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Selection of bearings between elastomeric pads&pot bearings

Selection of bearings between elastomeric pads&pot bearings

Selection of bearings between elastomeric pads&pot bearings

(OP)
  
   I want to know which type of bearing i.e.Elastomeric
   laminated pads or pot type of elastomeric bearings be
   selected in a railroad bridge  having prestressed
   concrete box girders having simply supported 48.5
   m spans over a seacreek if load on each of 4 bearings
   is  425 tonnes and longitudinal load on each bearing is
   50 tonnes. The effective span is 46m

RE: Selection of bearings between elastomeric pads&pot bearings

All things being equal, the POT bearings are expensive compared to the laminated elastomer bearings.  I would be surprised if a rail company also wanted the additional maintenance concerns with moving parts.  

So, if the elastomer works with the loads and temperature range then I would recommend its use.

RE: Selection of bearings between elastomeric pads&pot bearings

(OP)

   Thankyou Mr. QSHAKE
   In our design we are getting 105mm thikness at free
   end and 65mm thickness at the fixed end. The fixedity and
   at one end and free longitudinal movement at the other
   end have been provided by a steel central bearings at
   either end. Is 105mm thickness alarming in any way

RE: Selection of bearings between elastomeric pads&pot bearings

Not at all.  In my practice I've seen taller bearings used and without short or long term problems.

RE: Selection of bearings between elastomeric pads&pot bearings

(OP)

   Dear Qshake
           On our recent visit to a bridge under
   almost identical conditions bearings have been  found to be having slightly bulging elastomeric laminated
   pads just after 7-8 years of in service. With the
 kind of loads in our case under Indian condions I feel
 it may be prudent to go for Pot PTFE bearings for such
 large simply supported spans which may not be used
 in developed nations  so frequently.It must be more of
 continous spans    
                                        Yours thankfully
                                             nkg

RE: Selection of bearings between elastomeric pads&pot bearings

Hi, nkgarg.

If your observed bulging is really only "slight", then I do not believe that represents any problem.  ALL elastomer pads should be expected to bulge slightly under load, even when brand new.  

One of the very basic assumptions in designing elastomeric bearings is that the elastomer behaves as a totally incompressible fluid.  This, as each lamination is compressed slightly under vertical load, it must bulge at the perimeter to maintain constant total volume.

The Australian bridge design code specifies a maximum total shear strain, which is calculated as the sum of the effects of applied (horizontal) shear forces together with the effects of vertical compression plus rotation.

RE: Selection of bearings between elastomeric pads&pot bearings

(OP)

  Hi! Austim
      Thanks for responding,The bulging is definately
      unusual as compared to the similar bearings at
      other bridges.The inspecting bridge personnel
       are also concerned.They are not sure these
      will last long.Our confi dence has definately
      been shaken.

                                   nkg

RE: Selection of bearings between elastomeric pads&pot bearings

   I want to know which type of bearing i.e.Elastomeric
   laminated pads or pot type of elastomeric bearings be
   selected in a prefabricated bridge,composite bridge(steel beams and concerete slab)and hollow bridges  having simply supported 20-30 meter spans
please send me some catalog.

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