Eurocodes: Drafts of the UK National Annexes to the Eurocode suggest a convergence with the current British Standard

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Eurocodes ‘But not long after there arose against it a tempestuous wind, called Euroclydon.’ Drafts of the UK National Annexes to the Eurocode suggest a convergence with the current British Standard. There will however be significant changes concerning determination of wind loads, which the Steel Construction Institute’s David Brown highlights. This article offers a preliminary overview of EN1991- have changed, but only to point out what significant 1-4, which covers wind actions. For UK designers, changes designers might expect in the process having only relatively recently managed the change of determining the wind loads. This can only be to BS 6399-2, the quotation from the Acts of the an introduction, as the National Annex is not yet Apostles may seem particularly appropriate as we available for public comment. Designers will have open the Eurocode. However, the main conclusion to work with the published documents as they The process will draft UK National Annex introduction!become available rather than the information in thisof this article is that the be familiar, even if appears to move the There is no equivalent to the ‘directional method’ Eurocode fairly close found in BS 6399-2. The Eurocode approach is the nomenclature to the British Standard similar to the ‘Standard method’ but will allow has changed – meaning that the an equivalent to the ‘hybrid method’ where the process will be familiar, pressure is calculated by considering different even if the nomenclature directional factors, terrain categories and distances has changed. EN1991-1-4 is already published, and from the sea around the site. its National Annex is expected to be published for public comment this year. Basic wind velocity The map has modest differences, as it is based on a Wind loads will be (even more) important 10-minute mean wind speed. BS 6399-2 was based Under the Eurocode regime, wind loads will on an hourly mean, and CP3 on 3-second gusts. For be a more important part of the design load users, this has no impact on the process. combinations than previous practice. This is not the responsibility of EN1991-1-4, which simply Terrain Categories defines the wind loads, but arises from EN1990, Although there are five categories in the Eurocode, which describes load combinations and their the National Annex reduces this to the three familiar factors. Table 1 shows a limited comparison to UK designers – sea, country and town. Designers between combinations to the British Standards may regret the opportunities denied for the and Eurocode, (including some assumptions about intermediate categories, but may rejoice that the UK the factors likely to be in the National Annex for position will be simple and familiar. EN1990.) Altitude factor Table 1: Comparison of Dead Imposed Wind The Eurocode introduces a reduction factor into the probable load combination calculation of the altitude factor, of the form (10/z)0.3 factors between British British Standards 1.4 1.6 — where z can be the height of the structure (it seems). and European Standards 1.2 1.2 1.2 This applies where z > 10m. A structure of 40m Eurocodes 1.35 1.5 0.75 height will produce a reduction factor of 0.66, which will appeal to UK designers. 1.35 1.05 1.5 When designing to the Eurocodes, wind loads will Mean wind velocity appear in most load combinations, and may have a The size of the National Annex is due in some significant impact on ‘lightweight’ designs, such as degree to the very extensive guidance to calculate portal frames. For other structures, such as offices, the mean wind velocity. Graphs are provided for the effect will be modest, and only make an impact sites in country and town terrain that will shortcut on the bracing and stability systems. the approach taken in the Eurocode. Again, the idea will be familiar to UK designers, who are now used Key Changes to discriminating between sites, allowing for the This article does not attempt to explain why things distance from the sea and allowing for the distance 32 NSC March 2006


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