For simple or continuous spans exceeding about 85 ft, plate girders may be the most economical type of construction. Used as stringers instead of rolled beams, they may be economical even for long spans (350 ft or more). Design of such bridges closely resembles that for bridges with rolled-beam stringers (Arts. 12.1 and 12.2). Important exceptions are noted in this and following articles.
The decision whether to use plate girders often hinges on local fabrication costs and limitations imposed on the depth of the bridge. For shorter spans, unrestricted depth favors plate girders over rolled beams. For long spans, unrestricted depth favors deck trusses or arches. But even then, cable-supported girders may be competitive in cost. Stringent depth restrictions, however, favor through trusses or arches.
Composite construction significantly improves the economy and performance of plate girders and should be used wherever feasible. (See also Art. 12.1.) Advantage also should be taken of continuity wherever possible, for the same reasons.
Spacing. For stringer bridges with spans up to about 175 ft, two lanes may be economically carried on four girders. Where there are more than two lanes, five or more girders should be used at spacings of 7 ft or more. With increase in span, economy improves with wider girder spacing, because of the increase in load-carrying capacity with increase in depth for the same total girder area.
For stringer bridges with spans exceeding 175 ft, girders should be spaced about 14 ft apart. Consequently, this type of construction is more advantageous where roadway widths exceed about 40 ft. For two-lane bridges in this span range, box girders may be less costly.
Steel Grades. In spans under about 100 ft, Grade 36 steel often will be more economical than higher-strength steels. For longer spans, however, designers should consider use of stronger steels, because some offer maintenance benefits as well as a favorable strength-cost ratio. But in small quantities, these steels may be expensive or unavailable. So where only a few girders are required, it may be uneconomical to use a high-strength steel for a light flange plate extending only part of the length of a girder.
In spans between 100 and 175 ft, hybrid girders, with stronger steels in the flanges than in the web (Art. 11.19), often will be more economical than girders completely of Grade 36 steel. For longer spans, economy usually is improved by making the web of higher-strength steels than Grade 36. In such cases, the cost of a thin web with stiffeners should be compared with that of a thicker web with fewer stiffeners and thus lower fabrication costs. Though high-strength steels may be used in flanges and web, other components, such as stiffeners, bracing, and connection details, should be of Grade 36 steel, because size is not determined by strength.
Haunches. In continuous spans, bending moments over interior supports are considerably larger than maximum positive bending moments. Hence, theoretically, it is advantageous to make continuous girders deeper at interior supports than at midspan. This usually is done by providing a haunch, usually a deepening of the girders along a pleasing curve in the vicinity of those supports.
For spans under about 175 ft, however, girders with straight soffits may be less costly than with haunches. The expense of fabricating the haunches may more than offset savings in steel obtained with greater depth. With long spans, the cost of haunching may be further increased by the necessity of providing horizontal splices, which may not be needed with straight soffits. So before specifying a haunch, designers should make cost estimates to determine whether its use will reduce costs.
Web. In spans up to about 100 ft, designers may have the option of specifying a web with stiffeners or a thicker web without stiffeners. For example, a 5â„16-in-thick stiffened plate or a 7â„16-in-thick unstiffened plate often will satisfy shear and buckling requirements in that span range. A girder with the thinner web, however, may cost more than with the thicker web, because fabrication costs may more than offset savings in steel. But if the unstiffened plate had to be thicker than 7â„16 in, the girder with stiffeners probably would cost less.
For spans over 100 ft, transverse stiffeners are necessary. Longitudinal stiffeners, with the thinner webs they permit, may be economical for Grade 36 as well as for high-strength steels.
Flanges. In composite construction, plate girders offer greater flexibility than rolled beams, and thus can yield considerable savings in steel. Flange sizes of plate girders, for example, can be more closely adjusted to variations in bending stress along the span. Also, the grade of steel used in the flanges can be changed to improve economy. Furthermore, changes may be made where stresses theoretically permit a weaker flange, whereas with cover-plated rolled beams, the cover plate must be extended beyond the theoretical cutoff location.
Adjoining flange plates are spliced with a groove weld. It is capable of developing the full strength of the weaker plate when a gradual transition is provided between groovewelded material of different width or thickness. AASHTO specifies transition details that must be followed.
Designers should avoid making an excessive number of changes in sizes and grades of flange material. Although steel weight may be reduced to a minimum in that manner, fabrication costs may more than offset the savings in steel.
For simply supported, composite girders in spans under 100 ft, it may be uneconomical to make changes in the top flange. For spans between 100 and 175 ft, a single reduction in thickness of the top flange on either side of midspan may be economical. Over 175 ft, a reduction in width as well as thickness may prove worthwhile. More frequent changes are economically justified in the bottom flange, however, because it is more sensitive to stress changes along the span. In simply supported spans up to about 175 ft, the bottom flange may consist of three plates of two sizes a center plate extending over about the middle 60% of the span and two thinner plates extending to the supports. (See Art. 11.17).
Note that even though high-strength steels may be specified for the bottom flange of a composite girder, the steel in the top flange need not be of higher strength than that in the web. In a continuous girder, however, if the section is not composite in negative-moment regions, the section should be symmetrical about the neutral axis.
In continuous spans, sizes of top and bottom flanges may be changed economically once or twice in a negative-moment region, depending on whether only thickness need be changed or both width and thickness have to be decreased. Some designers prefer to decrease thickness first and then narrow the flange at another location. But a constant-width flange should be used between flange splices. In positive-moment regions, the flanges may be treated in the same way as flanges of simply supported spans.
Welding of stiffeners or other attachments to a tension flange usually should be avoided.
Transverse stiffeners used as cross-frame connections, should be connected to both girder flanges (Art. 11.12.6). The flange stress should not exceed the allowable fatigue stress for base metal adjacent to or connected by fillet welds. Stiffeners, however, should be welded to the compression flange. Though not required for structural reasons, these welded connections increase lateral rigidity of a girder, which is a desirable property for transportation and erection.
Bracing. Intermediate cross frames usually are placed in all bays and at intervals as close to 25 ft as practical, but no farther apart than 25 ft. Consisting of minimum-size angles, these frames provide a horizontal angle near the bottom flange and V bracing (Fig. 12.12) or X bracing. The angles usually are field-bolted to connection plates welded to each girder web. Eliminating gusset plates and bolting directly to stiffeners is often economical.
Cross frames also are required at supports. Those at interior supports of continuous girders usually are about the same as the intermediate cross frames. At end supports, however, provision must be made to support the end of the concrete deck. For the purpose, a horizontal channel of minimum weight, consistent with concrete edge-beam requirements, often is used near the top flange, with V or X bracing, and a horizontal angle near the bottom flange.
Lateral bracing in a horizontal plane near the bottom flange is sometimes required. The need for such bracing must be investigated, based on a wind pressure of 50 psf. (Spans with nonrigid decks may also require a top lateral system.) This bracing usually consists of crossing diagonal angles and the bottom angles of the cross frames.
Bearings. Laminated elastomeric pads may be used economically as bearings for girder spans up to about 175 ft. Welded steel rockers or rollers are an alternative for all spans but may not meet seismic requirements. Seismic attenuation bearings, pot bearings, or spherical bearings with teflon guided surfaces for expansion are other alternatives.
Camber. Plate girders should be cambered to compensate for dead-load deflections. When the roadway is on a grade, the camber should be adjusted so that the girder flanges will parallel the profile grade line. For the purpose, designers should calculate dead-load deflections at sufficient points along each span to indicate to the fabricator the desired shape for the unloaded stringer.