Characteristic loads

The anticipated load levels due to self weight, contents and users, snow and wind are called characteristic loads.

Buildings will be subject to loads from various sources. The principal ones can be classified as dead load, imposed load and wind load, as described below. In some cases structures may be subject to other loads, such as those due to earthquakes or pressures from retained material. The expected maximum magnitude of each is referred to as the characteristic load.

Dead loads are those representing the self weight of the building; their magnitude can be estimated on the basis of material densities and component sizes.

Dead loads are those due to the self weight of the structure and any permanent fittings and finishes. It is generally possible to quantify the magnitude of dead loads with a reasonable degree of confidence. This is based upon the size of various components and the density of different materials as scheduled in BS 6339.

Imposed loads are those associated with occupation and use of the building; their magnitude is less clearly defined and is generally related to the use of the building.

Imposed loads are those which arise as the result of the use of the building. Personnel, storage, furniture all constitute loads under this heading. The magnitude of such loads cannot be quantified with any precision and it is therefore necessary to make some allowance based upon the type of activity to be performed within the building. Details are listed in BS 6339.

Wind loads can be calculated based on maximum likely wind speeds and the size and shape of the building

The magnitude of wind loads is closely related to wind speed but also depends upon a number of factors associated with the shape and dimensions of the building and its local environment. The design wind speed represents the maximum likely speed at a particular geographical location and is based upon statistical data. This may be modified if, for instance, the building site is particularly sheltered, such as may be the case in a city centre, or exposed, such as on the coast. The effect of this wind condition in terms of forces on the structure is related to the building geometry and is expressed as wind pressure coefficients for the walls and roof. Details of the method for determining both design wind speed and pressure coefficients are contained in BS6399 Part 2.