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Load and Resistance Factor Design (LRFD) and Its Impact on Your Project
Up until the early 1970's, service level design was used for all structures. Under service level design, the stresses in a structure caused by design loads were compared to allowable stresses. These allowable stresses were sometimes increased for unusual loads like earthquakes. With this approach, all loads have the same factor of safety applied to them. This approach does not take into account the variability of the loads or the variability of the strength of the member.
In the 1970's a load factor approach was adopted in concrete design. Steel design also developed an alternative plastic design approach. The basic concept of these approaches was to look at the failure mode of a structure and compare this failure mode to factored loads applied to the structure. In concrete design, different load factors were applied to different types of loads. A smaller load factor was applied to permanent loads than was applied to live loads. In steel design, permanent and live loads had the same load factor. Earthquake loads had a different load factor applied to them. The load and resistance factor design method improves on these previous methods of design to create a design that has more reliability.
When a structure is designed, there are many factors that enter into the safety of the structure. These include the variability of the loads applied to the structure, the variability of the strength of the materials, and the variability of the construction. The reliability of a structure is a measure of its safety taking into account all of the above variables. When the reliabilities of structures were compared for structures designed according to the service load method, the results were not consistent. When the same reliabilities were checked for structures designed according to the load factor method, the consistency was better than the service load method, but still not that good.
With the concept of reliability developed, code developers took a different approach to the writing of the LRFD specification. They first determined what reliability was acceptable for a structure. They then searched for methods that could be easily implemented and achieve a consistent reliability. An approach similar to the load factor approach seemed most promising since different load factors could be applied to the different loads, and different resistance factors could be assigned to different failure mechanisms and material properties. Then, they determined what factors had to be applied to the loads and what factors needed to be applied to the strength of a member to achieve the desired reliability. The results look very much like the load factor design method, but with different factors applied to what are called limit states.
A limit state is a criteria that we want a structure to satisfy. Beyond the limit state, the structure or component ceases to satisfy the provisions for which it was designed. Examples are:- Service Limit State - This defines deflection, deformation, and crack width criteria.
- Fatigue Limit State - This defines stress range criteria due to live loads.
- Strength Limit State - This defines criteria for the ultimate capacity of the structure.
- Extreme Event Limit State - This defines criteria for extreme events such as floods, earthquakes, and hurricanes.
A structure and all of its components must satisfy all limit state requirements under the LRFD design criteria.
For structures that are designed according to the LRFD method, the consistency of the safety of the structures should be improved. There should also be an improvement between the consistency of the safety provided by different types of structures, such as steel and concrete structures. There will be both increases and decreases in the amount of materials used for the construction of structures, depending on how it satisfies the limit state criteria.
To date, there has been varying progress in the development of the LRFD method. The American Institute of Steel Construction (responsible for developing steel design specifications) has developed an LRFD code for the design of steel buildings. The American Concrete Institute (responsible for developing concrete design specifications) has not recalibrated their load factor method to meet the required reliability criteria of the LRFD method. The Uniform Building Code has recently adopted a load factor design method for masonry. Wood has not had an LRFD approach developed yet. The American Association of State Highway and Transportation Officials (AASHTO) has developed a LRFD code for the design of bridges. This was written for steel, concrete, aluminum, and wood structures. Therefore, we can conclude that the LRFD method is still in the development stages.
The steel LRFD code was first published in 1986 and the AASHTO LRFD code was first published in 1994. Most structural engineers have not adopted the LRFD code in steel design. The reason for this has to do with relearning how to apply the code. It also means updating computer programs to work with the LRFD code. This has slowed the adoption of the code.
Currently, AASHTO has two accepted codes for the design of bridges. One is the old Standard Specifications for Highway Bridges and the other is the LRFD Bridge Design Specifications. Factors that are slowing the adoption of the LRFD Specification as the only specification include computer program adaptation and education. But, within the next few years, the LRFD Specification should be adopted.
The time it takes to design structures will not be much different than before with this design procedure. As we learn more and more about structure and component behavior, we find that engineers should be responsible for checking and designing more items. This continues to add to the time and expense that it takes to design a structure. We try making this as efficient as possible with design aids and the use of computers.
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