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Bridge Engineering

Why type of prestressing is better, external prestressing or internal prestressing?

At several locations in the span (i.e. third or quarter points) the tendons are deviated to the correct tendon profile by concrete deviators in external prestressing. The advantages of external prestressing are listed below:

(i) Owing the absence of bond between the tendon and structure, external prestressing allows the removal and replacement of one or two tendon at one time so that the bridge could be retrofitted in the event of deterioration and their capacity could be increased easily. This is essential for bridges in urban areas where traffic disruption is undesirable.

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What is the difference between dry joint and wet joint in precast segmental bridges?

Dry joints have been properly used in the past in which the bridge segments are formed by match casting. The prevalence in the past is due to it lower cost and time for construction. There is no gluing material to seal up the joint. As such, leakage through the joint into the box culvert occurs from time to time and this may affect the durability of external post-tensioning tendons. Moreover, owing to the effect of seismic, temperature and creep, the joints are found to open under these conditions. Spalling of top concrete slab at bridge joint was also reported.

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In precast segmental box girder bridges, the bridge segments are usually formed by match casting. It is sometimes observed that a gap is formed between adjacent bridge segments. Why?

To enhance perfect fitting of bridge segments in precast segmental box girder bridges, segments are usually constructed by match casting so that it would not impair the serviceability and load bearing ability of the bridge. The end face of completed segment is adopted as formwork for the new segment. During the concrete hardening process, the hydration effect of new segment induces a temperature rise and develops a temperature gradient in the completed segment. Hence, the completed segment bows temporarily and the new segment sticks to this bowed shape when hardened. After match casting, the completed segment retains its original shape after cooling down while the new segment obtains the profile of bowed shape. Such bowing effect is even more significant for slender segments with large height to width ratio.
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What are the main design considerations for temporary nose in incremental launching?

There are two main design considerations for temporary nose:

(i) Maximum sagging moment
The maximum sagging moment at the point of connecting the nose to superstructure occurs when the superstructure is launched far from the pier. It is estimated to occur at about 75% of the span length.

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Which of the following methods to reduce cantilever moment is better in incremental launching, (i) temporary nose, (ii) mast or (iii) auxiliary piers?

The use of mast is an alternative to temporary nose. From practical point of view, the use of mast requires continual adjustment of forces in the guys when the superstructure is pushed forward. On the other hand, the implementation of temporary nose system does not require much attention during operation.

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