Should Structural Engineers Adopt A New Design Method? LRFD vs. ASD. The question of what method to use when designing structural members for

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Collin Ross
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1 Michelle Mentzer English 202C.24 February 22, 2001 Should Structural Engineers Adopt A New Design Method? LRFD vs. ASD The question of what method to use when designing structural members for buildings seems like it should be purely a technical one, but when it affects the standards to which professionals in the field are accustomed, it becomes much more involved. The current standard for industry use in steel design is the Allowable Stress Design (ASD). However, the future of the field seems to be headed in the direction of the Load and Resistance Factor Design (LRFD). ASD has successfully served the profession since 1923, so this leaves many established structural engineers wondering why a change is necessary. While their apprehension is understandable, movement toward LRFD will only benefit the field in the future. To better understand the controversy surrounding the proposed switch from ASD to LRFD, it is helpful to examine some of the history behind the two design methods. The ASD method began in the 1920s and has been continuously updated and used effectively ever since then. [3] The industry has made many advances in material since ASD was originally developed. At that time, most materials behaved elastically, but now many common materials behave inelastically or plastically. Engineers have developed ways of looking at the new situations through this method. They developed safety factors to ensure that the structure will still be safe if the members do not behave elastically. However, the engineers are limited in how well they can incorporate these new properties 1

2 into the ASD calculations. Since ASD was not initially developed with these types of behaviors in mind, the results, while on the safe side, are not the most accurate. The safety factors generally overcompensate for the difference in strength, so the members are overdesigned. These limitations were the incentive for developing a new method of steel design, LRFD. LRFD is being developed by the American Institute of Steel Construction (AISC) as a replacement for ASD, which is becoming outdated. LRFD takes material differences into account and is much more accurate. While it is still safe, it is more economical than ASD. Currently, the AISC board is continuing to develop LRFD while maintaining ASD for use in the field. The controversy stems from the fact that any time a change of any kind is proposed, there is strong opposition from those who have grown accustomed to the old way and feel that it is still adequate. Many established Structural Engineers fall into this category. These people have successfully performed their job for years, so they see no reason for change. They look at the new design method as an inconvenience, an unnecessary complication to a procedure that has become second nature to them. While the ASD method is simple to anyone who just finished school and learned steel design using LRFD, the methodology of LRFD is difficult to grasp for those accustomed to ASD. The older professionals may also fear that younger engineers just entering the field may have an extra edge over the experienced engineers if LRFD is adopted. Additionally, it appears that, at least in the short term, there is no problem with continuing to use ASD. The reason for adopting LRFD is to allow further development in the design of steel structures. However, this may not seem like an appealing idea to the older professionals in the field either. Since people always tend to resist change, 2

3 especially once they get comfortable with the old way of doing something, they have no desire to change the design method simply the overall design can be changed. Despite the resistance of many structural engineers, there is still a strong group pushing for the adoption of LRFD in place of ASD. Educators and researchers lead this group. They are the people who have the greatest interest in seeing improvements in the field. No one in this group would deny that a lot has been accomplished with ASD, but they see that in order for design progress to continue, some fundamental changes must be made. Innovative progress is required for continued success in any industry. An example of an industry in the U.S. that tried to stay competitive without this kind of progress and failed is the automobile industry. Since then, many important and innovative changes have continuously been made in that industry to accommodate changing needs. Examples include seat belts, fuel injection, antilock breaks, and airbags. [3] When ASD was developed, only elastically behaving materials were considered. Since that time, new materials have been developed many of which behave inelastically. LRFD offers a solution to inelastic design that ASD is incapable of providing. Instead of addressing the root of the differences in material, ASD uses a general safety factor that has little to do with the actual properties of the material. While this is suitable for current building design, it does not leave room for progress in the way that buildings are designed. [2] Topics such as allowable stress and seismic design are not addressed sufficiently in ASD. LRFD takes care of this already and still is being developed and improved. [1] The conflict between the structural engineers who are set in their ways and those who see the need for continued progress to keep the field competitive and successful has 3

4 a possible solution in sight. Those who wish to make LRFD the industry standard are as determined to see that happen as others are to resist the change. Since LRFD is backed so strongly by educators, they will do whatever they can to make the transition easier. They have scheduled conferences around the country to present LRFD through a threeday tutorial. Professors in favor of the change are willing to train all interested structural engineers in the field so that they are as familiar with the method as they would be if they had just graduated from school. [4] Experienced engineers may fear that this will cause them lose their edge over new engineers. The fear stems from the fact that an engineer just out of school would not generally be as comfortable with designing steel as someone who has several years of experience. However, in most cases the values given by the LRFD calculations will be very similar to those given by ASD, so an experienced engineer will still know if an error has occurred. [2] Therefore, the experienced engineers learning the new method will still have the benefit of their time in the field. Adopting the new method will not jeopardize their seniority. This similarity of the values produced by the two methods is why many people say that there is no need for the change. It is important to realize that although the design changes have little impact on current traditional designs, the only way for the field to progress is to adopt a more accurate design method. In addition to educating practicing engineers in LRFD, that is the method that most universities are currently teaching. The new engineers entering the field will be qualified to use this method but will most likely be forced to use ASD since most companies have not yet switched to LRFD. Eventually, when older engineers learn the new method, the younger engineers will be well prepared for the change. Therefore, it is in the engineer s best interest to take advantage of the conferences and learn LRFD 4

5 as soon as possible so that when their company finally decides to adopt the change, they will not be left behind. While steps are being taken in the right direction, eventually firms are going to have to take it upon themselves to adopt a change from ASD to LRFD. ASD has served the field well in the past, but it has reached its limit of development. Most people agree that the future of the field lies with LRFD; the conflict is in the short term since either method appears to be acceptable. Experienced engineers are never going to be happy about changing their procedures, but at some point the change will have to be made. This is the only way that the field of structural design in this country can continue to have the success that it has always known in the past. Resources are currently available to educate engineers in the new method and make the switch. The question then arises as to when it would be appropriate to adopt the change. Why delay the progress of structural design rather than take just three days to learn a new method? Works Cited [1] American Institute of Steel Construction. Load & Resistance Factor Design: Manual of Steel Construction. Vol. 1 Chicago: AISC, 1998 ed. [2] Carter, Charles J. What s Up With the New Design Procedures?: LRFD and the Steel Detailer. Modern Steel Construction. May [3] Carter, Charles J. The Better (Not Bitter) Truth. Steelstuff. February Accessed 2/15/01. < [4] Melnick, Scott. No Gamble Involved, LRFD Is Here To Stay. Modern Steel Construction. January

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