Chris Hendy - Steel concrete composite bridge construction - an example
CH Chris Hendy
CH There are more documents that you need to use, unfortunately, but you do get used to that very quickly and as I mentioned earlier they are structured in such a way that if you find your one way around one of the documents you will be able to find your way around the others because the section headings all tend to be the same. I think the most extreme example of needing a lot of codes of practice is still concrete composite bridge construction and the reason for that is obviously you need the standards for concrete and for steel and then also the steel concrete code which ties it all together. In that example you do need typically around 20 codes of practice. Just very quickly giving you an idea of the sort of codes that we have to use, EN [?] 1990, which is basis of design, is required no matter what material you’re designing because that sets out the basic requirements for limit states and also the way you go about combining your actions together. It also sets out a lot of terminology that you need to understand for using the rest of the Eurocodes.
We then need Eurocode 1, which comes in multiple parts and I think for bridge design you typically need eight of those parts which cover things like self weights and material, thermal effects, wind loading, actions during construction, snow loading, etc. Although there are a lot of parts there the scope is really very much the same as we used to have in British practice in a smaller number of documents but the detail it goes into is greater. For example, the wind code contains all the information on quasi static gust pressures that our old standards used to have but it also contains a lot of useful information on bridge aerodynamics which you used to have to go to Highways Agency documents or research papers for; it’s now all nicely tied up in one code of practice. That’s the loading or the actions, as I should say, in Eurocode 1.
Then you’ll need Eurocode 2, for a bridge, you’ll need two parts of Eurocode 2, typically, Eurocode 2 part 1.1, which is the general rules and rules for buildings and actually that’s where most of the rules for member design that you need come from even if you’re a British designer and that’s the way the codes are structured. You need Eurocode 2 part 2 for bridges but that is really a bolt on document and it makes some bridge specific amendments, quite small amendments, but some bridge specific amendments to the general rules and then adds in rules for structures that you don’t typically find in buildings projects such as box girders and the amendments that it makes to the general rules really tend to be to do with scale effects. In a lot of the rules in the general part for buildings have been put together by testing relatively small specimens and a little bit of caution creeps in in the bridges part just because we’re extrapolating potentially up to much larger structures.
On the steel side there are a lot more parts in Eurocode 3 than there are in Eurocode 2, which is, again, a function of the number of project teams that have been used to compile the documents. Again, it’s around about eight parts that you need and once again the structure is very much similar to the concrete though in that those are general rules and rules for buildings, Eurocode 3 part 1.1. Again, that covers most of the basic member design rules that you need. We have a Eurocode 3 part 2 for bridges and in the same ways for concrete that adds in some specific rules for bridges for types of structures that aren’t covered by the buildings part and again makes some specific modifications.
Then in between those two parts, whether you’re designing a building or a bridge there are a whole series of other Eurocode parts of Eurocode 3 that you need to take note of. For example, if you’re designing a stiffener on a plate girder then you’ll require Eurocode 315, Plated Structures because that’s where the rules for stiffeners are contained. If you’ve got a bolt or a weld, as you’re always going to have in a bridge design, then you’d need Eurocode 3 part 1.8. Similarly, there are separate parts for fatigue and other aspects of design.
Finally, for resistances, if we’re doing a steel composite design then we need Eurocode 4 as well. Specifically for bridges we need Eurocode 4 part 2, which is the bridges part. In this one situation, and one situation only, Eurocode 4 part 2 is actually a stand-alone document and you don’t need to refer to Eurocode 411, which is the general rules and rules for buildings. That’s because all of the rules are duplicated from the buildings part into the bridges part and that was done especially because otherwise there were just too many cross-references in Eurocode 4 part 2 because it needed to cross refer to Eurocode 4 part 1.1 and then also to the concrete codes and to the steel codes. There were one or two clauses, which became unmanageable with cross-references so a special dispensation was given for Eurocode 4 part 2 to be a stand-alone document. Finally, finally, we need Eurocode 7 for geo-technical design and of course we need Eurocode 7 regardless of what material we’re designing. A lot more parts but they are very logically put together and once you’re familiar with one generic part you’ll very easily be able to find your way around the other parts.
You’ll find that when you start using Eurocodes they’re set out to deal with things by behaviour rather than by element type. In the old British Standards we were used to finding clauses that dealt with beams, columns, slabs. You won’t find that so much in the Eurocodes; you’ll find they’re set out by behaviour so the clauses will be bending, shear, torsion. There’s a slight difference there. In my personal opinion I think it’s actually better that way round because there used to be some confusion sometimes trying to define [?] what it was you were actually designing. In an integral bridge where maybe you have a beam but it’s heavily loaded in axial force because of the earth pressure from the embankments then what you had was normally a beam but with a considerable amount of axial force in it so was it actually a column or was it a beam? It doesn’t really matter in the Eurocodes, that distinction, it’s just an element with an axial force and a bending element [?] in so actually a lot of the time it’s easier to interpret.
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