Pipe Clamping Force with Four Bolt Connection 4

[swengi]

Hi

This is my first post on this Forum. I'm interested to see what responses there are to my query as I'm having difficulty getting an answer from Dr Google.

My problem is as follows:

We have been asked to certify a clamp. The clamp is a steel collar around a 2800mm DIA reinforced concrete pier, with a four (4) bolt configuration each side of the clamp. Each of the four bolts are proposed to be tensioned to 200 kN. The collar is designed to resist vertical sliding on the pier.

My question is this: What is the sliding resistance of the clamp, assuming the coefficient of friction is 0.3? Is it 200 x 0.3, or is it 4 x 200 x 0.3?

I would have said that the sliding resistance is 200 x 0.3, as there is only 200 kN of total tension in the system (despite there being four bolts). However, other engineers have claimed that we should allow 4 x 200 x 0.3 (i.e. the bolt clamping forces are additive).

It would be great to hear people's opinions on how to design such a connection and also if there are any useful references available for pipe clamp design. As I mentioned, I am really struggling to find any reference material. I'm happy to provide more information if any of the above is unclear.

Thanks!

 

[LittleInch]

I think this simply goes to show the power of a drawing / sketch. If you had posted the drawing above we could have understood a bit more about what you meant and that is was a band type clamp and not a more rigid clamp like this, which is where I and a few others I think were coming from....

Certifying something in which you can't back up your findings by experience and calculation can be a little dangerous.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Lomarandil]

I know engineers don't like to rely on friction... but in the bridge construction world, these friction collars are tested and tried and still in widespread use.

----
The name is a long story -- just call me Lo.

 

[swengi]

Hi LittleInch

The clamp is like the rigid type in your picture and SlideRuleEra's diagram above. It is not the band clamp, which is pinned on one side. However, I have used this analogy in my calculations. I know it is not 100% technically correct. Apologies, this is my first time posting on this forum and I wasn't aware you could post a screenshot.

Thanks!

 

[swengi]

Hi Lomarandil

Thanks for the response. This is my first time coming across the friction collar. I do a bit of bridge formwork but not tons. It seems that there is no good published literature or design guides for these connections and engineers are relying solely on empirical advice.

Thanks!

 

[SlideRuleEra]

I do a bit of bridge formwork...
...no good published literature or design guides...
...engineers are relying solely on empirical advice.

Maybe for simple situations... otherwise, statics, dynamics, strength of materials, geometry, trigonometry, reasonable simplifying assumptions, manufacture's published data, info from industry organization (e.g. Southern Forest Products Association, Steel Deck Institute, American Wood Council, Forest Products Laboratory), etc.

...while complying with the spirit and intent of ACI, AISC, building codes... but not necessarily every detail.

To me, that is where the fun and challenge of practicing engineering takes place.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Lomarandil]

Exactly... there's a whole world out there that's outside the scope of "The Codes", but close enough that you adapt the engineering principles buried inside (and other places) to find a solution.

----
The name is a long story -- just call me Lo.

 

[swengi]

Thanks SlideRuleEra & Lomarandil.

I usually can find good sources for design online, in codes or other reference material. I have had to utilise critical thinking and combine different methods for designing non-standard items in the past, e.g. dolly for pile hammer under impact loading, which did not come from the code. I'm not afraid to go looking for answers to difficult problems rather than using cookie-cutter solutions.

However, this one had me stumped. I really could not find any references that were of any benefit, aside from the 'band clamp' example previously cited. Hence why I decided to post here and consult the 'brains trust'!

I have received some really helpful feedback, including some surprising things I didn't expect. For example the reference that LittleInch cited examines these types of connections with a neoprene liner for offshore use. The neoprene manufacturers suggest a coefficient of friction of 0.8, which I would have said was reasonable-ish for a rubber type material. However, the authors found that the actual coefficient of friction varied widely and was a LOT lower. They suggested using a coefficient of friction of 0.2 and a FOS of 1.7, i.e. an effective coefficient of friction of 0.117. That's almost 7 times less than the manufacturer quoted! This makes me question the calculations backing up this clamp design, notwithstanding their long history of use in bridge construction. It seems that friction is not always a straightforward or completely intuitive topic.

I guess it's a case of what we're comfortable signing off. Relying on friction for these types of connections definitely does not make me sleep easy, given that they are not backed up by technical refernces or manufacturer's data. Yes, it is fun and challenging but I just wish I had something a little more concrete to hang my hat on in this particular situation.

Thanks again!

 

[desertfox]

Hi swengi

I am surprised the manufacturer doesn't give any tightening information for the bolts that are supplied with the collar, you would think they would want to avoid the bolts/clamp being overstressed.

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

 

[BridgeSmith]

"The neoprene manufacturers suggest a coefficient of friction of 0.8...the authors found that the actual coefficient of friction varied widely and was a LOT lower."

Look carefully to be sure the authors you refer to were using the same type of material. Some rubbers, including some neoprene formulations will 'bleed' oils under pressure, which will lubricate the contact surfaces. We discovered this with steel reinforced elastomeric bridge bearings. They would literally 'squirt out' from between the concrete pier cap and the steel sole plate because they greased the surfaces.

You should investigate fabric (cotton duck) pads. They'll conform to the uneven surfaces and should offer a more consistent coefficient of friction. We still spec them in an 1/8" thickness between steel bearing plates and concrete seats.

 

[swengi]

Hi desertfox

The collar is a bespoke design so unfortunately, no manufacturer specs are available!

Thanks!

 

[swengi]

Hi HotRod10

Thanks for the suggestion. I haven't come across fabric (cotton duck) pads before. Do you have any idea of the coefficient of friction one could expect when using these pads?

Also, with the steel bearing plates and concrete seats, I assume that the loads are stabilising? In our situation, we are relying on friction to provide structural support and the loads are destabilising.

Thanks!

 

[SlideRuleEra]

swengi - I'm going to make a guess... if it wrong, please just disregard.

We have been asked to certify a clamp.

...we are certifying the design and not the original designer.

...we have limited scope to change it. I believe these clamps have already been fabricated.

The collar is a bespoke design so unfortunately, no manufacturer specs are available!

The word "bespoke" is new to me, but from the web it seems to mean "designed from scratch".

It sure sounds like your firm has been retained to "evaluate" and determine justifiable clamp performance (steel on concrete friction, only) and to establish procedures (bolt tightening pattern and sequencing, etc.) to support the results of your evaluation.

Not to suggest improvements as an excuse to avoid setting a load rating for steel on concrete.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BridgeSmith]

With further investigation, it seems the cotton duck bearing pads in AASHTO (termed CDP) are laminated with elastomers also, and so suffer from the same oil secretion problems as other elastomeric bearings, resulting in a similarly low coefficient of friction. If you could find some made strictly from layered fabric, they should have a much higher COF.

You may want to contact one of the engineering/manufacturing firms for bearing pads and ask them if there's something they manufacture, or could manufacture, something to suit your needs. I worked with Voss Engineering on a custom solution for a retaining wall recently, and they were very helpful. I also found Fabreeka, who does CDP; they might be able to come up with something for you.

"Also, with the steel bearing plates and concrete seats, I assume that the loads are stabilising?"

To a certain extent, but in cases where the deflection of the girders resulted in the sole plates and seats not being parallel, the uneven pressure caused them to squeeze out.

Anyway, that was the long explanation of what I think is the most likely reason for the discrepancy between the high COF for the elastomeric pads from the manufacturer and the low COF in the tests. The short version is you'll have to find a material that is flexible and compressible, but doesn't secrete oils under pressure.

Perhaps a urethane material of some kind, similar to what the pros use for car windshield replacements could work. It's like a caulking material hot out of the tube, but cures to a really hard rubber. Can the clamps be grooved on the inside to allow interlock between the material and the clamp, so that there is shear strength in addition to the bond and friction? It would require a couple extra steps, since the urethane would have to cure with the clamps snug tight and then fully tensioned after it cures. Installation might require more oversight than you're prepared to provide, but I thought I throw it out there.

 

[swengi]

Hi SlideRuleEra

Yes, you are correct. We are certifying and therefore not trying to change the design. Any suggestion of improvement is purely for my own edification, as I may have to design one of these clamps myself at some point.

Thanks!

 

[swengi]

Hi HotRod10

Thanks again for the helpful response. In this instance we cannot change the design. However, I may be able to consider this approach for future designs.

It's good to be aware of the potential issues with oil secretion for elastomeric pads and neoprene. Do you know if rubber load mats would have the same problem? I have used them in the past for transport purposes. The suggested coefficient of friction being 0.7.

Thanks!

 

[BridgeSmith]

I don't know anything about load mats. The bearing pads only seem to have the problem under heavier loads (around 500 psi and up), so even if they are of similar materials, depending on how heavily compressed the mats are, they may or may not exhibit similar issues.