Specification of bridge steelwork

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This article provides guidance on the specification of bridge steelwork in the UK, for bridges designed in accordance with the Eurocodes.

The purpose of a structural steelwork specification is to state what materials and products should be used and how work (fabrication and erection) should be carried out, in order to ensure that the completed structure meets the designer’s assumptions and the client’s needs.

The principal document for the technical requirements for bridge steelwork is BS EN 1090-2[1] but the complete ‘execution specification’ comprises a range of contractual documents, including drawings, a general specification with technical requirements for supply of materials and products and for workmanship, and project-specific requirements for the individual structure. Project-specific requirements may be influenced by client authority requirements such as the Specification for Highway Works[2]

Contents

[top] European standards for steelwork

Rules for the design of steelwork are given in Eurocode 3 Design of steel structures. Within Part 1-1[3] of that Eurocode, the following basic assumption is stated: "fabrication and erection complies with EN 1090" (this assumption is maintained in Eurocode 3 Part 2[4]).

BS EN 1090, Execution of steel structures and aluminium structures, consists of three parts, of which two are relevant for steel bridges:

  • Part 1: Requirements for conformity assessment of structural components[5]
  • Part 2: Technical requirements for steel structures[1]

Conformity with the requirements in Part 1[5] (which are general statements, referring to the other Parts for detailed technical requirements), when formally declared, permits CE marking of the structure.

The technical requirements in Part 2[1] are comprehensive, covering all types of structural steel (including stainless steel) and types of structure (buildings, bridges, masts, cranes etc.). It includes references to European standards for products, welding, testing and corrosion protection. Products manufactured in accordance with harmonized European Standards are individually CE-marked.

[top] Model Project Specification

Because BS EN 1090-2[1] is a comprehensive document, for all steel products and types of structure, it includes numerous instances where options need to be selected or alternatives specified. A summary list of all the options and alternatives is included in an Annex to that Part: many of them are irrelevant for bridge structures.

It is therefore necessary to draw up a set of project-specific requirements that supplement BS EN 1090-2[1]. For bridge structures in the UK, the Steel Bridge Group have developed a ‘Model Project Specification’ (MPS) that offers clauses suitable for most bridge projects; only minor adjustments to that are needed to that model for most bridge projects.

The clauses in the MPS include prompts for the inclusion of detailed project-specific information and each clause is accompanied by a commentary that informs the specifier of the need for the clause and what other considerations need to be made. Numerous references are made in the commentary to Guidance Notes within SCI P185; that publication was developed specifically to provide designers and specifiers with relevant information about the construction of steelwork in general and about bridges in particular.

[top] Particular requirements addressed within the MPS

[top] Execution class

BS EN 1090-2[1] specifies four different ‘execution classes’, for which the strictness of the requirements (particularly for welded details) increases from EXC1 to EXC4. The classes may be defined either for the structure as a whole or for parts of the structure.

For most bridge structures, the MPS recommends the adoption of EXC3 for the whole structure. This is adequate for most details in ‘ordinary’ highway bridges and the adoption of a universal class offers simplicity, clarity and economy.

[top] Constituent products

The MPS expects that the grade and quality of most products will be given on drawings, with reference to the product standards that are listed within the reference standards in BS EN 1090-2[1]. Some particular requirements, such as surface conditions, are recommended, along with requirements for traceability (bridge steelwork generally requires traceability by piece for the principal components).

[top] Preparation and assembly

Preparation and assembly requirements are of special concern for fatigue-susceptible structures such as bridges. The MPS gives requirements related to the condition of surfaces and edges.

[top] Welding

Weld sizes and types are generally specified on drawings by symbols in accordance with the relevant standard. Welding is a relatively expensive operation and weld sizes should not be over-specified.

The MPS clarifies where pre-production tests are needed, such as for shear stud connectors, the requirements for tack welds and temporary attachment (which can have a detrimental affect on fatigue life) and where additional operations such as grinding flush are required. Generally, the weld acceptance levels for EXC3 are sufficient for most fatigue details but should more onerous criteria be needed, provision is made for adding appropriate requirements. Guidance on fitness-for-purpose specification of workmanship and acceptance criteria are given in PD 6705-2[6].

[top] Bolting

Preloaded bolts are almost always used in bridges. Where non-preloaded bolts are used, the MPS requires the use of locking devices.

BS EN 1090-2[1] does not provide requirements for one of the tightening methods commonly used in the UK, the part-turn method (the ‘requirements’ for the 'combined method’ require a higher bedding torque and require greater classification of bolt properties than in commonly available bolts in the UK and are thus not applicable for the part-turn method). The MPS includes additional requirements for the part-turn method, which have been validated for bolts to BS EN 14399-3[7].

BCSA publish a Model Specification for the Purchase of Structural Bolting Assemblies and Holding Down Bolts.

[top] Erection

For bridges, the erection method usually has a significant influence on the final state of stress of the structure, as well as necessitating consideration of the stability in the part-erected condition. An erection method statement is therefore an essential part of contract documentation. The MPS includes a number of clauses to clarify the requirements for temporary bracing and the dimensional control that is to be exercised.

General guidance on erection methods is given in the BCSA guide to the erection of steel bridges.

[top] Surface treatment

To protect against corrosion, bridge steelwork is usually either painted or weathering steel is used. In either case, BS EN 1090-2[1] covers the required surface preparation. The MPS confirms the preparation grade that is needed and offers additional requirements for sealing of spaces.

Where painting is called for, the requirements of the corrosion protection system are to be stated as performance requirements. Reference may be made to Series 1900 of the Specification for Highway Works[8] and the associated Notes for Guidance[9]

[top] Tolerances

Essential and functional tolerances are generally given in an Annex to BS EN 1090-2[1]. Specific additional requirements for bridge structures are given in the MPS.

[top] Inspection and testing

Generally, the scope of inspection for EXC3 will be sufficient but where additional testing is appropriate, the MPS provides the means to specify it. Production tests on headed shear stud connectors (not covered in BS EN 1090-2[1]) are specified.

[top] Quality assurance

BS EN 1090-2[1] lists a number of procedures that must be documented for classes EXC2 and higher. It also provides the option to specify a quality plan for the works. It is usual to require a quality plan for bridge structures and this is confirmed in the MPS.

BS EN 1090-1[5] states that conformity with the Standard (both Parts 1 and 2, in the case of steel structures) is demonstrated by initial type testing (ITT) and factory production control (FPC). Where these have been demonstrated, the steelwork may be CE marked. For bridge steelwork, the characteristics of the constituent products will normally have been demonstrated by production in accordance with product standards (and thus ITT has been demonstrated). A FPC system conforming to the requirements of BS EN ISO 9001[10] is considered satisfactory for conformity (although other systems could also be satisfactory).

BS EN ISO 9001[10] gives requirements for quality management systems. This Standard may be used in any industry. The Highways Agency (HA) has promoted development of a series of National Highways Sector Schemes (NHSS) to aid certification bodies in interpreting the requirements of BS EN ISO 9001[10] for particular sectors of activity for transportation infrastructure. The BCSA and the Steel Construction Certification Scheme (SCCS) have been active in assisting the HA in developing schemes for structural steelwork (NHSS 20[11]) and for corrosion protection (NHSS 19A[12]).

In addition, BCSA operates the Register of Qualified Steelwork Contractors for Bridgeworks (RQSC). A company qualifies for inclusion after being assessed by specialists who check the company’s financial and technical resources as well as track record and employed personnel. Companies are categorised in terms of the work they can execute technically, and the maximum size of contract they should handle.

For highway structures, the Highways Agency requires certification in accordance with NHSS 20[11]. The SCCS is accredited for this certification. In addition, on Highways Agency contracts it is mandatory that only companies listed on the RQSC for Bridgeworks for the type and value of work to be undertaken will be employed for the fabrication and erection of bridgeworks.

[top] References

  1. ^ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 BS EN 1090-2:2008+A1:2011 Execution of steel structures and aluminium structures. Technical requirements for steel structures. BSI
  2. ^ Manual of Contract Documents for Highway Works (MCHW). Volume 1: Specification for Highway Works. Series 1800 Structural Steelwork. The Highways Agency
  3. ^ BS EN 1993-1-1:2006. Eurocode 3: Design of steel structures, General rules and rules for buildings. BSI
  4. ^ BS EN 1993-2:2006. Eurocode 3: Design of steel structures, Steel Bridges. BSI
  5. ^ 5.0 5.1 5.2 BS EN 1090-1:2009+A1:2011 Execution of steel structures and aluminium structures. Requirements for conformity assessment of structural components. BSI
  6. ^ PD 6705-2:2010+A1:2013. Structural use of steel and aluminium. Part 2: Recommendations for the execution of steel bridges to BS EN 1090-2. BSI
  7. ^ BS EN 14399-3:2005. High-strength structural bolting for preloading. System HR. Hexagon bolt and nut assemblies. BSI
  8. ^ Manual of Contract Documents for Highway Works (MCHW). Volume 1: Specification for Highway Works. Series 1900 Protection of Steelwork Against Corrosion. The Highways Agency
  9. ^ Manual of Contract Documents for Highway Works (MCHW). Volume 2: Notes for Guidance on the Specification for Highway Works. Series NG 1900 Protection of Steelwork Against Corrosion. The Highways Agency
  10. ^ 10.0 10.1 10.2 BS EN ISO 9001:2008. Quality management systems. Requirements. BSI
  11. ^ 11.0 11.1 NHSS 20: National Highways Sector Schemes for Quality Management in Highway Works, 20. The execution of steelwork in transportation infrastructure assets, UKAS, Issue 5, July 2013
  12. ^ NHSS 19A: National Highways Sector Schemes for Quality Management in Highway Works, 19A. For corrosion protection of ferrous materials by industrial coatings. UKAS, Issue 7, November 2013

[top] Resources

[top] See also

[top] External Links