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??

 

[connect2]

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 ...'.

 

[structuralnerd]

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.

 

[csd72]

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.

 

[structuralnerd]

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.

 

[civilperson]

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.

 

[csd72]

thats 2.5" expansion each end.

 

[structuralnerd]

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?

 

[structuralnerd]

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?

 

[csd72]

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

 

[civilperson]

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).

 

[csd72]

civilperson,

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

csd

 

[dgkhan]

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

 

[hokie66]

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.

 

[structuralnerd]

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

 

[hokie66]

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.

 

[structuralnerd]

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?

 

[hokie66]

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.

 

[structuralnerd]

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

 

[hokie66]

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.