Structural Steel

Fabrication Tolerances

Variations from theoretical dimensions occur in hot-rolled structural steel because of the routine production process variations and the speed with which they must be rolled, wear and deflection of the rolls, human differences between mill operators, and differential cooling rates of the elements of a section. Also, mills cut rolled sections to length while they are still hot. Tolerances that must be met before structural steel can be shipped from mill to fabricator are listed in ASTM A6, ‘‘General Requirements for Delivery of Rolled Steel Plates, Shapes, Sheet Piling and Bars for Structural Use.’’
Tolerances are specified for the dimensions and straightness of plates, hot-rolled shapes, and bars. For example, flanges of rolled beams may not be perfectly square with the web and may not be perfectly centered on the web. There are also tolerances on surface quality of structural steel.
Specifications covering fabrication of structural steel do not, in general, require closer
tolerances than those in A6, but rather extend the definition of tolerances to fabricated members. Tolerances for the fabrication of structural steel, both hot-rolled and built-up members, can be found in standard codes, such as the AISC ‘‘Specification for Structural Steel Buildings,’’
both the ASD and LRFD editions; AISC ‘‘Code of Standard Practice for Steel Buildings and Bridges’’; AWS D1.1 ‘‘Structural Welding Code-Steel’’; AWS D1.5 ‘‘Bridge Welding Code’’; and AASHTO specifications.
The tolerance on length of material as delivered to the fabricator is one case where the tolerance as defined in A6 may not be suitable for the final member. For example, A6 allows wide flange beams 24 in or less deep to vary (plus or minus) from ordered length by 3⁄8 in plus an additional 1⁄16 in for each additional 5-ft increment over 30 ft. The AISC specification for length of fabricated steel, however, allows beams to vary from detailed length only 1⁄16 in for members 30 ft or less long and 1⁄8 in for members longer than 30 ft. For beams with framed or seated end connections, the fabricator can tolerate allowable variations in length by setting the end connections on the beam so as to not exceed the overall fabrication tolerance of 1⁄16 or 1⁄8 in. Members that must connect directly to other members, without framed or seated end connections, must be ordered from the mill with a little additional length to permit the fabricator to trim them to within 1⁄16 or 1⁄8 in of the desired length.
The AISC ‘‘Code of Standard Practice for Steel Buildings and Bridges’’ defines the clause ‘‘Architecturally Exposed Structural Steel’’ (AESS) with more restrictive tolerances than on steel not designated as AESS. The AESS section states that ‘‘permissible tolerances for outof- square or out-of-parallel, depth, width and symmetry of rolled shapes are as specified in ASTM Specification A6. No attempt to match abutting cross-sectional configurations is made unless specifically required by the contract documents. The as-fabricated straightness tolerances of members are one-half of the standard camber and sweep tolerances in ASTM A6.’’
It must be recognized the requirements of the AESS section of the Code of Standard Practice entail special shop processes and costs and they are not required on all steel exposed to public view. Therefore, members that are subject to the provisions of AESS must be designated on design drawings.
Designers should be familiar with the tolerances allowed by the specifications covering each job. If they require more restrictive tolerances, they must so specify on the drawings and must be prepared for possible higher costs of fabrication.
While restrictive tolerances may be one way to make parts of a structure fit, they often are not a simple matter of care and are not practical to achieve. A steel beam can be  fabricated at 65F and installed at 20F. If it is 50 ft in fabrication, it will be about 1⁄8 in short during installation. While 1⁄8 in may not be significant, a line of three or four of these beams in a row may produce unacceptable results. The alternative to restrictive tolerances may be adjustment in the structural steel or the parts attaching to it. Some conditions deserving consideration include parts that span vertically one or more stories, adjustment to properly set expansion joints, camber in cantilever pieces, and members that are supported some distance from primary columns.

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