Eurocode key design concepts

The key thing to remember!

In his courses Dr David Brown, Steel Construction Institute Associate Director, often reminds his students that "you can't make a steel beam stronger by changing code". If a beam deflects by 9mm under a 50kN UDL, it will deflect by 9mm under such a load regardless of the design code used. If a beam is happy carrying 30kN but collapses when the load is increased to 50kN, changing code won't change this. A different code may more accurately model what is happening in a member allowing the design load to be increased whilst maintaining an acceptable factor of safety but the beam isn't any stronger using the new code. Introduction to Eurocode 3 video lecture by David Brown here.

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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.

Comparison of load factors

Live load %
0% 25% 50% 75% 100%
BS5950 1.40 1.45 1.50 1.55 1.60
EC 6.10 1.350 1.389 1.425 1.463 1.500
EC 6.10a/b   1.350 a   1.312 b   1.374 b   1.437 b   1.500 b
As % of BS5950 96.4% 90.5% 91.6% 92.7% 93.8%

 

Simplifications incorporated in EuroBeam

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

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