 Bearings
thousands of roads, rail and river bridges in all climates in many
parts of the world, natural rubber bridge bearings are giving continuous
excellent service, demonstrating convincingly their efficiency and
durability. Natural rubber bearings possess distinct advantages
over alternative metal roller types, being easier to install, free
from corrosion and requiring no maintenance, factors important in
the economics of bridge building. In use, natural rubber has proved
superior to all other rubbers as bridge bearing material.
The design and positioning
of the bridge bearing allows the vertical loads (deck and traffic)
to be taken by the rubber in compression and horizontal deflections
(expansion and construction) by the rubber in shear. High vertical
stiffness is achieved by the insertion of horizontal metal plates,
which do not alter the required low horizontal stiffness. Under
maximum traffic and deck load, the amount of vertical deflection
must be limited and bearings can be precisely designed so that it
does not exceed a few millimeters, of which only a fraction will
be due to live load. Horizontal movements in more than one direction
can be accommodated and, if necessary, the horizontal stiffness
in two perpendicular directions can be varied independently. Some
end rotation can be allowed with rubber bearings without resort
to the complicated designs required with metal bearings.
Construction of the
rubber-metal plate sandwich is easily accomplished. Employing the
excellent bonding to metal properties of natural rubber; the bearing
may then be completely encased by a thin cover of rubber which protects
the plates from corrosion and also provides a bedding surface to
take up irregularities in the deck or supporting structure. Location
holes may be provided in the outer steel plates as a precaution
against slip.
Durability is ensured
as a result of the bulk of rubber in the product (low surface/volume
ratio) and is reinforced by the inclusion of protective chemicals,
which increase resistance to weathering. Experience has shown conclusively
that weathering of large components made of natural rubber does
not affect their service lives.
The performance of
natural rubber bearings is determined by the highly suitable physical
properties of natural rubber; it retains its designed stiffness
at low temperatures better than alternative, more expensive, synthetic
rubber and it has low creep, high strength and excellent fatigue
resistance.
Design Considerations
The performance data of Kossan Rubber elastomeric bridge
bearings presented in the following tables are calculated using
shear moduli value appropriate to the strain in the rubber and 200Mpa
for its bulk modulus. These are indicative values for bearing selection.
The maximum loads shown are calculated to the serviceability limit
state as required in MS 671 and BS 5400 Part 9, assuming a dead
load effect numerically equal to the live load effect.
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Specifications
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