huangyhg

turnbuckles and clevises for wind bracing
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".

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.
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.
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.
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.
why stress over rod sizes? it is one of the cheapest
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.
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
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

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