The steel-beam bridge uses rolled steel beams as shown in Fig. 4.8. Beam depths of 44, 40, and 36 in (1118, 1016, and 914 mm) are available, as well as shallower sections. Check with producers on current availability of the deeper sections from domestic sources because federal law applicable to federally aided projects, as well as many state laws, prohibits the use of foreign steel.

Steel beams may be made continuous by welding or bolting sections in the field. In the past, some states made welded connections at the piers, and currently at least

one state makes welded connections at contraflexure points, supporting the field sections temporarily and providing enclosures to shield the joint from wind. More commonly, field sections are spliced by high-strength bolts, using web-and-flange splice plates. Bolts may be installed using calibrated wrenches, by the turn-of-nut method, or by use of tension-indicating washers, depending on what the designer allows and what the erector prefers to use. With all methods of bolting, it is important to use a procedure and sequence of bolting that will compact the joint and prevent a bolt initially adequately tightened from losing tension when subse­quent bolts in the joint are tightened. Fasteners are generally ASTM A325 or A490 high-strength bolts.

To increase the span capacity of a rolled beam, or to permit a lighter beam to be used, cover plates may be added above the top flange and below the bottom flange in regions of high bending stress due to both positive and negative moments. The fatigue strength at the end of the cover plates, which is generally at a point of low maximum stress but high stress range, is much less than the fatigue strength of the unplated beam. Allowable fatigue stresses must not be exceeded, and this consideration may favor an unplated beam. However, an improved detail is available that uses bolts at the end of the plate, and the fatigue strength is somewhat higher. New Jersey DOT requires full-length cover plates, with termination about 2 ft (610 mm) from the end of the beam where the stress range is very small.

Prestressed-concrete beams of the basic I-shape, but with variations, can be used over approximately the same range of spans as steel beams. The deepest AASHTO standard prestressed beams (72 in or 1828 mm) have a somewhat greater simple-span capacity than 36-in-deep (914-mm) rolled steel beams, although deeper rolled beams are avail­able. This type of bridge is illustrated in Fig. 4.7.

Prestressed-concrete beams are heavier to transport and erect than steel beams, and require more care in handling. A prestressed-concrete beam can be destroyed if it is not maintained in an upright position.

I-beams may be standard AASHTO-PCI sections or conform to individual state standards. Depth varies from 28 in (711 mm) for the little-used AASHTO type I to 72 in (1828 mm) for the AASHTO type VI and BT-72 (1828 mm) bulb-tee. The basic differ­ence between the AASHTO type V and type VI beams and the bulb-tee beams, all of which have 3.5-ft-wide (1067-mm) top flanges, is that the bulb-tees have a thinner web (6 in instead of 8 in or 152 mm instead of 203 mm) and shallower top and bottom flanges. The bulb-tees have a flatter slope on the top of the bottom flange, as well. A variant of both is the modified AASHTO type VI, which uses the side forms for the AASHTO type VI beam but only a 6-in (152-mm) web. Individual analysis will determine which shape is best, but only shapes that are available from local precasters should be investigated unless the project is large enough to economically justify the purchase of special forms. Sometimes, bulb-tee sections are modified to have deeper web sections to increase their capacity, hence the span length.

As with prestressed-concrete box-beam bridges, the prestressing strands may include deflected or debonded strands. When strands are deflected and a number of beams are cast in line on a casting bed, resulting in many hold-down or hold-up points, stressing procedures should be used and verified that limit the maximum prestress loss due to friction to the amount permitted by specifications.

For very long bridges with repetitive spans over water, and where there is a precasting plant at a site from which the bridge units can be delivered by barge, the option of pre­cast deck units consisting of the beams, diaphragms, and deck slab cast monolithically should be considered.

The span range of a shallow bridge may be extended beyond the limits of a slab bridge by using precast prestressed-concrete box beams as illustrated in Fig. 4.6. The beams are prefabricated off-site. They are rectangular and, except for very shallow beams [12 in (305 mm)], which may be solid, have from one to three rectangular or circular voids. The void forms are either waterproofed cardboard or solid polystyrene foam and are left in the beams. Void drains must be provided to prevent entrapment of water. Prestressing strands are located on the bottom and in the sidewalls of the box, and may include debonded or deflected strands. The selection depends upon owner preference or, where the designer and owner allow the option, fabricator preference.

Wearing surface

This type of bridge can be constructed using adjacent beams or spread beams. In adjacent box-beam construction, prefabricated box beams are placed side by side, abutting each other. The box beams are connected by transverse tie rods or posttensioned tendons, or by welded connection of tie plates to plates embedded in the tops of the beams. Shear keys between beams are grouted. The combination of transverse connection and grouted shear keys is intended to make the beams act together as a unit and prevent relative movement and cracking at the longitudinal joints. This type of bridge can be erected quickly, and temporary traffic can be maintained on partially completed portions of the bridge. These features have made this a popular type of bridge, despite at least one shortcoming discussed below, and sometimes cause it to be selected over a com­petitive type when both types are viable candidates for a bridge of a given span length.

For low-traffic-count roads, the tops of the beams may constitute the riding surface, but on most bridges a topping will be used. This may be a composite concrete slab, which adds to the strength of the bridge, or an asphalt concrete overlay, which is used to smooth out any irregularities between beams, to compensate for difference between roadway profile and final camber of the beams, and sometimes to maintain continuity of pavement type when the adjacent roadway is asphalt concrete. A waterproofing mem­brane should be used with this type of construction, with special attention to the joints.

The elimination of movement at the longitudinal joints and the maintenance of a water­proof condition has not always been achieved, even when a composite concrete slab has been used. Leakage of roadway drainage containing deicing salt through longitudinal joints has sometimes resulted in corrosion of the prestressing strands. In some cases, wires have broken.

In spread-box construction, the beams are spaced apart, and a reinforced-concrete slab is constructed on top. The slab between the beams is formed, and stay-in-place steel forms are frequently used. This has been an economical type of construction in Pennsylvania. In bridges with end spans shorter than interior spans, the beams can be the same depth for aesthetic reasons, with the spacing between beams varied to meet structural requirements. However, diagonal and vertical cracks have been observed in the sidewalls of spread-box beams near supports in sharply skewed bridges. The fine diagonal cracks were most evident on the acute sides of the box beams.