The
shortcomings of working stress method led to the development of the ultimate
load design method. This method is also referred to as the load factor method
or the ultimate strength method. This method was introduced in the USA in 1956,
the UK in 1957, and as an alternative in the second revision of IS 456 in 1964.
In
this method, the non-linear stress-strain curves of concrete and steel and the
stress condition just before collapse are considered. Thus, the problems
associated with the concept of modular ratio are avoided in this method.
Sufficient safety is achieved by the use of a load factor, which is defined as
the ratio of the ultimate load (design load) to the working load. In this
method of design, different types of loads are assigned with different load
factors under combined loading conditions, thereby overcoming the related
shortcomings of working stress method. For example, a low load factor is used
for a load that is known more exactly (e.g. the dead load) and a higher load
factor for less-certain loads, (e.g. the live loads). Thus, in the ultimate
load design, the strength of the member must be more than the ultimate load
acting on the member.
Assumptions:
1) Plane
section normal to the axis of the member remain plane after bending. This means
the strain distribution across the depth is linear right up to failure.
2) Unless
otherwise specified the tensile strength of concrete is neglected in sections
subjected to bending.
3) At
ultimate strength, stresses and strains are not proportional and distribution
of compressive stress in a section subjected to bending is non-linear. The
diagram for compressive concrete stress distribution may be assumed to be a
rectangle, trapezoid, parabola or any other shape which results in ultimate
strength in reasonable agreement with tests.
4) Maximum
fibre stress in concrete does not exceed 0.68 fck where fck
is 28 days ultimate compressive strength of concrete.
Advantages:
1) Considering
the non-linearity of stress-strain relationship at high stress levels, this
method predicts the ultimate strength of a section in an acceptable manner.
2) Dead
loads for a structure are estimated more certainly than live loads. Hence it is
not justified to apply the same factor of safety to both dead and live loads.
3) Elastic
design in columns is a modification of ultimate strength design and is not
compatible with elastic design of flexural members.
Disadvantages:
1) The
performance at the normal service loads is not considered. Hence, it was
realized that the design approach that combines the best features of the
ultimate strength design and working stress design will result in better
structural performance in strength and serviceability.
