Threading a solid round bar

[SteelPE]

I have a project where the client needs to tap their own solid material to form a nut and then thread their own rods to go into these tapped nuts. The material we are using is A304 stainless steel. I am wondering how one would go about making sure these items are capable of resisting the design loads imposed by them? ie, if the thread on the rod will be long enough/strong enough/not eat into the rod to reduce the tensile capacity of the member with the same questions for the tapped material.

 

[Ron]

Compare the length of threads and the depth of the nut threads to standard nuts and bolts of rated capacity.

 

[SteelPE]

That was my fallback plan.

 

[MER3]

Use the dimensions of the nut and the thread pitch to check thread engagement of the nut, the Machinist's Handbook has a good method of checking thread engagement. Then, use the external thread dimensions to calculate the net area of the threaded rod, then check that tensile stress over the net area is less than the max allowable shear stress in the threads (internal and external) to verify that full thread engagement has been met.

 

[KootK]

To preserve 100% of the tension capacity, I'd think that you'd need upset threads or tapered threads similar to some rebar splicing products. The folks at Portland Bolt may be a good resource.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JStephen]

That general topic was pretty much a whole chapter in the Machine Design textbook.

 

[Tmoose]

What size is the thread, and the length of the bars to be threaded?

 

[SteelPE]

The bar is 1" dia ASTM A304. The length varies but is 10'-14' in length (it's part of a bracing system for a tower).

I'm thinking that maybe we should just increase the size of the rods by 1/4" across the board and not have to worry about it. Then again, I was a bit nervous about the project so these bracing members are only 40% stressed.... but I wouldn't be too enthused with cutting into my design FOS.

 

[JNieman]

You have a rod-to-rod connection with a preference toward threaded fastening, if I read correctly.

Have you thought about just buying coupling nuts and threading both ends of the rod?

/EDIT/ I accidentally a word. Fixed verb tense.

 

[Lomarandil]

JStephen, I was just about to say --

For all the times we say "just hire a structural engineer" on this part of the forum for a complex problem... you really should be talking to a MechE for this one.

 

[dhengr]

SteelPE:
You really should give some more detail, maybe some sketches, to explain what you are trying to do. You say you have a 1" dia., A304 stainless steel bar, which is part of a bracing system, but it is only stressed at about .4Fy. You should have no problem putting an adequate threading on a rod with a load of .4Fy, although SS can sometimes be tough to thread. Also, there can be galling problems with SS threaded parts. Then, the question becomes, what does the rod thread into, is this pl. A304 also, or mild steel? You may have to adjust the thickness of this plate to get enough female thread. Do you need a turnbuckle in the system to make up the final connection? As mentioned above various Machine Design texts and Machinery’s Handbook should show you how to do the analysis. This will not really be covered by one of our normal bldg. codes, the design is a matter of some engineering experience and judgement.

 

[SteelPE]

dhengr,

This project is a 100' tall tower in a coastal area. The owner originally wanted to build it out of mild steel (or carbon steel, I don't know the correct term) and after the bids came in they decided they wanted to switch it to stainless steel. The tower has 4 legs with bracing that runs in-between the legs. The SS fabricator switched every detail we gave them on the drawing in favor of details that were easy for them to construct. They also seem more focused on the aesthetics of the tower with the structural stability playing second fiddle. Some of the questions you asked are the same questions I sent back during their first submittal It has been a fight over the past 1.5 months that I did not anticipate when I put my proposal together.

Attached you will find a partial elevation of the tower. The X rods you can see are the bracing members I am talking about. In the specific elevation shown, the rods are 1/2" diameter. Also, my stresses are not 0.4Fy my unity equation is 0.4..... or I guess you could say 0.4Fy/omega.

 

[Teguci]

Carbon steel - rusts until failure, Alloy Steel - doesn't rust, Weathering Steel - forms coating of rust for protection. Mild steel refers to steel that hasn't been worked (OK, a little working) and treated to increase it's structural properties.

Structural stability never plays second fiddle and no one in their right mind would promote that. Take this as meaning the client has full faith in the structural engineer in making sure the final structure is stable.

How are you going to adjust the length of the threaded rod so that it fits the field conditions? If you have tapered threads on both ends of the rod, then you will have no adjustment = super bad idea. You also need to account for offsetting the rods (can't tell if you have or not). Just cut standard threads into both ends and design for the reduced section - KISS.

The simplest approach, in my mind, will be to provide clevises (with just a hole and no threads) at both ends (offset the holes for geometry) provide a jamb nut on the outside of the clevis and double nut the inside so the rods are tight. If they really want pretty, then they should get those gussets normal to the angle of the rods and line the rods up with the panel points, but alas, I am not an architect.

 

[Ron]

I would use turnbuckles for the rods. Also, the hem tensioners should be on the top, not the bottom as they will be more affected by water and water will sheet down the mesh. Less opportunity for a lot of water on the top, rather than the bottom.

 

[SlideRuleEra]

I agree with Teguci about the use of straight thread and Ron about using turnbuckles to make adjustment practical. Unless upset rods are used, turnbuckle strength is normally the limiting factor on allowable tension (not rod material, not rod treading).

Be careful when mixing carbon steel and stainless steel in outdoor use. Carbon steel members with stainless fasteners is "ok".
The opposite, stainless steel members with carbon steel fasteners is "bad".
See this article in Structure Magazine, March 2016.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldrunner]

You might look at rigging used for sail boats. Also I would check the connections for fatigue. Once had a 60 foot tower with a detail similar to yours but not as sophisticated where one of the tie rods failed and stabbed the deck of a bridge that the tower was cantilevered off the floor beam outriggers. Your tower is surely going to be resisting a lot of windage with the canvas infills. I imagine there will be a lot of shaking from the turbulence. Might be noisy as well.

 

[SteelPE]

What if I had them fabricate a connection and had them send it out for testing. I assume that would be acceptable? This fabricator is just plain useless when it comes to answering my questions in regards to this item (which is quite important).

My other option is to look at similar items (carbon bolts) and mimic what I see in terms of material sizes and thread lengths (and just be conservative) as stated above.

 

[desertfox]

Hi SteelPE

I think stainless threads on stainless threads is not a good idea, mainly because of galling as others have mentioned.

Regarding analysing the joint you need to check that your external force does not cause the screw threads to strip internally before the bolt physically breaks in tension, this condition shows you have enough thread engagement. If you have a fatigue situation then clearly the bolt needs to be checked for failure in fatigue as well as the joint.

One point about thread engagement, following the formula for thread engagement either on the Roy mech site and/or machinery's handbook will lead you to believe that the shear stress can be reduced indefinitely by increasing thread engagement which is not so, there is no benefit increasing thread engagement beyond a length of 1 to 2 times bolt diameter.

http://www.roymech.co.uk/Useful_Tables/Screws/index_screws.htm

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[KootK]

Could you talk them into using a Macalloy rod (Link)? Or perhaps you can emulate aspects of their product?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SteelPE]

dessertfox,

We are not using screws but rather using threaded and tapped members to form bolts and nuts (well not so much as nuts but rather clevises and sleeve/coupling nuts). I remember reading something in regards to bolt engagement of 1xbolt dia for bolting similar materials and 2xbolt diameter for different materials. This is where I came up with my 2xbolt dia requirement for this project.

Just so others are aware, this tower at this moment is 100% stainless. No carbon steel anywhere.

This project is really becoming a problem as you now have me concerned about this galling issue. ugh. I have asked the fabricator what his experience is with the issue. I am guessing that he has no experience with the problem.