Erection tolerances for steel structures: what is checked when erected columns, beams and poles are accepted

Essential and functional tolerances: how they differ

DSTU EN 1090-2:2019 divides geometrical tolerances into two types. Essential tolerances relate to strength and stability: these are the deviations on which the design calculation relies. Functional tolerances relate to fit-up, appearance and the working of joints. Both types are mandatory (cl. 11.1).

Essential tolerances are the same for everyone. Functional tolerances have two classes: class 1 is more lenient, class 2 stricter. If the execution specification does not name a class, class 1 applies (cl. 11.3.2). Class 2 is assigned to individual elements or parts of a structure, for example where a glazed facade is fixed to the frame and gaps and adjustment work must be reduced.

Not every table has both types. For towers and masts (table B.24) only essential tolerances are given, for beams in buildings (table B.16) only functional ones.

All the figures in the standard are permitted deviations that do not include elastic deformations from the self-weight of the components. If the client needs other values, they are set as special tolerances: the execution specification names the parameter, the permitted value and the elements it applies to (cl. 11.1).

Execution class and tolerance class are different parameters

The standard defines four execution classes, EXC1 to EXC4. Requirements grow from EXC1 to EXC3, and EXC4 builds on EXC3 and adds special design requirements. The class is named by the design in the execution specification (cl. 4.1.2).

Tolerance class 1 or 2 is a separate item of the same specification (cl. 4.1.1 and 4.1.4). The numbers in tables B.15 to B.25 depend on the tolerance class, and there is no EXC column. So EXC3 does not make a column straighter: if the tolerance class is not named, class 1 applies under EXC3 as well.

The execution class affects documentation. The results of the survey of the completed structure must be documented for EXC3 and EXC4 (cl. 12.7.3.1). For EXC2 to EXC4 the repair procedure for coating damaged during unloading, transport or erection is documented (cl. 9.6.3), and the contractor must have quality documentation with an inspection plan and a procedure for handling nonconformities (cl. 4.2.1). Detailed records of measurements at acceptance are required only when the specification says so.

Both parameters are set by the design or the client, not by the fabricator. We fabricate structures to the customer’s drawings, so the execution class and the tolerance class must be in the KM drawings or the specification before fabrication starts, not sorted out at acceptance.

What is measured from: coordinate system, control points, conditions

Deviations of erected elements are measured from their position points under ISO 4463. If no position points are set, they are measured from a secondary system. The standard distinguishes the primary system (usually covering the whole site), the secondary system (the grid of a particular structure) and position points of individual elements, such as columns (cl. 11.2.3.1).

The setting-out of the main axes of the structure is documented, and it serves as the reference for placing the steelwork and assessing support deviations. Site coordinates from the survey are considered reliable only if they meet the acceptance criteria of ISO 4463-1. The temperature to which setting-out and measurement are referred must be stated, and a temperature correction is applied to the results (cl. 9.4.1 and 12.7.3.1).

The position of a column or pole is measured at defined points: for an element within 10° of the vertical, the centre of the section at each end; for elements within 45° of the horizontal, the centre of the top surface at each end. For the rest, the erection drawings give the points (cl. 12.7.3.3).

Measurement is taken with only the self-weight acting on the structure, unless stated otherwise. The measuring conditions (wind, heating, temporary loads) must be defined in advance. Each individual value must be within tolerance, and the algebraic sum of the individual values may not exceed the tolerance for the structure as a whole (cl. 12.7.3.2 and 12.7.3.4).

Before erection: foundation, anchor bolts, supports

The supports are checked first: foundations, anchors, bearings. The standard requires them to be inspected and measured before erection starts, and nonconformities to be documented and corrected (cl. 9.5.1). Erection may begin only when the position and level of the supports meet the criteria of cl. 11.2 or the requirements have been amended accordingly, and the survey results for the supports are documented (cl. 9.5.2).

For a group of foundation bolts the essential tolerance is this: the centre of the group must not deviate from its specified position by more than ±6 mm relative to the secondary system of coordinates. A group of adjustable bolts is assessed from an adopted best-fit specified position, not from the outermost bolt (cl. 11.2.3.2).

Table B.23 adds tolerances for individual bolts, plates and the level of the foundation. They are functional and the same for both classes.

For bolts in a joint where adjustment is or is not provided the standard gives separate rows of the table. Which of the two applies is shown by the joint on the drawing.

Base plate holes and levelling nuts

Holes in base plates are given clearances that reconcile the tolerances of the foundation with those of the steelwork. Sometimes large-diameter washers are placed between the anchor nut and the plate for this (cl. 11.2.3.3).

If the position of the plate is adjusted with levelling nuts under the plate, they may be left in place as permanent unless the specification says otherwise. The nuts are chosen so that the structure is stable during erection and the bolt threads are not damaged (cl. 9.5.4). More on the joint is in our articles on anchor bolts, metric threads of anchors, anchor cages and foundation design inputs.

Columns and poles: inclination, position, straightness

For single-storey columns and storey-height columns in a frame the standard gives table B.17, for columns continuous over several storeys table B.18, and for column positions in plan table B.20. Below are only the rows usually needed at acceptance. h is the column height between the base level and the top, L is the distance between axes.

The rows are taken from tables B.15, B.17 and B.20. For columns continuous over several storeys table B.18 applies, with different formulae.

What this looks like in millimetres

For a column 6 m high, a tolerance of ±h/300 gives ±20 mm and ±h/500 gives ±12 mm. The two classes differ by almost a factor of two (20 mm against 12 mm), and the difference matters only where something is fixed to the column with a small clearance. Inclination is measured in two mutually perpendicular directions while the pole can still be adjusted: once the slings are off and the nuts tightened, correction is too late. The same is said in our article on acceptance of multi-sided poles.

For crane columns and bases see the article on tolerances of crane beams and stepped columns.

Beams: levels, straightness, camber

For beams almost all tolerances are functional: they decide whether joints and fit-up work out, not strength. The only essential tolerance is the straightness of a beam in bending and of an unrestrained compression member: ±L/750 (table B.25).

Tables B.15, B.16 and B.25. The essential tolerance applies to both classes.

Deflections, camber and temperature

Accuracy during construction is interpreted with regard to expected deflections, cambers, fit-up, elastic movements and thermal expansion (cl. 12.7.3.6). If significant movement of the structure is expected that affects the check (for example in prestressed structures), the range of permitted position values is set separately. For bolted connections of beams see the article on bolted connections of steel structures.

Masts and towers: table B.24

Towers and masts have a separate table, B.24: all its values are essential, with no functional tolerances. The rows describe lattice structures with legs, chords and bracing.

Twist is not checked on towers with permanent lateral loading. The verticality tolerances are default values: the specification may replace them with less stringent ones, provided they conform to the design assumptions for verticality.

What about multi-sided poles?

Table B.24 is written for lattice towers and masts. For tubular poles, multi-sided ones included, verticality and position in plan are set by the design; the same is said in our article on acceptance of multi-sided poles. The order of lifting and fixing is in the article on assembly and erection of poles, and HV substation portals are in the article on portal erection.

Full contact bearing and shims

If the design specifies a surface with full contact bearing, after alignment the erected surfaces must meet as follows (essential tolerance, classes 1 and 2): a local angular deviation of no more than 1/500, with a gap of 0.5 mm over at least two thirds of the area and up to 1.0 mm at most locally (table B.19, cl. 11.2.3.5).

In a bolted lap splice where the gap exceeds the set limits after initial bolting-up, it is reduced with shims, unless the specification forbids it. Shims are made of mild steel sheet to EN 10025-2, up to 3 mm thick, and no more than three are used at any point of the splice. If necessary the shims are held in place by a fillet weld or a partial penetration butt weld along the edges so that they cannot fall out (fig. 5 of the standard).

Shims have the same durability as the structure; if the steel is coated, they are protected in the same way (cl. 9.6.5.3). Permanent connections are not made until alignment, level and plumb have been checked on a sufficient part of the structure and temporary connections hold the elements in place.

The survey check: how and with what

Surveying is done from the setting-out coordinate system of the structure: before erection, of the supports; after it, of the completed structure. The locations and frequency of measurements go into the inspection plan (cl. 12.7.3.4). Checks of critical dimensions to which special tolerances apply are added to the inspection and testing plan separately.

Methods and equipment are chosen from the lists in ISO 7976-1 and ISO 7976-2, and accuracy is assessed under the relevant part of ISO 17123. Laser scanning methods are permitted. What matters is that the chosen method gives the accuracy needed for the acceptance criteria (cl. 12.7.3.1).

What goes into the as-built drawing

The standard gives no form for an as-built drawing; it requires results to be documented. The fields are settled by the design and the inspection plan; the drawing usually carries:

• the axes and marks of the setting-out system from which measurement was made, and the temperature to which setting-out is referred;
• for each anchor bolt the measured and design position in plan, the protrusion and the inclination, and for a group the shift of its centre;
• levels of base plates and the method of adjustment: levelling nuts, shims, grout thickness;
• inclination of each column or pole in two directions, with date, time and conditions of measurement;
• levels and positions of beams and joints, and truss camber where the design requires it;
• the method and the instrument used;
• deviations beyond tolerance and what was done about them: correction, recalculation or a decision by the designer;
• the list of uncorrected nonconformities, if the structure is accepted with them.

The informative checklist for the quality plan (annex C) counts among the execution records the survey results before erection, the survey results of the geometry of the structure with records of actions on nonconformities, and the certified acts of completion of erection and handover. Records are kept for at least five years, or longer if the design requires.

If a deviation exceeds the tolerance

An essential tolerance measured beyond the permitted value is treated as a nonconformity. Sometimes the deviation can be accepted if it is taken into account in a recalculation of the structure; if not, the nonconformity must be corrected (cl. 11.2.1).

The general order is this (cl. 12.3). First it is corrected by methods the standard allows and checked again. If correction fails, it is assessed whether the nonconformity is acceptable given the function of the component. Next, under the nonconformity procedure, the structure may be modified. In all other cases the component is not used.

Mismatched holes in bolted splices on site are preferably corrected by reaming or milling; other cutting methods are permitted only with a separate check (cl. 9.6.5.3). If insufficient fit-up cannot be corrected with shims, individual elements may be replaced, taking into account excessive forces in lattice components. If the structure is handed over with uncorrected nonconformities, they are entered in a list (cl. 12.7.3.7).

What depends on the fabricator and what to put on the drawings

The erection tolerance is made up of manufacturing, setting-out and erection tolerances, and fabrication comes first in this balance: for the position of connections the manufacturing tolerances are decisive (cl. 12.7.3.2). What is checked in the shop is later seen on site: lengths, position and diameter of holes, squareness of flanges to the axis, straightness. The shop inspection routine is covered in the article on quality control of steel structures.

We do trial assembly of joints in the shop; the standard gives its purpose as making sure the components fit each other (cl. 9.6.4 and 6.10). Erection marking and an orientation mark where the shape does not make it obvious are required by cl. 9.6.2; for elements over 5 t the mass is shown on the drawings (cl. 9.6.1). We provide for these in the KMD.

In the specification and drawings before fabrication it is worth stating:

• the execution class;
• tolerance class 1 or 2 for the whole structure or for individual parts;
• special tolerances, if needed: the parameter, the value and the elements they apply to;
• the coordinate system, support marks and the temperature to which measurements are referred;
• the extent and locations of measurements, and whether detailed documentation is needed;
• whether shims are permitted in full contact bearing splices.

KM and KMD drawings are prepared by us or sent by the customer: KM and KMD design, steel fabrication. Erection is done by a licensed partner construction company under the work execution plan. If the structure is already standing and there are doubts about its position or condition, a technical inspection will help.

Typical mistakes at acceptance

• anchors are measured only after the column is placed: the foundation and bolts are checked before the equipment is called, and forcing a flange or plate into place on site shifts the error onto the structure;
• anchor deviation is taken from the design position of the group alone and not from the setting-out grid: the group looks perfect relative to itself but is shifted relative to the axes;
• the tolerance class is not named: class 1 applies by default, while the client expected class 2 accuracy;
• execution class is confused with tolerance class: EXC does not make the tolerances stricter, it changes the requirements for documentation and inspection;
• verticality is measured in wind or in sun: a shaft heated on one side moves, so the standard requires the conditions to be stated, and the verticality of a mast is measured in still air;
• a column is left on its anchors alone without guys: the standard forbids securing columns against overturning with foundation bolts only, unless this mode of working has been verified (cl. 9.6.5.1);
• a gap in a splice is closed with shims beyond what is allowed: no more than three shims up to 3 mm thick;
• the drawing says only «within tolerance» with no figures: in a dispute the measured values are needed, not a signature.

Standards and validity

Erection tolerances, the order of the survey check and the requirements for supports are set out in DSTU EN 1090-2:2019 «Execution of steel structures and aluminium structures. Part 2. Technical requirements for steel structures» (EN 1090-2:2018, IDT). The standard was adopted by order of SE «UkrNDNC» of 10.05.2019 No. 117, has been in force since 01.07.2020 and replaced DSTU B EN 1090-2:2014.

ISO 4463, ISO 7976-1, ISO 7976-2 and ISO 17123 are international standards to which DSTU EN 1090-2 refers for setting-out and measurement; whether their national versions are in force is checked before they are cited in project documents. The figures in this article are for orientation: on site the execution class, tolerance class and special tolerances stated in the design apply, together with the current text of the standard.