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According to Eurocodes, we multiply designed loads by safety factors to account for non idealities and uncertainties. When multiple load types affect a structure, the corrected loads are summed and the sum is multiplied again by a load combination specific factor. Why do we need to correct the sum as well?

For example, we may have a live load and a snow load on a structure. Both have their own safety factors, but why does their combination have one as well? Aren't the uncertainties included already in the individual factors for each load?

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  • $\begingroup$ Can you please clarify specifically which factor, you are referring to? There is nothing in EN1990 (6.10b) that matches your description, so maybe you are referring to a NA-specific factor? $\endgroup$
    – ingenørd
    Jun 6 '20 at 14:17
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    $\begingroup$ Purely conceptually (I have not worked with the Eurocodes only the USA LRFD), load factors and combinations get into probabilities. It's quite likely that at some point in its service life a building will experience high live load. Same for a high snow load. It's less likely that peak live and peak snow load will happen simultaneously. Hence, adjusted factors. $\endgroup$
    – CableStay
    Jun 6 '20 at 18:30
  • $\begingroup$ @CableStay, in the Eurocodes that is handled by the $\psi$-factors which are specific to each load type and therefore multiplied unto each load before summing them all, so they don't match the description in the question. $\endgroup$
    – ingenørd
    Jun 7 '20 at 4:45
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Your question seems not specific however what I understand that you may be referring to the additional factors applied to variable actions or accompanying actions as mentioned in equation 6.10, 6.10a or b of EN 1990.

In this regard, you may please refer the said equations wherein you may see an additional factor termed as representative value factor denoted by “psi” used to calculate design value of variable/ accidental actions as shown below:

Design value of action=(partial safety factor) X (representative value of action)

Further, Representative value of action=(representative value factor) X (characteristic value of action)

Representative value factor is denoted by “psi” in all equations and the value for same may be picked from Table A1.1 of Annex A1 of EN1990

Due to above reason, you may see an additional factor

If above is not the case then please specify your query

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