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.
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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.
