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expansion in HSS truss

expansion in HSS truss

expansion in HSS truss

(OP)
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??

RE: expansion in HSS truss

It's so difficult to answer this question, so first I'll ask the most obivious.  Why do you think that birds won't nest on a 'Tube" type truss that is 450 feet long with supports at +/- 80 foot centers along its length?  What did you mean by '...Welded square HSS pipe ...'.

RE: expansion in HSS truss

(OP)
The client doesn't want birds to nest in the flanges of a wide flange shape so they wanted tubes instead.  Sorry to confuse you about the type of support...it's a typical square HSS truss that is supporting a pipe line with fluid running through them.  This is a giant pipe rack basically.

RE: expansion in HSS truss

If your columns are pinned top and bottom then they will just tilt slightly under the expansion/contraction. Calculate what this tilt is and allow for it.

RE: expansion in HSS truss

(OP)
I guess that is part of my question...how do you allow for this tilt at each column?  You don't want the entire 400 foot length to tilt.

RE: expansion in HSS truss

Pick a point,(center column works best), and fix it with diagonal braces.  Let the expansion/contraction occur outside at the ends.  The column base anchor bolts will be the pivot points.

RE: expansion in HSS truss

thats 2.5" expansion each end.

RE: expansion in HSS truss

(OP)
csd72-

Is that what the expansion is if I allow for it at each support at 80 feet on center, or if I do what civilperson suggests?

RE: expansion in HSS truss

(OP)
I guess my main question is...do I need to provide room for expansion at each support?  If I do, then how does the axial load from the truss top and bottom chord get into the column if I use a slotted bolted connection?  And how do I "allow" for tilt?  For instance, at an interior column, how can it tilt without pushing on the beam next to it, and causing the next truss section to tilt into the next column?  Do I really want that to happen for 400 feet?

RE: expansion in HSS truss

The 2.5" is the theoretical expansion at each end if the center is fixed and the ends are allowed to move (i.e. tube will expand by 5" total).

This will tilt the column and result in a lateral component to the column loads (easily calculated by vector geometry).

Then you just need to ensure that your truss can take this additional load.

csd

RE: expansion in HSS truss

5/8" expansion per hundred degrees of temperature change per hundred feet of length is much less than 2.5" expansion quoted by csd72 if the steel is erected at 45 degree temperature.   225 feet is the half length, 75 degrees or 65 degrees is the change from erected temperature, thus expansion would be 0.91 inches and contraction would be 1.05 inches (at the extreme ends of truss secured in the center).

RE: expansion in HSS truss

civilperson,

as per the OP it is 140 degree change. I assumed worst case using the AISC tabulated values.

csd

RE: expansion in HSS truss

80 feet is preety decent span. Using bearings. Place fixed bearing on center column and expansion on all others.

RE: expansion in HSS truss

If you don't want the columns to tilt, they each have to be braced to the ground or cantilevered, then use slide bearings on all but one as advised by civilperson.  It is normal practice on conveyor supports, pipe galleries, etc. to anchor them at one point and allow the base of the other supports to rotate, providing longitudinal movement joints only at longer distances, probably at about the 450 ft which is your whole length.

RE: expansion in HSS truss

(OP)
Can you explain a slide bearing?  I'm not familiar with that term for anchoring a column.  How do you detail that?

RE: expansion in HSS truss

A slide bearing is not for anchoring a column.  It is to allow the horizontal structure to slide relative to the top of the column.  They are proprietary items, normally consisting of a pair of steel plates, each with a layer of low friction plastic (like Teflon).  The plates are welded or bolted to the steel members.  They are normally "guided" with retainer plates to prevent vertical separation.  Suggest you do a google search for suppliers in your area.  Don't try to detail them yourself, get the manufacturer's data and pick the ones to suit your parameters.

RE: expansion in HSS truss

(OP)
In a truss, don't you rely on the axial force from the bottom and top chords to transfer into the columns?  How does this happen when you are using a slide bearing connection on the ends?

RE: expansion in HSS truss

No, truss axial forces do not transfer into columns.  The column carries the vertical load, which is transmitted to the column by the truss web members.  If the truss is continuous over the column, the axial forces are transmitted into the next span.

RE: expansion in HSS truss

(OP)
The truss is not continuous over the columns.  They span from column support to column support.

RE: expansion in HSS truss

That means you will have twice as many slide bearings if you choose to go that route.  I still think you should let the bases of the columns rotate except at the anchor point.

RE: expansion in HSS truss

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

RE: expansion in HSS truss

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.

RE: expansion in HSS truss

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.

RE: expansion in HSS truss

(OP)
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?

RE: expansion in HSS truss

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.

RE: expansion in HSS truss

(OP)
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.

RE: expansion in HSS truss

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

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