Evaluate bolt preload

[marulanda]

Hi, I need help with bolt preload design.
It's a simple problem, but I don't have experience with this argument.
I've two plates joined with two M5,

plates are s235 fy=235MPa steel thk 3mm and 2mm, bolt 8.8 fy=600Mpa.

The design preload of the bolt is 6300 N, evaluated with meccanical property of the bolt.

Is it too much, considering the difference of the of the fy and that the plates are tiny?

Thank you very much.

 

[rb1957]

preload should be designed around the usage of the bolt, shear or tension (probably something like 50% yield). better is to design the preload based on the tension loads expected.

another day in paradise, or is paradise one day closer ?

 

[TLHS]

If you know what your preload is, it doesn't sound like you need to do the design... it sounds like you need a tensioning procedure?

If they're structural bolts, see this document:

http://www.boltcouncil.org/files/2004RCSCSpecification.pdf

 

[dik]

You might consider using hardened washers, too.

Dik

 

[marulanda]

I've not a preload of the bolts, I have to find it starting from the value of the specifications (6300 N).

In this case I've calculated that with 6300 N the stress induced in the plates is too high, but I' m not sure that my calc is ok.

So the questions are two;

I would like to know if is commune to apply preload different from the specifications, considering the max tension load for exsemple (as rb1957 suggest)?
Is in my situation possible that the stress is too high (with 6300)?

I'm thinking to apply 3000 N, max tension load 350 N.

thanks all (sorry for my english)

 

[klaus.]

According to Eurocode the preload ( prestress) of a bolt is 0.7 times the breaking load of the bolt
F = 0.7 x fub x As
But steel structure design the world starts at M12 ....and not at M5

Here is a table that might be helpful....in German only
klick

For M5 grade 8.8 the table gives aboput 9kN preload ( Vorspannkraft)


best regards
Klaus

 

[rb1957]

how is the preload affecting the plates ? wouldn't it just compress under the head/nut ?

if the plates do not touch (if there's a gap between them), fill with shim.

I don't know the way you guys work, if the code spells out "thou shalt preload to 70% breaking load, not 69% unless you continue to 70%, not 71%" I'd've thought that preload should be a feature of the design, and the code would say "no gapping under limit load". if it says 70% then that's the number and design for it.

another day in paradise, or is paradise one day closer ?

 

[dhengr]

Marulanda:
Klaus’ preload (23NOV, 12:03) of [(.7)(Fult)(Abolt)] is some codified max. preload for that bolt, not necessarily your design preload, which can be some percentage less than near bolt failure. And, Rb1957 says this same thing in his several posts. Certainly, your preload should not yield the joined matr’l or your joint has softened and the preload decreases. At least, this has to be taken into account in your design, maybe with hardened washers to spread the bearing stress. But, then, we don’t know how you’ve made this calc. given the confusion your posts suggest. Your actual preload should be based on your actual loads and the joint functioning properly, and this takes some real engineering experience and judgement.