Pretensioned Bolts and Assemblies 6

[volcomrr]

thread507-253463

I normally just browse through eng-tips and rarely ever post (this may be my first post!)

Looking through threads, I saw one from 8 years ago that touched on a questionable debate we were having in the office last week. The question was, if you have pretensioned bolts supporting a hanger, and tension load is applied to the hanger, do the pretensioned bolts see more tension? My thoughts were the bolts would see more tension, 2 other engineers said the bolts wouldn't see more tension, and the last engineer was on the fence. By the end of the discussion, everyone agreed that tension applied to an assembly fastened by pretensioned bolts would create more tension in the bolts.

Then I saw in the above thread, slickdeals quoted someone from Modern Steel or AISC saying that the bolts would not see more tension until the assembly begins to separate. I believe this to be incorrect. I agree the assembly won't separate until the tension force on the hanger overcomes the pretension of the bolts; however, you're adding more tension load to the bolts that are already tensioned. Does anyone have any insight to shed on this? Thanks.

 

[BAretired]

Welcome to eng-tips, volcomrr.

When a bolt is tightened with force P, the assembly is compressed by force P while the bolt is stretched by the force P. The strain in the assembly is PL/A[sub]a[/sub].E[sub]a[/sub]. Simultaneously, the strain in the bolt is PL/A[sub]b[/sub].E[sub]b[/sub] where subscripts 'a' and 'b' refer to assembly and bolt respectively. Edit: During the tightening process, the nut travels a distance of the sum of these two strains relative to the bolt head.

If an external tensile force is applied to the bolt, the process reverses and, when the external force equals P, the strain in the assembly is zero but the strain in the bolt remains PL/A[sub]b[/sub].E[sub]b[/sub]. If the external force exceeds P, the bolt force increases but not until then.

BA

 

[ENGINEER92]

I remember having this same question a while back and arguing with an older engineer in our office about it. I never fully got my head wrapped around the concept, so my explanation may not be the best. What was explained to me was that when a bolt is pretensioned and tension is then added to the bolt, for every kip of tension added it relieves a kip of pretension. So it is not until the tension added exceeds the pretension does the bolt have an increase in tension.

Again, I accept the idea, just never fully understood it.

 

[BUGGAR]

Just substitute coil springs for the bolt and each of the connected elements and it all makes sense.

 

[CANPRO]

Consider this - you have two plates bolted together with a single bolt, with a pretension of T. There is a pipe welded to each plate and loaded with a force F. There is a force C which is the clamping force between the two plates as a result of the bolt pretension. Consider the free body diagram of a single plate. When F = 0, C = T. When F is greater than or equal to T, C = 0 and the total tension in the bolt has to increase to maintain equilibrium. The clamping force is reduced because of the additional load F, but the tension in the bolt doesn't increase until the force exceeds the pretension.

 

[volcomrr]

All of the responses make complete sense. The fact that the assembly doesn’t open up until the pretension is overcome means no elongation in the bolts until that force is reached. Will share this with the office. Have a nice Thanksgiving.

 

[klaus.]

this is a typical Question I like to pose to my young fresh from school colleagues.....

 

[CANPRO]

If an external tensile force is applied to the bolt, the process reverses and, when the external force equals P, the strain in the assembly is zero but the strain in the bolt remains PL/Ab.Eb. If the external force exceeds P, the bolt force increases but not until then.

BA, I believe you may have a typo in the above quote that makes a fairly significant difference in the discussion. If the external force is applied directly to the bolt, I believe you will in fact increase the bolt tension by applying additional load less than the pretension. I believe the external force has to be applied to the assembly in order for the bolt tension to be unaffected by external forces less than the pretension. Your explanation using strain within the assembly and the bolt make perfect sense to me, but the portion of your post that I quoted above doesn't align with your explanation. Perhaps I'm just being picky about words, but I can't help it sometimes

 

[BAretired]

That's okay CANPRO. In engineering discussions, we should be picky about words because otherwise, it is very easy to misinterpret the meaning.

Having said that, I would reiterate my previous statement. If a tensile force, say T/2 is applied to both ends of the bolt after the bolt was tightened to T (T instead of P to indicate tension), the tension in the bolt is still T with T/2 coming from compression in the assembly and T/2 coming from the external force. If, as you suggest, the force in the bolt exceeds T, i.e. goes to 1.5T, then bolt length L exceeds the assembly length. If there is no contact between bolt and assembly, there is no compression in the assembly. If there is no contact between bolt and assembly, it is not possible to generate a tension of 1.5T by applying a tensile force of T/2.



BA

 

[Shotzie]

Here's a good post from JGard1985 about this topic:

https://www.eng-tips.com/viewthread.cfm?qid=426226

 

[BridgeSmith]

Actually CANPRO, I believe BA has it right. Tensile force applied to the bolt reduces the force between the nut on the side where the tension is applied and the plate or assembly by the same amount as the tension force applied, so the force on the bolt in the pretensioned zone doesn't change until the applied tension exceeds the pretension force. If it helps, increase the pretensioned length of the bolt to some ridiculously long length and remember that stress is proportional to strain, so there has to be a change in bolt length to accompany any change in stress.

If you apply the force to a plate between the bolt head and nut, the force on the nut or bolt head increases and the tension in the bolt increases also.

 

[CANPRO]

HotRod10, I think we’re more or less all on the same page here. My first post in this thread basically said the same thing as BA’s first post, I just explained my understanding using forces rather than strain. My point was, I thought it mattered whether the external force was applied to assembly or directly to the bolt. I gave it some more thought on the drive home, and I agree that it doesn’t really matter. Using my example above, I could have drawn a free body diagram of the the bolt head with an external force applied directly to the bolt and the concept is still the same - external force reduces the clamping force until it exceeds the pretension.

BA, I believe this has happened before where I think I have you on a technicality and I end up being corrected. In my relatively short engineering career, I’ve learned that respectful disagreements can be extremely productive/educational. So, if you don’t mind, I’ll continue to speak up when I think I have you on a technicality, if for other reason than to be corrected.

 

[BAretired]

CANPRO, I agree with you 100% and would encourage you to speak up whenever you disagree or have something to add to the conversation.

BA

 

[Istructeuk]

Well, in the contrast, I don't see any tension load from the hanger added to the pre-loaded bolt because the tension load actually just occurs on the plate surface in the area around the bolts (pre-loaded area), the load path is different from normal bolts.

 

[BridgeSmith]

This time it appears I'm the one who was partially incorrect. I was thinking the force between the underside of the nut and the plate (and therefore the total force on the bolt) would be P+T, but Istructeuk's diagram showed the flaw in my thinking. Until T exceeds P, the force in the bolt is just P, regardless of whether the force is applied to the bolt or assembly.

 

[CANPRO]

Using my example above, I could have drawn a free body diagram of the the bolt head with an external force applied directly to the bolt and the concept is still the same - external force reduces the clamping force until it exceeds the pretension.

Until T exceeds P, the force in the bolt is just P, regardless of whether the force is applied to the bolt or assembly.

I had the same misunderstanding. Not often, if ever, you'd have a pretensioned bolt with an external force applied directly to the bolt, so maybe a bit of an academic discussion at this point. It is a very simple concept once you get your head wrapped around it, but it seems to be a concept that trips up a lot of engineers.

 

[BUGGAR]

The study of automotive connecting rod bolts is relevant here. They include fatigue loading as well.

 

[BAretired]

Not often, if ever, you'd have a pretensioned bolt with an external force applied directly to the bolt, so maybe a bit of an academic discussion at this point.

A similar situation occurs with post-tensioned tendons or bars when the end anchorage needs to be removed to make adjustments as was the case in the recent collapse of a pedestrian bridge in Miami, FL. Tension is applied to the tendon or bar but no movement occurs and no change in tension occurs until prestress force T is exceeded.

BA

 

[desertfox]

Hi volcomrr

I agree when you load a bolted joint there is an increase in bolt tension, however most of the external load goes into the clamped components to reduce the compression of the faces.
The load is proportioned depending on the stiffness of the bolt and clamped components see this link

http://www.boltscience.com/pages/basics5.htm

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[BridgeSmith]

"The load is proportioned depending on the stiffness of the bolt and clamped components..."

It seems as if we've been oversimplifying it a bit. I have to agree with desertfox's statement here. Strain compatibility would dictate that there would be at least a small (granted, typically very small) increase in bolt stress when compression on the plates decreases, since the bolt has to elongate ever so slightly to accommodate the elastic 'rebound' of the plate thickness with the reduced compression stress.