Pre Tension in Bolt 2

[Applet123]

I am desiging a bolting system that is subjected to waves loads. I specified a pretension force of 21 kips on a 1 inch diameter bolt with locknuts

However, AISC specification for structural steel buildings - allowable stress design and plastic design, Table J3.7 says that the bolts need to be pretensioned up to 70% of its tensile strength.

This means that the minimun pre tension force is 51 kips. Is it required to pretension the bolts up to 51 kips?

 

[diamondjim]

That looks about right for
120000 psi tensile bolts.

 

[chichuck]

Applet123,

The AISC Spec is for structural steel connections. The bolts they cover are ASTM A325 and A490 bolt materials. (these are known as high-strength bolts.) If that is indeed the appropriate code, then your 51k numberis for the A325 bolts. It is required under certain circumstances by the AISC Spec. If you have one of those situations, then you need to provide that pretension. It should be noted that the tension will have to be verified by either testing the turn of the nut method, by calibrated wrench, both of which require a test result 5% larger than the 51k. Or else by using load indicating washers or "alternate design bolts" (tension-control bolts or something similar.)

Regards,


chichuck

 

[Applet123]

Thanks for the help!

From my understanding, the bolts need to be pre tensioned if there is stress reversal of stress, how does the stress reversal relate to the need for pre tensioing of the bolts?

Also, how did AISC obtain the magnitude of this pre tension force?

 

[CoryPad]

You should educate yourself on the basic's of bolted joints. Try these sources from AISC:

http://www.boltcouncil.org/guide1.htm

http://www.boltcouncil.org/download RCSC Specification.htm

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Torquester]

The strength of the 1" fastener you intend to use is an important variable. The 51 kip target tension is for 120 ksi structural bolts of heavy hex pattern in slip-critical connections.

A normal hex pattern automotive fastener of the same strength level (SAE J429 Grade 5 or ASTM A449) would also typically be tensioned to much higher values than 21 kips. A common target tension of close to 42 kips would be common.

It is generally not a good practice to pretension higher strength fasteners to compartively low levels of tension. If subjected to vibration they are candidates for loosening and perhaps fatique failure.

Comparably high clamping force is correlated with resistance to loosening, and is in part, behind why we don't require the use of locking elements on structural fasteners on bridges or elsewhere. Lastly, locking elements function much better at higher proportions of bolt capacity, regardless of strength level.

A good source of information would be a techncial rep from either a manufacturer like SPS Technologies or a supplier of locking elements like Nylok or Loctite.

Good luck.