Turnbuckles and Clevises for Wind Bracing

[ironmon]

The company I work for likes using round rods for bracing low rise industrial building structures.
Using A307 Rod, the turnbuckle becomes the weak link, per the Safe Working Loads provided in the AISC, 9th edition.
They site a 5:1 safety factor and claim it due to rigging and dynamic loading.
If you reduce it to 3:1 you get compatable allowable tensile loads to the rod itself.
I know this type of bracing is used in pre-engineered buildings all the time. What criteria do they follow?
Anybody reduce the 5:1 safety factor when turnbuckles are used in wind bracing?
Our construction guys think I'm nuts when I bump the rod sizes up over 1" diameter because of these 5:1 loads. And I can't say I blame them....it looks like "typical engineering over-kill".

 

[twinnell]

I have used rod bracing for wind and seismic loadings. We never beefed up the rod diameter to meet the clevis or turnbuckle capacity. We would specify the clevis or turnbuckle that was required for the load and the fabricator would have to shoulder up the rod. We did this quite a bit and never heard one word from the contractor about it. When I say shoulder up, I mean that the threaded part of the rod has a larger diameter than the main part of the rod.

 

[swearingen]

Turnbuckles and shackles are designed to higher safety factors for rigging use as noted above. A 3:1 factor is not unreasonable, but you are shouldering the liability should anything fail. If you do what twinnell says and shoulder up the rod, then your back is covered. You can point to a published rating for the hardware and a calculated rating for the rod - easy to back up in court.

Please remember to design your connections for stiffness as well as strength for rod bracing. I've seen many details where bracing was connected to the center of a column web with nothing bringing the load to the flanges. There have been a good many documented collapses because of this problem. Even if the web is strong enough, the web may not be stiff enough for proper bracing.

 

[JAE]

We have used rods with turnbuckles in lots of industrial type buildings (in non-seismic areas) and were confronted with the 5:1 safety factor issue. We resolved to use 3:1 on everything as this was much more consistent with the safety factors on other elements of the building. We also didn't use anything greater than 1 1/8" dia. as heavier sizes were expensive and hard to find.

 

[ironmon]

Thanks Everyone, Good input!

I agrree with your logic JAE, that is resonable and consistant with the design of the other building elements.

The 3:1 is what I'm gonna go with.

It is still conservative since overall lateral stiffness and stability has many other contibutors that are ignored for simplicity.

 

[csd72]

Why stress over rod sizes? It is one of the cheapest members in the building so be a bit conservative.

I have used up to about 1.5 inch rods on an industrial building .

The guys on site always complain that things are overdesigned but they have no point of reference.

The only thing to note is that the bigger the rod, the more it is going to pull things inward when it is tensioned up. Many of the typical connections dont work for large rods either.

 

[sdz]

The Australian Code AS2319 specifies different quality grades for rigging screws and turnbuckles. I base capacity on
Minimum destructive test force x 0.8(Capacity reduction factor)
From this I find that a quality grade P turnbuckle is acceptable for structural purposes for matching mild steel rod size.
When designing rods remember to design as a member not as a connector. i.e. use yield of the rod rather than tension capacity of the treaded part to calculate design strength.

 

[dik]

Guess we're pretty much in the same ballpark... using a limit load of 1/2 the tabulated value with load factors of 1.25DL and 1.5 for LL... comes out to somewhere 2.8 to 3.0...

Dik

 

[j19]

Here's another issue that you are faced with. A307 is not an AISC approved material spec for rods, therefore you are stepping outside the boundaries of the AISC Specifications. Unless you consider your rods to be bolts.

Ref: A3 in the 2005 Specifications