tension for turnbuckles in steel building 2

[Bill Martin]

Hi-
I'm building a 40x78x14 pre-engineered steel building with a 12-1 pitch roof. There are turnbuckles in the roof and between the I-beams on the sides. Unfortunately, the factory does not supply any recommendations for the desired tension on the turnbuckles. Does anyone have any ideas on this?
thanks,
Bill

 

[SlideRuleEra]

I looked into this for the bracing in pre engineered metal buildings at our electric generating stations. All locations are in high wind / high seismic areas. Came to the following conclusions:

1. Too tight is worse than too loose. A turnbuckle is not a very strong connector - usually much weaker than the cables/rods that it holds together. If the brace has too much pretensioning, just a little more wind/seismic or other load can cause failure. A loose brace may allow excess movement, but that is better than failure.

Edit: Conclusion #1 is wrong for two reasons. Thanks to XR250 and hokie66 for pointing this out

2. Determining the proper tension for an actual building if futile - too many variables. Using engineering judgement, based on simplified statics, is probably better. For example if the center of a reasonably tight brace, say 30' (360") long, is displaced 2" to one side, by hand with a force estimated to be 10 pounds. That's about 450 lb. additional tension in the brace. Seems loose, but probably about right for a 30' long brace.

3. Tighten all the braces in the building more or less simultaneously. Don't fully tighten one then go to the next one. If all braces can't be tightened together (that could take a lot of people), give each one a couple of full turns, or so, then move on to the next one. Keep going like this as many rounds as it takes.

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[hokie66]

I am not a big fan of rods for bracing, but for relatively small buildings like this, they do make some sense.

I would disagree with SRE on a couple of points. If a rod is tight, and more tension is applied, it may yield, but that is not failure. The other point is that while off the shelf turnbuckles are normally weaker than the rods they splice, load rated turnbuckles are available, or can easily be fabricated from bar stock.

So my answer is that rods should be tightened enough so that they don't sag, or sag very little. If you can see the sag, that is too much. SRE's point about staging the tightening is well taken, and has the additional benefit of helping to properly align the flimsy PEB frames.

 

[XR250]

Too tight is worse than too loose. A turnbuckle is not a very strong connector - usually much weaker than the cables/rods that it holds together. If the brace has too much pretensioning, just a little more wind/seismic or other load can cause failure.

Maybe I am wrong here, but wouldn't the wind/seismic load have to be more than the pre-tension load before it added more load to the turnbuckle?

 

[Bill Martin]

Thank you to all! Right now, the turnbuckles are only "hand tight" but hopefully that isn't a concern because there is a 12x12 opening (garage door to be installed in spring) and 14x28 opening on the leeward end wall (to be completed in spring), so i'm guessing the structure will "winter-thru" and any strong winds will blow through rather than blow the building over...it is set against a sheltering hillside. When we get back there (Hampshire Co, WV) in the spring I will tighten them slowly in sequence.
Bill

 

[BUGGAR]

Turnbuckles mean cables for bracing, not rods? They behave differently. Depending on their design, they may be spec'd. for a higher load than rods and thus be more flexible. Also, the use of thimbles is important in cable flex.

 

[SlideRuleEra]

XR250 - Thank you. I worked with my paper/pencil model this morning - you are right. I'll edit my original post to mark out my error.

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[hokie66]

BUGGAR,
I think he is using rods, which are commonly connected with turnbuckles in metal buildings. Cables don't work very well, because of mechanical flexibility as you implied.

 

[XR250]

Cables don't work very well, because of mechanical flexibility as you implied

I have seen both used. I don't know if the cables are any worse than the less-than-stellar metal strap bracing used in CFS buildings.