Civil Design of Structures -WSM & LSM

How civil structures are designed? What are the methodologies to design a civil structure? If you wonder the same, you are at the right place to quench your quest.
The Working Stress Method (WSM) and Limit State Method (LSM) are explained in the easiest terms with their respective assumptions.
By the end, a comparative statement shall further strengthen your concepts.

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Civilverse Admin 11 July 2020
Civil Design of Structures- WSM & LSM
civilverse.org/civil-design
Table of Contents for Civil Design–
1. Introduction
2. Design Philosophies
3. Working Stress Method (WSM)
4. Limit State Method (LSM)
5. Scope of Civil Design Methods
6. Comparison of WSM v/s LSM
Civil design deals with the design of structural members like beams, columns, slabs,
footings, retaining walls, etc. A number of loads and their combination acts on the
structure during its lifetime. The loads are dead load, live load, seismic load, wind load,
lateral thrusts, etc. The individual loads and load combinations are considered for civil
design.
The civil structure design can be broadly classified into- RCC Structure Design and Steel
Structure Design. The choice of the structure depends upon various factors. Such factors
are the availability of raw material, availability of skilled manpower, design data, cost
control, the time duration of the project, etc. The RCC structures are generally bulkier
than the steel structures.
IS 456:2000 can be referred for RCC design and IS 800:2008 for steel design.
Design Philosophies-

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The civil structures can be designed by any of the following methods-
Civil Design Philosophies
1. Working Stress Method (WSM)
2. Limit State Method (LSM)
Working Stress Method-
The Working Stress Method utilizes the permissible stresses in the material as a design
criterion. The Hook’s Law remains valid and the stress-strain graph is linear. In other
words, the stress is directly proportional to the strain. The elements are designed in such
a way that the working loads don’t induce the stresses more than the permissible limit.
The permissible stress is the maximum stress a material attains in the elastic limit. The
permissible stress accounts for a small part of the ultimate stress of the material. Thus,
the structural member designed by WSM is bulky as more area is required for the same
quantum of the load. The heavy structures lead to an increase in the cost of the project.
However, WSM is safer.
Limit State Method-
The Limit State Method is a modern approach to civil structure design. It utilizes the full
strength of the material in design. Thus, for the same size, the members designed by LSM
can carry a heavier load than the member designed by WSM. The LSM members are more
optimized from the project cost perspective.
In LSM, the members are subjected to the yield stress of the material under the working
load.
The LSM can be further divided into-

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LSM Breakup
Limit State of Collapse- It deals with the design of members for safety. It can be
further divided into following-
1. Flexure- Deals with moment carrying capacity of the beams.
2. Shear- Deals with the shear resisting capacity of the structural member.
3. Bond- It helps in designing aspect considering the bond between steel and
concrete.
4. Torsion- Members subjected to torsion are designed as per this head.
5. Compression- Members like columns and pedestals are designed under this head.
Limit State of Serviceability- It deals with the durability of the structure. It helps in
designing the structure for deflection, creep, cracks, etc.
Assumptions of RCC Civil Design-
1. The material is homogeneous and isotropic. It means the properties of the material
such as density, Young’s Modulus, etc. are the same at all points and in all
directions.
2. The plane section remains plane before and after bending. It means the stress is
directly proportional to strain.
3. The bond between concrete and steel is perfect and no slipping occurs between
them.
4. Steel takes all of the tensile stress. Since the tensile stress of concrete is very low as
compared to steel and hence it can be neglected.
Scope of Civil Design Methods-
WSM- The WSM is not used in India for now due to cost ineffectiveness. However, some
structures like bridges, chimneys, water tanks are designed by WSM.
LSM- The LSM is frequently used in India for the design of residential buildings,
commercial buildings, industrial buildings, etc.
Comparison of WSM v/s LSM-

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Comparison of WSM vs LSM
For More
Working Stress Method- https://en.wikipedia.org/wiki/Permissible_stress_design
Limit State Method- https://en.wikipedia.org/wiki/Limit_state_design
Happy Engineering!