Eccentricity of framing into the web. 1

[WARose]

Got some interesting feedback from a client reviewing my calcs (something I've thought of myself but rarely included): he asked that I include an offset from a steel beam framing into the web of a column. (I.e. bolted through.) My reply was that there really wasn't any eccentricity.....but theoretically, since the bearing stresses are almost certainly non-uniform, I guess there could be some. So I threw in a "offset" of about 1/4 of the web thickness.

Thoughts on this? We aren't building Swiss watches here, but I've worried some on this myself.

 

[msquared48]

Is he saying that the.moment due to the eccentricity needs to be added to any moment the connection sees?

Mike McCann, PE, SE (WA)

 

[KootK]

- Sure, it's true.

- It's probably more than just 1/4 of the web thickness. As the clip angles flex to relieve moment in the beam end, the same angles will exert a moment on the web. A conservative estimate is to consider the eccentricity to be the distance to the bolt line.

- I don't bother with any of this. I consider it to be of no practical significance and outside of commonly accepted practice. Beside, at some point, we gotta have enough approximation leeway that this job is actually doable by a human with a finite attention span.

- When last I looked into this, I'm pretty sure that Australia and some other places actually force you to consider some eccentricity at your shear only connections. Good for them. Seriously.

- I've got a paper someplace by Socrates Ionnides that studies the interplay between shear eccentricity on the one hand and the effect of the beam kind of fixing the column top on the other. To an extent, those things balance out.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

I consider 3" of eccentricity but only when a column has a beam framing in from 1 side only. If there's beams on either side then I feel that it cancels out enough to ignore it. Rarely does it make a significant change in the column size.

 

[WARose]

@msquared48 He is saying the moment needs to be added to column interaction (stresses) and for the purposes of any lateral drift of the frame.

 

[KootK]

and for the purposes of any lateral drift of the frame.

Seems reasonable to assume that you're dealing with somebody who doesn't understand our craft. Weave a little BS and say that notional loads take care of that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WARose]

[blue](Kootk)[/blue]

Seems reasonable to assume that you're dealing with somebody who doesn't understand our craft. Weave a little BS and say that notional loads take care of that.

I think this got on their check list because of a incident at the plant where a braced but flexible frame had this situation, and the drift po'd everyone.

 

[canwesteng]

I've been of the opinion that clip angles have enough flexural capacity to take the small end moment to the bolt line. At yield, the bearing stress will be uniform. So there is a lower bound solution that matches your modeling so no extra work is needed. If we were dealing with brittle materials, no question I would check an envelope of both condition (effectively what this client is asking for, since I doubt you offset the beam ends the 1/4" to reduce your moments). Ductile materials are smarter than we are, or a whole lot more buildings would be in the dirt.

 

[Mjkkb2]

it is ignored because AISC did enough testing to show that the eccentricity is resolved within the connected elements. Hell if they let you ignore it for a standard bolted connection (to the bolt lines), why would a 1/4" be of any significance to worry about.....