Slip Critical Connection 4

[onlym112]

Hello,

I am designing slip critical connections for the first time and was wondering if someone could help me verify a few things:

1) If you want to prevent slip on a connection that is subject to load reversals, fatigue, etc., you would design for the slip conditions using specified loads(service loads) and check the bearing limit states using factored loads as an additional precautionary measure. Please correct me if I'm wrong, but my understanding is that slip critical connections will withstand loads solely based on the friction provided by the additional clamping force of the bolts on the connection for the lifetime of the structure and SHOULD NEVER withstand forces in bearing. If the connection goes into bearing for whatever reason, we ensure that all of its bearing limit states are sufficient to withstand the factored loads. Is it correct to say that slip critical connection is conceptually similar to that of a precambered beam in which the beam is precambered to a set deflection so that under regular service loads the beam is "flat", and any additional unexpected overload for whatever reason is accounted for by the regular limit states design of the beam?


2) When drawing the connection, is there additional information that needs to be added to my drawing other than specifying to use A325SC bolts by turn-of-nut method and its fraying surface class?

Thank you,

 

[JoshPlumSE]

Are you in the US, Canada, Europe, or what?

I ask because I'm only familiar with the US codes with respect to slip critical connections. And, they are not quite what you describe. Theoretically, they should be. But, they were simplified by AISC so that you could use the same LRFD level loading on slip critical that your other connections are designed for.

 

[BridgeSmith]

We don't allow tensioning of slip-critical bolted connections by turn-of-the-nut. we spec load indicating washers or twist-off (TC) bolts. For turn-of-the-nut tightening, the inspector has to observe the tightening of every bolt. For new construction, we get almost exclusively twist-off fasteners because they're easy to install, tension, and verify - if the splined end is sheared off, it's tensioned.

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

 

[onlym112]

I am in Canada but I rely on AISC a lot as our Canadian Code does not contain many of the information that AISC have. Regarding slip critical connections, the Canadian code in S16:19 Cl. 13.12.2.1 states "For a slip critical connection under the forces produced by service loads, slip of the assembly shall not occur. In addition, the effects of factored loads shall not exceed the resistances of the connection as specified in Cl. 13.12.1" which is the bearing limit states. Additionally, under the yellow pages of the code, it goes on to say "In addition to the slip resistance of the joints, their factored shear resistance as bearing-type joints under factored loads for all applicable ultimate limit states must also me checked."

There are no additional information on slip critical joints in the Canadian code that I am aware of.

 

[onlym112]

For new construction, we get almost exclusively twist-off fasteners because they're easy to install, tension, and verify

Are the twist-off bolts you are talking about F1852?

 

[BridgeSmith]

We spec F3125, which covers standard hex bolts in 120ksi (A325) and 150ksi (A490), and also twist-offs in 120ksi (F1852) and 150ksi (F2280). But yes, F1852, is the spec for twist-off bolts equivalent to A325.

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

 

[canwesteng]

In Canada, slip critical is only to be used for SLS loads, and the connection must be designed for bearing for ULS loads. Slip at the joint is considered a serviceability concern, not a failure condition. The connection still needs to be able to resist the ultimate loading safely and S16 will not let you use friction for that purpose.

 

[dik]

Only... you're correct and S16 clearly stipulates that the loading calculated is the slip load. The only inidcation that it is service loading is as follows. I would assume this is service design loading. I generally use factored loading for added slip resistance.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[onlym112]

Thank you everyone for your help. I really appreciate it.

 

[Veer007]

A thread that started by me earlier may help with a few of your questions, see below.

https://www.eng-tips.com/viewthread.cfm?qid=463988

Thanks in advance!!