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RE: Who's using ASD or LRFD?

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Title: RE: Who's using ASD or LRFD?

>... how much difference can there be?

Exactly! Steel doesn't know which method was used for the design. It only knows how to behave like steel! I think the real differences is better models in LRFD to assess the behavior, strength and performance. I think you pointed out yourself the differences in the models.

>>The 1999 LRFD Specification will have, among other things,
>>explicit treatment of srength and stiffness requirements for stability
>>bracing...
>Is this going beyond the current practice of specifying limiting
>width/thickness ratios to stave off elastic buckling or are you getting
>into post-buckling strength or improved correlations of buckling response
>of thin elements? Maybe a little more detail on bracing stiffness
>requirements?

Not exactly on the first part. Closer on the second part. It's an assembly of provisions like direct bracing and torsional bracing for beams. Also, similar stuff for column bracing and frame bracing. Gets into strength and stiffness.

>Tell me a little more about the fatigue improvements.

A broader selection of details to assess fatigue categories. A better explanation of what details fall into what categories and why. The ability to calculate permitted stress ranges rather than just using the old tabulated stair-step approach, which made for a hairy time when your numbers fell just to the left or just to the right of a step.

>The sorts of changes (especially fatigue and bracing
>provisions) you mentioned look like general improvements which aren't
>peculiar to LRFD and would be advisable for ASD design like the machinery
>and the associated structures I do. Which will likely always be based on
>elastic response.

True. LRFD is the one that's undergoing active development. ASD isn't, except for safety-related changes, and hasn't since the 1978 version. As you can tell by reading it, the 1989 version is just a reorganization of 1978 with some safety-related changes to things like the provisions for heavy shapes. That's too simplistic a statement, but it's mostly true.

Charlie

P.S. For those of you that want to get up to speed on LRFD, there is a short course at the AISC North American Steel Construction Conference in February. You can get info on this at http://www.aisc.org.

P.P.S. There is also a similar course we're doing with NCSEA in March. You can get info on it at http://www.ncsea.com/engineers/institute/.