Selection of the typology of eccentric bracings

There are many possible typologies of eccentric bracings, involving seismic links, which can be either short or long.

The choice between short and long links is partly determined by the following considerations:

• Short links provide more stiffness than long ones.

• Shear deformations essentially are in-plane deformations of the web of the link section, without a marked tendency to lateral torsional buckling.

• Long links mean strong bending effects and plastic hinges in bending with buckling of flanges, which distorts the symmetry of sections under cyclic load conditions, with the potential for lateral torsional buckling. This effect should be prevented, which requires strong lateral restraints of both the upper and the lower flange of the profile.

The choice between various typologies is influenced by many factors, including architecture, which can impose a choice by requiring certain openings.

Structural considerations may also intervene:

• It has been recalled above (with reference to clause 6.8.2(7) in EN 1998-1) that there is a requirement for 'tuning up' of the strength of the dissipative zone to the heightwise distribution of the seismic shear in the building, in order to make the spread of yielding over the height of the structure uniform.

• If the seismic links are in beams, while the beam sections are determined by design checks other than those of the ULS under seismic conditions, the above 'tuning up' may require an important overstrength of beams and consequently of all other structural components, due to their capacity design to beam strength. Frames with V or inverted-V eccentric bracings in which the Vs have a flat horizontal tip correspond to this situation.

• One way to escape this penalizing situation is to select a typology which forces all seismic links to yield simultaneously, such as the evolution of the frame with eccentric V bracings in Fig. 6.17.

9 Vertical seismic links as in the typology of Fig. 6.15 can more easily be designed as specific 'ductile fuses', because gravity loading subjects them essentially to axial forces, which do not interact significantly with their bending and/or shear resistance.

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