Torque of austenitic 304 bolts in aluminum beams of geodesic dome 8

[ll23]

Greetings, I hope somebody can help me with this one, I am in a project involving the erection of a geodesic aluminum roof, is necesary to aply torque to the stainless steel fastners of the estructure of the dome, this is the node and the beam, both of aluminum, AA 5454 H34 and AA 6061 T6, the bolts are A2-70 1/2".

 

[SnTMan]

Consult the owners manual?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[MintJulep]

37

Maybe 38

 

[EdStainless]

Just curious, what are you using for an anti-seize?
These had better thread in very smoothly.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[ll23]

First and all, thanks so much for the response,SnTman, the owner's manual doesn't mention torque the bolts, its more a constructors form to have a quantitative way to know how much we can tight the bolts, the dome fabricator manual is silent in this matter; MintJulep, those values are in N.m or lb.ft, EdStainless, we are not using anti-seize, at least the owner doesn't requested, and sincerely I don't know if this is a "must" or a "shall be" option for torquing SS bolts, I gratefully accept further information on this point.
Again thank you all

 

[Screwman1]

If you have no other way to determine a torque setting, take at least 10 bolts and torque them in a virgin joint with all the components that will be used until something fails in the joint (whatever that may be) and then set your seating torque at 60% of the average of the failure torque.
This sounds like a critical joint that contains mixed materials; for the manufacturer or designer to not tell you what the assembly torque should be is nearly criminal.

 

[MJCronin]

I agree with Screwman1 ....

Sometimes the cheapest and most empirical thing that you can do is to make up a few of these joints and break them !

I suggest a formal letter to the dome vendor asking the torque question. Someone is paying for his product and he has a responsibility to answer it !!!

MJCronin
Sr. Process Engineer

 

Austenitic fasteners are problematic because of their low and quite variable yield point.
For a critical application I would sacrifice one bolt per lot to determine actual mechanicals. I would base my torqueing procedure on that, and I would have the connection validated by a 3rd party.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[IFRs]

Never ever buy again from that dome vendor. They are either withholding information they have or they don't know the answer, either option would take them off my approved vendors list permanently. Many dome vendors went to lock bolts but they may be a little more expensive and although they solve some problems like yours, they can cause or exacerbate others. The owner may have bought the least expensive, now they may regret that decision, and they may have unpleasant future surprises. The fasteners you have mentioned primarily see single shear, possibly with threads in the shear plane if that dome vendor is not paying attention to details. I would not consider them slip critical unless they go into slotted holes. I'd make the plate tight to the beam flange, then go another 1/2 turn. You are going to have a hard time measuring torque due to the nature of the joint and how it was assembled (SS bolts / SS nuts and SS nuts on AL plates with or without lubricant). If you are making the nuts sung tight by hand or a slow impact tool, you may get away without anti-seize. Try a few without anything but nut and bolt. You can see if the lock up with your process. Since you are not the owner who spent their good money on the materials, perhaps the owner can influence the dome vendor and get some criteria to tighten these joints. It's not an unusual or difficult question. I'd also review your warranty and evaluate your risk based on the unwillingness or inability of dome vendor to give you this information.

Remember - you read this on the World Wild Web !!

 

[drawoh]

ll23,

If you tell us the size of your screws, we can work out a clamping torque on solid aluminium. If you are clamping to a hollow tube, all bets are off. Some bolts in building are methodically not clamped down, so that joints can expand and contract when the temperature changes.

How catastrophic will it be if this roof collapses? There must be a professional engineer somewhere whose license is on the line if you are not given proper installation instructions.

--
JHG

 

[klaus.]

basically you cannot torque ( prestress) SS Bolts

 

Does the building code not enter into this at some point?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[btrueblood]

klaus,

Hunh? We torque/prestress ss bolts and cap screws all the time, just like we do with more typical steel fasteners. It all depends on the spec. the bolts were made to.

 

Fastener failures are one of my most popular income sources, and I can vouch for Klaus's comment.
If I am a mechanical designer I would try very hard to avoid austenitic SS fasteners for any critical application.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[klaus.]

btrueblood
You cab torque them...but you cannot relay on the prestress because SS material will creep away

 

[CWB1]

I don't recall ever using 304 stainless fasteners, but have used 316 without issue in safety critical, highly cyclical applications many times and always analyzed them the same as any other bolted joint.

 

Just curious, what are you using for an anti-seize?

Fastening dry SS bolts causes galling issues and greater uncertainty about pre-stress.
Using anti-seize only facilitates yielding of SS bolts.



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[CWB1]

Using anti-seize only facilitates yielding of SS bolts.

Am I missing something from a material science perspective? Any competent engineer designing bolted joints should understand and be able to quantify the effect of lubricants and adhesives on the joint in question.

 

Am I missing something from a material science perspective?

°
Have you observed how torquing (an indirect and at best crude measure of pre-stress) is often done on the ground? The detailed science underlying the tightening of a bolt group (e.g., pipe flange) is extremely complex.
There are good reasons I do not permit tire installers to use their pneumatic or other weapons on the lug nuts of my vehicle.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[btrueblood]

"cannot relay on the prestress because SS material will creep away "

I've never used stainless fasteners that exhibited any creep, unless they were operating at relatively high temperatures (where they shouldn't be operated). I have seen fasteners that exhibited creep, but they were used on rocket engines at very high operating temperatures, and they were not stainless, but a super-alloy. Over-torqued or overloaded and stretched stainless fasteners are pretty common, as they typically have a lot more ductility than a high strength alloy fastener...but that is not creep per se, that is just idiot installers using the "grunt" method for estimating torque.

"Using anti-seize only facilitates yielding of SS bolts. "

No, come off it. Stainless and other alloys are used daily in piping systems, we use them for our products by the thousands and rarely yield them, and they have never been the cause of a product failure - but the torques we used, and the rated proof load of the fasteners, are substantially lower than what you see for a grade 5 or grade 8 or similar alloy steel fastener. You need to do a proper bolt joint calculation (see Shigley), prescribe a bolt torque based on best guess for the friction coeff. (lubricate them and the error due to friction becomes manageable), and then run some tests. Oh, and find a fastener supplier who will sell you bolts that are made to a spec., like ASTM F837 for ss and alloy cap screws, or A193 or F593, or similar Euro standards that specify the min. proof strength or yield strength of the fasteners.

Again, what facilitates yielding of any bolt is an un-trained installer and poor QC.