Fatigue Loading on Simple Shear Connections

[jmggks]

I have a basic question.
My understanding is that one of the ways that simple shear connections accommodate rotations is through localize yielding, for example at bolt holes.
My understanding is also that cyclic loading that is sufficient in magnitude to cause yielding will cause crack growth and fatigue failure.
So can a beam that is subject to high cycle bending loading loads be designed as simply supported, and if so what specific end connections (e.g. double clip angles) are appropriate?

Thanks for your answers.

 

[lexpatrie]

Normal gravity load connections don't get fatigue in that sense. I also think the deformation you're referring to is largely at the bolt holes. These connections aren't exactly simple, but they are adequately safe when designed as simple support connections.

I'd also encourage you to contact the steel solutions center, they'll have a more thorough explanation.

https://www.aisc.org/steel-solutions-center/

Regards,
Brian

 

[jmggks]

Thanks Brian. The steel solutions center is a good idea.

This is a beam that supports runways for an underhung automated crane system, and cranes will pass by every 3 minutes, so definitely a fatigue situation.

 

[straub46]

A prime example of why more information = better answers.

For crane systems, you should be looking at some kind of bearing connection at the end of the crane beams, whether they sit on a bracket off a building column or an independent column. Holes are a major stress concentration area; I don't remember which crane guide it's from but there's a comment that non-bearing shear connections should be avoided in cranes in high fatigue situations as to eliminate one source of possible failure in the beam. Since your cranes are running all the time, I would highly recommend looking into a bearing end connection of the beams.

Go Bucks!

 

[jmggks]

It's an underhung crane system, so bearing supports are not an option. Similar previous systems have been installed with runways hung from beams that have full moment end connections (shear tabs and either flange plates or field welded flanges). There are reasons in this particular situation why it would be advantageous to use simple shear connections rather than full moment connections.

 

[straub46]

Are you concerned about the runway beam or the beams that support it? I would have no qualms with a roof beam having a simple end connection that's supporting an underhung crane. Were the moment connections provided for stiffness/provide a lateral load path?

Go Bucks!

 

[lexpatrie]

Crane rail supporting elements are kind of a classic (potential) fatigue situation. Stress range plays a role, and the type of connection selected.

 

[jmggks]

Going back to straub46's reply, I'm concerned about the beams that support the runways. BTW, this is a structure inside a building, not the building itself.

Previous systems had fully restrained beam to column connections to provide lateral stiffness and lateral force resistance. The system is 2 bays wide. Automated cranes carry loads forward down 1 bay and return empty down the second bay. If both bays are moment frames, a great deal of load (and cyclic load range) needs to pass through the center column. The system has sufficient lateral stiffness and strength with 1 rigid bay and the other bay lean-on for lateral strength/stiffness. Doing this simplifies the analysis and results in much lower forces at the rigid beam to center column connection.

I think you've answered my question when you say that you would have no qualms about a roof beam with simple shear connections that supports an underhung crane, but I'm just trying to understand how fatigue applies to simple shear connections. The bolts in any such shear connection would be designed as slip-critical, right? How are slip critical bolts consistent with the simple shear assumption of minimal resistance to rotation? And as I said originally, I've always understood that simple shear connections typically involve some self-limiting local plastic deformation, but is that deformation really self-limiting if the load is applied in a cyclic manner?

 

[lexpatrie]

I wouldn't ignore fatigue loading for anything supporting a crane. Normal beams don't usually see enough load cycles to produce fatigue despite the stress ranges. You can calc them out but it's probably not going to control to the point AISC has mentioned it several times (i.e. fatigue for wind doesn't really exist, not in steel construction).


Fatigue and Wind, Steel Interchange, Modern Steel Construction, May 2004

Keep in mind this is 2004 and they may have a newer answer somewhere I'm not aware of. If you find anything more recent or contradictory, do please let us know.

Also, it seems like the Australians use fatigue a bit differently so there's a fair array of "wind fatigue on metal roofs" kind of publications, I'm not convinced this is actually fatigue in the traditional sense, but rather low-cycle plastic fracture.

 

[SWComposites]

jmggks - please post a picture of your “simple shear connection”. And also what you mean by “accommodate rotations”.

Us aerospace structures types deal with fatigue of bolted joints all the time.

Fatigue in metallic bolted joints is typically in the form of cracking in the net section area under tensile stress. Bearing stress typically does not result in a fatigue unless end distances are too short.

 

[jmggks]

SWComposites,

I appreciate you wanting to help with this, but this is more of a structural steel question. When modeling a steel structure, connections are usually defined as simple or moment resisting. There are various standard types of simple shear connections (shear tabs, double clip angles etc.). They must all have the ability to accommodate some rotation without developing significant moments. Otherwise, it would not be appropriate to model them as pin connected.

 

[SWComposites]

Ok, presumably you are referring to in plane rotations of the joint.
And presumably you are referring to joints with more than one fastener (if only one fastener then its pinned by definition).
So if you have fatigue loading that cause bearing yield or net section yield, then it’s likely a problem for fatigue cracking. I don’t see how you can consider a multiple fastener joint to be pinned under fatigue loads; it only makes sense to me to maybe consider it (sort of) pinned under ultimate loading conditions.

 

[canwesteng]

What is the actual load in the connection? I think the concern is valid, but the load in these connections should be quite low. Might make sense to look at the stress between the bolt and the hole to see if it is negligible first. FWIW, I have seen cracks in bolts holes in ~25 year old structures under bins (constant loading and unloading), but stress at the bolt hole would be much higher than this.

 

[jmggks]

I am looking at a W18x50 beam. ASD design loads at the ends are 9.5k shear and +/- 5.4k axial load (there is no diaphragm, so horizontal bracing is provided and this leads to some axial loading). For fatigue, the end connections see a range of 6.6k shear.