Bolted connections in steel structures: ordinary, high-strength, tightening and inspection

Types of bolted connections and how they transfer force

A bolt works in one of three ways, and sometimes in two at once. In shear and bearing: the plates slide until the wall of the hole bears on the shank. In tension: the bolt is loaded along its axis, as in a column flange. In friction: the plates are clamped so tightly that friction between them stops the slip, and the shank hardly works in shear.

In an ordinary shear connection there is a clearance in the hole, so the plates first shift slightly and only then bear on the bolt. For static loads this is usually acceptable. If slip is not acceptable, a preloaded connection is needed.

DBN V.2.6-198 requires slip-resistant connections in structures that directly carry moving, vibrating or dynamic loads, and where the connection has increased requirements on limited deformability (cl. 16.3.1). The execution standard also names preloading as a means of resisting fatigue and seismic action.

Shear, bearing, tension: what the calculation checks

DBN gives the design force of one bolt separately for shear, bearing on the hole wall and tension. The number of bolts in a connection is found from the smaller of the shear and bearing values. If a bolt is in shear and tension at once, it is also checked by a combined formula (cl. 16.2.9, 16.2.10, 16.2.13). The formulas themselves are taken from the current text of DBN.

There are corrections that are often skipped. With packings or a one-sided cover plate the number of bolts is increased by 10 %. If a short stub is fixed to the leg of an angle or channel, 50 % more bolts than calculated are used. When the length of the connection along the force exceeds 16 diameters, the number of bolts is increased too, by a factor that depends on the length (cl. 16.2.14, 16.2.10).

Thread in the shear plane: the section through the thread is smaller than through the plain shank. In structural lattice structures, power-line poles, open switchgear and wherever the outer element is thinner than 8 mm, the thread must stay outside the plate pack. In other cases it may run into the hole by no more than half the thickness of the outer element on the nut side, or 5 mm (cl. 16.2.5). Bolts with different diameters along the unthreaded part are not used in shear (cl. 16.2.4).

The bolt length is chosen so that after tightening the end of the shank projects beyond the nut by at least one thread pitch. In non-preloaded connections at least one full thread must remain between the bearing face of the nut and the plain part of the shank (DSTU EN 1090-2, cl. 8.2.2). The minimum bolt diameter in load-bearing members under this standard is M12 unless the project says otherwise.

Slip-resistant connections and preloading

In a slip-resistant connection the bolt is tightened to a stated force, and one plate is held on the other by friction. So three things matter here that an ordinary connection does not have: the class and completeness of the bolt assembly, the method and level of preload, and the state of the contact surfaces. If one of the three does not match the project, the load capacity of the connection is not confirmed.

Under DBN V.2.6-198, bolts of class 10.9 or higher are used in slip-resistant connections and in preloaded flange connections (cl. 7.8 as amended by Amendment No. 1). The bolt diameter in a slip-resistant connection is, as a rule, not less than the thickness of the thickest element of the pack. The distance from the hole centre to the edge in such a connection is at least 1.3d for any edge and any direction of force, and the difference between the hole and bolt diameters is from 1 to 6 mm depending on the assembly conditions (cl. 16.3.2, 16.3.7).

The design tensile resistance of a bolt in a slip-resistant connection is 0.7 of the lowest characteristic strength of the bolt steel (cl. 7.8). The force carried by one friction plane depends on the preload, the friction coefficient and a reliability factor. It is multiplied by a working-condition factor of the connection: 0.8 if there are fewer than five bolts, 0.9 with five to nine bolts and 1.0 from ten (cl. 16.3.4).

If part of the force in a connection is also carried by welds, this is allowed only with special justification, and welding may be done only after the bolts are finally tightened (cl. 16.1.13). Free access to slip-resistant bolts must be left for a torque wrench or power wrench (cl. 16.3.9). This is checked on the joint drawing, not on site.

Minimum preload force

DSTU EN 1090-2:2019 takes the minimum preload force Fp,C as 0.7·fub·As: 0.7 of the nominal strength of the bolt material multiplied by the tensile stress area of the bolt. It is used for all slip-resistant connections unless the project sets a lower level. In substance this is the same as DBN: 0.7 of the tensile strength.

Table 18 of DSTU EN 1090-2:2019. The table gives both classes, while DBN V.2.6-198 requires class 10.9 or higher for slip-resistant and preloaded flange connections, so for such joints the 10.9 column is used.

Holes: kinds, clearances, making

A bolt hole may be a normal round hole, an oversize round hole, a short slotted hole or a long slotted hole. The normal hole leaves the smallest clearance and gives the smallest slip. Oversize and slotted holes are needed to absorb erection tolerances, but they are chosen in the project deliberately: the slip is larger, and a connection with such holes calls for plate washers.

The nominal clearance is the difference between the hole diameter and the nominal bolt diameter. The table gives the values for M12–M36 bolts under DSTU EN 1090-2:2019 (Table 11).

For M12 and M14 bolts a 2 mm clearance is also allowed if the project states it. For coated fasteners the 1 mm clearance may be increased by the coating thickness. In towers and masts the nominal clearance of normal round holes is reduced by 0.5 mm unless stated otherwise. Long slotted holes and close-tolerance bolts (hole equal to the shank diameter) are covered in the standard itself.

What else is said about holes

DBN takes the hole diameter equal to the bolt diameter for accuracy class A, and for classes B and C in power-line poles, open switchgear and transport contact-network structures as the bolt diameter plus 1 mm. In other structures it is the bolt diameter plus 1, 2 or 3 mm (note 2 to Table 16.3). Clause 16.2.8 of DBN, which referred to DSTU B V.2.6-199 for the rules on making holes, was deleted by Amendment No. 1; the making of holes is covered by DSTU EN 1090-2:2019.

A hole may be drilled, punched or cut thermally (laser or plasma) provided the finished hole meets the requirements for edge hardening and cut-surface quality and the bolt enters the assembled pack easily, at right angles to the contact surfaces. The standard allows punching when the thickness of the part does not exceed 1.4 times the hole diameter; beyond that the hole is drilled. Thermally cut holes have a tolerance of ±0.5 mm on the mean diameter.

Punching without reaming or drilling out is usually not suitable for connections under cyclic or seismic loading, for slip-resistant ones and for lap joints with bolts above class 8.8. Burrs are removed from holes before assembly. For connections with slotted and oversize holes the plate washers are stated in the project with their size and steel grade, at least 4 mm thick.

Spacing between bolts and to the edge

In calculated connections the bolts are placed at the minimum distances, in constructional ones, which simply join elements, as a rule at the maximum (cl. 16.2.2). The minimum values from Table 16.3 of DBN are given below; d here is the hole diameter.

Maximum distances (from the edge and between bolts in outer and inner rows) are also given in Table 16.3 of DBN. Ryn is the characteristic strength of the rolled steel being joined.

Accuracy classes, washers and locking

The accuracy class relates to how tightly the bolt fits the hole. Class A bolts go in holes drilled to the design diameter in the assembled pack or through a jig, or drilled or punched undersize and reamed in the assembled state (cl. 16.2.3). Class B and C bolts in multi-bolt connections are permitted by DBN for steel with a yield strength up to 390 N/mm².

Where the deformation of the connection must be limited, class A bolts are used. Where the bolts work mainly in tension, class B and C or high-strength bolts are used as a rule. For structures of communication antenna towers DBN recommends that erection connections carrying design forces be made with class B and high-strength bolts; under alternating forces, with high-strength bolts or erection welding. Erection welding or class A is agreed with the organisation that will erect the structure (cl. 20.15).

After Amendment No. 1, DBN names the standards for the bolts, nuts and washers themselves in general terms («current normative documents»). So the project states a specific standard for each part of the assembly, not just «M20 bolt».

In non-preloaded connections with normal round holes washers are not mandatory unless the project requires them, but in single-shear connections with one row of bolts a washer goes under both the head and the nut. On sloping surfaces, such as the inner faces of I-beam and channel flanges, taper washers are needed: the standard calls for them at a slope above 1/20 for bolts up to 20 mm and above 1/30 for larger ones. In preloading assemblies washers are taken to the assembly standards: a chamfered washer under the bolt head, chamfer towards the head; for bolts 8.8 the washer goes under the part that is turned, for 10.9 under both head and nut, and the standard sets out the exceptions separately.

Against self-loosening, in calculated connections with class A, B and C bolts, except in auxiliary structures, DBN requires measures to be provided: spring washers, extra nuts and others (cl. 16.2.6). The standard forbids adding locking devices to preloading assemblies unless the project states otherwise. Bolts and nuts are not welded, except special weld nuts and weld studs.

Tightening methods

The reference torque Mr is determined for each combination of bolt and nut. It is taken either from the k value declared by the assembly manufacturer (for class K2 the torque is k multiplied by the bolt diameter and the preload force; for K1 the factor is 0.125), or from on-site calibration under Annex H of the standard. A value «from memory» will not do: the torque depends on the bolt-nut pair, the lubrication and the coating.

DSTU EN 1090-2:2019 names four methods and allows each of them unless the project restricts the choice. Tightening goes gradually from the stiffest part of the connection to the least stiff; several cycles may be needed to even out the preload. Before preloading the residual gap between the steel parts must not exceed 2 mm. In Table 16.5 DBN provides for checking the preload by tightening torque and by nut rotation angle.

After cl. 8.5 and Table 19 of DSTU EN 1090-2:2019. The K class is the calibration state of the assembly as delivered (EN 14399). Final torque and angle values in a project are taken from the assembly manufacturer’s documents and the standard’s tables.

Thickness difference, packings and re-tightening

Individual elements of the pack must not differ in thickness by more than 2 mm, and in preloaded connections by more than 1 mm. If the difference is removed with steel packings, they must be at least 1 mm thick, and no more than three are used. The packings are of steel whose corrosion behaviour and strength are no worse than those of the plates being joined.

An assembly that has been tightened to the minimum preload and then loosened is removed and scrapped as a whole. Bolts that were only used to fit the connection up without being tightened to the minimum value are suitable for final assembly. Preloading assemblies are used as delivered, without changing the lubricant, unless DTI is chosen or calibration is done under Annex H.

Contact surfaces: preparation and friction coefficient

Friction depends on the state of the metal under the bolts: an as-rolled surface gives the lowest coefficient, blast cleaning the highest. So the project states not «clean» but the treatment method and the area of the contact surfaces. Below are two sets of values from two documents. They belong to different systems: the friction coefficient in the calculation is taken from the document the calculation was done by, and the treatment method in the KMD is written to match it.

DBN V.2.6-198, Table 16.5, preload checked by tightening torque. When checked by nut rotation angle γh is multiplied by 0.9. Other treatments are permitted if they give a friction coefficient not lower than those shown.

Friction surface classes under DSTU EN 1090-2

The execution standard divides surfaces into classes A–D. The treatments that give a minimum friction coefficient without testing are collected in the table. For treatments not in it, the coefficient is determined by testing under Annex G.

DSTU EN 1090-2:2019, Table 17. Unless an equivalent abrasive process is confirmed, hot-dip galvanized surfaces are blast-cleaned to EN 15773.

What is done to the surface before assembly

Contact surfaces are cleaned of oil, dirt and paint, and burrs that stop the parts fitting tightly are removed. Uncoated surfaces are cleaned of rust layers and loose deposits without smoothing the roughness. Zones around the perimeter of the connection are left untreated until the inspection is complete. There must be no thick-film coatings between the washers and the contact surfaces.

If the contact surfaces are to be galvanized, say so before galvanizing. Lap joints and narrow gaps need venting during galvanizing, see design for hot-dip galvanizing and preparation for galvanizing; treatment of the contact surfaces after galvanizing is agreed separately.

Galvanized bolts and assemblies

Bolts and nuts galvanized together with the structure and preloading assemblies galvanized by the assembly manufacturer relate to the joint differently. The thread, classes 8.8, 10.9 and 12.9, the hydrogen risk and galvanizing to ISO 10684 are covered in the article Fastener strength classes. Here only the consequences for the joint.

The design resistances of bolts 8.8 and 10.9 in DBN are given for uncoated bolts, so a galvanized bolt is justified separately in the calculation. The execution standard requires hot-dip galvanized coatings on fasteners to meet EN ISO 10684 and electroplated ones EN ISO 4042, and separately warns of the risk of hydrogen embrittlement of class 10.9 assemblies during electroplating and hot-dip galvanizing.

The calibration of an assembly (K class) applies to the bolt-nut pair as delivered. So assemblies are not taken apart, nuts from different batches are not swapped, the lubricant is not changed. A joint with galvanized parts is assembled after galvanizing, not before, as described in the article on design for galvanizing.

Inspection of tightening

All non-preloaded connections are visually inspected after tightening in the design position. If bolts are missing, the fit of the elements is checked after they are installed. If the cause is a thickness difference in the pack beyond the limit, the connection is reworked.

In preloaded connections the contact surfaces are inspected immediately before assembly, and the pack after the initial tightening and before preloading. For execution classes EXC2–EXC4 the installation procedure and the calibration certificates of the torque wrenches are checked. The execution class is set by the project under Annex U of DBN: it depends on the consequence class, the category of the structure and the type of loading.

After tightening, bolts are inspected by sampling. For EXC2 this is 5 % for the second stage (torque method, combined method, DTI). For EXC3 and EXC4 it is 5 % for the first and 10 % for the second stage of the combined method, and 10 % for the second stage of the torque method and DTI. The sample must cover different connection types, batches, equipment and operators; if a defect is found, all bolts of the subgroup are checked.

After cl. 12.5 of DSTU EN 1090-2:2019. The test torque and angle limits in the standard’s table are supplemented by the assembly manufacturer’s documents.

Common mistakes

• mixed assemblies: a bolt with a nut of another system or manufacturer, an HR assembly with an HV nut, an 8.8 bolt with a lower-class nut. For preloading, an assembly from one batch as delivered is used, and the nut class is matched to the bolt;
• tightening «by eye»: no torque, angle or indicator, the wrench is not calibrated. Overtightening snaps the bolt or crushes the plates, undertightening leaves the connection without its design friction;
• reusing high-strength bolts: an assembly that has been brought to the minimum preload and loosened does not go back into service. Bolts that were only used for fitting up are fine;
• washers: none under the nut or head, a chamfered washer turned the wrong way, plate washers in oversize holes thinner than 4 mm, no taper washers on I-beam and channel flanges;
• an unground weld or burr near a hole: the head or washer sits crooked, the pack is not clamped over its full area, and in a slip-resistant connection the contact area is lost. Welds near holes and on contact surfaces are dressed before assembly, burrs are removed from holes;
• thread in the shear plane: the bolt length was not checked against the pack thickness, and the thread fell into the shear;
• punched holes without reaming in a slip-resistant connection, slotted or oversize holes without plate washers, a hole that missed by a few millimetres and a bolt driven in by force. The bolt must enter the assembled pack easily;
• a gap in the pack above 2 mm before preloading or a thickness difference between plates without packings;
• a nut welded on «so that it does not turn»: the standard does not allow this unless the project says so.

What to state in the KMD

The fabricator sees a bolted joint only as it is drawn. DBN requires the working documentation (KM) to state the strength and accuracy class of the bolts, the method of preparing contact surfaces for slip-resistant connections, the controlled preload value, which connections are made in the shop and which on site, and where needed the sequence of installing the bolts and the inspection methods (cl. 5.1.3). Amendment No. 1 adds the requirement to state the execution class EXC (cl. 5.1.5, Annex U).

Related material

How schedules and designations are drawn up on drawings is covered in the article Drawing rules for KM and KMD. Sizes and masses of fasteners are in the fastener range tables, anchor bolts in the article «Anchor bolts for poles and masts», welded connections in the article on weld joint types, the steel grade in the article on steel grade selection, shop checks in the article Quality control of steel structures.

We prepare KM and KMD drawings (design) and fabricate steel structures to the customer’s drawings (fabrication). In the shop we check the position and diameters of holes and the squareness of flanges. We supply the fasteners for the joints together with the structures; the class, assembly standard, diameters and quantity are taken from the schedule of erection items. Erection, including bolt tightening and preload inspection, is done by our partner construction company following the project instructions.

Standards and validity

Bolted connections are designed under DBN V.2.6-198:2014 «Steel structures. Design standards» with Amendment No. 1 (approved by order of the Ministry of Regional Development of 30.12.2021 No. 365, in force from 01.09.2022; sections 5, 6, 7, 16 and 20, Tables 16.3 and 16.5, Annex U). The execution of structures, including holes, bolting assemblies, preloading and inspection, is covered by DSTU EN 1090-2:2019 «Execution of steel structures and aluminium structures. Part 2: Technical requirements for steel structures» (EN 1090-2:2018, IDT; in force from 01.07.2020, replaced DSTU B EN 1090-2:2014).

DSTU B V.2.6-199:2014 (fabrication), DSTU B V.2.6-200:2014 (erection) and DSTU-N B A.3.1-21:2013 (erection connections on high-strength bolts) were cancelled from 02.02.2026 (order of SE «UkrNDNC» of 29.01.2026 No. 12). References to them in old projects and in the original text of DBN no longer apply; execution and erection are oriented on DSTU EN 1090-2:2019.

Preloading assemblies are covered by the EN 14399 series, non-preloaded assemblies by the EN 15048 series; DSTU EN 1090-2:2019 refers to both. The Ukrainian editions of the parts of these series are checked before they are cited in a project; parts DSTU EN 14399-1, -3 and -4 are named in the article on fastener strength classes. The figures in this article are for orientation; a project uses the current text of DBN and DSTU EN 1090-2:2019.