Page 33 - Dynamic Loading and Design of Structures

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Table 1.3 Classification of actions.

Origin Variation in time Variation in space Limiting value Nature/Structural response
Direct Permanent Fixed Bounded Static
Indirect Variable Free Unbounded Dynamic
Accidental Quasi-static

base shear and top storey lateral deflection may represent action effects on whole structures.
An action should be described by a model, comprising one or more basic variables. For
example, the magnitude and direction of an action can both be defined as basic variables.
Sometimes an action may be introduced as a function of basic variables, in which case the
function is called an action model.
From a probabilistic point of view, the classification of actions according to their variation
in time plays an important role, and is examined in detail in the following section dealing with
the specification of characteristic and other representative values. Table 1.4 presents, in
qualitative terms, the criteria for classifying actions according to time characteristics
(Eurocode 1.1 Project Team, 1996). The variability is usually represented by the coefficient of
variation (CoV), i.e. the ratio of the standard deviation to the mean value, of the point-in-time
distribution of the action. Figure 1.9 shows schematically the three different types of action.
The distinction between static and dynamic actions is made according to the way in which a
structure responds to the action, the former being actions not causing significant acceleration
of the structure or structural elements, whereas the opposite is valid for the latter. In many
cases of codified design, the dynamic actions can be treated as static actions by taking into
account the dynamic effects by an appropriate increase in the magnitude of the quasi-static
component or by the choice of an equivalent static force. When this is not the case,
corresponding dynamic models are used to assess the response of the structure; inertia forces
are then not included in the action model but are determined by analysis (ISO, 1998).

Table 1.4 Action classification according to time characteristics.

Action Permanent Variable Accidental

Probability of occurrence during 1 year Certain Substantial Small
Variability in time Small Large Usually large