Strategies of seismic damage mitigation for infilled RC frames: Shake-table tests

摘要

Unreinforced masonry panels are widely used as interior or exterior partitions in reinforced concrete frame structures around the world. In most design codes, infill panels are considered as non-structural elements and commonly neglected in the structural design process. The conventional tight-fit infill walls have some favourable effects in increasing the frame's in-plane stiffness and are beneficial for resisting minor earthquakes; however, under strong earthquake excitation, the severe interaction between infill walls and bounding frame can introduce brittle shear failure in RC columns and thereby lead to catastrophic failures and collapse. It has been extensively reported that in every disastrous earthquake during the past few decades, severe damage and poor seismic performance of masonry infilled RC frames have been observed, including many newly designed ones, particularly in the 1999 Izmit earthquake in Turkey, the 2008 Wenchuan earthquake in China, and the 2015 Pokhara earthquake in Nepal. It is indisputable that inherent problems relating to analysis and design methods for tight-fit infilled frame structures have not yet been solved, and some design guidelines provided in different countries for evaluation of infilled RC frames are recognized as being far from satisfactory in terms of completeness and reliability.;The primary objective of the reported research is to propose and test an innovative flexible connection detailing method which could effectively mitigate undesirable interaction damage for infilled RC frame structures and minimize the life-safety hazard under potential earthquake excitation. This proposed strategy isolates the infill panel from bounding columns with finite width vertical gaps during the construction phase, and steel wire connections are deployed in mortar layers and anchored to columns. To evaluate the effectiveness and adaptability of the proposed seismic mitigation strategy, extensive shake-table tests and numerical investigations are conducted, based on which, the performance criteria and comprehensive design recommendations could be developed and proposed.;Taking into account the similitude requirements, a total of nine one-third scale, singlestorey single-bay RC frames with different masonry configurations and flexible connection details were carefully designed and tested on a unilateral shake-table in HKUST. Three real earthquake records are selected and scaled to ascending intensity levels and used as input signals. A series of thorough investigations including dynamic characteristics, hysteretic behaviour, failure mechanism, out-of-plane vulnerabilities, the effect of connection length, and the effect of different gap filling materials and load transfer mechanisms are rigorously studied. A discrete modelling approach employing a surfaced-based interaction modelling technique to simulate fracture, crack propagation, sliding and separation, and post-fracture behaviour of mortar joints is also developed and verified by finite element software ABAQUS. Dynamic and monotonic push simulations are carried out and compared with the experimental observations. The numerical and experimental results indicate that the proposed seismic damage mitigation concept could considerably reduce undesirable interaction between infill panels and bounding frame, protect the columns from direct shear failure at an early stage, and provide structural redundancy at high levels of excitation. Globally, the structural stability and integrity, displacement ductility, and energy dissipation capacity of infilled RC frame are remarkably improved.;Hong Kong has recently been classified as a region of moderate seismicity with an earthquake intensity of VII and peak ground acceleration of 0.15g. In view of the large number of masonry-infilled frame buildings in Hong Kong, the presented research is expected to provide crucial guidance for seismic assessment and the design of safer frame structures. The test results and design guidelines/recommendations from the proposed research are also expected to benefit the infrastructural development in other countries that are threatened by earthquakes.