Civil Design of Structures- WSM & LSM

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-

The civil structures can be designed by any of the following methods-

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-

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-

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!