The Development and Validation of a Non-tearing Floor Precast Concrete Structural System for Seismic Regions.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
Traditional seismic design philosophy for reinforced concrete seismic frame structures localises damage and inelastic deformation to regions of significant plasticity within the beam (plastic hinge zones) during a severe earthquake event. Collapse prevention of the frame is applied through capacity design methods, requiring the maximum expected flexural strength of the beam plastic hinges to be reliably assessed in order to design for, and ensure, the predominantly elastic flexural response of the columns in the frame.
Previous experimental and numerical investigations have shown that significant and detrimental damage to the frame and floor system occurs due to the formation and elongation of ductile beam plastic hinges; requiring extensive post-earthquake repair or demolition with likely loss of function of the building. This poses significant economic consequences to occupiers of the building, as the time required to reinstate the integrity of the structural and non-structural building components is often lengthy.
More importantly, it has been highlighted that the interaction between elongating ductile plastic hinges and the accompanying floor system enhances the flexural strength of the beam hinges, altering the distribution of forces in the seismic frame compared to that assumed during capacity design. Research has shown that the consideration of frame-floor interaction in current New Zealand design codes significantly underestimates the flexural strength enhancement of beam plastic hinges, threatening the hierarchy of strength and collapse prevention mechanisms employed in capacity design.
Recent research has introduced change in the design philosophy of precast concrete seismic frames. Rather than designing for localised damage in the frame, unique Non-tearing (of the floor) connection details have been developed which provide a gap or slot between the end of the beam and column face and force connection rotation to occur about a shallow hinge located at the top of the beam, thereby avoiding the formation of plastic hinges and associated beam elongation effects altogether. Research investigations have shown that Non-tearing connections successfully minimise damage to the structural frame and floor, while providing seismic energy dissipation characteristics at least comparable to that of traditional reinforced concrete connections.
In this research, the mechanics of different non-tearing connection arrangements were investigated and original theory introduced for the aspects of connection behaviour which diverged from fundamental reinforced concrete design. A variety of precast concrete non-tearing connection details were developed, with the design focus placed on economic and construction efficiency in order to encourage the rapid implementation of non-tearing connection technology into New Zealand construction industry.
The performance of the developed connection details were explored and assessed experimentally and analytically. A two bay precast concrete frame with precast floor system was tested under a demanding reversed cyclic, quasi-static loading protocol using displacement control. The seismic response of the non-tearing connection details employed in the test frame successfully minimised damage to the frame and floor systems. Only minor repair of one primary crack at each connection between the floor diaphragm and supporting beam would be required after a design level earthquake. Issues encountered with buckling of the longitudinal reinforcement in the bottom of the beam reduced the connection performance at high levels of drift. However, detailing measures were successfully employed in successive tests which improved the drift capacity of the connections. Detailing improvements to enhance the seismic response of the developed non-tearing connections were recommended based observations from the frame test.
Numerical analysis of the non-tearing connection details was performed using simple rotational and compound spring models, with the key features of the experimental response captured with excellent accuracy. The analytical models were constructed using engineering theory, rather than by calibration with experimental observations. The modelling assumptions and principles adopted in the analysis have been presented for use in design offices or future research programmes when designing and analysing seismic frames using non-tearing connections.
This research successfully contributed to the development and progression of non-tearing frame technology. With further research and the refinement of construction details, non-tearing floor connections exhibit impressive potential for providing superior seismic safety, performance and efficiency in precast concrete seismic frame buildings.