The steel shapes, plates, and bars that make up most of the materials used for structural steel are produced by mills as hot-rolled products. These products are made in a batch process; each production run of steel comes from a heat. The specific grade of steel in all mill products is identified by reference to the heat number.
Through universal acceptance of ASTM specifications (Table 7.1), mill materials have uniform physical and quality characteristics. There is no significant metallurgical or physical difference between products ordered to a specific ASTM specification and rolled by any U.S. structural mill.
Grades of Steel
Structural steel grades are referred to by their corresponding ASTM designation.
For example, the most commonly used grade of structural steel is A36, which is produced to meet the requirements of the ASTM A36 specification. This grade offers a good mix of strength, weldability, and cost. In many designs, this specification alone will satisfy designers needs. Other specifications, such as A53 for pipe, provide an equivalent grade of steel for that type of product. However, as loads on the structural elements becomes larger, other grades of steel may become more economical because of dimensional limitations or simpler fabrication. These grades provide greater strength levels at somewhat higher costs per unit weight.
AISC recommends certain grades of steel, all of which have desirable characteristics, such as weldability and cost-effectiveness, for use where higher strength levels are required. The specifications covering these grades are listed in Table 7.2. Several steels have more than one level of tensile strength and yield stress, the levels being dependent on thickness of material. The listed thicknesses are precise for plates and nearly correct for shapes. To obtain the precise value for shapes, refer to an AISC Manual of Steel Construction (ASD or LRFD) or to mill catalogs.
Weathering Steels. The A242 and A588 grades of steel offer enhanced corrosion resistance relative to A36 material. These steels, called weathering steels, form a thin oxidation film on the surfaces that inhibits further corrosion in ordinary atmospheric conditions. However, special treatment of construction details is required.
Because of such constraints, and because these grades are more expensive, utilization of weathering steels in building construction is limited. These grades are more commonly used in bridge construction.
Steel Grade Identification. Because of the several grades of steel in use, ASTM specifications require that each piece of hot-rolled steel be properly identified with vital information, including the heat number. The AISC specifications for structural steel buildings require fabricators to be prepared to demonstrate, by written procedure and by actual practice, the visible identification of all main stress-carrying elements at least through shop assembly. Steel identification include ASTM designation, heat number (if required), and mill test reports when specifically ordered.
Availability. Because structural steel is produced in a batch process, the less commonly used shapes and the higher-strength grades are produced less frequently than commonly used A36 shapes. Furthermore, steel service centers stock the smaller A36 shapes. As a result, availability of steels can affect construction schedules.
Consequently, steel designers should be aware of the impact of specifying less commonly used materials and shapes if the project has a tight schedule. Fabricator representatives can provide needed information.
Steel mills have a standard classification for the many products they make, one of which is structural shapes (heavy). By definition this classification takes in all shapes having at least one cross-sectional dimension of 3 in or more. Shapes of lesser size are classified as structural shapes (light) or, more specifically, bars.
Shapes are identified by their cross-sectional characteristics angles, channels, beams, columns, tees, pipe, tubing, and piles. For convenience, structural shapes are simply identified by letter symbols as indicated in Table 7.3. The industry recommended standard (adopted 1970) for indicating a specific size of beam or column-type shape on designs, purchase orders, shop drawings, etc., specifies listing of symbol, depth, and weight, in that order. For example, W14 x 30 identifies a wide-flange shape with nominal depth of 14 in and weight of 30 lb / lin ft. The , read as by, is merely a separation.
Each shape has its particular functional use, but the workhorse of building construction is the wide-flange W section. For all practical purposes, W shapes have parallel flange surfaces. The profile of a W shape of a given nominal depth and weight available from different producers is essentially the same, except for the size of fillets between web and flanges.
Tolerances for Structural Shapes and Plates
Mills are granted a tolerance because of variations peculiar to working of hot steel and wear of equipment. Limitations for such variations are established by ASTM specification A6.
Wide-flange beams or columns, for example, may vary in depth by as much as 1â„2 in, i.e., 1â„4 in over and under the nominal depth. The designer should always keep this in mind. Fillers, shims, and extra weld metal installed during erection may not be desirable, but often they are the only practical solution to dimensional variations from nominal.
Cocked flanges on column members are particularly troublesome to the erector for it is not until the steel is erected in the field that the full extent of mill variations becomes evident. This is particularly true for a long series of spans or bays, where the accumulating effect of dimensional variation of many columns may require major adjustment. Fortunately, the average variation usually is negligible and nominal erection clearance allowed for by the fabricator will suffice.
Mill tolerances also apply to beams ordered from the mills cut to length. Where close tolerance may be desired, as sometimes required for welded connections, it may be necessary to order the beams long and then finish the ends in the fabricating shop to precise dimensions. This is primarily the concern of structural detailers.
Frequently, designers want long-span beams slightly arched (cambered) to offset deflection under load and to prevent a flat or saggy appearance. Such beams may be procured from the mills, the required camber being applied to cold steel. The AISC Manuals give the maximum cambers that mills can obtain and their prediction of the minimum cambers likely to remain permanent. Smaller cambers than these minimums may be specified, but their permanency cannot be guaranteed. Nearly all beams will have some camber as permitted by the tolerance for straightness, and advantage may be taken of such camber in shop fabrication.
A method of cambering, not dependent on mill facilities, is to employ heat. In welded construction, it is commonplace to flame-straighten members that have become distorted. By the same procedure, it is possible to distort or camber a beam to desired dimensions.
Used by fabricators to manufacture built-up structural members, such as columns and girders, and for detail connection material, plates are identified by the symbol PL. Cross-sectional dimensions are given in inches (or millimeters). A plate 1â„2 in thick and 2 ft wide is billed as PL 1â„2x 24. Plates may also be specified by weight, although this is unusual in building construction work.
Mill tolerances for plate products for structural applications are also defined by ASTM specification A6. There are provisions for thickness, crown, camber, and length. Consideration of these characteristics are primarily the responsibility of fabricators. However, steel designers should be aware of how these tolerances affect the fabricators work and permit the design to accommodate these characteristics.
Pipe and Tubular Sections
Pipe meeting the requirements of ASTM specification A53, Types E and S, Grade B, is comparable to A36 steel, with yield strength Fy = 36 ksi. It comes in three weight classification: standard, extra strong, and double extra strong, and in diameters ranging up to 26 in.
Several mills produce square and rectangular tubing, known as hollow structural sections, in sizes from 3 x 2 and 2 x 2 to 12 x 8 and 10 x 10 in, with wall thickness up to 5â„8 in. These flat-sided shapes afford easier connections than pipes, not only for connecting beams but also for such items as window and door frames.
The main strength properties of several grades of steel used for pipe and tubular sections are summarized in Table 7.4.
Cautionary Note. Hollow structural sections are not produced to meet the requirements of ASTM specification A6. Because of this characteristic, the AISC and the Steel Tube Institute of North America recommended that the nominal wall thickness of such sections be reduced by 7% when calculating the section properties of these sections, (area, section modulus, and moment of inertia) so as to maintain a factor of safety equivalent to that present in other structural steel shapes.