Neoprene bearing shear stiffness 2

[bkal]

Considering neoprene bearings of a rectangular / square shape on plan - does its shear stiffness depend on the direction of the applied horizontal load? I did quite a lot of internet research but could not find any information so any pointers would be greatly appreciated.

I am not designing / specifying the bearings but I am assessing an existing system.

 

[rb1957]

is it a purchased design ? ask the vendor.
is it your (company's) design ? how was this assessed before ?

why is the question being asked now ?

I think a simple test would give you the most reliable results.

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

 

[bkal]

Thanks rb1957,

the bearings are old, have been in place for over 30 years and it is not possible to ask the vendor. This was not assessed before as we are considering a new load case which might load them at any horizontal direction. While the bearings are in a good condition, it is not possible to take a sample and test.

I was hoping that somebody else might have come across a similar issue and have conducted tests. I am not after exact numbers, just an indication if this is an issue or not.

 

[dik]

Are they fibre reinforced? Is it possible to remove one and take it to a lab to determine the duro or Shore hardness? Are you replacing all bearings? Do you have load or deformation requirements for the new replacements or the old bearings. This info is often contained on drawings. do you have drawings? specs?

Dik

 

[bkal]

Apologies for probably being to brief in the first instance.

The bearings are not reinforced, just plain neoprene. A small sample has been taken recently and tested for hardness; it was within the original tolerance. We are not replacing them, we are just checking them for a new loading condition. The original design has assumed they would be loaded only parallel to the sides (on plan), the new load case can load them at any angle and I am interested if the stiffness depends on the loading direction (in the horizontal plane).

 

[SAIL3]

if it is just a plain old rectangular pad, I would doubt if it's performance is directional based...just imagine the possible screw-ups and handling in the field to get the so-called direction right...would the pad itself have to have visible arrow indicating the correct direction?....would the pad supplier require in advance the pad location and direction of the load?...years ago, I specified many of these pads for col bases and after calculating the size and type, one would place the order to whatever supplier one was dealing with....I would advise the OP to contact any current supplier of these pads and run the question by them....

 

[bridgebuster]

Are you familiar with the Dupont publication "Engineering Properties of neoprene Bridge Bearings"? Perhaps it could be of help; it's what we used back in the day 30+ years ago; there wasn't anything else.

Link

 

[dhengr]

Bkal:
The link that Bridgebuster gave is a darn good place to start in understand neoprene bearings and the neoprene itself. As I recall, the material doesn’t particularly have a grain orientation (stress/strain properties orientation) in the plane of the pad matr’l. And, I don’t think the mechanical properties are appreciable different in the through thickness direction either. What you will be dealing with is the difference btwn. bulk compression through the thickness and shear stress/strain in the plane of the bearing pad, and exactly how the matr’l. reacts to these two loading conditions. In the first case you will have a hardness/durometer which relates to inches per inch of thk. vs. the compressive load in lbs./sq.in. and deflection, and also a max. allowable compressive (squashing) stress. In the second case you will have some shear force/shear strain per inch of thk. vs. some length in the direction of loading relating to the shear loading and max. allowable shear stress on the matr’l. And, in this latter case geometry (plan orientation) of the pad changes the total length of application of the shearing load. In both cases, there will likely be some edge distance consideration (thk. relating to allowable edge dist.) where the full mech. props. are not applicable. You might also want to investigate the effects of aging, after 30 years, on the mech. props. I would talk to a few neoprene suppliers for general info. on 30yr. old neoprene and its likely mech. props.

 

[bkal]

Thanks to everybody, especially dhengr; and also to SlideRuleEra who has uploaded this document to his web site.

 

[BridgeSmith]

Looks like you may have already resolved this, but in case you still have doubts, the shear force is independent of the direction. It's just the shear modulus (G) multiplied by the deformation and multiplied by the plan area.

If I understand correctly, the lateral movements are changing. If that is the case, you should be aware that for PEP (plain elastomeric pads), the deformation limit is half of the height. The pads are fairly forgiving, but if your expected deformation is going to exceed the height of the pad, or it's sustained beyond half the height, they may not last very long.

From AASHTO LRFD Bridge design spec., Article C14.7.6.3.4: "The deformation in PEP and FGP are limited to ±0.5 hrt because these movements are the maximum tolerable for repeated and long-term strains in the elastomer. These limits are intended to ensure serviceable bearings with no deterioration of performance..."