Bridge Engineering

When the piers are built directly into deck without bearings, the monolithic construction creates a portal structure which modifies the bending moment envelope in the deck when compared with bridges with bearings. For instance, hogging moments are increased in supports with the decrease in sagging moments in mid-span of bridge deck. On the other hand, the shear stiffness of piers is a major concern because it tends to resist length changes of bridge deck which could not expand and contract readily.

Read More

Bridges constructed by incremental launching method are usually low in span depth ratio and typical values are 14 to17. With low span depth ratio, the bridge segments are stiff in bending and torsion which is essential to cater for the launching process.

Read More

The cross sectional area of duct is normally 2.5 times that of the area of prestressing steel. The size of ducts should be not designed to be too small because of the followings:

Read More

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.

Read More

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.

Read More

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.
Read More

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.

Read More

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.

Read More

When the superstructure is pushed forward, a temporary nose is usually adopted at the front end of the superstructure to reduce the cantilever moment for which the central prestress is designed. The length of temporary nose is about 60-65% of bridge span.

The bending moment of self-weight for internal spans (equal span) of long bridge is -0.0833WL²
at piers and +0.0417W L² at mid-span (W = unit weight of deck and L = span length). However, without the use of temporary nose, the bending moment in the leading pier when the deck has to cantilever from one pier to another would be -0.5WL², which is 6 times higher than normal values at support.

Read More

After the completion of launching process, the superstructure has to be lifted up to allow for installation of bearings. This is usually achieved by means of jacks to raise 5-10mm successively at each pier.

Read More