Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
(OP)
For slip critical connection, I always call off a "NO PAINT" requirement at the bolt line to ensure the appropriate surface class is maintained. When dealing with connections attaching to existing steel, I always provided horizontal long slots for erection allowance. If I were to have a hz long slotted shear connection where the bolts are subject to eccentricity effects, does the connection get forced into a slip critical condition (i.e. requiring the "NO PAINT" requirement)? I guess the question becomes is the connection still defined as bearing, as a moment due to eccentricity imposes a force couple on the bolts. The vector sum of the force is then not perpendicular to the longitudinal axis of the long slot. How's my reasoning on this? To be on the safe side, a "NO PAINT" labelling should be included, but is it truly required? (NOTE that when either slip critical or long slotted connections are used, the bolts are always pretensioned). Feel free to point me to any literature on the subject.






RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
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.
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
My original question pertains to applying the Instantaneous Centre of Rotation Method for eccentric bolt groups in conjunction with long slots.
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
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.
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
Its my understanding that failure to resist moment @ the bolt line would then introduce moments into the supporting member (which is not an encouraged practice). The Instantaneous Centre of Rotation method (ICR) essentially comes down to the fact that the bolts are resisting a shear load in addition to a moment derived from the shear load at a distance 'e' from the supporting member.
I've found a clause in the 3rd ed of the AISC Manual stating that slip critical joints are not required if the load is "approximately" normal to the direction of the slot (within 80 to 100 degrees). As this version is the only one accessible to me, I'm not sure if it has been updated in terms of verbiage. (Disclaimer, I'm Canadian and only refer to the AISC periodically; the CISC seems more restrictive by not including the "approximate" wording in their clause).
Combining the AISC wording of "approximately" normal with ICR would suggest that slip critical conditions in some scenarios could be avoided. Of course, its safer to design for slip critical, but this does come at the cost of a less economical design (i.e. additional labour time to mask off NO PAINT regions on members and connection plates).
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
I don't think it's a big deal so long as you account for it. Every moment frame ever dumps moment into the columns. Additionally, even with an extended shear tab, you're still putting moment into the support. It's just capped at the moment at which the shear plate would plastify in flexure.
I see designating the bolt group as moment resisting as being a choice. Pick a path and deal with the associated issues. It would help here to know the details of what you're dealing with. The geometry of your connection and such. In these situations, it is common to slot the shear tab holes but still not go to an extended shear plate connection. It's more eccentricity but not usually that much more.
I would avoid the slip critical connections if at all possible. Proper inspection of those adds real cost. If your geometry is such that you really need to do an extended shear tab, you could do it with two columns of bolts so that you can develop a moment capacity without requiring bolt forces parallel to the slots. I've never done that but it seems rational.
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.
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
My question stemmed from doing work for retrofit jobs. Connecting into existing steel almost immediately requires the use of long slots (unless extensive surveying is conducted and even then does not guarantee successful bolted connections). When connecting a new beam to an existing beam that runs perpendicular, we often like to stop the new beam short of the toes of the existing beam flange for ease of erection (i.e. new beam is not coped to fit inside existing beam). Working for a steel fabricator (and especially for retrofit jobs) we are not always privy to all loads in the structure, so intentionally developing moments and torsion by not accounting for moment resistance @ the bolt line is neither conservative nor good practice.
As a side note, I loathe slip critical connections. They're an overly used and often unnecessary part of connection design (unless you're building some kind of quantum microscope facility where structure movement can't be tolerated)
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
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.
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
This is not documented anywhere in the AISC literature that I know of, but it is my understanding of the requirements of the shear tab procedure. I did ask Dr. Pat Fortney in person this question at the steel conference a few years back, and he confirmed that this is how to consider long-slotted holes in shear tabs.
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
"Slip-Critical Joints
Slip critical joints are only required in the following applications involving shear or combined shear and tension:
...
(3)Joints that utilize slotted holes, except those with applied load approximately normal (within 80 to 100 degrees) to the direction of the long dimension of the slot"
This gives rise to interpretation, but it can be argued that if the ratio of shear normal to the slot (i.e. gravity loads) to the shear parallel to the slot (i.e. moment at boltline resolved into a force couple) is high, enough so the resultant vector force applied to each bolt is within the 80-100 degrees specified, then the use of slip critical conditioning is not mandated.
Again, the main question I was seeking to answer was not if long slotted holes in eccentrically loaded shear connections "should" be treated as slip critical, but whether it "shall" (to use the code terminology) be treated as slip critical
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
If for whatever reason the holes didn't align and the fabricator wanted to weld the shear tab to the beam web, we would all argue that there is no rotational ductility in the connection. Now with SC bolts we're OK with that?
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity
RE: Surface Preparation for Long-Slotted Shear Connections Subjected to Eccentricity