The working stress method
is developed in Europe during the early part of 20th century. Working
stress method is based on the Hook’s law. It is also known as elastic method,
as the sections are designed in accordance with the elastic theory of bending
assuming that both materials obey Hook’s law.
The
working stress method is one of the traditional method of design based on
classical elastic theory used. Other than R.C.C structures it is also used for
timber and steel structures. The conceptual basis of working stress method is
simple. This method basically assumes that the structural material behaves in a
linear elastic manner and that adequate safety can be insured by suitable
restricting the stresses in the material due to the expected working loads on
the structure.
The stresses under the working loads are obtained by
applying the methods of ‘strength of materials’ like the simple bending theory.
The limitations due to non-linearity (geometric as well as material) and
buckling are neglected.
In
working stress method it will be assumed that concrete and steel are elastic
and they are subjected to such stresses that the components remain elastic and
the maximum stresses induced in the components do not exceed the allowable
stresses. This method has certain shortcomings. For concrete, the relation
between stress and strain is not linear but follows a curve.
Though
the stress-strain relation is linear for mild steel it is not so in the case of
high yield strength deformed bars which are most commonly used in practice.
This method does not provide a true factor of safety against failure or
objectionable deformation. The method ignores the effect of creep and shrinkage
of concrete.
A reinforced concrete member shall be designed for all
conditions of stresses that may occur and in accordance with the principles of
mechanics.
The characteristic property of a reinforced concrete member
is that its components namely concrete and steel act together as a single unit
as long as they remain in the elastic condition i.e. the two components are
bound together so that there can be no relative displacement between them.
Assumptions of working stress design method:
1) A
section which is plane before bending remains plane after bending (leads to linear
strain diagram)
2) Bond
between steel and concrete is perfect with in elastic limit of steel
3) Principle
of cracked section – All tensile stresses are taken only by reinforcement and
none by the concrete except as otherwise specially permitted.
(“Un-cracked moment” – Concrete takes the
tensile stress without failure.
– The material should be in homogenous.)
4) The
stress – strain relationship of steel and concrete under working loads is a
straight line (i.e.) modulus of elasticity is constant.
5) The
modular ratio (m) has the value based on the permissible bending compressive stress in
concrete.
6) The
steel area is assumed to be concentrated at the centroid of steel.
Merits of working stress method:
1) It
is simple in concept and application.
2) The
structure designed by this method are large and therefore gives better
serviceability performance (i.e. less deflection and less crack width).
3) Knowledge
of working stress method is essential since it forms a part of limit state
method for serviceability condition.
Demerits of working stress method:
1) The
concrete is assumed as elastic which is not true as the concrete behaves
inelastic even on low level stresses.
2) The
factor of safety is used only for stresses and not for loads. Thus this method
does not give any true factor of safety with respect to loads because the
failure load is unknown.
3) As
it does not provide any factor of safety with respect to loads, there is no
provision for the uncertainties associated with the estimation of loads.
4) It
does not take care of shrinkage and creep which are time dependent and plastic
in nature.
5) As
the sections are very large, it leads to uneconomical.
6) No
special care is taken to the condition that arises at the time of collapse.
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