TB Bolts - Overstressing possibility?

[BenAustralia]

Hey Everyone,

First post here! Names Ben, structural engineer from Australia! (name gives that away)

My question is more help with understanding the concept of locked in tension/stresses. Best example is as follows:

Say I have a bolted end plate moment connection, or a bolted moment base plate connection.

If, for example, the bolts use were TB Bolts (Tensioned bolts) that were tensioned to 100kN each. If a moment is then applied to the system and each bolt on the tension side is required to resist another 100kN, is the tension in the bolts 200kN or 100kN?

Looking at it from a logical point of view, if I held the bolt and pulled it apart to 100kN, then applied another 100kN to it, there would be 200kN tension in the bolt.

Anyone shed some light on this? Its just something I've have recently thought about but have never been told about or cautioned on.

Thanks guys!

 

[slickdeals]

Welcome to ENG-TIPS.

The answer is No. It is counter-intuitive. The links below explain it well.

http://machinedesign.com/article/how-external-loads-affect-bolted-joints-0420

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

 

[hokie66]

Welcome from me as well. Slickdeals is correct. Take a bit of time to get your head around it, and read about prestressing. Maybe you have a better feel for how prestressing works in concrete structures. Take the case of a post-tensioned slab...the tensioned strands place the concrete in compression...bending the slab decreases the compression, but does not increase the strand tension while everything is elastic.

 

[dik]

Can you post your example... it is possible to stress a bolt beyond the pre-load force. You can put enough moment on the connection to actually fail it.

Dik

 

[BenAustralia]

Thanks guys, I did think about it both ways but was unsure on the correct.

So my understanding now is that;

Tightening of a bolt induces an elongation in the bolt and a shrinkage in the plate. Applying an additional load merely starts to relieve the clamping forces in the plate (and to some degree further elongation of the bolt but that is proportional to the relative stiffnesses) and reduce its shrinkage. Once the additional load exceeds the initial tension load, then the bolt will experience additional tension?

Am I on the right track? (it's late at night here, don't think i'm thinking straight).

Questions:

1. Given our TB bolts are typically 830 MPa, and our steel is 250 MPa, would increasing the steel plate strength to match that of the bolt mean that more of the applied load is taken by the bolt prior to it exceeding the preload?
2. So in my original example. If the bolt is tightened to 100 kN, then additional loading applied 100 kN, the actaul tension in the bolt would be quite a low number? Dependent on the relative stiffnesses of the bolt and plate.

This really isn't something they teach at University here, and I'm yet to have come across it unfortunately.

 

[dhengr]

Ben:
In keeping with Hokie’s suggestion of thinking in terms of prestressing in conc. structures, try thinking of it this way, it’s essentially the same thing:

Your initial bolt tightening strains the bolt and pulls the base plate down on the conc. pier or footing, and there is a compressive force btwn. the base plate and pier. With a very stiff base plate, until you overcome this initial compressive force you are not adding any new strain (thus stress, or force) to the bolts which are in tension. Once you overcome this initial compressive force you are finally adding new tension to the bolts. Of course, the relative stiffness of the various parts of the entire system come into play, and with a fairly flexible base plate you can induce plate bending which causes a new prying force on the bolts, over and above the basic tension.

 

[BenAustralia]

Alright, might have to go read up on prestressed concrete a bit more. Still doing my head in a bit!

I guess any moment would need to pull the plate away to a point where there is no compression between it and the concrete (or other plate) before the bolt can start to stretch more.

Its one of those ones that my understanding changes depending on how I try to visualise it in my mind.

 

[dik]

The answer to question 1 is: not likely, increasing the yield strength of the plate may reduce the thickness a tad but may cause problems with welding the plate to whatever is connecting to it or may cause added prying action forces because of the reduced stiffness of the plate...

I don't quite understand question 2... if the bolt is preloaded to 100 kN, then as the load is applied... the 100 kN load remains until sufficient moment is applied to balance this tensile load... at this point as the moment increases, the bolt gains additional tensile load... the relative stiffness of the bolt and the plate has little if anything to do with it, except in some instances if yielding or prying action comes into play...

Dik

 

[BenAustralia]

Ok, sorry, poorly worded Q2.

100kN preload remains. As moment increases to a point that the tension force required is above 100kN, the bolt tension remains at 100kN. When the tension from the applied moment exceeds the preload, the tension in the bolt is whatever the applied tension is.

The part doing my head in is visualising it, which I tend to do with most problems and concepts. Does the theory behind this assume that the plate does not compress? I.e. No change in thickness?