long span joist welded at both ends 1

[WWTEng]

Consider a single story precast building with 60'LH joists. Would there be any issues if the joist is welded at both ends rather then welded @ one and bolted at other? i.e any thermal movement issues?

Personally I think there shouldn't be any problem because the wall panels are precast with regular and more efficient (as opposed to cmu) control joints plus the pre-engineered camber should some how help too, but I am not sure.

FYI-building dim: 60x100.

 

[SteelPE]

Can't say I ever worried about thermal issues with a 60'x100' building. usually I only begin to consider thermal issues around 300' or so (depending on the framing materials).

I am currently designing a building that is 125'x125' clear span with the joists welded on each end (actually each end is both welded and bolted because OSHA requires joists over 40' to be bolted). If you run into any issues let me know.

 

[WWTEng]

SteelPE:

I have come up with less then 1/2" of change in length over a 180 (+/-90) degree range for the 60' span. So certainly doesn't seem like it should be an issue.

 

[SteelPE]

180 degree range. That seems like a lot to me. Is your structure open to the elements or closed with conditioned air?

 

[WWTEng]

Its enclosed field-house, air-conditioned/heated. Yes 180 is high, from my curtain wall design days I used the number somewhat blindly. Should be more like 120.

 

[SteelPE]

I would think even 120 would be high. In my area the difference in temperature I would use would be around 61 degrees or so. Therefore I would end up with a change in length of 1/3 what you are calculating. Remember, the sun is not beating down on your joists like it is your building facade.

Check out this paper, hopefully the link will work.

http://www.nap.edu/openbook.php?record_id=9801

At the end of the paper they give temperature changes for various cities in the US.

 

[JAE]

We have had some problems with the concrete cracking at a joist seat when the joist was very near a building corner. In the middle of the building the walls just rotate in and out with any thermal changes in the steel length.

But near the corners, the walls can't rotate as easy due to the intersecting concrete wall that is parallel to the joist.

I agree that the thermal loads aren't that much but they are a sort of irresistable force. If the thermal deflection is 3/16" to 1/4" then that might be the crack width you see at the seat at one end.

 

[WWTEng]

@ Steel: Yup, I agree 120 is high, I was just being conservative, 60 for indoors is the norm from what I know.

@ JAE: Thank you for the informative post. I am just wondering how a detail would look like over a precast wall? Would you use 2 headed studs in the precast angle that the joists seat can sit on?

 

[spats]

"I have come up with less then 1/2" of change in length over a 180 (+/-90) degree range for the 60' span. So certainly doesn't seem like it should be an issue."...

Good lord! If you though about the axial load developed to represent a 1/2" change of length if fully restrained, you wouldn't be making such a statement. Of course, in all likelihood, that kind of restraint would only happen near building corners as pointed out by JAE, and the joist will bow and relieve some of the stresses... but to make such a statement is not to recognize the tremendous forces that can be developed due to thermal.

 

[MiketheEngineer]

I live in St. Louis. Temps have ranged from -15F to 108F in one year - so 120 degrees is not out of the realm of possibility!! And yes I have seen concrete cracked by thermals expansion of I-beams - not necessarily joists. The beams were cut so they fit "exactly" in the pockets with no "give" whatsoever. Cracked the walls! Also were about 200' long!!

I would think joists would probably bow or buckle a bit given their light I or mass!!

 

[SteelPE]

Mike,

The paper I referenced give a delta T of 61 degrees for St Louis. I understand the temperature swings that can happen in a give region however, you have to realize that you are not going to weld the joists to the support structure on the -15F day and expect the job to still be exposed to the elements on the 108F day... well maybe for a very large project.

 

[MiketheEngineer]

SteelPE -

While I cannot disagree with your argument - this one beam was installed in the dead of winter - say at 10F. Then at 100 F it "blew" out later in the summer!!

True story!! and well documented!!

Go work in Fort McMurray Canada or Minneapolis, MN ---- You will be AMAZED by what can happen!!