Selection of bearings between elastomeric pads&pot bearings 1

[nkgarg]

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

 

[Qshake]

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.

 

[nkgarg]

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

 

[Qshake]

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

 

[nkgarg]

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

 

[austim]

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.

 

[nkgarg]

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

 

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.