Braced Steel Frame

Development of a Novel Self-Centering Concentrically Braced Steel Frame System

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Description:

Resistance to seismic loading in steel structures is often provided by the use of concentrically braced frames (CBFs), which are designed to undergo numerous cycles of inelastic deformation through the tensile yielding and inelastic global buckling of its bracing members. This inelastic behaviour leads to the possibility that structures designed according to current codified approaches are likely to have residual deformations after a major seismic event, meaning the structure may have not collapsed, but large permanent deformations exist in the structure. These residual deformations represent a major problem as they can often render a structure unusable, or cause significant delays such that the downtime associated with rectifying these residual deformations incur significant economic losses.

The aim of this project is to develop and test a novel CBF system in order to eliminate the occurrence of these residual deformations following major seismic events. This is done by combining the existing CBF system with a post-tensioning arrangement to give a self-centring CBF (SC-CBF). This post-tensioning arrangement consists of a series of strands running parallel to the beam members and between the beam flanges to be anchored at the exterior columns of the frame. This arrangement provides a recentering system for the frame which ensures the self-centring behaviour of the novel system.

The mechanics of the SC-CBF are developed and the general behaviour is described. An experimental test setup is then designed to investigate a variety of SC-CBFs under quasi-static cyclic loading. This testing, conducted using the strong floor and reaction frame in the structures laboratory at the Engineering Building in NUI Galway, demonstrates both the self-centring nature of the SC-CBF, and also the added energy dissipation provided by the brace members. Using the results of the nine experimental tests carried out, a numerical model is developed and verified using OpenSees. The model is initially developed based on previous research in the modelling of CBFs and self-centring systems, and is then developed further for the specific arrangement of the SC-CBF. The model is validated using the results of the tests carried out, where both physical testing and numerical modelling are shown to be in good agreement. Finally, the development of the SC-CBF into a performance-based design (PBD) framework is presented briefly. This outlines how the definition of the seismic hazard for a given site can be defined, together with the selection of suitable ground motions, to accurately represent the hazard levels associated with that specific site. The definition of performance goals for each level of seismic hazard is discussed, where these goals correspond to the expected damage state for each of the hazard levels considered. The design methods that can use the definition of seismic hazard for a given site, and produce a SC-CBF design that achieves the performance goals outlined for the structure are then reviewed.

The outcome of this research is the proposal of a novel SC-CBF system to be used in seismically active regions. The mechanical properties of this SC-CBF have been identified theoretically and compared to both experimental test results and numerical modelling simulation, which both validates the concept of the frame and also validates the ability of the numerical model to capture the behaviour of the SC-CBF. In light of this, a methodology is proposed which integrates the design of such a system into a PBD framework such that SC-CBF’s can be designed to incorporate different design goals corresponding to different seismic events of various return period.

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