Dimensional variations in the field often are a consequence of permissible variations in rolling of steel and in shop fabrication. Limits for mill variations are prescribed in ASTM A6, General Requirements for Delivery of Rolled Steel Plates, Shapes, Sheet Piling, and Bars for Structural Use. For example, wide-flange beams are considered straight, vertically or laterally, if they are within 1â„8 in for each 10 ft of length. Similarly, columns are straight if the deviation is within 1â„8 in per 10 ft, with a maximum deviation of 3â„8 in.
It is standard practice to compensate in shop details for certain mill variations.
The adjustments are made in the field, usually with clearances and shims.
Shop-fabrication tolerance for straightness of columns and other compression members often is expressed as a ratio, 1:1000, between points of lateral support.
(This should be recognized as approximately the equivalent of 1â„8 in per 10 ft, and since such members rarely exceed 30 ft in length, between lateral supports, the 3â„8- in maximum deviation prevails.) Length of fabricated beams have a tolerance of 1â„16 in up to 30 ft and 1â„8 in over 30 ft. Length of columns finished to bear on their ends have a tolerance of 1â„32 in.
Erected beams are considered level and aligned if the deviation does not exceed 1:500. Similarly, columns are plumb and aligned if the deviation of individual pieces, between splices in the usual multistory building, does not exceed 1:500.
The total or accumulative displacement for multistory columns cannot exceed the limits prescribed in the American Institute of Steel Construction Code of Standard Practice. For convenience, these are indicated in Fig. 7.62. Control is placed only on the exterior columns and those in the elevator shaft.
Field measurements to determine whether columns are plumb should always be made at night or on cloudy days, never in sunshine. Solar radiation induces differential thermal strains, which cause the structure to curl away from the sun by an amount that renders plumbing measurements useless.
If beam flanges are to be field welded (Fig. 7.56a) and the shear connection is a high-strength-bolted, slip-critical joint, the holes should be made oversize or horizontal slotted (Art. 7.3.1), thus providing some built-in adjustment to accommodate mill and shop tolerances for beams and columns.
Similarly, for beams with framed connections (Fig. 7.46 and 7.47) that will be field bolted to columns, allowance should be made in the details for finger-type shims, to be used where needed for column alignment.
Because of several variables, bearing of column joints is seldom in perfect contact across the entire cross-sectional area. The AISC recommends acceptance if gaps between the bearing surfaces do not exceed 1â„16 in. Should a gap exceed 1â„16 in and an engineering investigation shows need for more contact area, the gap may be filled with mild steel shims.
Tolerance for placing machinery directly on top of several beams is another problem occasionally encountered in the field. The elevation of beam flanges will vary because of permissible variations for mill rolling, fabrication, and erection.
This should be anticipated and adequate shims provided for field adjustments.
