Q.: What is size effect of concrete? What is its role in determining the material strength of concrete?***
A.: The characteristic strength of concrete is determined from the results of tests conducted on cube specimens of 150 mm dimension. The dimensions of concrete structures or in members of structure are different and widely varying. This has an effect on the strength of concrete in the structure. This is known as size effect.
Due to the size effect, the characteristic strength of concrete is reduced to 2/3 of its value and then further divided by the partial safety factor of the concrete (γm = 1.5) to get the design strength of concrete (fd)..
fd = (2/3)fck/ 1.5 =0.45fck
Q.: Give four reasons to justify the design of structures by limit state method.
A.: The four reasons are:
(i) Concept of separate partial safety factors of loads of different combinations in the two limit state methods.
(ii) Concept of separate partial safety factors of materials depending on their quality control during preparation. Thus, γm for concrete is 1.5 and the same for steel is 1.15. This is more logical than one arbitrary value in the name of safety factor.
(iii) A structure designed by employing limit state method of collapse and checked for other limit states will ensure the strength and stability requirements at the collapse under the design loads and also deflection and cracking at the limit state of serviceability. This will help to achieve the structure with acceptable probabilities that the structure will not become unfit for the use for which it is intended.
(iv) The stress block represents in a more realistic manner when the structure is at the collapsing stage (limit state of collapse) subjected to design loads.
Q.: Why the partial safety factor of concrete is taken (1.5) more than that of steel (1.15)?
A.: Because the steel for reinforcement is produced in steel plants and commercially available in specific diameters with expected better quality control than that of concrete (size effect of concrete).........
A.: The characteristic strength of concrete is determined from the results of tests conducted on cube specimens of 150 mm dimension. The dimensions of concrete structures or in members of structure are different and widely varying. This has an effect on the strength of concrete in the structure. This is known as size effect.
Due to the size effect, the characteristic strength of concrete is reduced to 2/3 of its value and then further divided by the partial safety factor of the concrete (γm = 1.5) to get the design strength of concrete (fd)..
fd = (2/3)fck/ 1.5 =0.45fck
Q.: Give four reasons to justify the design of structures by limit state method.
A.: The four reasons are:
(i) Concept of separate partial safety factors of loads of different combinations in the two limit state methods.
(ii) Concept of separate partial safety factors of materials depending on their quality control during preparation. Thus, γm for concrete is 1.5 and the same for steel is 1.15. This is more logical than one arbitrary value in the name of safety factor.
(iii) A structure designed by employing limit state method of collapse and checked for other limit states will ensure the strength and stability requirements at the collapse under the design loads and also deflection and cracking at the limit state of serviceability. This will help to achieve the structure with acceptable probabilities that the structure will not become unfit for the use for which it is intended.
(iv) The stress block represents in a more realistic manner when the structure is at the collapsing stage (limit state of collapse) subjected to design loads.
Q.: Why the partial safety factor of concrete is taken (1.5) more than that of steel (1.15)?
A.: Because the steel for reinforcement is produced in steel plants and commercially available in specific diameters with expected better quality control than that of concrete (size effect of concrete).........
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