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Very Short Slotted Connection

Very Short Slotted Connection

Very Short Slotted Connection

(OP)
We have a set of shop drawings that came in where every connection, every one, has a 13/16" x 1" horizontal slot at each bolt.  The project involves numerous independent column/beam/column segments that span across areas where adjacent bearing walls are terminated.

We called for A325N (bearing) connections so the slots were a surprise.  We were told that this is just a field tolerance thing, that its no big deal and that it is a common practice.

But I've never seen a case where a fabricator, on their own initiative, simply used slots at every bolt without notifying the engineer.

Do you see these very short slots used in straight-up bearing connections in your own practice?

RE: Very Short Slotted Connection

Yes.  For typical shear connections resisting vertical forces a horizontal slot will not have any effect on the strength of the connection.  

If they were slip critical to begin with then yes the strength would decrease with the slot.  

RE: Very Short Slotted Connection

(OP)
Yes, but do you want to introduce the horizontal "slop" in your connections?  That seems wrong.  And with the slop comes potential movement and cracking in slabs, stress in other features/elements that bridge the connection, etc.

I know I'd never do it at a brace, but even at other connections it seems wrong.

RE: Very Short Slotted Connection

I have this come up quite a bit.  A long as the slot is normal to the load direction then I allow it.  AISC uses no reduction for this (see Table 1-D 9th edition).  I also often get the request for parallel to load direction slots.  I will not allow this unless the connection has been designed accordingly with slip critical bolts.  As far as the horizontal slop I see your point but I find it unlikely that your issues will happen.  I think that you have to consider the friction of the steel on the bolts from vertical load.  Say for mu = 0.7 (dry steel on steel) then for a bearing connection with you could safely say you have 70% of the dead load bolt load allowed for lateral load.  This should cover most situations with plenty to spare.  If you are using pretensioned connections then you have the slip critical values.  Not sure if this will convince you, but certainly something to think about.

RE: Very Short Slotted Connection

(OP)
Thanks for the replies....I just have gone through 25 years or so of structural design - never seen this come up to this extent before.  And that tells me that the slots aren't all that critical to have for erection purposes...I was just suspecting that this is just another one of those "lazy" things that fabricators and/or contractors ask for to allow them to be more sloppy in their plumbing of columns, etc....sort of like the concrete finisher who keeps slopping water on the slab to ease the troweling...

RE: Very Short Slotted Connection

Most fabricators I have dealt with for the past 25 years have a list of "Standard Clip angles" in their set of fabrication/detailing standards.  These standard clip angles are used to resist shear and/or axial loads and have horizontal short slots that are normal to the beam end reaction.  This is not a "lazy" thing that fabricators due to be more sloppy in the column plumbing. The reason they do this is to allow the same clip with the same gage to be used on beams with differing web thicknesses and still keep a typical center to center bolt spacing on end connections.  They can pre-fab these angles ahead of time during slow shop times so that when the work load builds up these are already made and ready to weld up to the beam ends.

RE: Very Short Slotted Connection

I have never seen this before either, so maybe it depends what region you are in.  Maybe the AISC Solutions Center would have something interesting to say about this.  If you are worried about potential horizontal slip, maybe you could have them use A325 TC bolts and have them fully tensioned.  These bolts are commonly used in my area.  Maybe you wouldn't get the full effect of a slip critical connection due to faying surfaces being painted, but you probably would develop significant friction that might prevent movement.

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