Limit state, working stress, ultimate load method - Detailed Concept

Table of Content
What are the Various Design Philosophies?
Working Stress Method
Assumptions of Working Stress Method
Limitations of Working Stress Method
Ultimate Load Design
Assumptions of Ultimate Load Method
Limitations of Ultimate Load Method
Limit State Method
What are the different types of limit state of design?
Assumptions of Limit State Method

Introduction
Whereas Ultimate Load Method was developed in the 1950s as a
alternative to Working Stress Method. ULM got accepted as an alternative to
WSM in ACI code in 1956 and in British code in 1957. ULM was based on
the ultimate load carrying capacity of reinforced concrete on collapse.

What are the Various Design
Philosophies?
Working Stress
Method
Ultimate Load
Method
limit State
Method

Working Stress Method
Working Stress Method was the traditional method of design for RCC
structures, steel structures, and timber structures also.
The basic concept of working stress method is adequate safety can be
achieved by restricting the stresses in the material due to working loads
on the structures, if we assume that structural materials in a linear elastic
manner.

Assumptions of Working
Stress Method
• The plane section before bending remains plane after
bending at any cross-section.
•
•
• σ

Limitations of Working
Stress Method
• The assumption that the stresses under working loads can be
kept within permissible stresses is unrealistic or not practical.
•
•

Ultimate Load Design
The ultimate load design method also termed as load factor
method or the ultimate strength method. This method was
developed in the 1950s with overcoming shortcomings of WSM
and introduced in the USA in 1956, the UK in 1957, and also in
1964 revision of IS 456.
In the ultimate load method, the nonlinear stress-strain curves of
steel and concrete also the stress conditions just before the
collapse are considered so as to avoid the problems arisen due to
the imaginary concept of modular ratio (m).

Assumptions of Ultimate
Load Method
• Stress in reinforcement varies linearly with
strain up to specified yield strength.
•
•

Limitations of Ultimate
Load Method
• Performance under normal service load is not considered,
that is why it was realized that the design approach that
combines the very best features of the working stress
method and the ultimate load method will result in better
structural stability and performance.
•
•

Limit State Method
In the limit state method of design, the structure should be
designed to withstand safely all the loads acting on it throughout
its life. Also, the structure should satisfy the serviceability
requirements, like limitations of deflection and cracking.
WSM was based on calculations on service load conditions only,
ULM was based on calculations on ultimate load conditions only
and now LSM provides comprehensive and rational solutions to
the design problems by considering safety at ultimate loads and
serviceability at working loads.

WHAT ARE THE DIFFERENT
TYPES OF LIMIT STATE DESIGN?
1
COLLAPSE 1.2 DURABI LI T Y 1.3 SERVI CEABI LI T Y
1.1.A F L E X URE 1.3.A DE F L E CT I O N
1.1.B CO MP RE S S I O N 1.3.B CRA CK I NG
1.1.C S HE A R
1.1.D TO RS I O N
LIMIT STATE OF
1.1
WWW. CI V I L I NS IDER. CO
1.4
FI RE
RESI STANCE

Assumptions of Limit
State Method
• A section that is plane normal to the axis of
structural components remains plane after
bending.
•
•

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