Sign Up

Sign Up to The Constructor to ask questions, answer people’s questions, write articles, and connect with other people. VIP members get additional benefits.

Sign In

Login to The Constructor to ask questions, answer people’s questions, write articles & connect with other people. VIP members get additional benefits.

Forgot Password

Lost your password? Please enter your email address. You will receive a link and will create a new password via email.

Sorry, you do not have permission to ask a question, You must login to ask question. Become VIP Member

Get More Features, Sign Up Now. Become VIP Member

Print, PDF & Email

Load and Resistance Factor Design Format

Of the many multiple safety factor formats in vogue, perhaps the simplest to understand is the Load and Resistance Factor Design (LRFD) format, which is adopted by the ACI Code [Ref. 3.5, 3.8, 3.9]. Applying the LRFD concept to the classical reliability model, adequate safety requires the following condition to be satisfied :

where Rn and Sn denote the nominal or characteristic values of resistance R and load effect S respectively; image and image denote the resistance factor and load factor respectively. The resistance factor image accounts for ‘under-strength’, i.e., possible shortfall in the computed ‘nominal’ resistance, owing to uncertainties related to material strengths, dimensions, theoretical assumptions, etc., and accordingly, it is less than unity. On the contrary, the load factor image, which accounts for ‘overloading’ and the uncertainties associated with Sn, is generally greater than unity.

Eq. 1 may be rearranged as
which is representative of the safety concept underlying WSM,  image here denoting the ‘factor of safety’ applied to the material strength, in order to arrive at the permissible stress for design. Alternatively, Eq. 1 may be rearranged as

image

which is representative of the safety concept underlying ULM, image here denoting the so-called ‘load factor’ (ULM terminology) applied to the load in order to arrive at the ultimate load for design.

Gopal Mishra

Gopal Mishra

Gopal Mishra is a Civil Engineer from NIT Calicut and has more than 10 years of experience in Civil Engineering and Construction. He is the founder of The Constructor.

Related Articles

1 Comment

Leave a comment

You must login to add a new comment.