Seismic behaviour factor in combined frame and braced structures

In seismic design of light steel structures according to Eurocode 8 the behaviour factor is accounting for the ability of the structure to dissipate energy by yielding. Applying dissipative design, cross sectional class, joint detailing and management of possible overstrength of material enable behaviour factors up to 8. Thereby, proper choice of the behaviour factor depends on the bracing system, in which all bracing elements which restrain the structure in the same direction should go plastic at the same time. This rule can be easily fulfilled with buildings where identical braces are in parallel walls which provide equal stiffness and equal strength.

However, e.g. in plant construction type and position of the braces often need to be irregular due to operational reasons. So the use of only one type of bracing system does not fulfil the special demands and bracing systems are mixed up.

The common engineering procedure would now imply to calculate on the safe side and therefore to take the lower behaviour factor. Large jumps of the values of the behaviour factors as well as the “black or white” classification leave little space left for the designer. ... mehrA more sensitive approach might be a linear interpolation between the corresponding behaviour factors depending on the percentage of the bracing system of the whole structure in one main direction.

The paper reports on a study, where the dynamic behaviour of simple combined frame and braced structures is investigated by elastic-plastic time-history analyses. Design recommendations for the amendment of EC8 will be proposed.