Eurocode key design concepts
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The material-specific Eurocodes are all based on a common foundation found in EC0 and EC1. Steel designers have been using BS5950 since 1985 so limit state design is nothing new, but for timber design it is. In the permissible stress codes BS449/BS5268 used in SuperBeam, a factory of safety is provided by setting permissible stresses that are well below those at which failure would occur. In BS5950, in use for limit state steel design since 1985, the stresses uses approximate to those at which failure would occur and a factor (generally 1.4 for dead loads, 1.6 for live loads) is applied to each loads to provide a factor of safety.
Eurocode 0 provides two alternative approaches. The more conservative one uses equation 6.10 - dead loads are factored by 1.35 and live loads by 1.50. Alternatively the factors set out in 6.10a or 6.10b can be calculated - the higher being used in calculations. Both will give a lower design load than 6.10 if the member is subject to a mixture of dead and live loads: typically the factoring will be (1.35 dead + 1.05 live) [6.10a] or (1.25 dead + 1.50 live) [6.10b] so most designers will opt for this second option. EuroBeam offers a project option of designing to 6.10 or 6.10a/b; if the latter is selected the worst case is calculated by the program.
Simplifications incorporated in EuroBeam
- Loadings: All live loads are treated as being primary loads. This is conservative: Eurocode equations 6.10a/b allow a reduction factor to be applied where there is more than one type of imposed load on a member and both/all loads are not likely to act at one time (e.q. snow and maintenance access).
- Beam deflection: To simplify calculations and to make checking easier, all beam span deflections are now calculated at mid-span.
What effect does this have on member sizes?
Designing to Eurocodes using equations 6.10a/b may reduce the required member size as a lower factor is being applied to loads (usually 6.10b 1.25/1.5 instead of 1.4/1.6 [BS5950] or around 1.5 [BS449/BS5268]). Note that changing code doesn't make the member any stronger: for beams, applying a load of x will cause a deflection of y so reducing member size may require closer attention to deflection. Codes compared
