LRFD and ASD

LRFD and ASD might be two confusing term for some of you. To be honest, when I was in university I never found out what are the meaning of these technical terms. In this article I want to explain them in really simple terms.

ASD is an abbreviation for Allowable Stress Design. LRFD refers to Load Resistance Factor Design. The main difference between ASD and LRFD how to define and apply a safety factor. In simple words, ASD provides a constant factor of safety for all designs regardless of the load type, while in LRFD method we apply a higher factor of safety on the loads that are less well defined. Dead load can be perfectly calculated so that LRFD consider a lower factor of safety for them. On the other hand since live loads are more unpredictable LRFD assign a higher factor of safety for them to remain on the safe side.

So LRFD method adjust the value of safety factor based on the uncertainty of different failure modes. As a result it is generally more economical and more effective.

What are the uncertainties in construction?

No matter how precise the constructors accomplish their duties, a degree of uncertainty need to be consider due to :

• Design errors
• Construction errors
• Unpredictable nature of some phenomenon such as earthquake and flood

A mathematical comparison between LRFD and ASD is performed in the table below:

ASD	LRFD
The general form for ASD is:	The general form for LRFD is:
where: Rn= nominal resistance Qd= nominal dead load effect Qt1, Qt2= nominal transient load effects γ = load combination factor FS = Factor of Safety	where Rn = nominal resistance Qd = nominal dead load effect Qt1, Qt2 = nominal transient load effects γ1 = load factor associated with the ith load effect Φ = resistance factor