Abstract:
Conventional ductile structures rely on system ductility involving inelastic action in selected components in their lateral load resisting systems to dissipate earthquake energy. When subjected to severe earthquakes, the lateral load resisting systems are expected to undergo large deformations without losing their strength and structural integrity. However, the extent of damage caused by these severe earthquakes can be considerable, requiring extensive structural repairs or demolitions and causing significant ongoing social and economic disruption. In order to minimise the disruption, a low damage design philosophy is adopted. Low damage structures are expected to withstand severe earthquakes without requiring extensive structural repairs, using either or both isolating systems or sacrificial systems which either do not need repair or are readily replaceable. In order to respond the performance targets for low damage design, an innovative Centralised Rocking Concentrically Braced Frame (CRCBF) system has been developed in this thesis. The CRCBF system utilises a free-base V-braced Concentrically Braced Frame and a centralised rocking system with Ringfeder® - Friction ring springs as its energy dissipation devices. The CRCBF system does not rock under gravity loading, Ultimate Limit State (ULS) wind loading, and Serviceability Limit State (SLS) earthquake loading, remains essentially elastic under ULS earthquake loading by undergoing controlled rocking, and dependably returns to its initial position following the ULS earthquake. As the CRCBF system rocks back-and-forth about the central base of the CRCBF, the ring springs are arranged to work as a double acting system to accommodate cyclic vertical movements of the CRCBF columns and to dissipate earthquake energy during rocking. Additionally, the ring springs are partially prestressed to provide a high initial stiffness for the CRCBF system in order to prevent rocking at the SLS earthquake level. Two designs of double acting ring springs systems have been developed for the CRCBF system. The first design (Type I) of the double acting ring springs system generates a parallelogram-shaped hysteresis response, whereas the second design (Type II) of the double acting ring springs system generates a flag-shaped hysteresis response. Upon the lock-up of the ring springs in the MCE event, structural fuses, such as machined-down threaded rods and - ii - column baseplates, are designed to yield. The yielding of the machined-down threaded rods and the column baseplates prevents yielding of the other CRCBF components, which have less dependable inelastic behaviour. Then, after the MCE event, the yielded machined-down threaded rods can be replaced and the deformed column baseplates are repaired if necessary, allowing a structure designed with the CRCBF system to be fully operational. To maintain the lateral stiffness of the CRCBF and to transfer internal forces between those components at all time, all joint connections for the CRCBF system are designed as rigid connections, especially for the beam-brace-column joint connections. Therefore, two advanced pass-through beam-brace-column connections are developed for the CRCBF system. A Square Hollow Section (SHS) column has been designed with pass-through beam and brace slots, allowing a collector beam and a brace to pass through the SHS column. The collector beam and the brace are then welded to the SHS column faces. Finally, the SHS column is filled with concrete. These joint connections are not only cost-effective, when compared to bolted connections, but also easy to fabricate with modern equipment. The behaviour of the CRCBFs has been validated by numerical analyses and a series of experimental tests. Two CRCBF models were developed with the Type II double acting ring springs system and two different types of pass-through beam-brace-column connections where each CRCBF model represented each type of pass-through beam-brace-column connections. The numerical analyses were conducted using SAP2000 and Abaqus/CAE. Linear static analysis, non-linear pushover analysis, and non-linear time history analyses were undertaken using SAP2000, while finite element analyses (FEA) for investigating the complex behaviour of the two pass-through beam-brace-columns were undertaken using Abaqus. Then, a series of experimental tests were undertaken, comprising component testing, energy dissipation device testing, and centralised rocking frame testing. Lastly, the test results were compared with the analysis results derived from SAP2000 and Abaqus. The numerical analyses and experimental testing of the CRCBF systems showed that the CRCBF systems exhibit stable and repeatable flag-shaped hysteresis responses. A good agreement in axial forces is achieved between the SAP2000 analysis results and the test results. Also, a good agreement in Von Mises stresses in the beam-brace-column panel zones is achieved between the Abaqus FEA results and the test results. Lastly, the Abaqus FEA results showed that column baseplates, which were optimised with mild steel square supports, - iii - perform well as structural fuses for the CRCBF systems. The column baseplates developed a yieldline when the CRCBF columns are in compression. The results derived from the experimental testing and the numerical analyses have determined the behaviour of the CRCBF systems and have shown that the CRCBF systems meet the performance criteria required for a low damage system.