Nuts and Bolts, but mosly nuts 1

[NS4U]

Say I have an 85 ksi, 1 in. dia. (A=0.785 in^2) threaded tension bar which I need to add a nut to. Pn (of the bar)=67 kips

So, how do I select the proper nut that will develop bar?

My thought: I go into ASTM A563, see that for a 1 in. nut with UNC threads, the tensile stress area is 0.606 in^2. From A563, for a Grade C heavy hex nut, the “proof” stress is 144 ksi. Therefore is the capacity of the nut is= 87 kips? Shouldn’t there be a resistance factor on this? How do I find this factor? Does AISC give any guidance on this?

Also What is the difference between nut grades (A,B,C,DH…)? What is the difference between hex, heavy hex, and hex thick? Size? Chemistry? Mechanical properties?

thanks

 

[Lion06]

I'm sure there are other differences, but the difference that I'm aware of (just from ACI APP. D) is that the different nut types (Hex, Heavy Hex, etc) have different plan areas and shapes.

There are a few other observations I would make regarding your question.

1) I don't believe the tensile capacity of the nut is what you're after, unless you're trying to use the nut as a coupler and thread a bolt in from each side.

2) If you are after the tensile capacity of the nut, I don't believe you would want to use the full cross sectional area. A nut is wide compared to its thickness and I would imagine that you would have serious shear lag effects. I would liken this to trying to put a 12" square x 3" thick plate in tension across its thickness by pulling it apart in a small area in the middle - the load wouldn't reach the outer areas of the plate.

3) I imagine the real load transfer mechanism is shearing across the threads. This is what I would look at, but I swear I heard somewhere that these are designed to develop the tensile strength of the rod without shearing off the threads.

 

[NS4U]

yes that's what I'm after... think of a nut-bolt assembly... I pull on the nut (tensioning the bolt- shearing the threads at the nuts)... how do I know (using ASTMs/AISC) that the threads of the nut don't fail before the bolt yields?

I ask becuase I have a an irregular threaded rod that is being tensioned and I need to spec a nut that has a sufficent thread size and pitch that will not "slip" off when the rod is loaded in tension

 

[Lion06]

That's a good question. I'm sure there's some literature out there that discusses this, but here's my two cents and why I'd be surprised if it didn't develop the nut.

For ACI App. D, ACI, PCA, PCI, and all papers that I've read on the subject make no mention of checking this. For the failure mode of concrete breakout in tension, the entire load is being transferred from the rod to the nut via shear in the threads, then from the nut to the concrete in bearing. I'm sure this was thought about, but there is no mention of it anywhere, which leads me to conclude that it's nothing we need to concern ourselves with.

If, however, you have an odd thread pitch or something else that isn't standard, it would be worth looking into a little more.

 

[Stillerz]

What is the exact application, may I ask?

Often times in the case of pullout of anchor rods in pier stems or footings, I weld a plate to the rod (a big square plate washer) then thread a nut on and also tack the nut in place.

 

[SlideRuleEra]

See the AISC "Guide to Design Criteria for Bolted and Riveted Joints, 2nd Ed."

It can be purchased directly from AISC, here:

http://www.aisc.org/store/p-1622-guide-to-design-criteria-for-bolted-and-riveted-joints-2nd.aspx

or, downloaded (free .pdf) from the Research Council on Structural Connections, here:

http://www.boltcouncil.org/Publications.html

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[desertfox]

hi NSU4


Have a look at this reference, it shows how to calculate the shear area of the nut and how to ensure it won't fail before the bolt or stud yields although its not to a specific code as such it is the procedure is used throughout industry.
Your interested in this from about page 17 onward.

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 28.pdf

 

[JStephen]

For actual bolts, the bolt spec will tell you what nuts to use with that bolt; that's why this is not normally a design consideration.

 

[NS4U]

this is not an anchor rod or a bolt....it is an A36 threaded rod, therefore the RCSC stuff doesn't apply

 

[desertfox]

Hi NSU4U

Have you tried the link the theory still applies

desertfox

 

[NS4U]

Yes, DF it does thanks... I looked through it and it makes sense, but it'd be nice if AISC offered some insight on nut "capacity"

 

[SlideRuleEra]

Then, try this AISC stuff:

http://www.modernsteel.com/steelinterchange_details.php?id=99

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[apsix]

If you can find a bolt/nut assembly that gives the capacity required, then you've found the nut that has the required capacity. For extra comfort double nut it.

In metric, a Grade 10.9 nut would be required, by my estimation.

 

[JStephen]

I believe ASTM A307 Gr. C bolts are A36 steel, if that would help any.

 

[dhengr]

NS4U:

Most any full sized nut of a better grade (that is, higher yield and tensile strength) than your A36 rod should serve your purpose. As StructuralEIT suggests 19NOV,15:05, 3) shear through the threads is the load transfer mechanism, and the higher grade nut will cause the male threads on the rod to yield first. The height of the nut, or length of thread engagement is to assure that you get tensile failure in the rod before you get the shear failure at the threads. Double nuts properly tightened is another possibility to pick up thread engagement length, if that helps you sleep better at night. You can mess around with thread class and series a bit to improve connection strength, but except for special conditions this is usually too costly.

SlideRuleEra and desertfox have provided excellent links on the subject matter. The AISC links and the boltcouncil.org link pretty well cover AISC’s philosophy and that of the fastener industry in general. The utm.edu link shows you the general method of calculation and their design philosophy. This same general approach can be found in any number of other Theory of Elasticity and Machine design refs. The above pretty much encompasses AISC’s thinking on the matter. And most of the nut/bolt combinations for connection systems are intended to avoid the need for over-thinking the subject, they want to, try to, keep it relatively simple.

Your want to fully understand your problem is not over-thinking, instead it is good thorough engineering. Congrats. Your want for ‘insight’ from AISC on nut capacity, is, well..., nuts; that could come back to haunt us. You got your insight from the links above, we just can’t have them all in the codes. With the exponential expansion of code complexity, and their printed volume; I can imagine it now; an eight volume addition to the already existing AISC volumes, all you ever wanted to know about nut and bolts. They would each be revised at least every three years, but never together so you could work with one set or actually learn the contents of that edition. They would all refer liberally to ASTM, ANSI so we would need to own these also. I can remember when I could hold the codes for steel, reinf. conc., masonry, and timber design, in one hand, between my thumb and fingers. I actually felt I had a pretty good handle on what was in each code; I could riffle through to find the exact section; and I had some feel for the thinking and research which led to the code verbiage. I’m kinda thinking of getting out of the engineering business and into the code publishing business where the real volume and money are.