Width to Thickness Ratio

[ToadJones]

I am analyzing an old built-up girder made with a web plate and pairs of equal leg angles for the top and bottom flanges.
It is quite a sizable girder (about 7' deep) but only has a 7/16" web.

I was wondering how out of line it would be to check the web slenderness h/tw according to AISC chapter B, by taking the height between the toes of the flange angles rather than the clear flange distance. The angles are 8x8's. If I checked the web between the vertical toes it would reduce the "h" value 16".

I cant see the web buckling where it is sandwiched in between the flange angles.

Anyone have any thoughts?

 

[Ron]

TJ...I agree. Web won't buckle in the "sandwiched" area.

 

[ToadJones]

Ron-
You don't know of any code provisions qualifying something like this, do you?
This just doesn't fall neatly into AISC's nice little examples chart in Chapter B of the spec.
I have wondered about this in the past too. This job has several girders like the one I described and I am trying to avoid the dreaded slender elements provisions if possible.
If it has to be done, so be it

 

[JAE]

Or maybe just to be a bit conservative - go 1/3 of the way past the toe of each angle?

 

[ToadJones]

Yea-Maybe I could just work it backwards and see how far into the angles the limiting ratio puts me?

Back to the original question though....don't you think that using the distance between the angles is more in keeping with the intent of "limiting width-thickness ratios" as they are stated in the code?

 

[JAE]

Assuming the angles are a bit thicker than the web yes. Otherwise, if they are the same thickness or only slightly thicker, I think the answer is somewhere between the tips and the heel of the angles.

The angles can bend under a buckled web condition to some extent right? This is analogous to the stepped column situation for bridge crane columns where the lower section of column is deeper than the upper section of column and the engineer wonders...what's the real k value and what I should I use?

 

[ToadJones]

I agree JAE-
In this case the each angle is twice the thickness of the web...(7/16 web sandwiched between 7/8" angles).

JAE- I take it you have had the fine pleasure of going through figuring out K-factors for stepped columns....quite fun eh?

 

[JAE]

I just stay far away from them....or go conservative and ignore the deeper part. To much time saving a few dollars of steel to justify the extra effort on smaller projects.

 

[ToadJones]

They can be a pain in the rear...
I analyze them quite often...thankfully I have some FEM techniques handed down to me from some guys who helped write the Mill Bldg spec that make them a little bit easier.
I agree...I like an independent crane shaft.

 

[miecz]

I believe this geometry is often used for railway bridges. You might find some guidance in an AREMA Design Manual.

 

[ToadJones]

This geometry was used in industrial applications for years and years as well. The problem is, I have to use the Spec that the project calls for....maybe AREMA has some commentary, but I don't have any AREMA literature.

 

[mikesg]

In our local code for steel structures the effective web height is taken as the distance between the lines of bolts connecting the angles to the web. I believe it is the same in the Russian SNIP.

 

[bootlegend]

I think mikesg has stated it correctly per AISC's view. Use the hole to hole dimension if bolted or the inside weld-to-weld dimension for a welded situation. The glossary in the AISC 13th addition spells it out pretty well.

 

[ToadJones]

bootlegend-
In the glossary?

 

[bootlegend]

Well, actually it is defined in the symbols section right before the glossary. In the AISC 13th edition, pg 16.1-xxxviii and pg 16.1-xxxix define h and h-sub-c.

 

[firai]

As mikesg and bootlegend pointed out, AISC's definition of h for built-ups is the distance between adjacent line of fasteners or the clear distance between flanges for welded sections.

On a side note, regarding JAE and ToadJones's discussion about stepped columns, there's an article by Suresh T. Dalal named "Some Non-Conventional Cases of Column Design" that gives guidance into determining buckling loads for stepped columns. Dr. White at Georgia Tech also has an AISC Design Guide coming out soon that talks about designing with web-tapered members (such as PEMB frame configurations).

Structural Design Engineer
New York, NY

 

[dik]

If you have a properly rivetted connection with two angles twice the thickness of the web, for modelling, you might consider the web to be 'fixed' at the toe of the angle or at least at the line of fasteners.

Dik