Highway Bridges – Composite Deck Construction.

Composite deck construction (Fig. 4.9) should be used wherever the construction depth will permit. If possible, multiple spans should be made continuous over the intermediate supports, so reducing the number of bearings and expansion joints.

Continuity gives economies throughout the structure and reduces traffic disruption arising from the maintenance needs of these vulnerable elements. A number of options are available for maintaining continuity over intermediate supports. If ground conditions are poor such that the predicted differential settlement of the supports is significant (say, exceeding (span/1000)) then to avoid overstress the structure should be made statically determinate by use of simply-supported spans.

Highway bridges – composite deck construction
Fig. 4.9 Highway bridges – composite deck construction

Cantilever and suspended spans are alternative options which retain some of the advantages of continuity. A girder depth of (span/20) (girder depth excludes floor slab) is generally economic although shallow girders can be used down to a depth of (span/30) or less.

Rolled sections are appropriate for short spans up to 25m span. For continuous spans exceeding about 22m, fabricated plate girders will show economy because lighter flanges and webs can be inserted in the mid-span regions.Automated manu- facture of plate girders means that they are highly economic when compared with box girders. Normally a girder spacing of 2.5–3.5m is optimum with a floor slab of about 220–250mm thick. Edge cantilevers should not exceed 50% of the beam spacing and to simplify falsework should where possible be less than 1.5m.An even number of girders (i.e. 2,4, 6, 8, etc.) achieves better optimization for material ordering and permits girders to be braced in pairs for erection.

For medium spans exceeding 40m where adequate construction depth is available it may be economic to use twin girders only. A number of variants are available as shown in Fig. 4.9, using a thickened haunched slab, longitudinal stringers and cross girders to support the slab intermediately. For narrow bridges the complete precasting of composite floors may offer advantages in speed of construction. Use of girders with a curved soffit becomes economical for medium spans exceeding  45m and efficiently achieves maximum headroom if required over the central portions of a span (see Fig. 4.9). Plate girder flanges should be proportioned so as to be as wide as possible consistent with outstand limit to reduce the number of intermediate bracings. For practical reasons a desirable minimum flange width is about 40mm to accommodate shear connections and to permit the possible use of permanent formwork. A maximum flange thickness of 75mm is recommended as a guide to avoid heavy butt welds, but thicker flanges can be used where necessary, to 100mm or greater.

For long spans, box girders (see Fig. 4.10) are more suitable than plate girders, for which flange sizes would be excessive. Other reasons for using box girders for long spans may include a need to improve aerodynamic stability, the presence of severe plan curvature, a requirement for single column supports or very limited construction depth.However, for short and medium spans box girders are generally less economic because, although a reduction in flange sizes may be possible due to superior load distribution properties, this is more than offset by the amount of internal diaphragms and stiffening and the extra costs in manufacture. Fabrication costs are higher because the assembly and welding processes are less amenable to automation than with plate girders. Also access must be permitted inside box girders for welding, protective treatment processes, and permanent inspection. However, erection of box girders is often easier because they require minimal external bracing to maintain overall stability.Multiple compact section box girders can be economic for spans of up to 50m in particular situations, and enable longitudinal stiffeners to be eliminated.Open-top trapezoidal box girders (known as ‘bath tubs’) are widely used in North America and possess some of the advantages of plate girders. They have seen same use in the UK but temporary bracing is required during construction to maintain shape and relative twist of the sections until the concrete slab is placed and the full torsional rigidity achieved.

Highway bridges – box girders. (a) Twin box and cross girders (spans 40–150m); (b) open top box (spans 40–100m); (c) multiple box (spans 30–60m)
Fig. 4.10 Highway bridges – box girders. (a) Twin box and cross girders (spans 40–150m);
(b) open top box (spans 40–100m); (c) multiple box (spans 30–60m)

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