expansion in HSS truss

[structuralnerd]

I'm designing a welded square HSS truss pipe support that is approximately 450 feet long with HSS column supports at approximately 80 feet on center. The client has insisted on using tubes because they do not want birds nesting in or on the structure. However, the truss is outside and it will be exposed to maximum 110 degree F and minimum -30 degree F temps. Since it is so long, I think I need to allow for expansion in the truss bottom and top chords. Does anyone know of a good connection detail at the column supports that allows for expansion? Any help is greatly appreciated. When I mentioned this issue to my supervisor, the only thing he said was, "yes, this is something you need to consider"....not much help. Is there anything else I could be missing??

 

[structuralnerd]

Instead of using the slide bearing support, could I use long slotted bolt holes or is that too much friction?

 

[hokie66]

Don't even think about it! Slotted holes with gravity load on the connection create lots of problems which you don't want. They are only useful in vertically slotted connections.

Back to my previous post, I am wrong about simple spans meaning you need twice as many slide bearings. You would only need to allow for movement on one end.

 

[civilperson]

Note: Steel is not erected at minus 30 or at 110 degrees. (I earlier assumed 45 degrees to make roughly equal expansion and contraction.) If the columns are 25 feet tall, the maximum tilt due to temperature change at the extreme outside column will be in the range of L/300, (1.1"/ 300 inches or 0.2 degree rotation). This amount of tilt or bend is easily accommodated with standard 4 anchor rods and a welded base plate. No other provision needs to be made, i.e. no slide bearings, no slotted holes, etc. Make the truss continuous over the column tops with fixed connections. Until the columns get very short, the expansion will be nominal.

 

[structuralnerd]

civilperson -

I'm a bit confused now... you are recommending that I don't need to do the slide connections, and that the tilt is actually very small in the columns? What would you do differently to the base of the column to allow for more tilt? And why is it important to make the truss continuous over the top of the column? Could I just do a standard pinned connection?

hokie66 - Could you tell me some of the problems horizontally slotted holes have when under gravity loads?

 

[gwynn]

If the columns are tall enough the thermal expansion can be accomodated for by either rotation at the base of the columns or elastic deflection of the columns themselves. Check the deflections against the actual column height to see what sort of rotations and/or column bending you have. If the movement does not cause distress in the columns or base assembly, then you do not need any sort of expansion joint at the top of the columns - the columns themselves are the expansion joint. If using column deformation you also need to check the added compression in the truss.

Not sure off hand what you could do to allow for even more movement if the columns and bases cannot combine to allow for the needed movement, outside of a sliding interface.

Standard pinned connection at the top of the columns are fine. Civilperson will probably correct me if I'm wrong, but I believe the comment about continuity was in regards to the thermal expansion (in other words, sliding bearings may not be needed).

What sort of bearing are you dealing with at the piers? I often see pipe bridges requiring elastomeric bearing pads etc. at each pier (thought that's with 200' spans). If a sliding surface is needed, the details may depend on any bearing requirements.

 

[structuralnerd]

The amount of expansion at the truss chord ends at each column is about a half an inch, and the deflection due to wind of the columns is a little less than an inch. Making the total expansion at each end almost 2 inches - approx. 0.35 degrees of rotation. I like the idea suggested to fix the base of a center column and make the rest pinned, and simply do a pinned connection at each truss chord end. One more question...in an HSS column, you cannot detail the baseplate like a wide flange where you put the anchor bolts to the inside of the flanges to do a pinned connection. How do you do that for a tube? Put them on two sides instead of a square pattern? Thanks so much for all your help everybody. It has been extremely valuable.

 

[csd72]

Just make the plate flexible, no gussets, just strong enough to take any uplift there may be.

The rotation required would be a very small fraction of an inch.

csd