|Abstract:||Steel concentrically-braced frames (CBFs) are widely used lateral force resisting systems with high strength, stiffness, and material efficiency. Special concentrically-braced frame (SCBF) systems are a type of CBF commonly used in high seismic regions because they are designed to permit large inelastic drifts. Ductile detailing and capacity design requirements ensure braces are the main source of energy dissipation in the system. While the design of SCBFs in high seismic regions is prevalent and substantiated by previous research, SCBFs are not frequently designed when seismic demands are lower. In moderate seismic regions, it is typical to design low-ductility CBFs because of their design simplicity and economy. Low-ductility CBFs do not have the same detailing or proportioning requirements as SCBFs, which typically results in lighter system weights but has the consequence of non-ductile frame behavior during earthquake response. The lateral force resisting behavior of low-ductility CBFs relies on reserve capacity, or secondary stiffness and strength, following initial brittle limit states.
This study investigates the behavior of SCBFs designed for moderate seismic regions, which is an area unaddressed in previous evaluations of CBFs. The SCBFs designed for this study are compared to recent assessments of the frame behavior and economy of widely used low-ductility CBFs, namely the R=3 CBF and the ordinary concentrically-braced frame (OCBF). Numerical models developed in OpenSees capture the member behaviors and limit states pertinent to multistory CBFs. The suite of frame models analyzed in this work consider variations in frame type, frame height, and brace configuration. Seismic stability of SCBFs is evaluated including the influence of gravity column continuity.
A seismic performance assessment is conducted using the frame models to performing nonlinear static and nonlinear dynamic analyses. Nonlinear static analyses are conducted as a preliminary assessment of the behavior of CBFs, and they are employed to identify the sources of lateral load resistance and ductility.
Nonlinear dynamic analyses are conducted according to the incremental dynamic procedure (IDA) to perform a collapse performance assessment based on the FEMA P965 framework (FEMA, 2009). The numerical models used for the dynamic analyses have the increased capability to model the degradation of components through load reversal.
There are three primary research objectives addressed in this work: (1) to compare the system design, behavior and economy of low-ductility CBFs and SCBFs, (2) to evaluate the collapse performance of SCBFs designed for moderate seismic regions, and (3) to investigate the seismic stability of SCBFs including the influence of gravity column continuity. The results of the nonlinear static analyses highlighted that SCBFs had considerable overstrength beyond the design level as a consequence of the capacity design procedure. The characteristic ductile pushover response for SCBFs was observed and three regions of secondary stiffness were defined and used to evaluate frame behavior. The secondary stiffness term that describes the first region of negative stiffness indicated the predominant behavior for each design variation (e.g. the split-x configuration relying on continuous column contribution more to resist the destabilizing effects of P-Δ). Compared to the low-ductility CBFs, SCBFs exhibited similar levels of elastic stiffness and higher levels of post-elastic stiffness which contributed to the improved ductility capacity.
The dynamic analyses expanded upon the pushover analysis results by capturing structural degradation resulting from cyclic loading, modeling finite ductility capacity of the braces (e.g. the limit state of brace fracture), and the effects of higher modes in the response of the system. The chevron configuration consistently exhibited superior dynamic performance compared to the split-x. While this initially contrasted with the pushover results, the dynamic results were seen as a more realistic inelastic response of the systems. The chevon configurations engaged frame action to provide a combination of lateral resistance from brace inelasticity and column flexural strength, while the split-x distributed inelasticity across the braces of many levels, and did not have the member proportioning to engage frame action. The seismic performance was compared between low-ductility and SCBFs, and all SCBFs exhibited adequate performance for application in moderate seismic regions.