What structural engineers need to know about mill, fabrication, and field practices
Contractors, including structural steel fabricators, take design drawings and turn them into buildings. How does the engineer know where the structural steel will actually be located when it is erected? Part of the answer is tolerances.
The majority of tolerances in structural steel construction for fabrication and erection are outlined in the American Institute of Steel Construction’s (AISC) Code of Standard Practice for Steel Buildings and Bridges (COSP). Tolerances for the manufacture of rolled shapes and plates are found in Standard Specification for General Requirements for Rolled Structural Steel Bars, Plates, Shapes, and Sheet Piling (ASTM A6). Tolerances for hollow structural shapes, such as HSS and pipe sections, are found in ASTM A500 and ASTM A53. The American Welding Society (AWS) D1.1 Structural Welding Code-Steel document also specifies tolerances for fit-up of welded parts. However, the details of the A500, A53, and AWS provisions will not be discussed in this article.
The COSP represents the existing practices considered to be the standard of custom and usage within the structural steel industry. The COSP tolerances provide an envelope of the location and orientation of erected steel members relative to column lines and elevations rather than the exact location of a point on a piece of steel in space. Tolerances are subject, in certain instances, to total cumulative maximums.
Tolerances of steel construction will be discussed in three general areas—mill tolerances, fabrication tolerances, and field tolerances.
Mill tolerances
Mill tolerances for hot-rolled structural steel shapes are given in ASTM A6, incorporated as part of the COSP by reference. ASTM A6 stipulates tolerances for items such as cross-section dimensions, weight per foot, and the variations from straightness of the member as produced by the mill. The cross-section parameters that are limited for wide flange shapes are illustrated in Figure 1. For the actual tolerances, see ASTM A6 or see Tables 1-22 through 1-29 in the AISC Steel Construction Manual, 13th edition. Variations from straightness, known as sweep and camber, are illustrated in Figure 2.
Figure 1
Figure 2
Mill tolerances usually only affect the mill producing the steel and the steel fabricator or service center placing the order. The fabricator is allowed, but not required, to rework members following the provisions of the COSP to correct any out-of-tolerance variations. Fabricators often use details that reduce the impact of most mill tolerances. Pieces with severe variations may need to be replaced by the mill. ASTM A6 also limits the variation in length and end-squareness, but these tolerances are rarely critical because most pieces will be cut by the fabricator.
Fabrication tolerances
Fabrication tolerances for rolled shapes, covered in Section 6.4 of the COSP, facilitate the erection fit-up and positioning of the members by controlling the dimensions of fabricated pieces. Tolerances for welded, built-up members such as plate girders are in AWS D1.1
The tolerances on the length of the member fall into two categories—members with both ends finished for contact bearing (generally columns) and members that frame into other steel members (generally beams and braces). See Figure 3 for limits on columns and Figure 4 for limits on other members.
Figure 3
Figure 4
The variation in straightness of members is measured in the same manner as the ASTM A6 mill tolerances. Again, there are two categories—members without specified camber (straight) and members with specified camber. Straight members not intended to be used as compression members have no tolerance requirements beyond ASTM A6. Straight members to be used as compression members have a limit on camber and sweep specified as the member length divided by 1,000. This limit is incorporated into compression design equations in the AISC Specification for Structural Steel Buildings.
Beams with specified camber have a tolerance on the variation in camber of plus 1/2 inch for members shorter than 50 feet (with an additional 1/8-inch for every 10 feet of member length over 50 feet) and minus 0 inches. Camber is measured in the fabrication shop in the unstressed condition. At that point, the camber must not be less than the specified camber. The tolerances for sweep in members with specified camber are the same as for straight members. All completed members are to be free of twists, bends, open joints, and sharp kinks.
Field tolerances
Field tolerances can be divided into two basic categories—site preparation and erection tolerances.
Site preparation is generally the responsibility of the owner’s designated representative for construction, usually the general contractor. This involves the accurate positioning of foundations, piers, and abutments; the accurate location of building lines and benchmarks; and the accurate installation of anchor rods, foundation bolts, and other embedded items. The tolerances for these items are stipulated in the COSP sections 7.2 through 7.6. Many tolerance issues occur at the interface of two or more trades of construction materials.
Anchor-rod tolerances, covered in section 7.5 of the COSP, and improper anchor-rod placement are common sources of questions for AISC’s Steel Solutions Center. Table 14-2 of the AISC manual recommends base plate hole sizes that are compatible with the tolerances established for setting anchor rods and actually allow for some variation in location in excess of the COSP. Common anchor-rod placement issues may be reduced by including the more stringent COSP tolerances for anchor rods in the concrete specifications.
Erection tolerances are covered in Section 7.13 of the COSP. Until now, the discussion of tolerances has involved only individual members that are easily measured and controlled. Erection tolerances include not only the tolerance of members, but also the orientation and placement of the individual members in the frame. Section 7.12 of the COSP states, "The accumulation of mill tolerances and fabrication tolerances shall not cause the erection tolerances to be exceeded." The commentary for this section adds, however, "accumulations of mill tolerances and fabrication tolerances generally occur between the locations at which erection tolerances are applied, and not at the same locations." In other words, erection tolerances generally limit the locations of the work points at the end of each member, while mill and fabrication tolerances limit the variations between those work points.
Structural steel erection is a balance of many factors, including fabricated member length, connection material location, temperature, and changing loads during construction. The erection process has been refined over many decades, and practical erection tolerances have been developed during that time to help ensure high-quality structures. Additional information is given in the commentary to Section 7.13 on the historical development of the criteria.
In typical building construction, setting and plumbing the columns is the most critical step. Section 7.13.1.1 stipulates the allowable angular variation of the work line from plumb. For individual column pieces, permissible angular variation is generally 1/500 of the distance between working points. This out-of-plumbness is included in the stability design requirements of the AISC specification. Additional limitations apply to columns adjacent to elevator shafts and for exterior building columns. These tolerances are illustrated in the COSP commentary. The graphic from COSP Figure C-7.5 illustrates the envelope of the column work lines at an exterior column. This envelope includes the 1/4-inch tolerance on the location on the anchor rod group at the column base from COSP Section 7.5. Figures C-7.3 and C-7.4 in the COSP illustrate the possible effects of the accumulation of mill, fabrication, and erection tolerances on the column location.
Straight members other than columns will generally be considered acceptable if the variation of alignment is caused solely by variations in column alignment and/or primary supporting member alignment. There are additional limitations on members connected to columns, cantilever members, members containing field splices, members that consist of irregular shape, and members that are identified as adjustable items.
Members that are critical in terms of their location can be identified by the engineer (the owner’s designated representative for design) in the contract documents. When adjustable items are specified, the engineer is required to indicate the total adjustability necessary for the proper alignment of the piece. The fabricator will provide adjustable connections and the erector will erect the items in accordance with the tolerances of Section 7.13.1.3.
Conclusion
While the numerical values of tolerances can be obtained by examining the references in this article, the key to working with tolerances is knowing how to use them to achieve the structure that contract documents describe. Communicating critical tolerance expectations early in the design and construction process is crucial. Fabricators and erectors can help designers identify practical and economical tolerance requirements. In critical locations, tighter tolerances are not always the answer. Providing details that allow for adjustment or identifying items as "adjustable items" can alleviate many fit-up complications in the field. Getting all parties to talk early and often will help make tolerances tolerable on your next project.
Jason Ericksen, S.E., is the director of the American Institute of Steel Construction’s Steel Solutions Center in Chicago. He can be reached at 1-312-670-5429 or ericksen@aisc.org.