Do Bolts Transfer Compression Loads?

[Burner2k]

Lets say I have a structure with loading as shown in the below figure.

My understanding is along the line of what Flabel says in his book. I am posting excerpt from Chap. 5 "Shear & Tension Clip Supports" Example 5-6, Page 349-350.

The bending Moment M determined there (that is, at the centroid of the fastener pattern) is further distributed as a horizontal couple-force between the upper bolt attachments and the lower base flange portion of the bracket: one force acting along the centreline of the upper bolts and the other acting along the center of bearing pressure of the lower base flange, as shown below. The contact stresses depicted in this figure are referred to as bearing stresses, not bolt compression forces. Hence, the lower bolts do not actually carry this load. Here, the bearing stresses are entirely carried by the lower base flange.

Image illustrating the FBD as Flabel draws it.

A couple of days ago, I was asked in an interview a question along the above lines i.e. how best to transfer moments between two structures. Although, I did not think of this joint arrangement, the interviewer brought this up and I guess he was expecting to hear my knowledge about moment load path. Two plates joined by 2 rows of bolts, subjected to an end transverse load.

I gave the above answer i.e. the "compression" part of the couple will be transferred through bearing contact of the plates and bolts won't transfer it. He disagreed and I mentioned I got this info from Flabel's book but he did not seem convinced. He maintained that bolt will carry the compression part of the couple and my scenario plays out if there is only one bolt (to carry tension) and the compression part will be carried via "Heel-Toe" effect between plates.

Anyways, I am posting here to get clarity on my understanding. What do folks think about bolt carrying compression in a joint like above? I plan to do a 2D FE analysis with CBUSH Springs (bolts) & Contact between plates some time in the next week to see how FEM reports the load path.

Regards...

 

[dik]

Through the plates, through the bolts, through the plates, through the bolts, and through the plates. Seems simple enough...

Dik

 

[SwinnyGG]

What do you mean by the above statement, Sir?

I mean that in none of your examples are there any bolts loaded in axial compression

 

[WKTaylor]

OK... for clarity...

'Bolts*' are designed to carry [pure] shear, shear-tension, tension-shear or [pure] tension loads.

When installed with pre-load torque, bolts/nuts generate axial tensile stress internally which applies 'steady-state**' compressive/clamp-up forces on the mating structure/mechanical parts.

With the rare exception of up-stop/travel-stop bolts, or bolts/threaded-rods acting in push-pull mechanism capacity, they never*** carry true axial compression.

*Definition. Bolts have a head, plain shank [varying lengths] and a threaded-end on the shank [varying length]. Screws typically have a head and a full-threaded shank [some variations to this rule permitted for really long screws].
** will vary with thermal effects for the alloys; and structural strain [tension-thinning or compression-thickening or prying forces] at the fastener hole.
***OK, I should never-say-never.



Regards, Wil Taylor

o Trust - But Verify!
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[Worldtraveller]

Burner2k, you are technically correct, and the figures you share from the text are correct when it comes to the pure physics of what's "actually" happening.

I don't know the background of your interviewer. Maybe he wasn't really an engineer, or if he's a manager, hasn't done engineering work in a long time. Or, alternately, that's the only method he learned at whatever company he works/worked for. Producing a simple couple based on the fastener spacing is fast an easy, and is often used as the primary method of determining the tension load in the critical fasteners. (Assuming that it is realized that the other fastener, as noted, does not actually experience compression load).

My first boss (I miss that guy) used to say that he preferred hiring young college grad engineers to experienced engineers because then they would only learn his bad habits and not come in with lots of their own that they might have to unlearn.

Most times, fast and easy is good enough for engineering.

 

[banag]

One more extended query to this problem:

It is understood that the bolt doesn't transfer the compression loading whereas the compression is transferred through contact between plates in a particular region. The region through which the contact is transferred can be evaluated using the heel-toe approach. Question is: Does the preload in the bolt affect the contact region? In cases of more accurate contact region CG as interest, is it needed to account the preload into consideration? The question raises from the argument that the pre-loaded region of the plates is relatively in more contact (clamped) than the surrounding. The Flabel's drawing (OP) illustrates the contact reaction is maximum at fitting's bottom edge and linearly drops to the bolt hole. It means, there is no impact of bolt preload to the contact region?

 

[rb1957]

"Does the preload in the bolt affect the contact region?" ... yes, the more preload, the more contact.

I interpret Flabel's drawing as an expedient way to analyze the joint, and one that represents a lot of what's going on, but not the Truth.

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

 

[lusosa]

You are right that the bolt will not carry compression, but it is under shear. The orange plate is deforming for extension and the blue plate is deforming for compression, in that intersection, so both bolts will also have a shear force if you isolate them.

In a more general case, I don't really see how a bolt could carry compression...I mean, it obviously could if the two things it is uniting are separated, but why would anyone use a bolt like that?