Historically, steel fabrication found a workable compromise, making large manufactured products using workshop techniques that assured their efficient assembly at a remote site – tolerancing was not part of that process as a rule it was implicit in much of the work.

In contrast, civil engineering construction has largely ignored the concept of tolerances, depending on the calibration of its metrology to build the product satisfactorily in situ. No mechanical drawing is complete without tolerances on all dimensions, limits and fits on mating parts, and flatness tolerances on surfaces. It is achieved by specifying tolerances – limits to the deviation from nominal dimension. The control of dimensions is fundamental to the mechanical engineering discipline, and without which no mechanism could work, no parts would be interchangeable. Interface with substructures and foundations, where adjustment has to be provided to accommodate the different accuracies of steelwork and substructure – for example, the provision of large pockets for holding down bolts and variable grout layers beneath bearings and base plates.Accuracy of assembly at site, where the steelwork must be assembled without having to apply unintended forces to connections and without deforming the structure from its intended geometry (thus inhibiting, for example, construction of the correct concrete slab thickness).

For example, buckling resistance is less for an out-of straight slender strut.

Deviation from flatness or straightness, which affects the stiffness and strength of components.In a bolted site splice, for example, relative position of holes is crucial for inserting the bolts but the positional accuracy of individual bolts has very little effect on the strength of the connection. Fit-up of fabricated members, which is essential for efficient assembly.Mechanical fit, which is vital, for example, for functioning between nut and bolt, between bearing and girder, and between machined abutting faces of compression members.(Image courtesy of Kiernan Structural Steel Ltd.)

Fillet weld tolerance