Looking for preload procedure: torque + turn of the nut

[kingnero]

I'm looking for a desscription on how to design an anchor rod connection.
My client has received a value for the preload, and the design bureau does not want to give a procedure, however they are willing to look at it if a procedure is submitted to them for review.
While this is not something I am familiar with, circumstances here force me to either look at it myself, or subcontract it if I don't feel conformtable enough with it.
Just to give you an idea of the situation, require force is 250 kN per anchor rod ("free" length ~1m in the concrete), 6-8 rods per base plate, size is around M30 and they are stainless steel rods (exact details not yet known).
If somebody could link me a document or book where I could find some more information, it would be much appreciated!

 

[klaus.]

You cannot prestress stainless steel.....
I quess the rods are not designed with prestress....they probably apply prestress only constructively ...as usual

 

[kingnero]

What is the difference between "no prestress" and "prestress construtively" ?
And why can you not prestress stainless steel? Just asking here, I honestly have no idea.

Is this something I should run away from?

 

[klaus.]

'real' prestress is considered in the design...e.g friction connection and you need to use height strengh bolts ( at least in Europe)

constructive prestress is applied to secure the bolts or nuts not to get loose ...
every nut and bolt has to be tighend ...so you can call this also 'prestress'

Stainless steel cannot ( better said should not) be prestressed because SS can creep and SS usually does not have the requered strengh
but...you can of coures apply a certain 'prestress' or torque to secure the bolts

 

[kingnero]

Thanks for the clarification.
Threaded rods should be of quality A4/80, if that makes any difference.

Still hoping for someone to point me in the right direction (pdf, document, book, ...).

 

[ldeem]

There are torque to tension relationship charts/equations but they are notoriously inaccurate. I would first ask why a prestress is being specified and try to understand how critical is the prestress. If not too critical maybe toque tension equations are ok. If prestress is highly critical then I would talk with some inspection companies to see if they have a non-destructive test method that may work. If in between you could make a mock up of the situation and use a load cell to calibrate a procedure to the desired tension.

 

[Agent666]

Yeah I'd think you need to do a mockup and determine your own procedure that they can calibrate against on site. There will be guidance albeit related to high strength steel bolts in steel standards. But at least it would give you some idea especially regarding calibration via torque (refer to AISC for example as I believe it has guidance around the calibration aspects and the testing required). They key is it has to be reproducible if the same procedure is used on site, I believe in AISC you need to do tests daily because of the changes in environmental conditions, effect on lubrication, etc.

Unlike steel bolts the stainless material doesn't have a defined yield plateau. So in steel you hit this and the actual load is more or less constant in the bolt for a fully tensioned bolt in steel to steel connections. With stainless a yield force isn't so easy to hit because of the nature of the stress/strain curve.

Maybe load indicating washers are an option, I don't think you can get them calibrated to any old force though. To be honest not even sure you can get them in stainless varieties.