Mitigation of Higher Mode Effects in Base-Rocking Systems by using Multiple Rocking Sections

FUORI CATALOGO

Base-rocking walls have been suggested as an improvement over base-yielding walls for seismic applications because structural forces in a rocking system are limited but structural damage is practically eliminated. In both systems, however, forces can increase after the specified base moment is achieved because of higher mode effects. This report demonstrates that these effects may be reduced by providing multiple rocking sections over the height of a rocking wall.

The program MURO is developed to perform the nonlinear time-history analysis of wall systems modelled using simplified input parameters. MURO is then used to conduct an initial parametric study, which suggests that rocking sections above the base limit the peak bend­ing moments in rocking walls without increasing the peak displacements. This study also shows reduced moments and shears for a base-yielding system with a secondary hinge.

The parametric study is followed by a statistical study of the response of 24 walls, from 4 to 20 storeys tall and with a variety of joint configurations, to 40 earthquakes. The peak bending moments above the base are significantly reduced by providing multiple rocking joints, while the peak displacements do not increase or become more variable with additional joints.

The results of the first two studies are confirmed by a case study of a 12-storey building in Los Angeles. Two alternative Direct Displacement-Based Designs are conducted, one using base-rocking walls and the other using base-yielding walls. Both are mod­elled, as well as an elastic wall and two base-rocking walls with additional rocking joints above the base. For all systems, the design displacement envelope is achieved on average for a suite of 20 design level earthquakes. At both the design and the maximum considered earthquake level, the moments and shears are lower in the walls with multiple rocking sec­tions than in the other systems.