ACCURACY OF STEEL FABRICATION
The dimensions of any item may vary from those defined by the designer.
Such variations stem from the nature and behaviour of the material as much as from the process of making it.
Modern steel fabrication involves the manufacture of large and often complex welded assemblies of rolled steel products. High temperature processes are used to make the steel products, to form the components and to join them together, so dimensional variation is inherent and unavoidable.
This behaviour has implications for the designer, for the steelwork contractor, and for the builder of supporting and adjoining structures. In carrying out their roles, each has to anticipate the variations.
The important questions are:
1. Which dimensional variations are significant;
2. What limits must be put on those variations which are significant; and
3. How should variations be managed,
to ensure that the design is implemented to meet its performance requirements without delay?
In steel construction, dimensional variation is significant in a number of ways, for it involves structural steelwork manufactured remote from the site, civil engineering works at the site, and sometimes even precise mechanical components. These interface with each other and yet their precision varies from the high accuracy of mechanical components to the inaccuracies inherent in placing concrete. It is convenient to distinguish between the following:
1. 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.
2. FIT-UP OF FABRICATED MEMBERS, which is essential for efficient assembly. 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.
3. DEVIATION FROM FLATNESS OR STRAIGHTNESS, which affects the stiffness and strength of components. For example, buckling resistance is less for an out-of straight slender strut.
4. 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).
5. 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.
The control of dimensions is fundamental to the mechanical engineering discipline, and without which no mechanism could work, no parts would be interchangeable. It is achieved by specifying tolerances – limits to the deviation from nominal dimension.
No mechanical drawing is complete without tolerances on all dimensions, limits and fits on mating parts, and flatness tolerances on surfaces. 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.
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.
The level of accuracy common to a mechanical engineering workshop is generally unnecessary for constructional steelwork – for which it would have to be justified because such accuracy comes at a substantial cost and needs special facilities, including machining.
For example, the variation of flatness and thickness of a steel plate from the rolling mill is perfectly satisfactory for a girder, but it would be unacceptable for a machine part.
With the widespread use of automated processes from the 1980s for cutting, hole drilling, girder assembly and welding, the geometrical accuracy to which steel fabrication can be made has much improved: this has been driven by the economics of practicable manufacture and the replacement of labour intensive traditional practice.
For each new project, the steelwork contractor will assess the design to determine how best to undertake the fabrication and how to control dimensions to ensure proper fit-up and assembly at site. For large roof trusses, box girders, and for large bridges with steel decks, this may well include a project specific regime of dimensional tolerances on sub-assemblies; these would be compatible with the tolerances set by the designer for the finished building or bridge.
Geometrical tolerances are specified in Annex B of BS EN 1090-2. Tolerances are grouped three distinct categories:
ESSENTIAL TOLERANCES. These are the limits of permissible deviation for the mechanical resistance and stability of the structure and are used to support conformity assessment to BS EN 1090-1].
FUNCTIONAL TOLERANCES. These are the limits of permissible deviation for fit-up and appearance. Two classes of deviation are given, class 1 being the less onerous and is the default for routine fabrication. Class 2 requires more expensive and special measures through fabrication and erection.
SPECIAL TOLERANCES. Individual projects may specify special tolerances, either as a modification of the essential or functional tolerances or for aspects not already covered. There is a need for certain additional tolerances on most bridge structures, and these should generally be implemented via the Specification for Highway Works. Guidance on geometrical tolerances for bridges is available in GN5.03
The values of the permitted deviations for both essential and functional manufacturing tolerances are given in tables BS EN 1090-2 B.1 through to B.14.
Essential and functional erection tolerances are given in Tables B.15 to B.25. As an alternative to the functional tolerances in Annex B, BS EN 1090-2 does allow the use of BS EN ISO 13920; this is more likely to be used in cases of heavily welded structures where distortion from welding is the dominant factor in determining the dimensions and shape.
That standard specifies general tolerances for linear and angular dimensions and for shape and position of welded structures.