Self-weight and imposed load

Self-weight

The weight of the structure and the layers on it. One of the best-known actions, so it gets a small partial factor.

Where it is an advantage

• Foundations: less load, a smaller foundation, a cheaper solution — a serious item on poor ground.
• Rooftop additions: an existing building can often take exactly as much as a light structure adds. Rooftop additions.
• Seismic action: the force from an earthquake is proportional to mass, so it is smaller for a light building.

Where it has to be managed

• Uplift: self-weight would work against wind suction, but there is little of it. Hence the need for strong anchorage.
• Floor vibration: low mass gives a more sensitive floor.
• Sound insulation: mass is one of the bases of airborne sound reduction, so what the structure does not give has to be made up in the build-up. Sound insulation basics.

Imposed load

The load from use: people, furniture, stored goods. The standard gives values by use — different for dwellings, offices and storage.

What to know in practice:

A change of use is a change of load

If a living room becomes an office, a store or a plant room, the load can change. That is a structural question as well as a permitting one.

Concentrated loads are a separate question

A heavy piece of furniture, an aquarium, a plant item is not covered by the general imposed load. If such a thing is expected, say so in advance.

The weight of the walls

The weight of partitions is a load too — and if the occupant later builds a wall, the structure did not necessarily allow for it.

What matters in this for the occupant

A light gauge steel building is not “weaker”, but the actions are defined more precisely, because there is no large reserve from mass. So ask the designer before a major alteration or a heavy installation.

The point

The low self-weight is one of the technology’s greatest advantages — and precisely why uplift and vibration need separate attention.