Steel for bridges is available in several different strength levels, each of which may be specified under ASTM A709, Standard Specification for Structural Steel for Bridges.

The grade designations are indicated in Table 4.1, as well as some alternative specifications that may be more familiar. The grade designation indicates the specified minimum yield stress in kips per square inch, and a “W” indicates that it is a weathering steel composition. ASTM A709 contains supplementary requirements for notch toughness and other items that are available but apply only when specified by the purchaser. When such supplemen­tary requirements are specified, they exceed the requirements of the basic specifications such as A36 or A572.

The HPS designations indicate that the materials are high-performance steels. They are so designated because they possess superior weldability and toughness compared to conventional steels of similar strength. Grades 36, 50, and 50W are available either as structural shapes or as plates. The other grades are available only as plates. Grades 36, 50, and 50W are the most frequently used materials. In general, compared with A36 steel, where other limitations such as deflection or stiffness do not override, the extra unit cost of the higher-strength grades (50 or 50W) is more than offset by the higher – yield strength. Grades 70W and 100/100W have proven economical in longer-span structures, or the higher-stressed portions of medium-span structures. The AASHTO publication Guide Specification for Highway Bridge Fabrication with HPS-70W Steel suggests that economies can be achieved by combining the use of HPS-70W and Grade 50W steels in a structure. In a 181-ft (55-m) span bridge for the New York State Thruway Authority, the use of HPS-70W steel reduced the number of girders in the cross section from five to four, enabling a savings of 28 percent in weight and 18 percent in cost.

Weathering grades (50W, 70W, and 100W) have chemical compositions that pro­vide enhanced resistance to atmospheric corrosion. They can be used in the bare (unpainted) condition for bridges in many cases (see Art. 4.13). The savings on cost of painting and repainting frequently makes them an economical choice.

Although prices vary widely due to demand and availability, in reference to the unit price of grade 36 steel, the relative material price of the other steels in plate grades is approximately as follows:

As indicated, these are only price factors and do not consider the reduced quantity of steel that may be required as the yield strength increases. For structural shapes, grade 50 steel can usually be obtained for about the same price as grade 36 steel, but there would usually be some additional cost for grade 50W.

The cost of fabrication and erection for members of grade 36 and grade 50 or 50W steel is approximately the same. Thus, in preliminary cost studies, only the cost of the mill material for the members selected need be compared. Fabrication costs for grade 70W, grade 100, and grade 100W tend to be higher than those of the as-rolled products, and thus, the cost comparisons must include those costs.

Steels with greater strength than grade 36 tend to be economical for beams and girders in many cases, and are particularly attractive under the following conditions:

• When dead load is a major part of total load

• When deflection limits do not control

• When deflections can be reduced (composite design, continuous structure, etc.)

• When weight reduction cuts cost of foundations, shipping, etc.

• When selection avoids use of built-up members (cover plates, fabricated girder versus rolled beam, etc.)

The higher-strength steels often show advantage for tension members of trusses because the higher strength is used more effectively (for the entire depth of the member, because there is no stress gradient). The same is true for compression members of trusses where the member slenderness ratio is small to moderate (ratio of length to radius of gyration of about 80 or less, depending on grade).

The basic materials most often used to construct bridges are concrete and steel. Timber is occasionally used for deck construction and sometimes for short-span bridges.

4.5.1 Concrete

High strength is desirable for bridge concrete to reduce member size and weight, but durability is equally or more important. Component materials must be compatible with each other, and the concrete must have low permeability.

A long-term destroyer of concrete from within is alkali-silica reaction. While material specifications for concrete have been developed to preclude use of cement and aggre­gates that will produce alkali-silica reaction, the best prevention of this problem is the use of cements and aggregates from sources that have a known history of absence of this problem.

Given that required strength can be obtained by mix design with relative ease, low permeability becomes one of the most desirable properties, because bridge concrete is reinforced or prestressed and prevention of corrosion of the embedded steel is essential for long-term durability. Reduced permeability will also reduce carbonation, alkali-silica reaction, and freeze-thaw damage. Permeability can be reduced by proper mix design, by maintenance of a low water/cement ratio during concrete placement, by use of admix­tures, by compactive effort, by use of specialty concretes, or by application of concrete sealers or coatings. Often a combination of these procedures is employed.

Another desirable quality for the prevention of reinforcing steel corrosion, along with low permeability, is resistance to cracking. The cracking of bridge decks and of bridge deck overlays has been a persistent problem. The use of shrinkage-compensating con­crete using type K cement has been found to be effective in reduction of cracking, and some agencies mandate its use in construction of bridge decks. One property of shrinkage – compensating concrete that is different from regular concrete is the need for adequate amounts of mix water to cause the chemical reaction necessary for the development of the expansion. The normal rules of low water-cement ratio do not apply and must not be enforced. Another difference is that bleed water cannot be expected to appear. Waiting for bleed water to appear will result in the start of concrete hardening, making finishing very difficult. Agencies that have adopted shrinkage-compensating concrete have also adopted strict specifications for the production, placement, and curing of the concrete.

These specifications address the requirements peculiar to shrinkage-compensating concrete, but also include many requirements that are applicable to normal concrete as well. Perhaps the most important requirement is that prior to placement of the deck concrete, a preconstruction meeting be held, and that all participants in the cement manufacturing and concrete mixing, delivery, placement, finishing, curing, and inspection be required to take part. This meeting gives the type K cement manufacturer the opportunity to instruct the other participants in special requirements, and to correct any misconceptions that exist. Such meetings in themselves go a long way toward improving the quality of the concrete.

Admixtures. Various admixtures are available to enhance the properties of concrete made with the basic ingredients: coarse aggregate, fine aggregate, portland cement, and water. Admixtures may be classified as chemical admixtures such as air-entraining, water-reducing, set-retarding, accelerating, or superplasticizer; and mineral admixtures such as fly ash, silica fume, or slags. Mineral admixtures are usually added to concrete to improve workability, and resistance to thermal cracking and sulfate attack; and to reduce cement content whereas the chemical admixtures are added for entrainment of air, reduction of water or cement content, plasticization of the mixture, or control of setting time.

Air-entraining admixtures produce a distribution of bubbles that become permanent tiny voids in the concrete. This system of voids makes the concrete resistant to scaling, a surface failure that became frequent when deicing salts came into use. Air entrain­ment has virtually eliminated scaling. The use of air entrainment is recommended even with high-strength concrete.

Water-reducing admixtures make concrete mixtures workable at a lower water/cement ratio than is possible with use of “water of convenience” (water in excess of that required for hydration of the cement) alone. High-range water reducers provide great workability at very low water/cement ratios, and have been developed to provide reasonable control of duration of extra fluidity.

Dense concrete is a concrete developed by the Iowa Department of Transportation for overlayment of new and existing concrete bridge decks. It has a low water/cement ratio and relies on special compactive effort imparted by vibrating screeds to produce a dense concrete with reduced permeability.

Latex-modified concrete uses an admixture of latex, generally liquid styrene butadiene with a minimum solids content of 40 percent. It achieves a reduced permeability equivalent to dense concrete at a lesser thickness. This quality is important to the via­bility of latex-modified concrete as an alternative option to dense concrete because the cost of latex-modified concrete is higher on a volume unit measure basis.

Silica fume concrete, or microsilica concrete, incorporates extremely fine particles of microsilica. Added to concrete in powder or liquid form, it densifies the concrete, increases strength, and reduces permeability. Where silica fume concrete has been allowed as a contractor’s alternative option to latex-modified or dense concrete, it has rapidly replaced these other specialty concretes.

Calcium nitrite concrete contains calcium nitrite, a widely used inorganic corrosion inhibitor that acts at the surface of the steel reinforcement to limit the electrochemical reaction involved in the corrosion process. The calcium nitrite is added in liquid form to the concrete at a rate of 3 to 5 gal/yd3 (15 to 25 L/m3), depending on the quality of the concrete, the level of chlorides expected, and the life required for the concrete. (See Manual for Corrosion Protection of Concrete Components in Bridges, Task Force 32 Report, February 19, 1992, AASHTO, Washington, D. C.)

High-Performance Concrete. High-performance concrete (HPC) is defined as con­crete that meets special combinations of performance and uniformity requirements that cannot be achieved routinely using conventional ingredients, normal mixing and placing procedures, and typical curing practices. HPC offers many strength-related improve­ments such as higher compressive strength and modulus of elasticity, and lower creep and shrinkage. It also offers ductility-related improvements such as increased resistance to freeze-thaw, abrasion, and scaling, as well as reduced permeability. Potential benefits to owners include lower initial and life cycle costs as a result of lower construction costs, less required maintenance, longer structure life, and elimination of additional pro­tective systems. Also, there should be less disruption to the public due to the decreased maintenance requirements and longer periods between major rehabilitations.

HPC qualifies for the special federal funding allocated by the U. S. Congress (fiscal 1998 to 2003) for repair, rehabilitation, replacement, and new construction of bridges or structures that demonstrate the application of innovative materials. Missouri, Nebraska, New Hampshire, Texas, Virginia, and Washington are included in the AASHTO Strategic Highway Research Program (SHRP) lead state team for HPC implementation. HPC program bridges have already been constructed in several states including Texas, Nebraska, Virginia, Washington, New Hampshire, and Colorado. HPC bridge decks have been designed and either built or scheduled to be constructed in Ohio, New Jersey, and Puerto Rico.

HPC mixes must use pozzolan materials such as silica fines and fly ash. Pozzolans make concrete denser, thereby increasing durability. Silica fines eliminate the detri­mental effects of fly ash on concrete mechanical properties. Fly ash reduces the heat of hydration and thereby reduces plastic shrinkage. Mix design requirements vary, depending upon the application. For bridge decks, states have specified a 28-day com­pressive strength from 4000 to 8000 lb/in2 (27.6 to 55.2 MPa), and from less than 750 up to 2000°C for chloride permeability. For prestressed concrete beams, states have specified a 28-day compressive strength from 6000 to 10,000 lb/in2 (41.4 to 69 MPa), and from less than 1000 up to 2000°C for chloride permeability. The minimum specified for silica fines has ranged from 5 to 10 percent. Curing requirements have included a wet burlap covering within 10 min of finishing, and a 7-day wet cure followed by application of a curing compound.

HPC specifications typically also include production, placement, and curing trials. Testing programs include evaluations of strength, permeability, scaling, freeze-thaw, abrasion, elasticity, creep, and shrinkage. Not all tests are required for each application, but because many of the tests are of long duration, mix design may take 6 months or more.

Lightweight Concrete. Although the concrete most often used in bridge construction is normal-weight (hardrock) concrete having a unit weight, reinforced, of 150 lb/ft3 (24 kN/m3), lightweight concrete can be produced from manufactured aggregate that is available from several sources around the United States. The coarse aggregate is produced by heating shale in a kiln, which expands it. Lightweight fine aggregate can also be pro­duced but is not recommended for bridge concrete. Use of lightweight coarse aggregate can reduce the weight of reinforced concrete to 115 lb/ft3 (18 kN/m3). While the history of lightweight concrete for bridges includes premature failures, it also includes successful applications in both deck slabs and beams. It is important to recognize the different (greater) creep characteristics of lightweight concrete in structures where long-term deflec­tions are a significant design factor.

Bridge Width. Roadway width on bridges is the inside measurement to the bottom of the sidewalk curb or the bottom of the safety barrier. For bridges on roads where sidewalks are not provided, the bridge width is made equal to the approach roadway width including shoulders, so that the bottom of the barrier curb or the near face of the railing is aligned with the face of the barrier rail at the outside edge of the shoulder.

In the past, policy did not always permit full shoulders to be accommodated on bridges. Often the roadway was made narrower, particularly on longer bridges. This was done strictly to reduce bridge cost. From the traffic operations standpoint, however, it was an unwise practice. Disabled vehicles could not find refuge on the shoulder, and a full shoulder was not available for temporary maintenance of traffic during road rehabilitation or repaving. It is now recognized that a bridge is an integral part of a highway system when it comes to roadway width. The FHWA requires a minimum shoulder width of 8 ft (2.4 m) on each side of the roadway on federally funded projects.

Bridge Horizontal Clearance. For bridges over streams, the location of substructure units, and therefore the length of spans, is controlled by hydraulic requirements and by nav­igation clearance requirements established by agencies such as the U. S. Coast Guard and the U. S. Army Corps of Engineers. For bridges over navigable waters, the bridge designer should also consider the possibility of collisions from vessels. Refer to AASHTO’s Guide Specifications and Commentary for Vessel Collision Design of Highway Bridges.

For crossings of highways, the bridge columns or pier walls should clear the traveled way, shoulders, ditches where required, barrier rail, and any additional width required to provide a safe clear zone from edge of pavement. A minimum clearance of 30 ft (9.1 m) from edge of pavement is required except where this clearance is impractical, in which case the pier or wall may be placed closer to the edge of pavement, with barrier rail 2′-0" (610 mm) minimum from edge of shoulder, and pier or wall 2′-0" (610 mm) minimum from face of barrier rail. The barrier rail offset from face of pier or wall will be further controlled by the dynamic deflection of the particular system used. (See Chap. 6 for additional information.)

For crossings over railroads, the horizontal clearance requirements are usually set by the railroad company or by the state public utilities commission. In addition to clearance for safe operation of trains, including allowance for accidentally overhanging cargo, rail­road companies are cognizant of the importance of trackside drainage and require that drainage ditches be accommodated where present. In addition, a maintenance roadway for off-track equipment is often required. A horizontal clearance of 25 ft (7.6 m) from the centerline of the track is desirable and will obviate the need for pier crash walls.

If a pier adjacent to a railroad track is located closer than what is considered to be an adequate distance to prevent derailed cars from striking the pier (generally 25 ft (7.6 mm) from centerline of railroad track), the pier is required to be of heavy construction, or a sub­stantial crash wall is required to be constructed to protect the pier and prevent catastrophic collapse of the bridge. This wall should be aligned with the pier. For additional details, refer to the American Railway Engineering and Maintenance-of-Way Association (AREMA) Manual for Railroad Engineering.

AASHTO LRFD Specifications require abutments and piers located closer than 30 ft (9.1 m) to the edge of the roadway or closer than 50 ft (15 m) to the centerline of a railway track to be designed for a vehicular collision load defined in the specifications, unless protected by an embankment or a structurally independent, crashworthy, ground – mounted barrier.

Bridge Vertical Clearance. Generally, a clearance of 16 ft (4.9 m) plus an allowance for resurfacing should be provided over major state, U. S., and interstate highways, over the entire width of roadway. Over less important highways, a clear­ance of 14 ft (4.3 m) should be provided. These are AASHTO requirements. Published state standards, if different from AASHTO, should be followed.

The above vertical clearances apply to vehicular bridges. Because pedestrian bridges are narrower and lighter in weight, and therefore more vulnerable to major damage or collapse in the event of collision from overheight vehicles passing under the bridge, states are beginning to require an additional clearance of 1 ft (300 mm) for pedestrian bridges. This additional clearance is also recommended for overhead sign structures.

Vertical clearance requirements over railroads, like horizontal clearances, are set by the railroad company or state public utilities commission. A minimum clearance of 23 ft (7.0 m) above high rail is common for new bridges over nonelectrified racks. If the tracks are electrified, an additional 1-ft (300-mm) minimum clearance is required for catenary wires. Widened or rehabilitated bridges will generally be allowed to maintain the existing clearance, but no less.

Vertical clearance requirements over navigable waterways are subject to bridge permits by U. S. Coast Guard (USCG). USCG has established guide clearances for particular waterways. They are not regulatory in nature and greater or lesser clearances meeting reasonable needs of navigation for a particular location may be required or approved by USCG.