AASHTO LFD edge distances

[HgTX]

In the old AASHTO code, a distinction was made for rolled edges between beams and channels on the one hand and other shapes on the other. Is this because there is something different about the edge of a beam or channel as opposed to other shapes (such as angles and I don't know what else), and thus a WT cut from a W-section would also have the same edge distance requirement as beams & channels?

Hg

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[HgTX]

...and a related question:

Planed edges have a lower edge distance requirement in AASHTO LFD than sheared or cut edges. How much would one need to plane off a thermally cut edge to be able to call it a planed edge? (My answer would be to forget all that and just go with the modern LRFD requirement, but that is not an option at the moment.)

Hg

Eng-Tips policies: faq731-376

 

[bridgebuster]

A sheared edged could potentially have a crack; a flame cut edge might become brittle.

If you're working on a DOT project, do they have a Steel Construction Manual? That could trump the AASHTO requirements. I don't see any problem with using the LRFD requirements in lieu of the Standard Specs.

 

[HgTX]

If LRFD doesn't trump it, the SCM definitely wouldn't.

I myself don't see a problem using the LRFD requirements either, but not everyone agrees with me.

As I recall, the changes were made in LRFD because it was decided that any additional brittleness in a thermal-cut edge was not going to be a problem solved by increased edge distance. (I thought I saw written commentary about this somewhere but I can't find it. Anyone know what I am talking about?)

What I don't know is (a) why the old AASHTO specs drew a distinction between "beams and channels" and other shapes and (b) just how much needs to come off the suspect edge for the "planed" tolerance to come into play.

Hg

Eng-Tips policies: faq731-376

 

[bridgebuster]

The distinction between "beams and channels" was probably due to the manufacturing process. How much needs to come off? Why not suggest the difference between the two edge distances in AASHTO?

In NY the SCM trumps AAHTO.

 

[HgTX]

Outside of NY the SCM doesn't exist.

Using the difference between the two edge distances doesn't buy me anything.

It's a pretty common fix, if one busts the (obsolete) 1-1/2" requirement for cut edges, to plane the edge to become eligible for the 1-1/4" distance. But taking off a full 1/8" from something already less than 1-1/2" would end up with something less than 1-1/4". I never really worried about it because my preferred solution was to just use the 1-1/8" LRFD tolerance and not plane anything, but at the moment I may not have that option.

But maybe I do. I shall find out soon. But it would be good to know for the next time I don't have that option. Part of the problem is, in reasoning this out, that it has been determined (hence the changes) that there was no good reason for the old requirement. Makes it hard, then, to apply reason...

Hg

Eng-Tips policies: faq731-376

 

[bridgebuster]

Out of curiousity, what exactly is the situation you're dealing with?

 

[HgTX]

The situation is one in which, at long last, I don't get to make the decision. Edge distance for 7/8" bolt is less than the old requirement of 1-1/2" but more than the new requirement of 1-1/8".

Them as do make the decision won't use LRFD edge distances unless the whole structure is reanalyzed using LRFD loading and methods. Which demonstrates an unfortunate lack of understanding of the edge distance requirements, but there is nothing I can do about that.

Hg

Eng-Tips policies: faq731-376

 

[bridgebuster]

I know the feeling. I have clients with a similar mentality.

Can you make the provisions of 10.24.7.2 work for your situation?

 

[tumbleleaves]

The minimum distances (based on my unsubstantiated guess) are based on construction tolerances and methods. The distance was likely reduced for thermal edges since this would imply work "in the field". Edge distance was possibly decreased for the flanges in channels and beams because this is used so often (lateral bracing) in secondary connections.

Agree with the general consensus that blindly and rigidly following code is an annoyance. Does anyone have some older versions of the standard specs on hand to see when this was set and if it changed over the years?

If you calculated the stresses and failure mechanisms and it is sufficiently safe in a sane world it would be done.