Application of partial factors

Forum for the book 'Decoding Eurocode 7' by Andrew Bond and Andrew Harris

Application of partial factors

Postby crossdma » Fri Jan 30, 2009 9:32 am

Dear Andrew

I recently attended one of your training courses in London (sheet piling design on the 11th December) which has been very useful. Since, I have been making reference to your book Decoding Eurocode 7 for further guidance.

I am working on the design of retaining structures and am confused by section 11.3.4 in your book (page 358). It suggests that an action (in this case from passive earth pressure) can be multiplied or divided by partial factors depending on whether it is treated as an action or a resistance. I cannot find the same claim in either Eurocode 0 or Eurocode 7. In Eurocode 7 section 2.4.7.3.3 Design Resistances, equations 2.7a to 2.7c, only characteristic material properties are divided by the relevant partial factor.

This concerns me as I have only ever assumed that characteristic actions can be multiplied by the relevant partial factor (e.g., favourable or unfavourable). Please can you provide me with the reference in the codes for the formulae provided in section 11.3.4 of your book.

Regards
David
crossdma
 
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Re: Application of partial factors

Postby Andrew Bond » Fri Jan 30, 2009 12:36 pm

David

your book (page 358) ... suggests that an action (in this case from passive earth pressure) can be multiplied or divided by partial factors depending on whether it is treated as an action or a resistance. I cannot find the same claim in either Eurocode 0 or Eurocode 7


You have misquoted the book - what we say is passive earth pressure could be treated as an action or a resistance - you have automatically assumed that it *is* an action. If you are right, it could be regarded as favourable or unfavourable. Most engineers' initial thought is that passive earth pressure is a resistance ... but as far as the wall is concerned it looks like an action ... and for compatibility with FE analysis, it is necessary to treat it as unfavourable (which is highly counter-intuitive). You are right - Eurocodes 0 and 7 are silent on this matter.

In some countries, e.g. Germany, passive earth pressure is treated as a resistance and divided by gammaR = 1.4 (according to Design Approach 2). This is similar to traditional UK practice with a factor on passive pressure.

In Design Approach 1 (used in the UK), gammaR = 1.0 but there are factors applied to actions. If you regard passive earth pressure as a favourable action, then it would be multiplied by gammaG,fav = 1.0. If you regard it as an unfavourable action (which we recommend in our book), then the multiplier depends on which
Combination you are looking at. For Combination 2, gammaG = 1.0, but for Combination 1, gammaG = 1.35.

Why isn't this discussued in Eurocode 7? Because there was insufficient agreement on how to do these calcs for a definitive statement to be included. Even now, so-called experts disagree on the 'right' way. Hence the long discussion of this issue in our book.

Our conlucsion is that, for compatibility with finite element analysis, it is necessary to treat passive earth pressure exactly the same way as active earth pressure (invoking the 'single source principle), i.e. as an unfavourable action.

The good news is it probably doesn't affect the design outcome too much anyway, since DA1 Combination 2 often governs, and for that gammaG = gammaG,fav = gammaR = 1.0.

I hope this adds clarity to a difficult subject!

Andrew
Andrew Bond
 
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