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Bridge Engineering

What are the purposes of waterproofing in bridge decks?

Waterproofing materials like membranes are applied on top of bridge deck surface because:

(i) Vehicular traffic (e.g. tanker) may carry dangerous chemicals and the leakage of such chemicals in the absence of waterproofing materials may endanger the life of bridges. The chemicals easily penetrate and cause the deterioration of concrete bridge decks.

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What is the purpose of overlays on concrete bridge deck?

After years of servicing, some overlays may be applied on the top surface of bridges. Overlays on concrete bridge decks achieve the following purposes:

(i) It aims to provide a smooth riding surface. Hence, it may be applied during the maintenance operation to hide the uneven and spalling deck surface and offers a smoother surface for road users.

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What are the problems of using transition slabs in bridges?

In some designs, transition slabs are provided on the approach to bridges. For instance, soils in embankment supporting the roads may settle due to insufficient compaction and sharp depressions would be developed at the junction with the relatively rigid end of bridge decks.

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How do engineer determine the number of cells for concrete box girder bridges?

If the depth of a box girder bridge exceeds 1/6 or 1/5 of the bridge width, then it is recommended to be designed as a single cell box girder bridge. However, if the bridge depth is smaller than 1/6 of the bridge width, then a twin-cell or multiple cell is a better choice as suggested by Jorg Schlaich & Hartmut Scheef (1982).

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

Diaphragms are adopted in concrete box girder bridges to transfer oads 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.

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What are the functions of diaphragms in bridges?

Diaphragm is a member that resists lateral forces and transfers loads to support. Some of the diaphragms are post-tensioned and some contain normal reinforcement. It is needed for lateral stability during erection and for resisting and transferring earthquake loads. Based on past research, diaphragms are ineffective in controlling deflections and reducing member stresses. Moreover, it is commonly accepted that diaphragms aided in the overall distribution of live loads in bridges.

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What are the advantages of assigning the central pier and the abutment as fixed piers?

(i) For abutment pier to be assigned as fixed pier while the bridge is quite long, the longitudinal loads due to earthquake are quite large. As the earthquake loads are resisted by fixed piers, the size of fixed piers will be large and massive. In this connection, for better aesthetic appearance, the selection of abutment as fixed piers could accommodate the large size and massiveness of piers. Normally abutments are relatively short in height and for the same horizontal force, the bending moment induced is smaller.

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How does the position of bearing affect the continuity of joints?

Expansion joints in a bridge structures cater for movements in transverse, longitudinal, vertical and rotational forms. The layout and position of expansion joins and bearings have to be carefully designed to minimize the future maintenance problem.

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How will inclined bridge deck affect joint continuity?

Bearings are usually designed to sit in a horizontal plane so as to avoid the effect of additional horizontal force and uneven pressure distribution resulting from non-horizontal placing of bearings [43]. For an inclined bridge deck subject to a large longitudinal movement, a sudden jump is induced at the expansion joint and discontinuity of joint results. To solve this problem, an inclined bearing instead of a truly horizontal bearing is adopted if the piers can take up the induced horizontal forces.

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Why are excessive movement joints undesirable in bridges?

Movement joints are normally added to bridge structures to accommodate movements due to dimensional changes arising from temperature variation, shrinkage, creep and effect of prestress. However, the provision of excessive movement joints should be avoided in design because movement joints always encounter problems giving rise to trouble in normal operation and this increases the cost of maintenance.

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