Background to loadings

1. INTRODUCTION

Structures are subject directly to loads from various sources. These loads are referred to as direct actions and include gravity and environmental effects, such as wind and snow. In addition, deformations may be imposed on a structure, for instance due to settlement or thermal expansion. These 'loads' are indirect actions. In applying any quantitative approach to structural analysis, the magnitudes of the actions need to be identified. Furthermore, if the structure is to perform satisfactorily throughout its design life, the nature of the loads should be understood and appropriate measures taken to avoid problems of, for instance, fatigue or vibration.

The magnitude of loads cannot be determined precisely. In some cases, for instance in considering loads due to the self-weight of the structure, it might be thought that values can be calculated fairly accurately. In other cases, such as wind loads, it is only possible to estimate likely levels of load. The estimate can be based on observation of previous conditions and applying a probabilistic approach to predict maximum effects which might occur within the design life of the structure. (In fact, the extensive wind records which are now available mean that wind loads can often be predicted with greater accuracy than self-weight). Loads associated with the use of the structure can only be estimated based on the nature of usage. Insufficient data is available in most cases for a fully statistical approach and nominal values are therefore assigned. In addition, problems of change of use and fashion can occur.

In analysing structures it is rare to consider all loadings acting simultaneously. This approach may be because the most severe condition for parts of the structure occurs when some other combination of load is considered. Alternatively, it may be that the possibility of such a condition actually occurring is extremely small. However, the risk of coexistence of apparently unrelated loads may be greater than is first imagined. Correlations can be produced from unexpected sources or from coincidences which, although physically unconnected, are temporarily connected. For example, floor and wind loads would normally be considered unrelated. However, in hurricane areas residents on the coast might be expected to move their ground floor contents to upper floors if a hurricane warning, with associated tidal surge, were given. This circumstance could very easily produce extreme floor loads in combination with extreme wind loads. This case may be a very special one, but there are others. The risk of fire may not be considered correlated with high wind loads, yet in many parts of the world high winds are more likely in winter, which is also the period of greatest fire risk.

For these reasons, it is convenient to consider loads under various categories. The categories can then be ascribed different safety factors and applied in various combinations as required. Traditionally, loadings have been classified as dead, superimposed, and environmental loads. These classes include a wide range of gravity effects, seismic action, pressures due to retained material or liquids, temperature induced movement, and, for marine structures, water movement. The Eurocodes on actions and steelwork design [1, 2, 3] classify loads and other actions as permanent, variable, and accidental. These classes of action will be considered in more detail in the following sections.

In limit state design, characteristic values of actions are used as the basis of all calculations. They are values which statistically have only a small probability of being exceeded during the life of the structure. To provide a margin of safety, particularly against collapse, partial safety factors are applied to these characteristic values to obtain design quantities. In principle, different partial safety factors can be applied depending on the degree of uncertainty or variability of a particular type of action. In practice, whilst this appears to be the case, the actual values of partial safety factors used incorporate significant elements of the global safety factor and do not represent a rigorous probabilistic treatment of the uncertainties of the actions.