Bearing and Slip Critical Connections.....

[Ron]

AISC says that if there is a significant stress reversal, the connection should be designed as slip critical. This is clear for connections that have wind uplift reversing gravity loads; however, should this also include lateral force reversals such as for shear walls? I have my thoughts on this, what are yours?

 

[Guastavino]

Ron,

Not gonna lie. I didn't know that. What part of spec is that in?

I will say this. In my career, I have never specified a SC connection. I may be wrong to not have done it due to ignorance.

Also, the firm I used to be at, I don't know of anyone that ever spec'ed SC connections.

I'm ready for the njlutzwe roast if it is needed!

 

[DETstru]

I'm not exactly sure what you're asking regarding shear walls but the load reversal comment in the specification is regarding a tension load turning into a compression load (or vice versa). Examples of this would be:

1. A brace member that can experience either tension or compression depending on the direction of lateral load on the building.

2. Flange plate moment connections that can experience load reversals depending on the direction of lateral load on the building.

 

[DETstru]

Here's a good article on the subject:

https://www.dropbox.com/s/zdjoaed1kw7uw0i/Slip Critical Connection - steelwise article.pdf?dl=0

 

[Ron]

njlutzwe.....look at AISC FAQ 6.5.3 and 6.5.4

 

[canwesteng]

That isn't my interpretation of AISC's guidance here, the bolts in connections for tension/compression braces are required pre tensioned but not slip critical. I think the infrequent occurrence of significant load reversal in wind cases means slip critical connections are not required.

I think the requirement for seismic is that they have a slip critical surface and the bolts be pre tensioned, but the connection isn't designed as slip critical. Don't quote me as I don't use AISC specs much and rarely do seismic design.

These are needed when under vibrating machinery that subjects the connection to load reversal (ie dynamic forces overcome dead load), or in oversized or slotted holes where slip would be a problem.

 

[JAE]

AISC Specification section J1.10 outlines when pretensioned bolts are required.

J3.1 also comments on it:

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[JAE]

Commentary on J1.10 and J3.1:

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[sandman21]

If your connection will see seismic loads it needs to comply with AISC 341-10 D2.2(4), which requires pretensioned bolts with a class A faying surface. The connection will need to be designed as if it a bearing connection even if you detail/design of SC.

 

[SlideRuleEra]

I used slip critical collections on a railcar unloader project that I designed (essentially a short ground elevation railroad bridge over an unloading pit). The connections have to support the weight of the largest locomotives and fully loaded coal cars. Simultaneously, there are significant longitudinal lateral loads from the start-stop motion as the locomotives move the entire train during the bottom-dump car unloading process. Slip critical connections were essential for both the vertical and lateral loads.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[nutte]

AISC says that if there is a significant stress reversal, the connection should be designed as slip critical.

I don't know that AISC says this. It's not in the FAQ sections you mentioned, or in the Specification sections noted by JAE. "Pretensioned bolts" does not mean "slip critical connection."

 

[Ron]

nutte....here is an excerpt from AISC Engineering FAQ 6.5.4...

In some cases, slip resistance is required. The AISC and RCSC Specifications list cases where connections must be designated by the Structural Engineer of Record as slip-critical:
•Connections with oversized holes
•Connections with slotted holes when the direction of the slot is not perpendicular to the direction of the load, unless slip is the intended function of the joint.
•Connections subject to fatigue or [highlight #FCE94F]significant load reversal.[/highlight]
•Connections in which welds and bolts share in transmitting shear loads at a common faying surface

 

[JedClampett]

I use SC anytime the loads are not insignificant. The reason is (and I'm ashamed of this) that I'm lazy. SC is SC. Capacity belongs to the bolts. for bearing connections, I've got to worry about connection piece thickness. Not a problem with the angle or shear tab, you give the thickness for those. But I have to look up the web thickness of the wide flange and channel sections. And they're sometimes pretty thin. That means a calculation to check the bearing vs. the thickness.

 

[WinelandV]

Jed,

Unfortunately, specifying an SC connection doesn't get you out of checking the bearing strength of the bolts. From AISC 13th edition, J3.8:

"High-strength bolts in slip-critical connections are permitted to be designed to prevent slip either as a serviceability limit state or at the required strength limit state. The connection must also be checked for shear strength in accordance with Sections J3.6 and J3.7 and bearing strength with Section J3.1 and J3.10."
(underline added by me)

 

[canwesteng]

Now, I may be way off base here, but what you are doing seems incorrect Jed. I thought slip critical joints are designed to resist service loads, but must also always also be designed as bearing connections for ultimate loads.

 

[nutte]

I think what Jed means is that by sizing the bolts for the much-lower slip critical strength, bearing of connected elements almost never controls. This method results in connections with more bolts than would otherwise be required, so it comes at a cost to the fabricator and erector.

Back to the original question, I don't think collectors, drag struts, shear walls, and other such elements necessarily need slip critical connections. Design wind load is an infrequent load. I believe the intent is that members that will have frequent live load reversals, like those supporting cranes or other types of machinery, should be designed as slip critical. If this were not the case, then every bolted vertical bracing connection would need slip critical bolts, and this is not the common practice.

 

[sbisteel]

The latest version of the RCSC requires pretensioned joints for load reversal, not slip critical:

 

[structSU10]

We use SC bolts in all LFRS members, be it braced frames or moment frames, in which there could be a load reversal. I didn't read all above, but it is due to the 'banging bolt' syndrome, in which at load reversal you may hear a bang when the connection shifts to engage the bolt in bearing in the other direction. This makes some people uneasy. So, AISC recommends the SC bolts.

 

[canwesteng]

I thought banging bolts was caused by the opposite - bolts pre tensioned when they were designed as bearing. Once the bolts slip and engage in bearing, you get the loud bang since the friction has stored energy sort of like a spring.

 

[KootK]

@Ron: glad you asked this. I've long wanted more clarity on it myself.

We use SC bolts in all LFRS members

Including collectors and drag struts? That's the big question for me as that takes you from a hand full of beams and braces to 1/3 of them.

From a cost perspective, SC connections in my area are a killer because of the costs associated with inspection/correction.

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.