Civil - Design of Reinforced Concrete Elements - Methods Of Design Of Concrete Structures
1. What are the assumptions made in the working stress method?
a) At any cross-section, plane sections before bending remain plain after bending.
b) All tensile stresses are taken up by reinforcement and none by concrete, except as otherwise specifically permitted.
c) The stress-strain relationship of steel and concrete, under working loads, is a straight line.
d) The modular ratio m has the value -280/3?bc.
2. Difference between Elastic method and limit state method.
Advantages of limit state method over the other methods
a. In the limit state method of analysis, the principles of both elastic as well as plastic theories used and hence suitable for concrete structures.
b. The structure designed by limit state method is safe and serviceable under design loads and at the same time it is ensured that the structure does not collapse even under the worst possible loading conditions.
c. The process of stress redistribution, moment redistribution etc., are considered in the analysis and more realistic factor of safety values are used in the design. Hence the design by limit state method is found to be more economical.
d. The overall sizes of flexural members (depth requirements) arrived by limit state method are less and hence they provide better appearance to the structure
e. Because of the modified assumptions regarding the maximum compressive strains in concrete and steel, the design of compressive reinforcement for double reinforced beams and eccentrically loaded columns by limit state method gives realistic valued which is not so in other methods.
3.Draw stress-strain curve for concrete in working stress design and mention the salient points.
4. Define characteristic strength in limit state method.
The term ‘characteristic strength’ means that value of the strength of the material below which not more than 5 percent of the test results are expected to fall.
5. What is meant by balanced section?
When the maximum stress in steel and concrete simultaneously reach their allowable values, the section is said to be balanced section.in this section the actual neutral axis depth is equal to the critical neutral axis.
6. Define : Limit state".
The acceptable limit for the safety and serviceability requirements before failure occurs is called a ‘limit state’. The aim of design is td achieve acceptable probabilities that the structure will not become unfit for the use for which it is intended, that is, that it will not reach a limit state.
7. What are the expressions recommended by the IS 456-2000 for Modulus of Elasticity and Flexural Strength?
· lexural strength f 0.7.fck /mm2
Where ‘fck’ is the characteristic cube compressive strength of concrete in N/mm2.
Where, E, is the short term static modulus of elasticity in N/mm2
8. Write the formula for the neutral axis depth factor 'K in working stress design.
Neutral axis depth factor ‘K’ ?bc.m/(?bc.m + ?st)
Where ?bc permissible stress in concrete. ?bc permissible stress in steel.
M = modular ratio.