Structural Engineer subscribers are seeking assurance that NBS is keeping pace with the release of Eurocodes. Consultant Engineer and NBS freelance author, Ted Wallis, strikes the NBS formwork and assesses the position as of May 2009.
Eurocodes affect the specification of building structure and structural cladding in two respects:
- Where detailed design is by the Contractor or supplier, the Employer's Engineer must provide design requirements to the appropriate Eurocode, with sufficient information concerning the structural use of the element and its environment to ensure its adequacy for the working life of the building
- For the completed work to be Eurocode compliant, structural materials and execution must comply with the appropriate Eurocode, whether the design is prepared by the Employer's Engineer, the Contractor or supplier.
Generally, as National Annexes for Eurocodes have become available, the main NBS structural sections have been updated to give the specifier the option of specifying to the existing British Standard or to Eurocode; also, CEN (BS EN) standards for materials and execution that replace British Standards have been incorporated into NBS as they are published. However, due to the extended time over which CEN documents have been prepared and the consequent unstructured way in which UK versions have become available, there are gaps in the NBS coverage.
The following steps are being taken to rectify this situation:
- All CEN standards referenced in Eurocodes, and the associated execution standards, are under review to identify those that include choices for the specifier. We then ensure that these choices can be specified in the appropriate NBS work sections
- A new NBS structural section is in preparation to cover general requirements of Eurocodes for design and construction. This will include relevant criteria from Eurocode 0 (BS EN 1990) – Basis of design and items such as loading from Eurocode 1 (BS EN 1991) – Actions on structures that may be referenced from a number of NBS work sections
- A 'Eurocode' clause will be included at the start of each relevant NBS work section, the use of which will make compliance with Eurocodes and the new NBS structural section mandatory.
The situation with respect to Eurocodes 2 to 8 is as follows:
Eurocode 2 (BS EN 1992) – Design of concrete structures: The execution standard for concrete (BS EN 13670) has yet to be published but, in the meantime, the background paper to Eurocode 2 (PD 6687) indicates that 'In the UK, the provisions of the 'National Structural Concrete Specification for Building Construction' (NSCS) are considered equivalent to those in DD ENV 13670 for tolerance class 1 and therefore could be used in conjunction with BS EN 1992'.
In NBS the E sections cover the requirements of concrete similarly to NSCS, and clause E05/290 gives as an insert for element shape and position 'To Section 7 of the 'National Structural Concrete Specification for Building Construction'. With this insert, the requirements of NBS should not be dissimilar to those of NSCS.
PD 6687 also indicates that 'DD ENV 13670 does not apply to the production of precast concrete elements made in accordance with product standards', the implication being that precast concrete should comply with the relevant product standard. Hence, where appropriate, NBS will make reference to these standards.
Eurocode 3 (BS EN 1993) – Design of steel structures: NBS section G10 relies upon the 'National Structural Steelwork Specification' (NSSS), and is therefore as current as the NSSS.
A number of reference standards in Eurocode 3 had not been published when the current 5th edition of NSSS was prepared, including the standard for execution BS EN 1090-2. However, if specifiers wish to give precedence to BS EN 1090-2 or substitute material reference standards for those currently in NSSS, this can be done in the item 'Additional requirements' in clause G10/125.
Other NBS sections specifying steel cladding, staircases, etc. are not to NSSS and will need more careful consideration.
Eurocode 4 (BS EN 1994) – Design of composite steel and concrete structures: This relies on Eurocodes 2 and 3 for material requirements, hence NBS currency with this Eurocode is as described above.
Eurocode 5 (BS EN 1995) – Design of timber structures: There is no execution standard associated with this code, but fabrication, erection and control requirements are given in Section 10. Generally, CEN material standards have been incorporated in NBS section G20.It is understood that when PD 6693 Complementary information for use with Eurocode 5 is available it will reference complementary non-contradictory information in BS 5268.
Eurocode 6 (BS EN 1996) – Design of masonry structures: Requirements for execution are included in Section 9. Generally, CEN material standards have been included in NBS F sections and Z21.
Eurocode 7 (BS EN 1997) – Geotechnical design: A significant change is the mandatory check of soil and rock properties during construction to justify the design assumptions. This has been covered briefly in relevant NBS work sections as they have been updated and more fully in section C11. These requirements will be transferred to the new NBS structural section, together with requirements for design and construction reports.
Standards for testing in NBS section C11 have generally been updated as the CEN replacements became available, as have material standards for piles. Reference is made where appropriate to standards for execution of special geotechnical work, but a full compatibility check has yet to be carried out.
Specification for piling in NBS sections D30, D40 and D50 relies upon 'Specification for piling and embedded retaining walls' (SPERW) and each section includes the opportunity for the specifier to substitute new CEN standards as they become available.
Eurocode 8 (BS EN 1998) – Design of structures for earthquake resistance: It is assumed that these requirements will only apply to a small number of buildings constructed in unfavourable situations where failure would have grave consequences for human life and society, or high economic and environmental cost. Hence, there are no plans to include these requirements in NBS.