What is W.S.M , U.L.M. & L.S.M
Introduction :-
The aim of design of R.C.C structureso that it fulfils the all the requirement it’s intended purpose during the adequate safety with intended lifetime like; Structural stability, structural integrity, strength, stiffness, durability and economy structural design.
Over the years, have evolved in different parts to reinforced concrete design. A design philosophies is built-up on the assumptions. The earliest codified design theory is the working stress method of design (W.S.M.) is traditional method of design, based on a hundred year old. The working stress method (WSM) is based on the linear elastic theory, this method is still surviving in some countries. But this method side-lined by the modern design limit statemethod (L.S.M.). In the recent 2000 revision the code IS-456 the provisions relating to the working stress method design process. So that more emphasis can be given to limit state design.
Historically, the design procedure to follow the working stress method of design was the ultimate load method (U.L.M.) of design and it is developed in 1950. Based on the strength of reinforcement concrete at ultimate loads. It evolved and generally gained acceptance. This method was introduced as an alternative to W.S.M. in the ACT Code 1956 and the British code in 1957 and subsequently in the Indian code IS – 456 in 1964.
Thus, the last several decades have seen an evolution in design philosophies from the traditional ‘working stress method’ to the ‘ultimate load method’ to modern ‘limit state method’ of design.
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Q. What is working stress method ?
Working stresses method :-
Working stress method (W.S.M.) is the traditional method for design of Reinforced cement concrete structure steel structure and timber design. W.S.M is first method for design of R.C.C structure. It is basically simple conceptual, Assumes that the structural materials behaves as a linear elastic manner that means it follow the Hook’s law and that adequate safety can be ensured by suitably restricting the stresses in the material induced by the expected working load / Service load on the structure.
As the specified permissible stresses are kept well below the material strength, the assumption of linear elastic behaviour is considered justifiable. The ratio of the strength of the material to the permissible stress is often referred to as the factor of safety.
Working stress method (W.S.M) is fail to differentiation between different types of load act simultaneously. It is does not provide realistic measure of the actual factor of safety underlying a design.
The design usually results in relatively large sections of structural members. Thereby resulting in better serviceability performance under the usual working load and it is very safe method for design of R.C.C structure and other way it is uneconomical method because it take very high factor of safety. W.S.M take 3 & 1.8 factor of safety for concrete & steel respectively.
Q. What is ultimate load method ?
Ultimate load method :-
Ultimate load method is based on the ultimate strengthof reinforced concrete at ultimate load is obtained by enhancing the service load by some factor called as load factor for giving a desired margin of safety. Hence the method is also referred to as the loads factor method or the ultimate strength method.
Ultimate load method is a alternative to working stress method. The ultimate load method is evolved in 1950. In the U.L.M, stress condition at the state of impending collapse of the structure is analysed, thus using the non-linear stress-strain curve of concrete and steel. In U.L.M the concept of modular ratio (m) problems are avoided entirely in this method. The safely measure in the design is obtained by the use of proper load factor. The satisfactory strength performance at ultimate loads does not guarantee satisfactory strength performance at ultimate loads does not guarantee satisfactory. Serviceability performance at normal service load.
Ultimate load method is more economical method of beam and columns compared to working stress method. The result is more slender section.
Q. What is limit state method ?
Limit state method :-
Limit state method (L.S.M) of design represents a definite advanced over the traditional method of design. Unlike
- W.S.M is based calculations on service load conditions alone.
- U.L.M is based ultimate load conditions calculation.
Limit states method is aims to solution the comprehensive and rational design problems. By considering factor of safety at ultimate loads and serviceability load.
Limit state method have multiple safety factor which attempts to provide adequate safety at ultimate load as well as adequate serviceability at service loads, by considering all possible ‘limit states’.
There are two distinct category of limit states :-
(a) Ultimate limit states / Collapse.
(b) Serviceability limit states.
(a) Ultimate limit states / Collapse – The limit state of collapse is reached when the structure as a whole or part of the structure collapses. This limit state corresponds to
(i) Flexure
(ii) Compression
(iii) Shear
(iv) Torsion
(b) Serviceability limit states – To ensure that satisfactory performance under service load conditions which implies providing adequate stiffness and reinforcement to contain deflections, crack-widths and vibrations within acceptable limits and also providing in-permeability and durability.
Serviceability limit state is deals with the discomfort to occupancy causes by cracking, deflection, leakage and durability.
Cracking of concrete is done when the tensile strength of concrete is exceed. The degree of cracking can be controlled through proper design. Cracking is considered undesirable, not only for obvious aesthetic reasons, but also because it is adversely affects durability and leads to corrosion of the embedded steel.
Various factor are affect for the cracking in R.C.C members.
· Tensile stress due to bending under applied loads
· Direct tensile stress under applied loads
· Causes of shear and torsion ( applied for diagonal tensile load)
Various factor are affect for deflection in flexural members.
· Aesthetic discomfort
· Combination of structural and non-structural elements
· Ponding in slab
· Crack width limitations.
Compare between working stress method (W.S.M.) & Limit state method (L.S.M.) for design –
| Sl.No. | Working stress method | Limit state method |
| 01. | It is based on the behaviour of structure at working load. | It is based on the behaviour of structure in different limit states ensuring adequate safety against each limit states. |
| 02. | It is referred to as deterministic (because it is assumed that loads, permissible stresses and factor of safety are known accurately). | It is referred to as nondeterministic (because it is based on predictions which depend upon the experience or on field date). |
| 03. | Factor of safety is applied to the yield or ultimate stress to get permissible stress. | Partial safety factors are applied to characteristics values to get design value. |
| 04. | It is based on linear stress distribution. | It is based on nonlinear stress distribution taking inelastic strain into consideration. |
| 05. | Actual safety against ultimate loads not known | It overcomes the drawbacks of working stress method. |
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