08-16-2017, 10:00 PM
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.
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 top the first span of a
bridge.
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.
(ii) 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.
Bursting Reinforcement
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 based on David J. Lee. 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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