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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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Sliding bearings
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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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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.

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Question: In the design of a simply supported skew bridge, which direction of reinforcement should be provided?


In the conventional design of steel reinforcement for a simply supported skew bridge, a set
of reinforcement is usually placed parallel to free edge while the other set is designed
parallel to the fixed edge. However, this kind of arrangement is not the most efficient way of placing the reinforcement. The reason is that in some parts of the bridge, the moment of resistance is provided by an obtuse angle formed by the reinforcement bars which is ineffective in resisting flexure. In fact, the most efficient way of the arrangement of reinforcement under most loading conditions is to place one set of bars perpendicular to the fixed edge while placing the other set parallel to the fixed end as recommended by
L. A. Clark (1970).
In this way, considerable savings would be obtained from the orthogonal arrangement of reinforcement.

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Question: What are the functions of grout inside tendon ducts?

Grout in prestressing works serves the following purposes:
(i) Protect the tendon against corrosion.
(ii) Improve the ultimate capacity of tendon.
(iii) Provide a bond between the structural member and the tendon.
(iv) In case of failure, the anchorage is not subject to all strain energy.

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A typical prestressing system
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Details on bubble Deck slab

https://t.me/evolvingcivilengineers/179

🎲 Quiz 'REINFORCED CONCRETE'
We are testing this new feature so please respond and give us suggestions in the discussion group Thank you for participating The questions are related to Indian standards
🖊 1 question · ⏱ 30 sec

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Question: What is sucker deck principle for variable depth bridge decks?

For a variable depth bridge deck, the depth of continuous multi-span bridge deck is
increased in pier supports and this absorbs sagging moments in the mid-span with the
consequent increase in hogging moments in pier supports. As a result, the mid-span depth
can be significantly reduced due to the reduction in sagging moment. In essence, this
sucker deck principle is applied in locations where headroom requirement is of great
concern. Moreover, in terms of structural performance, sucker decks are effective in
reducing dead loads than voided slab of equivalent uniform depth for span length between
20-40m. In terms of aesthetics point of view, the public tends to appreciate the structural
form of arches and curved soffit rather than boring uniform deck alignment.

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For bridges with continuous spans. the maximum sagging moment will be at mid span, and the maximum hogging moment will be above the pier.
By increasing the depth of the deck above the pier, this will result in increasing the hogging moment above the pier and sucking or reducing the sagging moment. therefore the depth will be reduced at the mid of the span.
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