Nuts Pulling Threads off of Threaded Rods

[bigmig]

I have a detail on a project where I am hanging a wood beam from a steel bent. The hanging mechanism is a group of four, 5/8" diameter A307 Threaded rods with nuts on both ends.

The comment the framer made was "wow, that is not a lot of thread holding that whole roof up". As I looked at it, the nuts did look "normal", albeit narrow. For a 5/8" diameter bolt,
the nut has 9/16" of width (parallel to the threaded rod). We will add another set of nuts for safekeeping, but it occurred to me that the design values I was using was for the rod.

Is there another set of values I should be looking at for the thread/nut interface? Are nuts sized so that the rod fails before the threads strip out?

 

[desertfox]

Hi bigmig

What grade of material are the nuts made from?
Normally if the nut and threaded fastener are the same grade or very close then the threaded rod should fail first.

desertfox

 

[DDBPE]

Yes, there is a standard for the nuts.
I used threaded rod for temporary supports for bridge falsework for years. The manufacturer of the threaded rod and nut should be able to give you the safe working loads. For example, Dayton/Richmond (could be another name now as it has been 10 years since I have worked with it), specified its safe work loads for 1/2 rod at 9,000 lb tension and the standard nut at 6,000 lb for 1 nut or 9,000 lb for two nuts and for a heavy coil nut, 9,000 lb for one and 18,000 lb for two.

Also, I used 4"x5"x5/16" washers for all the connections to distribute the loads to the supporting members better than a standard washer, but the manufacturer should be able to guide you with that also.

 

[XR250]

Wow, 9k seems like a real high working load. Must not have been A307

 

[racookpe1978]

problem you get with "threaded rod" is that the rod itself is not even Home Depot/Lowes/Ace Hardware store quality, but is "as-cheap-as-I-can-find-it" quality, particularly for field construction or temporary or one-off building fabrication. That is, almost more likely to be potmetal rather than specification-level steel supplied by a reputable supplier like Fastenal or the like.

 

[racookpe1978]

9,000 lbs seems like "a bit much" How did you calculate that for what grade metal?

for example, see this chart for both total strength ans pullout load from concrete:

http://www.strongtie.com/products/anchorsystems/adhesives/at/loads_rod_conctension.html#

 

[bigmig]

The reaction on a pair of rods from beam one is 5.5 kips. The reaction from beam 2 is 2.2 kips. The beams butt under the bent. The rods run from the bent, through the beams, to a 5/8" thick 5"x 12" long plate.
The Nut and washer are A307 and F844 washers. The properties can be found here:

http://www.portlandbolt.com/technicalinformation/astm/ASTM_A307.html

The loads given are allowable loads.

 

[bigmig]

Actually....sorry. There are no washers. Just nut on steel.

 

[smb4050]

The height of a 5/8" nut is 9/16". Check AISC steel manual for tension loads for a 5/8" bolt. A lock nut should be used on a tension connection, so a double nut would double the nut capacity and provide security. I've checked althread for use as pipe hangers, and the tension capacity drops dramatically when a small moment is added to the load. A pure tension connection implies no deflection between connect parts, prying action, thermal movement, etc. I'd assume some movement/moment is going to be exerted on the bolt, and suggest checking for adequate tension capacity using a combined loading condition. DDBPE made a good suggestion using a larger plate washer.

 

[racookpe1978]

Well a double nut won't really double the capacity of the rod at all: That stays the same in terms of lbs of force. (remember to use the minor diameter of the threaded rod).

But, if you torque a double nut against each other, the load between the two double nuts is actually increased. yes, there are more threads to engage the threaded rod overall, but ONLY if both nuts have the same force (load) on the rods. Make sense? If one is tighter than the other, then it will carry the load, the second has none. If the lower is torqued up hard against the upper, then you are actually pulling the upper down and the lower up, and so the local stress on the rod between the two is greater than the original single-nut condition.

If I really, really had to do this, I'd get an extra long nut like a coupling nut.

 

[DDBPE]

Here is the spec sheet for the threaded rod I listed earlier. This was and maybe still is a reputable supplier of bridge temporary support components in the US.