Multiple Choice Questions & Answers (MCQs) focuses on “Working Stress Method: Modular Ratio and Limitations”.
1. What is modular ratio in Working stress method?
a) Ratio of Modulus of elasticity of concrete and modulus of elasticity of steel
b) Ratio of Modulus of elasticity of steel and modulus of elasticity of concrete
c) Product of Modulus of elasticity of steel and modulus of elasticity of concrete
d) Sum of Modulus of elasticity of steel and modulus of elasticity of concrete
View Answer
Explanation: Modular ratio is the ratio of modulus of elasticity of steel and modulus of elasticity of concrete. Modulus of elasticity is the resistance offered by the material to deformation when some load is applied on it. Here in for reinforced concrete modulus of elasticity for both steel and concrete are considered. In a reinforced section steel is a tension member and concrete is a compression member.
2. Which of the following equation expresses the modular ratio (m) for working stress method?
a) (290/4 σcb)
b) (290/3 σcb)
c) (280/4 σcb)
d) (280/3 σcb)
View Answer
Explanation: In the working stress method the modular ratio (m) is given by the following equation:
m = (280/3 σcb)
Here,
(σcb) is the permissible compressive stress in bending.
The unit is N/mm2 or MPa.
The above value of modular ratio is also one assumption of working stress method of design. It can be taken equal to a constant value for all strength of concrete or may vary according to strength of steel and concrete. IS 456 recommends modular ratio of elasticity of concrete which varies with the strength of concrete.
3. Which of the following is correct in view of the effect of creep of concrete on modular ratio?
a) It increases the modular ratio
b) It decreases the modular ratio
c) It either increases or decreases the modular ratio
d) It has zero effect on modular ratio
View Answer
Explanation: The creep of concrete is the deformation of structure under sustained load. It can also be defined as the time-dependent part of strain resulting from stress. Also modular ratio is the ratio of modulus of elasticity of steel and modulus of elasticity of concrete. So the effect of creep of concrete on modular ratio will be such that creep will increase it. There are many factors affecting creep of concrete.
4. In working stress method it is impossible to determine the actual factor of safety with respect to the loads.
a) True
b) False
View Answer
Explanation: In working stress method since the factor of safety is on the stresses under working loads, there is no way to account different degrees of uncertainty associated with different types of loads. With elastic theory it is impossible to determine the actual factor of safety with respect to loads.
5. The main drawback of working stress method is that the concrete is elastic.
a) True
b) False
View Answer
Explanation: The concrete is not elastic. The inelastic behavior of concrete starts right from very low stresses. The actual stress distribution in a concrete section cannot be described by a triangular stress diagram. Also it is difficult to account for shrinkage and creep effects by using the working stress method for design.
6. Which of the following is not a disadvantage of Working stress method?
a) This method is uneconomical
b) This method assumes that stress- strain relationship is constant
c) This method considers the modes of failure
d) In this method concrete does not have a definite modulus of elasticity
View Answer
Explanation: The WSM gives larger section of structural members, thus it is uneconomical. It assumes that stress-strain relationship is constant for concrete, which is not true. It does not consider the modes of failure of the structure (brittle or ductile). Concrete does not have a definite modulus of elasticity because of creep and non-linear stress-strain relationship.
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