Partial factors and load combinations

Two separate things

Partial factors

They handle uncertainty. They increase the action and reduce the resistance, so that safety is maintained given the real scatter.

The size of the factor depends on how uncertain the quantity is: self-weight is better known than wind load, so it gets a smaller factor.

Combination factors

They handle the fact that actions do not reach their maxima at once. In a combination one action is the “leading” one, the others appear at reduced, combination values.

Why several combinations have to be examined

Because it is not known in advance which will govern. A different combination can be the most severe for one member than for its neighbour:

• for a floor joist the imposed load leads,
• for a rafter the snow,
• for a shear wall the wind,
• and for an anchorage often the combination of low self-weight and high wind suction.

That last one is characteristic of light gauge steel construction: the unfavourable case here is when there is little downward load. Resisting uplift in light structures.

Favourable and unfavourable effects

An action can also be favourable: self-weight works against uplift. The standard handles that by taking a favourable action at a smaller factor — in other words it must not be relied on in full.

That logic often surprises the layperson: the same action appears at a larger value in one check and a smaller one in another, depending on whether it helps or harms.

Serviceability combinations

Different, milder combinations apply to the serviceability checks — deflection is not checked with the same actions as strength.

What it means in practice

Forming the combinations is done by software, but the designer has to check that:

• every relevant action is included,
• the favourable/unfavourable treatment is right,
• the program is working with the national annex factors.

The point

Load combinations are not a formality but the substance of design. For a light structure the governing case is often the one with little load, not the one with a lot.