Resistance of 12ga brace wire

[skeletron]

12ga (0.1055" diameter) wire is commonly used for brace wires of suspended ceiling.
[ul]
[li]Manufacturers like Clark Dietrich give a break strength (375 lbs) and then a safe work load (94 lbs). The SWL is the break strength divided by 4. CD lists the tensile range as 50ksi to 65ksi[/li]
[li]USG also provides a yield point (424 lbs) and an ultimate load (+550 lbs), both slightly higher than the break strength from CD[/li]
[li]USG also provides a value for "wire tie failure/pullout failure" of the tight-3 wrap tie (358 lbs) which is ~95% of the break strength from CD[/li]
[li]Grabber Pro lists an even higher typical capacity (600 lbs) as well as a yield stress (59ksi) and tensile max stress (75ksi)[/li]
[/ul]

The ASTM standard for "galvanized carbon steel wire" is ASTM A641 and lists the following tensile strength for temper designation:
Soft = 70 ksi (max)
Medium = 65 ksi to 90 ksi
Hard = 80 ksi to 110 ksi

For vertical hanger wires, no problem...I'm using the SWL = 94 lbs with no problems and no reason to change.
For splay wires in a reasonably high seismic zone, the calculations are suggesting that 12ga shouldn't really work unless the FOS is modified to a somewhat low range.

My questions:
1. How is everyone arriving at the capacity of the 12ga splay wire, and is it different than the SWL or the capacity used for the vertical wires?
2. Is it reasonable to back calculate a "dynamic capacity" for the 12ga wire based on an ultimate stress or tensile strength? In this case, what is the stress that is appropriate?
3. Is this just a case of 9ga or 1/8" aircraft cables being the better solution than the FEMA E-74 generic detail?

I've come across some other competitors that have used 9ga wire (super hard to twist)and 1/8" cable (more expensive, but easier to inspect and install). But I've also come across another prominent firm using the 12ga wire for high importance structures, which doesn't make sense by my research. What says you anonymous 'geers?

 

[hawkaz]

I agree- In high seismic areas, 12 ga splay wire does not pencil out. Look at 8 ga. It's a little more common than 9 ga.

In reality- the code required 4 psf DL for a lay in ceiling is very conservative. I suspect that some municipalities that provide "approved standards" may take this into account when they show 12 ga splay braces.

 

[XR250]

Can you just use more 12ga braces?
I imagine the installers would prefer that.

 

[skeletron]

@hawkaz: Thanks for confirming my original thought re: 12ga wire. I spec'd 9ga on a couple of smaller jobs. They were able to supply and install it, but the contractor said it's a bit of a bugger to get the tight tie (3 wraps in 1.5"). Clark Dietrich lists 8ga wire as 0.162" ... so essentially the size of a 16D nail. I can't see that being any easier to install and tie tight. Ultimately, the wraps are what make or break the brace attachment. I'm not sure about the comment regarding 4psf DL for lay-in ceiling. Everywhere I've seen it listed as 3-5psf. You would have to reduce it to at least half of the listed weight for 12ga braces to work in higher seismic areas or upper storeys. I do agree that there is a bit of hand-waving regarding this issue because the detail is so common and, ultimately, it is "just t-bar ceiling".

@XR250: There are some issues with more braces in my opinion:
1. Often the plenum is jammed full.
2. The t-bar hole would ultimately govern the failure if more braces were attached to it. Putting braces further away would either have little effect, or require more compression posts.
3. I don't see the installers wanting to move away to a tighter spacing (6ft x 6ft or 4ft x 4ft) because of the above.

Going with aircraft cable becomes a lot more manageable and easier to inspect at some point. Higher material cost, but the installer just has to swage it correctly and then it's almost goof-proof!

We've avoided the original questions which I'll clarify: what is the capacity of the 12ga wire when used in a seismic brace condition? Is it reasonable to take a lower FOS (say 1.5 or 2.0) versus the listed ultimate break strength? Can the break strength be calculated based on a listed tensile stress and, if so, does the tensile stress *really* vary that substantially between manufacturers and the listed ASTM standard?

 

[skeletron]

Seems like DSA's IR 25-1 bulleting lays it out cleanly. Ultimate tensile stress = 70 ksi, define allowable tension capacity with FOS = 2.0 (min)

 

[Deker]

Yes, IR 25-1 for wire capacity and IR 25-2 (Link) for detailing. Similar to XR250, our preference is to follow IR 25-2 detailing and use 12ga splay bracing at a tighter spacing. This keeps the detailing standard to avoid installation errors / difficulties. Where I practice it's pretty common to need 8'x12' spacing, and in extreme cases you may need 8'x8', though I've never encountered that. You certainly shouldn't need to go as tight as 6'x6' or 4'x4'.

 

[Clerical Forensics]

We went through this a few months ago. Like many niches in our industry, the more you dive down the rabbit hole the more inconsistency and lack of information you find. We’re in the Pacific NW and asked our clients to send us the docs for the strength of the wire they were purchasing. About 4 of our clients replied and each of them is purchasing wire from the same place in the Seattle area. The provided document listed 80ksi maximum and a maximum break test of 735 lb for 12g wire.

As mentioned, the drop wire is generally not an issue. For splay wire connections to concrete, we decided to design for the ductility of the wire in order to avoid Ω allowing for a smaller anchor size. With that, we design splay wire connections for 80 ksi wire, with (3) turns within 1'' (approx. 0.65 reduction derived from USGS Seis. Tech. Doc. regarding hanger wires), and a FS of 2. We list a minimum break strength on our drawings and have discussed adding a maximum capacity along with a minimum and adjusting the wire turns (4 turns or 3 turns with 3''). Not sure if our clients read any of it though, maybe this is all for practice (queue AI interview).

 

[Christy314]

Hi Skeletron,

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