# Partial factors in standards and guidance

# Partial factors

This article attempts to summarise the partial factors referred to in assessment codes and guidance. It is a work in progress.

## Document history

- Initial version, 2022-08-29 - only covers ɣ
_{f3}

## Partial factors

Symbol | Used in | Partial Factor Name | Purpose |

ɣ _{f3} |
CS454, C800 | Load effects | To take account of inaccurate assessment of the variable action effects |

ɣ _{G} |
C800 | Permanent actions | |

ɣ _{Q} |
C800 | Variable unfavourable actions | |

ɣ _{IMP} |
C800 | Impact factor | |

ɣ _{m} |
CS454, C800 | Material (masonry) strength | |

ɣ _{fL,[SLS|ULS]} |
CS454, C800 |
Partial factor on "actions" at serviceability or ultimate limit state | Generally > 1.0 if and only if this gives a worse result than taking the value as 1.0. The same symbol is used for partial factors on all actions, without any accepted system for disambiguation. |

## Partial factor on assessment actions ɣ_{f3}

From CS454:

ɣ_{f3}is a factor that takes account of inaccurate assessment of the effects of actions such as unforeseen stress distribution in the structure, inherent inaccuracies in the calculation model, and variations in the dimensional accuracy from measured values.

Per CS454, the value of ɣ _{f3} for masonry bridges is to be 1.0 at both SLS and ULS.

Ciria C800 takes a different view, suggesting that ɣ _{f3} should be 1.0 at SLS (which it calls PLS), and 1.2 at ULS for "level 1" assessment. The values are then 1.1 and 1.0 for level 2 and level 3 assessment. No justification is offered for these numbers, or for the deviation from the recently released normative standard.

Ciria C800 also suggests values of 1.3 for level 1 and 1.2 for level 2 assessment of skew bridges with a skew angle greater than 30 degrees. It is not clear whether values are proposed for PLS as well as ULS. Note that CS454 advises that for skew angles *up to* 30 degrees, skew bridges can be assessed as a square bridge with the skew span (and, presumably, skew shape). Revision 0 of CS454 actually gave a factor that increased assessed capacity from this result. The additional factor in C800 allows for bridges with even greater skew angles to be assessed as square bridges with the skew span, but the factor works in the opposite direction (that is, it reduces assessed capacity). Is this factor suitable even for very high skew angles?!