Slip Critical Length Requirements

[Robs1121]

I am working on an analysis where a 1.5 inch pipe is sandwiched between two plates and the plates are welded to a base plate that is bolted into the ground. There are 2 thru bolts that connect the two outer plates to the pipe, the top bolt is the pivot bolt and the bottom is a slotted hole for a bolt. The purpose of the slotted hole is to let the pipe pivot to 20 degrees in each direction from vertical. There is extensive wind loading on the pipe and what is connected to it, so the bottom bolt will need be strong enough (slip critical) to hole the pipe into any position along the 40 degree rotation (20 each direction).

My question is: The specifications for slip critical joints are only for flat plates with enough overlap distance for the bolts to fully engage the plates, however in my case, only a portion of the plate is in contact with the pipe. Is there anywhere i can find specifications regarding reductions for edge distance limitations? Or 2" around the bolt needs to be in full contact with both plates to use the equations in AISC? Is it the same resistance if i clamp two plates with 20 inch or 0.25 inch overlap, since friction is independent of area?

 

[Agent666]

I don't get it, you say you want slip critical and need rotation? Slip critical means no movement/slip/rotation. You have a pin like arrangement with what you propose, with the assembly pivotting about the single bottom bolt and top bolt limits the total movement range once it comes into contact.

Won't the pipe just crush/squash if you have a thru bolt. You can of course built out the curved surface of the pipe to be flat and put a sleeve through the pipe to ensure you achieve the same criteria as the code requires for slip-critical connections. Any movement will expose the unprotected faying surfaces to corrosion if its outside. Basically you can't put the flat plate against the curved surface, i'd imagine for the code to be valid that you would at a minimum need the minimum edge distances in direct contact even if the friction is independent of area.

Can you share more about what is being supported, is it a 'pinned' connection or is it intended to take some moment (due to your slip critical assumption, and the limit on the movement to 20 degrees)? What is causing/allowing the structure to move 20 degrees.

 

[Robs1121]

The pipe is expected to support an antenna and it's a moment connection at the top of the pipe. They use the movement to direct the antenna up or down. Once the antenna is directed in the correct direction than they will clamp the pipe

 

[Agent666]

Ok got it, maybe use a square hollow section instead with a bracket at the top for the antenna?

Or fabricate something at the base of the pipe that gives you two parallel flat surfaces.

 

[Robs1121]

I was given this to analyze and see if it works, for my client a pipe would be optimal. So i want to show that it either works or it doesn't, but i want to make sure i am doing the analysis correctly. There is just no guidance as to how much splice length is required for clamping two surfaces together.

 

[EdStainless]

Given that you only have two lines of contact, and once you tighten the bolt just two spots of contact, I seriously doubt that you will get adequate clamping.
You may need flats on the sides of the pipe, either by mechanically flattening it or by welding plates to it.
The ones that I have used had square tube for the mast and large thick washers on the clamping bolt to assure spreading of the load.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[BridgeSmith]

Counting on friction at the small contact area between a pipe and plates is a dubious proposition.

If you can move the plates a little farther apart, you could sleeve the end of the pipe with a short piece of 2 1/2 x 2 1/2 x 1/4 square tubing (or 5/16 with a little forcing or grinding). That would give you something that has significantly more contact area and lots of crush resistance.

 

[Tmoose]

I'm thinking to keep it from slipping, the clamp bolts may eventually to turn the "pipe" into this -

http://www.brufnut.de/MONTY/HEADLIGHT/Headlight_05.JPG