Prestressed bolt + tension force

[Alvarobg]

I have to calculate a joint formed by a prestressed bolt which also is tensioned by external force.
I would like to know which partial coefficient for prestress I should use to calculate the bolt in ultimate limit state.
I have read a lot of normative and I have founded: 1.2, 1.1, and even 1.
Anybody could clafify my mind.
Thank you very much.

 

[Ron]

The bolt should be pre-tensioned to 70 percent of its tensile strength. Work it from that.

 

[Alvarobg]

Thanks, but I don't understand that way...
Working with numbers:
- External loads produce a tension force of 300 kN
- γ[sub]external ULS[/sub]=1.35
- Prestress= 70% tensile strength x Area = 500 kN
- γ[sub]P[/sub]= ¿?

Force in bolt in ULS = 1.35 x 300 + γ[sub]P[/sub] x 500

 

[Klitor]

Following your logic, if you pretension to 70%, you would only have those 30% left over to carry external load but it doesnt quite work like that.

Try this

http://www.boltscience.com/pages/basics2.htm

 

[rb1957]

do you want to prevent gapping ?
1) under all circumstances ? then preload should exceed the maximum load seen. I've worked on a project when the bolt could never, ever, gap so we had to consider the scatter in preload, the applied loads from thermal, etc ... a bunch of work !

2) under most circumstances, then 70% ftu is a typical practice; and the peak load in the bolt is the larger of preload or service load.

research "bolt load diagram" or "preloaded bolt loads". ideally the load in the bolt doesn't increase until the bolt gaps ... at loads less than preload the load in the bolt = preload, at applied loads greater than preload the joint gaps and all the load is carried by the bolt.

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

 

[Alvarobg]

Thank you for your comments, the web:

http://www.boltscience.com/pages/basics2.htm

is awesome.
In my case, I have a joint with n bolts. The gap has been produced in some of these bolts (the ammount depends on the preload (with factor).