Understanding Load and Resistance Factor Design is a guest blog by Aaron Simmons
Whether you’re an engineer or not, you’d probably agree that each structure built, regardless of how big it is, should be made with integrity, stability, and strength.
Do you want to know how engineers do it? Read further in this article to learn about one of the philosophies or concepts they use to ensure safety in structures: the LRFD method.
In this article, we are going to talk about Load and Resistance Factor Design, or LRFD, a design method used in structural engineering. We’ll also do an ASD vs LRFD comparison.
For ordinary people, the subject might seem as complicated as taking the SAT, and that it’s almost always necessary to enroll in SAT prep courses. Not for the folks deeply involved in structural engineering, however.
Let’s look at the science behind LRFD and appreciate its significance to the engineering world.
What Is LRFD?
LRFD stands for Load and Resistance Factor Design, a structural engineering design method used to ensure the stability of structures. Engineers and other professions use LRFD with the aim of satisfying this condition:
Φ and γ are factors applied to nominal values of resistance and load to take into account the possible effects of uncertainties with regard to the materials, dimensions, and other characteristics of the superstructure or substructure being constructed.
In simple terms, the resistance factor is multiplied to the nominal value to consider the deficiencies that might occur due to uncertainties. On the other side of the equation, the load factor is used to sort of “overestimate” the effects of loading in the structure, which are combined using load combinations.
Using the LRFD method ensures that the “underestimated” resistance in the structure still outweighs the “overestimated” effect of the loadings in it.
ASD vs LRFD
In business, emerging technologies and software are developed to generate better results. For example, many businesses are increasing their cloud-based services as a way to reduce spending on hardware.
Similarly, construction principles are continuously developed to make structures safer. For instance, LRFD is considered an improvement from ASD in RC or steel design.
Allowable Stress Design, or ASD, is an older method that is based on the principle that the induced stress on the structural members should not exceed a particular fraction of the elastic limit.
While LRFD separately takes into account the predictability of the load to be applied through the load factor and the uncertainties brought by materials and dimensions through the resistance factor, ASD combines the two factors into one, which is the factor of safety.
In simple terms, using LRFD is the more realistic approach, since it considers both the load and resistance of the structural member, rather than just its allowable stress.
Application of LRFD
Today, LRFD is more commonly used in designing different types of structure, compared to ASD. One good example of this is the AASHTO LRFD Bridge Design Specifications. This document is a set of specifications that should be used for the design, evaluation, and rehabilitation of bridges in the United States.
The content of the AASHTO LRFD Bridge Design Specifications is based on the principle of LRFD. The factors developed in the document are based on the statistical knowledge of loads and structural performances.
The book has 15 sections that include Aluminum Structures, Loads and Load Factors, Buried Structures and Tunnel Liners, Decks and Deck Systems, and Joints and Bearings.
The most recent edition of the AASHTO LRFD Bridge Design Specifications is the 9th edition, which was published in 2020. It shows that LRFD is among the presently used principles in structural engineering.
There you have it: the science behind Load and Resistance Factor Design.
Philosophies like LRFD have been indeed significant in building adequate structures globally.
At the end of the day, there will always be development on principles, theories, and standards for engineering practices. Ultimately, great leadership in the field and proper application of these methods will create an impact in ensuring the safety and stability of the structures we build.
About the Author:
Aaron Simmons is the creator of TestPrepGenie.com. He believes in studying smart rather than studying hard to be successful. On the blog, you’ll find him sharing tips and tricks on how to develop smarter study habits and learning to test prep the right way.
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Anthony Fasano, P.E.
Engineering Management Institute
Author of Engineer Your Own Success