Sag rod "double nutting"

[sundy]

Girt Bracing

Question
08/01/2007

I am designing a building with vertical siding, channel girts, and sag rods. I was told that the industry standard is to consider the channel braced at the sag rods. How is this possible, since the sag rods are neither at the compression flange nor able to act in compression? Is this the appropriate way to look at bracing the channel, or is there a more appropriate method?


Answer(s)
The siding attached to the tension flange of the girt acts like a torsional brace. Unfortunately, it may be an ineffective brace in terms of restraining the compression flange of the girt, since it is attached to the tension flange. One needs to calculate the adequacy of such a brace using Appendix 6 (torsional bracing) in the 2005 AISC specification (a free download from

http://www.aisc.org/2005spec).

Procedures in AISC Design Guide 7, Industrial Builidings—Roofs to Anchor Rods, 2nd edition, for the design of girts are outlined on pages 17 and 18 of the document (

http://www.aisc.org/epubs).

Note that item 6 mentions that the sag rod acting in conjunction with the siding should be designed to prevent the twist of the girt under suction loads in accordance with Chapter F of the AISC specification. As such, double nutting would be required to provide resistance to twisting.

This is from

http://www.aisc.org/MSCTemplate.cfm...tomSource/Faq/SteelInterchange.cfm&FaqID=2779

My question: what is double nutting? How does it resist twisting?

 

[jike]

Double Nutting is when you put a nut on each side of the girt.

 

[JAE]

Here's a similar thread (but not about double nutting - just about sag rods and their unbraced length):

thread507-115545