An Alternative Approach to Improve the Capacity Design Concept for Moment Resisting Reinforced Concrete (Rc) Frame Systems

Abstract

Capacity design concept for reinforced concrete frame systems is based on a hierarchy of the strength and stiffness properties of the structural elements so that the seismic energy dissipation mechanism occurs in a certain way. This theoretical concept of seismic energy dissipation by allowing the occurrence of plastic hinges at the end of the beams and the columns located at the ground floor was not observed / identified in post-earthquake inspection of damaged RC frame structures. On the other hand, various failure mechanisms were observed that are not compliant with theoretical considerations specified in the seismic design codes. The damages produced during the latest seismic events, (2020 Zagreb earthquake, 2020 Aegean Sea earthquake, 2020 Caribbean earthquake, 2020 Puerto Rico earthquake, 2020 Mexico earthquake, etc.) raised some concerns related to the theoretical ductile failure mechanism (Strong Column -Weak Beams, SCWB) versus the practical approach. Consequently, a possible improvement of the capacity design concept through the consideration of different values for the behaviour factor "q" applied to structural elements (beams and columns) was investigated and presented in this paper. The goal was to reach the expected theoretical structural degradation mechanism. Thus, by considering different values for the behaviour factor "q", at the design stage, for beams and columns, it was possible to reach a favourable value for the ratio Kc/Kb between the bending stiffness of columns (Kc) and beams (Kb). Consequently, a good correlation between the real seismic response and the theoretical mechanism of structural deformation was obtained.

Applicable Solutions Regarding Ductile Seismic Response Development in the Pure Moment Resisting (MR) Reinforced Concrete (RC) Frame System

“…The real seismic response of the Moment Resisting (MR) Reinforced Concrete (RC) frame structures required to on-site seismic action differs from the idealized and theorized seismic response specified in the current seismic design standards (P100-1, 2013), (EC8, 2004), (Sococol et al, 2021 a).…”

“…This structural system type records soft storey mechanisms with vertical superstructure segmentation in linearly elastic rigid boxes and fragile segments with rapid rupture in marginal areas of the RC columns (Sococol et al, 2021 a).…”

Analytical Study Regarding the Seismic Response of a Moment-Resisting (MR) Reinforced Concrete (RC) Frame System with Reduced Cross Sections of the RC Beams

“…Related to the analytical studies, in seismic design codes, several suggestions and conclusions regarding the capacity design concept of pure MR RC frame structures were observed: Thus, in the FEMA P-2012 norm [28,31], "two important suggestions regarding Strong Columns-Weak Beams (SCWB) seismic energy dissipation mechanism (Figure 1b) for pure Moment Resisting (MR) Reinforced Concrete (RC) frame systems are specified" [32]:…”

“…Thus, the steel columns are computed with a different set of lateral seismic forces (generated from the load groupings stage) than the ones for the frame beams, this represents a theoretical solution for the reduction of seismic effects on these types of structures (MR steel frames and MR RC frame systems) [32].…”