The
Allowable Stress Design (ASD) technique is the conventional method accepted for
the design of steel structures over many years. The method is based on pure elastic
theory and demands that the stresses produced in a component by the applied
loads must not exceed a stipulated allowable stress. In 1986, the American
Institute of Steel Construction (AISC) presented an alternative method called the
Load and Resistance Factor Design (LRFD) method. In this method, factored loads
are applied to a steel component to determine the required absolute strength
and this is evaluated with the component's assumed strength and a suitable
reduction factor.

 

Historically,
the Allowable Stress Method (ASD) has delivered safe and reliable steel and
composite structures; however, the method does not comprehend inconsistency of various
load effects (live load, dead load) and resistances (i.e. shear capacity,
bending, cracks, etc.). For this reason, the Load and Resistance Factor Design (LRFD)
is the preferred method of structural steel design. LRFD method has two principle
benefits over the ASD method. First, during limit state analysis, engineer does
not have to presume linearity between force and load, or stress and force. Second,
different load factors can be utilized to suggest the degree of uncertainty for
various loads (dead and live). Due to these benefits of LRFD, more consistent
reliability is achieved during the structural steel design process and in many
cases a more cost-effective steel structure results.


The principal advantage of the Load and Resistance Factor Design method is
that, by applying a statistical analysis to the random values of component
strengths and loads, a consistent factor of safety may be achieved for all
types of steel structures. LRFD models the behavior of the structure at definitive
loads and provides an accurate estimation of the strength of the steel or
composite structure. In recent years, LRFD method has been
successfully employed to the design of hot-rolled and cold-formed steel sections and components in United States,
United Kingdom and other countries.

Also when you need to evaluate
structure strength under seismic circumstances, a truly elastic design approach
is difficult to correlate with estimated structural response. The existing
alternative provisions for ASD are totally misleading because they utilize a
conservative load factor of 1.7 on all live and dead loads and a set of customized
ASD factors to determine permissible strengths. At some point, we are creating
more confusion and work by trying to use ASD for the intrinsic inelasticity of
seismic design.

A third area where LRFD offers
tremendous advantage is in the design of frames with PR steel connections. Steel
Designers will be able to discard the restraining assumptions of perfectly
fixed connection behavior. Modeling connections using their authentic strength and
stiffness may result in a more economic structural frame due to easy connection
details.

AISC�s LRFD method is a practical,
world-class and trusted design specification. What�s more, because a metric
version is now available, it is assumed that most international jurisdictions
will recognize it, either as an alternative to their own code or in the lack of
an established code. Even if this is not the case, the LRFD method would form an
economical and convenient base for the US engineers to get familiar with the
internationally established limit states design philosophy. While ASD may not
be beyond its usefulness today, there can be no doubt that LRFD will replace it
gradually as innovative ideas become normal practice for steel structures.

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