AS 5104:2005 pdf – General principles on reliability for structures.
The theory of elasticity may be regarded as a simphfication of a more general theory and may generally be used provided that forces and moments are limited to those values for which the behaviour of the structure is still considered as elastic, However, the theory of elasticity may also be used in other cases if it is applied as a conservative approximation.
Theories in which fully developed plasticity is assumed to occur in certain zones of the structure (plastic hinges in beams, yield lines in slabs, etc.) may also be used, provided that the deformations which are needed to ensure plastic behaviour occur before the ultimate limit state is reached. A second corfltion Is that the actions influencing these deformations should nol be repeated frequently. Thus theory of plasticity shouki be used with care to determine the load-carrying capacity of a structure, if this capacity Is limited by:
— brittle failure, or
— failure due to instability.
In cases when action effect models and resistance models are applied separately in design calculations, both these kinds of models should in principle be mutually consistent, However, in many cases this principle may be modified or simplified. Thus, for example, a bending moment (action effect) in a continuous beam may be calculated according to the theory of elasticity and the resistance according to a theory of plasticity. In other cases, especially for second-order and other non•Iinear effects, such calculations cannot be applied unless special precautions are taken.
7.2.4 Models for dynamic response
In most cases the dynamic response of a structure is caused by a rapid variation of the magnitude, position or direction of an action. However, a sudden change (decrease) of the stiffness or resistance of a structural element may also cause dynamic behaviour. Thus, for example. the removal of a structural element mentioned in 4.3 d) may produce dynamic effects.
Dynamic analysis can be performed in the time domain and in the frequency domain. If the load is desenbed in statistical terms. it is also the statistical description of the response that Is looked for, Based on this description, one may calculate the probability of having some Imit state being exceeded In a given reference period.
Structural properties may be time-dependent as well as time-independent. In a full probabilistic analysis. these effects will be taken into account.
The models for dynamic analysis consists of:
— a stiffness model,
— a damping model, and
— an inertia model.
The stiffness model is pnncipally the same as for static analysis. Due to dynan’Wc influences there might be an increase in stiffness; although repetitions may also cause deterioration and a decrease in stiffness. For nontnear material models, there is normally a strain rate dependent increase in yield strength.
Inertia forces result from acceleration of the mass of the structure, non-structural masses and the added mass of the surrounding fluid, air or soil. These additional mass contributions originate from interactions of the structure with its environment, It may be necessary to perform dynamic analysis. considering different mass contributions,
Damping may be the result of many different types of mechanism. The most important mechanisms are:
— material damping, for instance from elastic nature or trom plastic material behaviour:
— damping due to friction in connections.
AS 5104:2005 pdf – General principles on reliability for structures
