Developments of Advanced Solutions for Seismic Resisting Precast Concrete Frames
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Major advances have been observed during the last two decades in the field of seismic engineering with further refinements of performance-based seismic design philosophies and the subsequent definition of corresponding compliance criteria. Following the globally recognized expectation and ideal aim to provide a modern society with high (seismic) performance structures able to sustain a design level earthquake with limited or negligible damage, alternative solutions have been developed for high-performance, seismic resisting systems.
In the last two decades, an alternative approach in seismic design has been introduced for precast concrete buildings in seismic regions with the introduction of “dry” jointed ductile systems also called “hybrid” systems based on unbonded post-tensioned rocking connections. As a result structural systems with high seismic performance capabilities can be implemented, with the unique capability to undergo inelastic displacement similar to their traditional monolithic counterparts, while limiting the damage to the structural system and assuring full re-centring capabilities (negligible residual or permanent deformations).
The continuous and rapid development of jointed ductile connections for seismic resisting systems has resulted in the validation of a wide range of alternative arrangements, encompassed under the general umbrella of “hybrid” systems.
This research provides a comprehensive experimental and analytical investigations of 2- and 3-Dimensional, 2/3 scaled, exterior beam-column joints subjected both uni and bi-directional (four clove) quasic-static loading protocols into the behaviour, modelling, design and feasibility of new arrangements for “dry” jointed ductile systems for use in regions of high seismicity. In order to further emphasize the enhanced performance of these systems, a comparison with the experimental response and observed damage of 2-D and 3-D monolithic beam-column benchmark specimens is presented.
However, after a lot of attention given to the behaviour of the skeleton structure, more recently the focus of research in Earthquake Engineering has concentrated on the behaviour of the floor system within the overall 3D behaviour of the building and the effects of beam elongation. The effects of beam elongation in precast frame systems have been demonstrated to be a potential source of un-expected damage, unless adequate detailing is provided in order to account for displacement incompatibilities between the lateral resisting systems and the floor. Two contributions to beam elongation are typically recognized: a) the material contribution due to the cumulative residual strain within the steel, and b) the geometrical contribution due to the presence of a neutral axis and actual depth of the beam.
Regarding jointed ductile connections with re-centering characteristics, the extent of beam elongation is significantly reduced, being limited to solely the geometrical contribution. Furthermore, such effects could be minimized when a reduced depth of the beam is adopted due to the use of internal prestressing or external post-tensioning. However, damage to precast floor systems, resulting from a geometric elongation of the beam, has yet to be addressed in detail.
In order to emphasize the enhanced performance in controlling and minimizing the damage of the structural elements via the use of the proposed advanced hybrid solutions, this research presents via experimental and analytical validation of two alternative and innovative solutions to reduce the damage to the floor using 2 and 3-Dimensional, 2/3 scaled, exterior beam-column joints.
The first approach consists of using standard precast rocking/dissipative frame connections (herein referred to as “gapping”) in combination with an articulated or “jointed” floor. This system uses mechanical devices to connect the floor and the lateral beams which can accommodate the displacement incompatibilities in the connection. The second approach to reduce the floor damage investigates the implementation of a “non-gapping” connection, also called non-tearing-floor connection, using a top hinge at the beam-column interface, while still relying on more traditional floor-to-frame connections (i.e. topping and continuous starter bars). Additionally, further refinements and constructability issues for the non gapping connection are investigated under the experimental and analytical validation of a major 2-Dimensional, 2/3 scaled, two-story one-bay frame using non-tearing floor connections.
Based on the non-tearing floor connections, a series of parametric analysis for beam-column joints and frames are carried out. Furthermore, the analysis and design of two prototype frames using different solutions is presented. The frames are subjected to cyclic adaptive pushover and inelastic time history analysis in order to investigate analytically the response characteristics of hybrid frames using non-tearing connections, as well as how the beam growth affects the frame response under earthquake loading. Computational models for hybrid PRESSS frames and a conventional reinforced concrete frames are developed and compared with the ones using non-tearing connections.