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Question: In the design of elastomeric bearings, why are steel plates inserted inside the
bearings?

For elastomeric bearing to function as a soft spring, the bearing should be allowed for
bulging laterally and the compression stiffness can be increased by limiting the amount of lateral bulging. To increase the compression stiffness of elastomeric bearings, metal plates
are inserted. After the addition of steel plates, the freedom to bulge is restricted and the
deflection is reduced when compared with bearings without any steel plates under the same load. Tensile stresses are induced in these steel plates during their action in limiting the bulging of the elastomer. This in turn would limit the thickness of the steel plates.
However, the presence of metal plates does not affect the shear stiffness of the elastomeric
bearings.

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Elastomeric bearing
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Question: What are the advantages of piers constructed monolithically with the bridge deck over usage of bearings?

Basically, piers constructed monolithically with the bridge deck are advantageous in the
following ways:
(i) Movement of the bridge deck is achieved by the bending deformation of long andslender piers. In this way, it saves the construction cost of bearings by using monolithic construction between bridge deck and piers. Moreover, it is not necessary to spend extra effort to design for drainage details and access for bearing
replacement. On the other hand, in maintenance aspect substantial cost and time
savings could be obtained by using monolithic construction instead of using bearings as bridge articulation.

(ii) Monolithic construction possesses the shortest effective Euler buckling length for piers because they are fixed supports at the interface between bridge deck and piers.


Note: Monolithic construction means that piers are connected to bridge decks without any joints and bearings.

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Monolithic bridge without any joints for provision of bearings
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Monolithically constructed bridge
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Monolithically constructed bridge
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Monolithic bridge
You can observe there are no bearings between the bridge span and abutments.

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Question:Are diaphragms necessary in the design of concrete box girder bridges?

Diaphragms are adopted in concrete box girder bridges to transfer loads from bridge decks to bearings. Since the depth of diaphragms normally exceeds the width by two times, they are usually designed as deep beams. However, diaphragms may not be necessary in case bridge bearings are placed directly under the webs because loads in bridge decks can be
directly transferred to the bearings, based on Jorg Schlaich & Hartmut Scheef (1982). This arrangement suffers from the drawback that changing of bearings during future maintenance operation is more difficult.
In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck.

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Question: What is the advantage of sliding bearings over roller bearings?

In roller bearing for a given movement the roller bearing exhibit a change in pressure
centre from its original position by one-half of its movement.

However, with sliding bearing a sliding plate is attached to the upper superstructure and the
moving part of bearing element is built in the substructure. It follows that there is no
change in pressure center after the movement.

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Roller bearings in Bridges
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Question: What are the three major types of reinforcement used in prestressing?

(i) Spalling reinforcement:
Spalling stresses are established behind the loaded area of anchor blocks and this causes breaking away of surface concrete. These stresses are induced by strain incompatibility with Poisson’s effects or by the shape of stress trajectories.

(ii) Equilibrium reinforcement
Equilibrium reinforcement is required where there are several anchorages in which
prestressing loads are applied sequentially.

(iii) Bursting Reinforcement
Tensile stresses are induced during prestressing operation and the maximum bursting stress occurs where the stress trajectories are concave towards the line of action of the load.
Reinforcement is needed to resist these lateral tensile forces.

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https://t.me/evolvingcivilengineers/387
Topic related to expansion joints
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Question: Why is the span length ratio of end span/approach span to its neighboring inner
spans usually about 0.75?


From aesthetic point of view, an odd number of spans with a decrease in length in the
direction of abutment is desirable. Moreover, spans of equal length are found to be boring. However, the arrangement of irregular span lengths is not recommended because it gives a feeling of uneasiness.
From structural point of view,
for a multi-span bridge with equal span length, the sagging
moment at the mid-span of the end span/approach span is largest. In order to reduce this
moment, the span length of end span/approach span is designed to be 0.75 of inner spans.
However, this ratio should not be less than 0.40 because of the effect of uplifting at the end span/approach span support.

Note: End span refers to the last span in a continuous bridge while approach span refers to the first span of a bridge.