Conveyor Truss Corbel Pin - Expansion Slot & Bearing Capacity 1

[ARS97]

In the design of conveyor box trusses, it is common practice to provide a slotted connection on one end of the truss to allow for thermal expansion & contraction. Typically, this detail consists of a long slot in the plate attached to the truss. (This allows the slot to be aligned with the truss.)

Basically, it's a 2.4375" C1045 pin, a 3" x 7.5" slot, and a 1.5" thick plate. The maximum reaction (ASD) is likely around 100k.

Here's the question - what's the best way to check the bearing capacity of the pin on the slotted hole? I've done some research on my own in regards to Hertzian contact stress and have come to the conclusion that there is no consensus. So.....now what?

AISC J7(b) doesn't apply to this situation (as confirmed by AISC themselves) since it deals with a wide flat surface. This should be fairly similar to Roarke's Chapter 14. I also came across a paper written by Dekker "You Can't Trust Your Lift Ears" (June 1996, Chemical Engineering Magazine) that includes a similar check for lifting lugs. However, if you use any of these methods, the allowable bearing stress is GREATLY exceeded.

So how is this handled? Is a bearing failure (dishing) just assumed to be acceptable in this situation? This connection detail is used widely in the industry, so there's certainly plenty of empirical evidence showing that it "works". So, most likely, the calculation shortfall is likely due to a misunderstanding in what is actually happening.

Any thoughts?

 

[hokie66]

Maybe it is common practice in some areas or sections of the industry, but I have done a lot of conveyor trusses without ever using a sliding pin in a slotted hole.

 

[ARS97]

hokie66 - so how do you do it?

 

[hokie66]

Depends on the length of the conveyor system. Guided slide bearings is one way. If the system is not too long and supported on A frames, you can just let the A frames lean, with only one end anchored to something solid in the longitudinal direction.

 

[ARS97]

I'm not a big fan of letting the A-frames lean, unless it's a minor amount. However, I suppose if proper base details are provided (such as a pin) this concern would be mitigated.

As far as the guided slide bearings......any details for something like this?

 

[hokie66]

These are the people I go to for assistance with bearings in Australia. Not sure about suppliers where you are located.

http://www.granor.com.au/products/structural-bearings/series-bgsu-up-lift-restraint

 

[TLHS]

Andy, I don't see a problem with letting things deflect.

Personally, I let it happen on industrial structures as long as it keeps within drift limits and the rotations at member connections aren't crazy. Heck, it's pretty standard practice as far as I know to build outdoor racks and things with a braced anchor bay and just let everything deflect out away from it.

 

[ARS97]

hokie66 - thanks for the link.....I'll check that out.

TLHS - I've worked in the material handling (coal) industry for 11 years now as an engineer and I don't think I've ever seen a conveyor system that DIDN'T implement some type of slotted hole corbel for each & every truss. I'm not saying it can't be done.....it's just what the industry has typically demanded. Heck, it's standard to provide expansion joints for handrail & conduit as well. In addition, the main supporting buildings at the end of these conveyors typically support the internal belt tension from the conveyor system. Due to this high lateral load, these buildings are subjected to a reasonable amount of drift. If the conveyor truss are somewhat rigidly attached then that load is then partial resisted longitudinally by the trusses, which isn't desirable.

Interesting discussion guys......thanks for your input so far!

 

[aggman]

I have looked at this slotted pin on some of our designs and some other peoples designs. I think that the hertzian analysis is the correct one. ASSHTO also has a section on it as well that they use for highway bridges. Since most designs do not seem to use either of these two methods my thoughts are that in reality the bearing plates will dish until the stress is below some acceptable level. The truss locks into place until the thermal loads force the pin connection to slide by overcoming the dished pocket. For reference we had a long span conveyor truss that would make a "pop" every morning around 11:00 A.M and "pop" again every evening after the sun went down (wasn't my design). After investigation it was the slide bearing popping loose and expanding/ retracting with the sun and temperature. It works but not very scientific or smooth. You can buy neoprene bearing pads very cheap (cheaper than the pins and thick plates cost to fabricate) which will accept up to 1/2 degree of rotation by themselves and have a coefficient of friction of 0.06. This is our standard way to handle expansion. Otherwise we just anchor the truss at a certain point and allow the A-frames to bend if everything is within acceptable limits.

 

[ARS97]

aggman - that's exactly how I've always handled our designs with the slot & pin. (I'll have to check out AASHTO though.....wasn't aware of that one.) These neoprene pads.......do you have a link to these? Sounds like a good idea....mimics a pinned restrained as well as provide expansion.

 

[aggman]

We use SEP bearings a lot (

http://www.sepbearings.com/fluorogold-slide-bearings.html).

I am sure their are others but we have always had good delivery and availability from them. Technical staff is decent as well.

 

[ARS97]

thanks aggman.......I'll check it out