The dimension of cell could be controlled by prestressing. As the span and width increases the beams and bottom slabs are to be tied to keep the geometry which in turn leads to evolution box girder.
Any eccentric load will cause high torsional stresses which will be counter acted by the box section. The analysis of such sections are more complicated due combination of flexure, shear, torsion, distortion. But it is more efficient cross-section. It is used for larger spans with wide cross-section. It can be used for spans up to m depending upon the construction methods. Cantilever method of construction is preferred most. In recent years, single or multi-cell reinforced concrete box Girder Bridge have been proposed and widely used as economic aesthetic solution for the over crossings, under crossings, grade separation structure s and viaducts found in modern highway system.
In case of long span bridge s, large width of deck is available to accommodate prestressing cables at bottom flange level.
Single-cell box girder bridges from around the world | Structurae
Interiors of box girder bridge s can be used to accommodate service such as gas pipes, water mains etc. For large spans, bottom flange could be used as another deck accommodates traffic also. The maintenance of box girder is easier in interior space is directly accessible without use of scaffolding. Alternatively space is hermetically sealed and enclosed air may be dried to provide a non-corrosive atmosphere. One of the main disadvantages of box decks is that they are difficult to cast in-situ due to the inaccessibility of the bottom slab and the need to extract the internal shutter.
Either the box has to be designed so that the entire cross section may be cast in one continuous pour, or the cross section has to be cast in stages. It can cover a range of spans from 25 m up to the largest non-suspended concrete decks built; of the order of m. Single box girders may also carry decks up to 30 m wide. For the longer span beams, beyond about 50 m, they are practically the only feasible deck section. Below 30m precast beams or voided slab decks are more suitable while above 50ma single cell box arrangement is usually more economic. Single cell box -girder cast-in-situ are used for spans form 40m to m.
The box arrangement is done in order to give aesthetic appearance where the web of box will act as a slender appearance when combined with a slim parapet profile.
Single box arrangements are efficient for both the longitudinal and transverse designs, and they produce an economic solution for mot medium and long span structure s. This type of deck is constructed span-by-span, using full-height scaffolding or trusses, or as balanced cantilever using form travelers. Google Scholar. Crossref Google Scholar.
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Concrete Box Girder Bridges J. Schlaich
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