Towards ductility enhancement of RC structures in regions of moderate seismicity : Hong Kong case study
by Adeyemi Israel Atanda
THESIS
2001
Ph.D. Civil Engineering
xx, 261 leaves : ill. ; 30 cm
Abstract
The reinforcement detailing of reinforced concrete frame and wall-frame structures, in regions where the probability of a seismic event is not significant, is characterized by lack of special consideration for meeting ductile response requirements. The buildings are normally designed for the onerous combination of wind and gravity loads. In the event of an earthquake on such structures, the potential for damage and consequent loss of lives may be high. Therefore, to mitigate this potential hazard, this research examined the response of representative existing structures to low to medium seismic energy excitation (PGA = 0.15g). The seismic response behaviour factors were significantly improved through the proposed risk-level-compatible detailing techniques....[ Read more ]
The reinforcement detailing of reinforced concrete frame and wall-frame structures, in regions where the probability of a seismic event is not significant, is characterized by lack of special consideration for meeting ductile response requirements. The buildings are normally designed for the onerous combination of wind and gravity loads. In the event of an earthquake on such structures, the potential for damage and consequent loss of lives may be high. Therefore, to mitigate this potential hazard, this research examined the response of representative existing structures to low to medium seismic energy excitation (PGA = 0.15g). The seismic response behaviour factors were significantly improved through the proposed risk-level-compatible detailing techniques.
The behaviour of existing reinforced concrete frame and wall-frame buildings, primarily designed and detailed for resisting onerous combination of gravity and wind loads, was computer simulated for a condition of low to moderate seismic event. It was concluded that the ductility demand on the buildings is significant and that the seismic response behaviour modification factors available in codes were not applicable for moderate seismicity consideration. Appropriate elastic seismic response behaviour modification factors were developed and presented. The vulnerability of the buildings was established through the evaluation of the theoretical displacement and curvature ductility factors demands. Experimental investigation was then performed on third scale substitute structure specimens in order to determine the available ductility factors and the proposed ductility enhancement. The specimens were specially designed to simulate the first four storeys of a building.
For reinforced concrete rigid frame structure type, two specimens were tested. One was a control specimen; reinforced in accordance with standard non-seismic procedure. It was used for the verification of the available ductility factor in a frame. The other specimen was to investigate the enhancement in ductile response behaviour following modifications in reinforcement detailing specifications. Four simple modifications were made to the control specimen. The first involved reducing the spacing between the stirrups while still maintaining the transverse steel ratio (Asv/Sv) in beams. The other was the provision of double-U stirrup at the beam-column joint. The third was the provision of short bent-up bars at the support region of the beams. The fourth involved the provision of crossties in the column. For the study on wall-frame structures, two third scale sub-frame specimens were fabricated and tested. One specimen was a control specimen used to verify the available ductility of a non-seismically designed and detailed wall-frame building. The second specimen's reinforcement detail was modified to enhance ductile response. The beam and column sections including the reinforcement detailing were maintained the same as in the frame building for both the control and the improved specimen respectively. Similar to the frame specimen, double U-stirrups were provided at the beam-column joint of the improved specimen. In the wall of the improved specimen, the spacing between the longitudinal bars was reduced whilst still maintaining the longitudinal reinforcement equal to that of the control specimen. Additionally, the wall’s distribution bars ratio were also maintained but provided diagonally at opposing slope on both faces of the wall to form an X-shape pattern. The specimens were quasi-statically tested. It was concluded that frame and wall-frame buildings in non-seismic regions would experience ductility demand factors greater than their inherent ductility factors in the event of a moderate seismic event. It was found that the ductility factors of the building types could be vastly increased, beyond 400% in an instance, with modification in detailing specifications that do not result in radical changes in presently practiced design and detailing techniques or a significant increase in construction cost. The detailing recommendations in this study eliminate the need for seismic analysis and design in ensuring frame buildings’ satisfactory seismic response performance in low to moderate seismic regions.
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