Seismic Design Particularities of a Five Story Reinforced Concrete Structure, Irregular in Plan and Elevation

The design of structures subjected to torsional effects is highly complex, and becomes more difficult to assess when elevation irregularity is also present. This paper has as main objective to present the particularities of design for a plan as well as elevation irregular structure (11.0 x 15.0 m layout and 15.50 m total height), built in Bucharest (corner period of 1.6 s and high displacement demand). Architectural considerations, owners wishes and structural needs join together in a complex layout: uneven in plane distribution of vertical earthquake resistant structural elements (RC walls and frames); eccentric vertical stair case circulation (coupled RC walls); one structural wall present only at ground level; at the last level the layout shrinks to the staircase circulation perimeter. In order to meet the code requirements the design had to be based on modal analysis because of the interference of translational and torsional movements. As a praxis suitable computation tool static nonlinear analysis was performed (using the SAP software, CSI analysis reference manual, University Avenue. Computers and Structures Inc., Berkeley, 1995. RC walls are modeled as frame elements with point hinges. The structural performance is evaluated in terms of structural displacements (in the center of mass and on the floor edges). The order of plastic hinge formation and their damage degree is traced. The efficiency of the chosen structural layout with respect to the reduction of general torsion effects is pointed out and the behaviour factor option is outlined. Linear dynamic analysis is performed also, provididng an upper bound for the expected displacement amplification in the nonlinear range of behaviour.