Slip critical connection for braces with standard round holes 2

[VinuCG]

Dear All,
I have a condition on one of my projects that the connection engineer wants all bolts in braces to be 1" dia A325SC bolts. I have detailed the braces with oversized holes in gusset plates with 1" dia A325SC bolts. After reviewing the drawings, the engineer asked me whether the holes can be standard holes. Since if it requires oversized holes, some of the bracing connections may need an additional bolt row. Can anyone explain to me if standard holes are needed in connection, then why can't be a bearing connection instead of SC? Also, I have seen many design drawings/connection examples showing brace connections with SC joints that utilize oversized or slotted holes. Is there any reason engineer wants the standard holes to be used instead of oversized holes?

 

[DrZoidberWoop]

The EOR has the final say typically, but are usually open for discussion on reasonable proposals. In my opinion, standard-on-standard holes in braces are a giant headache for the erector. Adding a row of bolts to preserve/improve erectability seems like a reasonable trade-off.

From experience, I'd say you could use the bolts in a "bearing" style connection w/ standard-on-standard holes, but I wouldn't take this approach anymore. The recent trend in the fabrication industry is to make standard holes, for bolts 1" diameter & larger, 1/8" larger than the bolt. It used to be 1/16". That might not seem like a lot of slop, but if you expect cyclic loading or you're designing a vibration-sensitive building, like a lab or medical operation space, best to stick w/ slip-critical w/ STD-on-OVS holes and properly joined faying surfaces.

 

[Tmoose]

I predict that if the braces are made with standard clearance holes the installers are going to have to oversize or slot some holes to assemble them (the braces and bolts).
The methods used in the field, especially 2nd shift, to enlarge holes to permit assembly may not be what you or the "connection engineer" are expecting.

 

[dik]

If they are slip critical, does the extra 1/16" make any difference? For bracing, some engineers like slip critical connections anyway.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[DrZoidberWoop]

I haven't noticed the novel +1/16" making much of a difference, other than making shop-drawing review a living hell when the detailing didn't pay attention to the new code.

For calcs, it affects the relevant checks for braces, gussets, and connections, but it hasn't required any changes at the overall/conceptual level of connection design.

 

[dik]

Did the new code increase the allowable hole size? Since code is minimum; you can always resort back to the old requirements if you want. I suspect that with most connections, due to erection issues, they are likely in bearing before torquing.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

Not sure if there was a question about the necessity of the extra bolt row, but AASHTO specifies a reduction for the slip resistance at oversize holes in Table 6.13.2.8-2 (Kh = 0.85 vs 1.0 for standard holes). I would assume AISC has something similar.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[azcats]

AISC does have similar factor (hsc) w/ same values and 0.70 for long slots.

Commentary says reduction isn't actually due to reduced slip resistance, but because of the 'possible consequences of increased movement with these connections.'

"High-strength bolts properly installed in oversized and short-slotted holes using washers as specified in the RCSC Specification have the same resistance to slip and similar bolts in standard holes."

 

[dik]

The Canadian code has the same provision for long slots, only, that I'm aware of and I think the reduction is 75%. I'm not aware of reductions for oversized or slightly oversized (aka oversized) holes. Since the resistance is determined by clamping action, I don't see that a slight hole size difference is going to have any impact.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik