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LFD vs. LRFD Fatigue Design 1

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71corvette

71corvette

Structural
Feb 26, 2003
105
Some co-workers and I are currently working on a bridge project which involves the LRFD design of a three span continuous steel plate girder bridge.

Out of our own curiousity we re-ran the model using LFD and an HS20 design truck to see what the difference would be. After doing so we noticed that the required plate sizes for LFD were noticably bigger. Typically when we've compared LRFD and LFD results in the past we've found them to be 'fairly' similar. However, this does not appear to be the case for our fatigue analysis.

I realize the method of how fatigue requirements are calcluated between the two codes are quite different, but should they be yielding much different results? Our comparison shows LFD to be quite conservative compared to LRFD.

Thanks for the input!
 
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They shouldn't be that far apart, if at all. I attended a lecture by Dennis Mertz, PhD., where he offered some explanation of the development of the fatigue design requirements in the AASHTO Specs. This was a lecture in Baltimore covering HPS and the LRFD Specs. There is much misunderstanding about fatigue and I don't know specifically what happened in your circumstance. I wonder, however, how well the programmer(s) understand fatigue, LFD and LRFD. Many don't know the engineering behind the code formulas and sometimes misapply them, hopefully in a conservative way. But, IMHO, there is also such a thing as too conservative WRT structural analysis and design.

Good Luck - Dinosaur
 
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Thanks Dinosaur,

We've spent a fair amount of time verifying the computer output by hand and we think the difference is coming from the different distribution factors used by each code. LRFD uses a more sophisticated approach in calculating the distribtion factors and as a result is less conservative.
 
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