For short simple spans (up to 30 ft or 9.1 m) and for somewhat longer continuous spans (interior spans up to 55 ft or 16.8 m), reinforced-concrete flat slabs provide a minimum — depth bridge. Figure 4.5 shows a schematic of this bridge type. At a slab depth of about 2 ft (610 mm), the slab begins to become uneconomical, with too much of the section required to support itself.
Falsework is required to construct the slab. Where space is available beneath the structure, scaffolding may be used. If the bridge is over a stream, or over a highway or railroad
TABLE 4.2 Approximate Maximum Span for Various Types of Bridges
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maximum
Type span, ft (m)
Reinforced-concrete flat slab, continuous 55 (17)
Composite steel beam (36-in series), simple 100 (30)
Precast prestressed-concrete voided box beam 120 (37)
Precast prestressed-concrete beams (bulb-tee), simple 120 (37)
Composite steel beam (36-in series), continuous 125 (38)
Precast prestressed-concrete beams (bulb-tee), made-continuous 140 (43)
Composite steel plate girder, simple 230 (70)
Precast prestressed-concrete beams (bulb-tee) spliced 250 (76)
Cast-in-place (on falsework) posttensioned-concrete box girder, continuous 300 (91)
Precast posttensioned segmental concrete box girder, continuous, 400 (122)
balanced cantilever
Composite steel plate girder, continuous, parallel flange 460 (140)
Composite steel plate girder, continuous, haunched 540 (165)
Cast-in-place posttensioned segmental concrete box girder, continuous, 850 (259)
balanced cantilever
Steel arch (New River Gorge, Fayetteville, West Virginia, U. S.A.)[3] [4] 1700 (518)
Steel cantilever truss (Pont de Quebec, Canada)* 1800 (549)
Steel cable-stayed (Stonecutters, Hong Kong)* 3340 (1018)
Suspension (Akashi Kaiko, Japan)* 6529 (1990)
where traffic must be maintained, the falsework must include support beams to span over the feature crossed. In that case, camber should be built into the falsework to compensate for its deflection. Also, the falsework must provide for the vertical geometry of the bridge and for deflection of the slab after removal of the falsework.
Longer continuous-slab spans can be constructed if the slab is haunched, that is, made deeper over the piers or bents. However, the cost and difficulty of constructing the forms, and bending and placing the longitudinal reinforcing bars, often negates the advantage of haunched construction.
Another type of construction that can be used to extend the span capability of slab bridges is voided construction. Voids, similar to those used to fabricate prestressed- concrete box beams, are used to replace the relatively ineffective concrete at mid-depth of the slab, thereby reducing the weight of the slab. However, where this type of construction has been used, it has generally been found to be more expensive than competitive types of bridges. A principal reason is the cost of providing adequate hold-down devices to prevent the voids from floating when the concrete is placed.
For balanced design of continuous-slab bridges, the usual rule that the end span should be shorter than the adjacent interior span may not apply. In the design of a three — span continuous flat-slab bridge with three equal spans of 30 ft (9.1 m), considering an HS 25 live load (a load 25 percent greater than HS 20) and the AASHTO Alternate Military Loading, a good balance resulted between maximum positive and negative moments using equal span lengths.
