Selection of the behaviour factor for design purposes

The values of behaviour factors q provided in Table 6.2 of EN 1998-1 are the maximum Clause 6.3.2(1)

allowed values. The designer can always decide to work with a smaller q. This can be justified in some cases by the fact that, as mentioned in Section 6.2, the seismic design condition is not necessarily the most controlling factor for the structure, and trying to make use of the highest possible value of q may have no practical impact. It should be stressed that, whatever the value of q used in the analysis, the displacements of the structure are the same, due to the application of the equal displacement rule. If drift control criteria prevail in the design, increasing the value of q does not help. On the contrary, considering higher q values will require the use of more stocky steel sections (smaller bit values) due to the imposed relationship between global ductility (high q) and local ductility (prevention of buckling) expressed in Table 6.3 of EN 1998-1.

In practice, for flexible structures such as moment-resisting frames, the values of q to use in order to avoid iterations are often closer to 3 than to 6. If deformation criteria prevail, it may be preferable to select a stiffer typology of earthquake-resistant structure, e.g. frames with concentric or eccentric bracings, which are more likely to profit from q values close to those of Table 6.2.

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