For two or extra spans steady decks are the norm and steel composite decks, because of their superior buildability, ought to win over Y beam decks. When optimising span lengths in multi-span bridges, shorter spans will be extra economical.
During the concreting stage, the cross girders of ladder decks present torsional restraint to the principle girder flanges. In multi-girder decks, bracing between pairs of girders is commonly provided utilizing a system of torsional bracing. This can comprise both cross bracing made up from angles, or channel sections. Larger sections and sections in weather resistant steel may be economically fabricated from plate. For freeway bridges, integral development must be thought-about and designers should follow the recommendation given in PD . Note that 60m is not a restrict on most length for integral bridges, Highways England have accepted integral bridges with lengths larger than 100m. Nowadays for single span bridges, except for the shortest spans, steel composite decks compete with prestressed concrete Y-beam decks and customarily will show extra economical at the higher end of the Y-beam span vary.
Continuous spans will invariably be extra economical in steel weight than merely supported spans of the same length. With non-integral development provision of an abutment gallery is expensive and might impact on construction programme. Hence integral bridges will almost all the time be extra economical than non-integral, especially when whole life costing is taken into account as user delay prices related to deck joint upkeep/substitute are excessive. An integral bridge is necessarily continuous over intermediate helps however does not must be integral with the columns or piers beneath. Eliminating the bearings by casting in the main girders or steel crosshead girders doesn’t convey major advantages and does increase complexity in building; the tendency to draw second may develop potential fatigue problems.
However where piers are pricey , or the environmental sensitivity of the site requires minimal disturbance, longer spans with fewer foundations are prone to be more economical. The most common technique of erecting bridge girders is direct erection by mobile crane lifting the girders from the bottom onto the bridge substructure. Typically, girders for single span bridges shall be placed both singly or in braced pairs spanning full size between the top supports. For multiple spans the girders are erected in a span and cantilever sequence involving erection items that cantilever over the piers to the point of contraflexure within the subsequent span as shown under. For wide ladder decks with long cross girders, will probably be more economical brace the cross girders than to extend the flange thickness. This can be achieved by pairing the cross girders with channel bracing at midspan. Previous experience will often be the first information to choice of flange and web sizes for the primary girders and cross girders.