friction coeficient of bearing pad 6

[yjung]

Hi again,

do anyone know what is the friction coeficient of typical bearing pad?

i have calculate longitudinal force due to shear resistence of bearing pad. it gave me a lot of lateral load more then 20 % of dead load. could anyone tell me what to do now?

thanks

young-chul

 

[georam]

Are you thinking about factor of safety against sliding at the base of the foundation pad ?

Factor of safety (FOS) = Sum or Resisting Forces/ Sum of driving forces.

The sum of the driving forces may be the one you indicated as lateral loads. If it is a gravity wall... it is the load of soil and water pressure (if any) behind the wall.

The sum of the resisting load is usually considered as W tan(phi), where W = weight of the structure above the pad (concrete, dead loads, etc). phi = friction angle between the pad and the underlying soil.
Usually you take the lowest value between the two materials. Concrete to concrete has phi 45 to 50 deg. soil to soil has phi of their shear strength as defined from shear tests/triaxial test.
For concrete on sand.. phi can be about 30-35 degrees. depending on density of the sand, and how rough the contact area.
For concrete on clay... phi varies from 20 to 25 degrees... may be can be more, to 28 degrees.

If you said that the lateral load is only 20% more than vertical load. Wtan(phi) will be maximum W (if phi=45 degrees), but likely be a fraction of W. If phi= 26.5 deg, resisting force will only about 0.5W

FOS will be 0.5W/1.2W = 0.4 < 1... not stable !

I hope that this helps

 

[yjung]

hi thanks for kind response, sir.
my bearing pad is between superstructure and capbeam.
and i would like to know what is friction coeficient. it should be between 0.1 to 0.2.

thank you so much

 

[breaks]

In the NCDOT Specifications, their recommendations for the friction coefficient are as follows:

Steel bearing on Steel - 0.30
Stainless Steel bearing on Teflon - 0.10

This coefficient should be multiplied by the Dead Load to obtain the longitudinal force (as you already stated).

For elastomeric bearings, NCDOT uses the following equation to calculate the friction force:

F = [(Shear Modulus)x(Contact Area)x(Deflection due to temp.)] / (Thickness of effective rubber)

These values may vary depending on what specifications your client uses, but they seem to be fairly standard. Good luck.

 

[yjung]

Hi,

My bridge is box girder bridge. The calculated F = [(Shear Modulus)x(Contact Area)x(Deflection due to temp.)] / (Thickness of effective rubber) was so huge. so i would like to use some other F as friction force. I would like to use F as friction coeficient * axial dead load. the friction coeficient will be between concrete with neoprene strip. please tell me what is friction coeficient for this case?

Thank you a lot for the kind responses.

 

[breaks]

The reason NCDOT uses this equation is because a neoprene pad will distort or deflect rather than allow sliding of the girder (or box beam in this case). So a friction coefficient isn't applicable in this case.

The neoprene's Shear Modulus will be 110 psi for a typical 50-Hard bearing. Depending on the length of the bridge and the distance between expansion joints, the deflection due to temperature (expansion) should be between 0.5&quot; - 1.5&quot;. Of course, all of this depends on your bridge information. If the bridge is a large one, expect the value to be relatively large as well.

This shear force equation is also found in &quot;Design of Neoprene Bearing Pads&quot; (Published by DuPont).

Hope this helps!

 

[yjung]

If i want to use friction coeficient approach.
The friction coeficient as 0.2 is high or not?

 

[breaks]

A previous VDOT (Virginia) job I worked on used a friction coefficient of 0.06. This project used steel girders and elastomeric bearing pads. I'm not sure about how this would change when concrete girders replace the steel ones, but it would probably cause it to increase.

Using 0.2 would be conservative, though, if you choose to use it. Doing a bit more research would most likely decrease this number.

Sorry I couldn't be of more help!

 

[austim]

Hi, yjung

Unless I have not properly understood your proposal, I believe that you cannot do what you want. One of the reasons that you are having difficulty in finding a recommended coefficient of friction for concrete/elastomer is that using an elastomer pad as a sliding bearing is not a sound proposal.

If you are trying to use an elastomeric pad which is too thin to accomodate your design deck movements, then you will end up with movement occurring between the deck and the elastomeric pad, which is a strict no-no.

Ask yourself how long the elastomer will survive if you are rubbing a loaded concrete surface over it whenever the temperature changes? (Consider how long your car tyres last if you 'burn rubber' by spinning them every time you start up).

I strongly suggest that you discuss your proposals with any reputable supplier of elastomeric bridge bearings (who ought to be horrified by your suggestion).

Although you may not like the cost of a properly designed elastomeric bearing, the only reasonable alternative (ie a mechanical sliding bearing, possibly with provision for rotation) could be worse.

 

[Qshake]

Given the other questions you've posted in this forum which it is apparent that you are a novice bridge engineer. This question only serves to further that point. There are two things you should do at this point.

1. Discuss the problem with your supervisor, who hopefully has more bridge experience.

2. If you feel that the lateral loads were arrived at properly then design the substructure and move on with the design.

3. Provide for another bearing, based on the recommendation of the agency you are designing for, such as a PTFE bearing.

For many agencies (State DOTs or other) the design of bearings is reduced to some simple charts. This is done for several reasons. The most important reason is that the states have had some success with various bearings and would like to continue that success. In changing the bearing type or relying on a set of different rules you are tossing the performance history and level of comfort out the door. Bearings which are ill-suited for a give situation will cause a lifetime of problems for that bridge.

In many cases the need for certain bearings is based on an expected vertical load and lateral movement (which may tied into the span length). The charts show the appropriate bearings to be used for span length (thermal length) and movement. Further charts based on load and movement will determine the size of the bearing. The bearing section in AASHTO may be used to develop such charts.

For heavy vertical loads and large displacements you should use a PTFE bearing. And yes, they are expensive compared to plain elastomeric bearings and or elasotomeric bearings with shims. Since in many cases, the elastomer bearing is expected to be bonded to the concrete, you should not see any movement between the concrete surface and bearing. As &quot;austim&quot; said &quot;this is a strict no-no&quot;. NO BEARING SHOULD MOVE RELATIVE TO THE CONCRETE SURFACE IT SITS ON! The movement should take place within the components of the bearing itself.

 

[austim]

Hi, again, yjung,

I should have mentioned this first time, but wanted to check with my own friendly elastomeric specialist before doing so.

Friction between DRY concrete and elastomer can be greater than 1.0. (That is why rubber tyres can be pretty good at roadholding and stopping when they are needed to be).

So, even if you can persuade yourself (plus your client and supervisor) that long-term durability need not be part of your bridge bearing design criteria, you shouldn't expect to reduce the horizontal forces by sliding on elastomer anyway.

I would echo all that 'Qshake' has written.

Selection of appropriate bridge bearings is an absolutely fundamental part of the whole bridge design. You should dig out any relevant bridge design textbooks (as well as discussing your design problems with more experienced engineers) to add to any helpful comments that you can get from this forum.

When I first started as a wholly inexperienced graduate engineer with leading consultants, the partner in charge of our office frequently reminded us that the most common cause of bridge maintenance problems was poor bearing and expansion joint design. Mind you, that was in the days when construction supervision was taken seriously, and before widespread use of de-icing salts

 

[breaks]

austim and Qshake,

Thanks for setting me straight - I should've stuck by my first 2 posts and don't know why I didn't.

 

[austim]

A follow-up question to 'breaks' (I am intrigued by your 3rd. post):

Did the VDOT job to which you refer really use steel-to-elastomer sliding contact, or merely rubber pads underneath steel-teflon sliding bearings (perhaps to give some rotational resilience to conventional sliding bearings)?

If the sliding surfaces were in fact steel to elastomer, has anyone checked recently to see how they are going? (Who knows, there might be something going on there which would turn conventional bridge bearing design on its head )

 

[noelbc]

The coefficient of friction for rubber involves very complicated processes and I'm not sure that anyone really knows. The surface wetness and roughness, type and hardness of rubber, type of surface, dynamic or static.... too many variables. I'm sure you've heard all this.... You almost have to do your own test to determine YOUR value, for YOUR conditions. BUT, I have a few results which should help.... Dry SBR to smooth steel; 0.5 This is for a hard rubber... expect better values for softer materials... like 60A bearing pad would be higher. I think over 1.0

 

[breaks]

Austim, sorry it took so long to respond, but here's the info you requested regarding my 3rd post:

The elastomer pads were rubber pads underneath steel-teflon sliding bearings, as you supposed. The bridge had a very high skew and this approach was used to give some rotational resilience (as you stated). This was the only part of the bridge I did not have a hand in designing, so I glanced at the calculations and assumed a bit too much. Definitely a no-no.

Thanks for making me clarify that!!

 

[austim]

Hi, breaks.

Thanks for that. You really had me wondering for a while there.