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Do Bolts Transfer Compression Loads?

Do Bolts Transfer Compression Loads?

Do Bolts Transfer Compression Loads?

(OP)
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.

Quote:

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...

RE: Do Bolts Transfer Compression Loads?

I would suggest the interviewer was wrong (My wife insists I have no tact.). The moment just reduces the tension in the 'back' bolt, and the tension in the front bolt stays the same until the moment exceeds the initial moment resistance. (Don't know if I explained it correctly).

Dik

RE: Do Bolts Transfer Compression Loads?

It seems clear to me that in the examples you give it is not possible for the bolts to carry any compression load. They are in tension and/or shear only. Now anchor bolts for a light pole are often in compression where one end is embedded in concrete and the flanged base of the pole is trapped between two nuts. These bolts may or may not be grouted after the pole is installed.

It could be argued that shear loading does cause transverse compression of the bolt due to shear but I've never seen anyone make that argument.

RE: Do Bolts Transfer Compression Loads?

Quote (Compositepro)

Now anchor bolts for a light pole are often in compression where one end is embedded in concrete and the flanged base of the pole is trapped between two nuts.
...

Yup, where there is no initial preload.

Dik

RE: Do Bolts Transfer Compression Loads?

I agree that the interviewer is possibly not correct ... I can't see the blots in the sketch carrying compression. With preload as a consideration, then there'd be less tension in the bolts but I don't see the bolts in compression (ever).

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

FWIW (which ain't much) my wife says (accusingly) that I'm like a Vulcan ... and I reply either "thank you" or "what's wrong with that ?"

RE: Do Bolts Transfer Compression Loads?

rb1957: and when it comes to having your ears trimmed, you can say, "a little off the top."

Dik

RE: Do Bolts Transfer Compression Loads?

(OP)
Folks,
Thanks for the replies. To make it clear, the interviewer was referring to the 3rd sketch (plates connected with two rows of bolts)...NOT the first two images. Rereading my OP, I think there may be a chance of slight confusion about which structure my question refers to.

I hope your replies about bolts not usually carrying compression is based on the 3rd sketch.

RE: Do Bolts Transfer Compression Loads?

I think at no time (in usual construction) do bolts carry compression loads ... certainly in none of the sketches given.

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

RE: Do Bolts Transfer Compression Loads?

Quote (Burner2K)

To make it clear, the interviewer was referring to the 3rd sketch

He's still wrong... You can send him a copy of this comment.

Dik

RE: Do Bolts Transfer Compression Loads?

rb1957: Compositepro sums it up...

Dik

RE: Do Bolts Transfer Compression Loads?

(OP)
Folks,
Thanks for the replies. It makes me happy & sad at the same time. Happy to know that my understanding is not off...sad if I don't hear back from the company just because of the above (& also because I made a comment justifying my understanding). I don't need the job desperately but it was not a bad opportunity.

Anyways, keeping my fingers crossed.

RE: Do Bolts Transfer Compression Loads?

sometimes it's best not to win !?

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

RE: Do Bolts Transfer Compression Loads?

Look on the bright side. Think of all the time you're not going to have to waste fighting silly battles.

RE: Do Bolts Transfer Compression Loads?

good luck with other endeavours. You probably didn't want to work with a bunch of dummys, anyway.

Dik

RE: Do Bolts Transfer Compression Loads?

OK... whooooaaaa tigers... here’s my take on this problem...

The first illustration shown would be for a 'snug' nut... no substantial pre-torque after joint 'slop was barely removed'.

Let’s assume a practical bolted joint: [4-each] AN4-X bolts with NAS679-4 nuts [0.25-28UNF-3x] [plus washers... cause I like washers]. NOW torque the dry cad plated bolts/nuts 30-to-40-in#.

The compression clamp-up loads will invariably be centered over the bolt/nut/washer foot-prints.

Resultant clamp-up tension load, per fastener installation, is ~1200 to 1600# [simplified calculation per SAE J1701 K=0.2]... so in this case 4800-to-6400# Force clamps the ‘T’ bracket to the beam. Depending on material thickness [stiffness] the bolts will see vertical 125#-shear/bolt [assuming no shear/friction loads in this joint]... and the upper bolts will see an increase of 375#/bolt tension load... well below the threshold loads noted... while the lower bolts will [might] see a decrease in tension [pre-]load of ~[-]375#/bolt... but maybe not.

Alternately for the second example, assuming parallel and lateral 1/4"D bolt spacing of 1.00 inches [4D]... that would amount to 2400-to-3700# clamp-up per inch along the fastener row. Similar rationale applies to this example as applies to the prior example... except the moment load at the 2-row bolted joint is probably dramatically higher... based on scalar geometry of the example...



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Do Bolts Transfer Compression Loads?

WKTaylor... in none of the examples presented do any of the bolts take compression... his initial reaction was correct, and, the interviewer was wrong.

Dik

RE: Do Bolts Transfer Compression Loads?

dik...

I think You missed my point.

Bolted joints by definition provide joint clamp-up... obviously to varying degrees. Clamp-up provides an essential compression-preload/stability for off-shear capacity.

This is why in the first and second visual examples, there is a high potential for joint stability and load capacity.... for properly installed bolts [as I noted].

On-the-other-hand, solid driven rivets DO NOT provide joint clamp-up under any reliable circumstance. Joint clamp-up is established prior to fastening by external mechanical devices... clamps, Clecos, etc. After riveting, tension preload thru the rivet is essentially ZERO.

A general rule-of-thumb [ROT, I learned early in my career] is that rivet tension load capacity should [for reliable design] should be NO MORE THAN = [0.10] X [shear rating]... for that solid rivet. Several design manuals I use have NO allowed tensile ratings for rivets [although very old version of those manuals did show low tensile ratings, at one time (+40-years ago)]. This ROT applies to all solid driven rivet alloys... aluminum, monel, A286, Ti-Cb, steel, etc.

Getting back to the examples shown...

I said my piece about the 2-bolted joints styles.

However IF these 2-bolted joints styles were assembled with solid driven rivets, they would be highly unreliable/unsuitable for the loading scenarios shown.... unless the .10X tension load limit was not exceeded.then all-bets would be off-the table, for the reasons many of You noted. AND I would only recommend use in limited load reversal scenarios.

ONE LAST COMMENT.

F-15 [aluminum honeycomb] wing-tips are installed on the wing-structure end-ribs with a single-row of tension head screws, upper/lower surfaces. What a nightmare for stiffness and durability... especially for the high vibration encountered in high AOA buffet.

Visualize the scenario shown in the figure 5-61 or the bolted-plate example... but remove one-row of bolts entirely... and institute a 100% load reversal spectrum. It doesn't matter how robust the single row design is... it is doomed to have a short life.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Do Bolts Transfer Compression Loads?

Quote (WKTaylor)

Long post

What you are saying is correct.

But that doesn't change the face that in the examples shown, none of the bolts see any compression load- which is the point Dik was making.

Seeing compression load and applying compression load are not the same thing. All bolts in OP's examples see tension and apply compression.

RE: Do Bolts Transfer Compression Loads?

WKTaylor... understood, and, thanks.

Dik

RE: Do Bolts Transfer Compression Loads?

(OP)
jgKRI
"All bolts in OP's examples see tension and apply compression."
What do you mean by the above statement, Sir?

RE: Do Bolts Transfer Compression Loads?

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

Dik

RE: Do Bolts Transfer Compression Loads?

Quote (Burner2k)

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

RE: Do Bolts Transfer Compression Loads?

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!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Do Bolts Transfer Compression Loads?

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.

RE: Do Bolts Transfer Compression Loads?

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?

RE: Do Bolts Transfer Compression Loads?

"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 ?

RE: Do Bolts Transfer Compression Loads?

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?

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