Tempreature effect on steel truss

[dawn836]

Hi all
I have a hall formed of steel N truss with upper and lower cord curved with same radiuses (parallel to each other) the expected variation in temperature between outside of the hall and inside is 35C the truss spans
30 Ms.

I would like to include temperature gradient effect.
Should it be considered only for upper cord?
Is it significant?

The cladding is supposed to give some thermal isolation but I think Air condition system will be the only way responsible for cooling the hall

 

[BigInch]

I don't see how you can consider only the upper chord.

It may be significant in increasing the curvature of the truss. If the temp is 35Cº higher the truss will bow up, if 35Cº lower, the truss will bow down.

If the AC fails, you will get horizontal thermal expansion (or contraction) loads, maybe with friction components at the supports.

Geometrically the new shape will approximate,

Height change = 1/2 dT [α] Max_Truss_Height
Top Chord length change = 1/2 dT [α] Length_Top_Chord
Bottom Chord length change = dT [α] Length Bottom_Chord

If the truss is free to move in all directions, the stress effects will be minimal, but trying to restrain the truss from expanding/contracting can result in considerable forces.




Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Ron]

As BigInch noted, the restrain conditions will be key.

Radiused trusses/members can be a bit tricky to deal with loads due to the eccentricity relative to the connections. The same is true of thermal movement. In this case, as the truss elongates due to thermal gain, the radius will change slightly and anything attached to the truss will move laterally. Outside the radius you'll compress the affected parts...inside the radius you'll pull on them.

If your truss is inside a plenum, then the effect should be relatively uniform, though some gradient will exist from the roof downward to the depth of the truss. I would consider the worst case (likely the top chord) and apply to both top and bottom chords for affect. If the gradient is significant (cooled bottom chord, heated top chord), you have to consider loading in three planes simultaneously, in addition to your other structural loads.

I would certainly provide connections, both top and bottom chord bearing connections, to accommodate the expected movement. Further, you might want to consider the effect of thermal fatigue on this application. You'll be getting a couple of cycles a day, and depending on stress levels of the individual members, that could be significant over a 20 or 30 year life, particularly if you are designing the members to take relatively high stresses.

 

[dik]

I generally try to keep trusses within the building envelope and insulation on the outside. Minimal issues with thermal movement although the ceiling area can be quite a bit warmer than the rest of the room. Also limited issues with condensation on the colder surfaces.

Dik

 

[J1D]

dawn836,
I think it is more important to get better assumption of the abient temperature around top chords, web members and bottom chords. Other than the possible insulation function of roofing, there must be a gap between cladding and the top chords (through the purlins), the top chord temperature may be very close to the room temperature.