Highway-bridge decks usually are constructed of reinforced concrete. Often, this concrete is made with conventional aggregate and weighs about 150 lb per cu ft. Sometimes, it is made with lightweight aggregate, resulting in 100 to 110 lb per cu ft concrete. Lightweight aggregate normally consists of slag, expanded shale, or expanded clay.
In some concrete decks, the wearing surface is cast integrally with the structural slab. In others, a separate wearing surface, consisting of asphaltic concrete or conventional concrete, is added after the structural slab has been placed.
In instances where weight saving is important, particularly in movable spans, or in spans where aerodynamic stability is of concern, an open, steel-grid floor is specified. Where compromise is necessary, this grid is partly or completely filled with asphaltic or lightweight concrete to provide protection under the structure or to provide a more suitable riding surface.
For orthotropic-plate structures, it is necessary to provide over the steel deck a wearing surface on which traffic rides. These wearing surfaces are generally of three types: a layered system, stabilized mastic system, or thin combination coatings.
The layered system consists of a steel-deck prime coat, such as zinc metallizing, bituminous-base materials, or epoxy coatings. Over this coat is applied a copper or aluminum foil, or an asphalt mastic, followed by a leveling course of asphalt binder or stabilized mastic, and a surface course of stone-filled mastic asphalt or asphaltic concrete.
The stabilized mastic system consists of a prime coat on the steel, as in the layered system, followed by a layer of mastic, which is choked with rolled-in crushed rock.
Combination coatings contain filled epoxies or alkyd-resin binders in a single coating with silica sand.
A bridge deck serves as a beam on elastic foundations to transfer wheel loads to the supporting structural steel. In orthotropic bridges, the deck also contributes to the loadcarrying capacity of longitudinal and transverse structural framing. In composite construction, the concrete deck contributes to the load-carrying capacities of girders. In fulfilling these functions, decks are subject to widely varying stresses and strains, due not only to load but also to temperature changes and strains of the main structure.
In general, bridge decks are designed as flexural members spanning between longitudinal or transverse beams and supporting wheel loads. A wheel usually is considered a concentrated load on the span but uniformly distributed in the direction normal to the span.
Concrete Slabs. The effective span S, ft, for a concrete slab supported on steel beams should be taken as the distance between edges of flanges plus half the width of a beam flange.
Allowable Stresses. The allowable compressive stress for concrete in design of slabs is 0.4Æ’c , where Æ’c 28-day compressive strength of concrete, ksi. The allowable tensile stress for reinforcing bars for grade 40 is 20 ksi and for grade 60, 24 ksi. Slabs designed for bending moment in accordance with the following provisions may be considered satisfactory for bond and shear.
Bending Moment. Because of the complexity of determining the exact load distribution, AASHTO specifications permit use of a simple empirical method. The method requires use of formulas for maximum bending moment due to live load (impact not included). Two principal cases are treated depending on the direction in which main reinforcement is placed.
The formulas are summarized in Table 11.27. In these formulas, S is the effective span, ft, of the slab, as previously defined.
For rectangular slabs supported along all edges and reinforced in two directions perpendicular to the edges, the proportion of the load carried by the short span may be assumed for uniformly distributed loads as
where a length of short span of slab, ft, and b length of long span of slab, ft. If the length of slab exceeds 1.5 times the width, the entire load should be assumed carried by the reinforcement of the short span. The distribution width E, ft, for the load taken by either span should be determined as provided for other slabs in Table 11.27. Reinforcement determined for bending moments computed with these assumptions should be used in the center half of the short and long spans. Only 50% of this reinforcement need be used in the outer quarters. Supporting beams should be designed taking into account the nonuniform load distribution along their spans.
All slabs with main reinforcement parallel to traffic should be provided with edge beams.
They may consist of a slab section with additional reinforcement, a beam integral with but deeper than the slab, or an integral, reinforced section of slab and curb. Simply supported edge beams should be designed for a live-load moment, ft-kips, of 1.6S for HS20 loading
and 1.2S for HS15 loading, where S is the beam span, ft. For positive and negative moments in continuous beams, these values may be reduced 20%.
Distribution reinforcement is required in the bottom of all slabs transverse to the main reinforcement, for distribution of concentrated wheel loads. The minimum amounts to use are the following percentages of the main reinforcement steel required for positive moment:
where S effective span of slab, ft. When main reinforcing steel is perpendicular to traffic, the distribution reinforcement in the outer quarters of the slab span need be only 50% of the required distribution reinforcement.
Transverse unsupported edges of the slab, such as at ends of a bridge or expansion joints, should be supported by diaphragms, edge beams, or other means, designed to resist moments and shears produced by wheel loads.
The effective length, ft, of slab resisting post loadings may be taken as
E = 0.8x + 3.75
where no parapet is used, with x distance, ft, from center of post to point considered. If a parapet is used, E = 0.8x + 5.
Steel Grid Floors. For grid floors filled with concrete, the load distribution and bending moments should be determined as for concrete slabs. The strength of the composite steel and concrete slab should be computed by the transformed-area method (Art. 11.16). If necessary to ensure adequate load transference normal to the main grid elements, reinforcement should be welded transverse to the main steel.
For open-grid floors, a wheel load should be distributed normal to the main bars over a distance equal to twice the center-to-center spacing of main bars plus 20 in for H20 loading, or 15 in for H15 loading. The portion of the load assigned to each bar should be uniformly distributed over a length equal to the rear-tire width (20 in for H20 loading and 15 in for H15). The strength of the section should be determined by the moment-of inertia method (Art. 11.15). Supports should be provided for all edges of open-grid floors.