Structural Steel

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).