A Self-Learning Manual - Institution of Engineers Mauritius

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A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU Fig. Diaphragm. 3. Are diaphragms necessary in the design of concrete box girder bridges? (BS1) Diaphragms are adopted in concrete box girder bridges to transfer loads 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. In fact, diaphragms also contribute to the provision of torsional restraint to the bridge deck. 4. 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). However, one should note that even for wider bridges with small depths, the number of cells should be minimized because there is not much improvement in transverse load distribution when the number of cells of box girder is increased to three or more. 42
A Self-Learning Manual Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU 5. What are the problems of using transition slabs in bridges? (BS2) 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 [53]. This creates the problem of poor riding surfaces of carriageway and proper maintenance has to be carried out to rectify the situation. As a result, transition slabs are sometimes designed at these junctions to distribute the relative settlements between the approaching embankments and end of bridge decks so that the quality of riding surface between these junctions could be significantly improved and substantial savings could be obtained by requiring less maintenance. 6. 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 [8]: (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. (ii) The use of overlays can extend the life of the bridge deck. 7. 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. (ii) In some countries where very cold weather is frequently encountered, salt may be applied for defrosting purpose. In case waterproofing is not provided, the salt solution penetrates through the concrete cracks of the bridge and causes the corrosion of reinforcement. (iii) In the event of cracks appearing on concrete deck, water penetrates the bridge deck and brings about steel corrosion. 43
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A Self-Learning Manual - Institution of Engineers Mauritius

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