Reinforced concrete frames infilled with masonry walls are the most popular building in the world. They are often built offices, schools, hospitals, and dwellings, and other kinds of residential and public buildings. However, the excessive damages from past earthquakes have repeatedly proved the instability of this building system and alerted us to make an effective response. In fact, most of infilled frame failures are attributed to the unexpected interaction between bounding frames and masonry infills. This interaction will not only destroy the structural integrity of walls, but also substantially affect the inherent ductile behaviour of frames.

Unfortunately, it seems that people still do not fully understand the precise role of masonry walls in the seismic performance of overa...[ Read more ]

Reinforced concrete frames infilled with masonry walls are the most popular building in the world. They are often built offices, schools, hospitals, and dwellings, and other kinds of residential and public buildings. However, the excessive damages from past earthquakes have repeatedly proved the instability of this building system and alerted us to make an effective response. In fact, most of infilled frame failures are attributed to the unexpected interaction between bounding frames and masonry infills. This interaction will not only destroy the structural integrity of walls, but also substantially affect the inherent ductile behaviour of frames.

Unfortunately, it seems that people still do not fully understand the precise role of masonry walls in the seismic performance of overall structures, and there is still not a unified consensus on how to design the masonry infills in most of the seismic design codes and practices. In general, masonry infills are recognised as the non-structural elements in the design phase, but shall be in full contact with bounding frames as detailing requirements, such as in Eurocode 8. This controversial design has resulted in the interactions. Currently, some alternatives have been proposed upon the conventional design, specified as the weakly interacted or fully isolated infill conditions, which are proposed by the codes of practice, such as GB 50011-2010, MSJC (2011), and NZS 4230:2004, to mitigate impacts from the serious frame-infill interaction.

The major objective of this research is to figure out the fundamental problem by whether the masonry infills can be favourable to structural system, and to explore a more effective way by column-isolated infill connection with frames to make them function together well. A total of ten large-scale, single-storey and single-bay masonry-infilled RC frame specimens are tested under reversed-cyclic loading. The different infill-to-frame connections, different masonry-infill configurations, different column-panel connectors, and different infills with openings have been investigated. The different failure modes and corresponding hysteretic behaviour of specimens are analysed and compared. A statistical analysis with 13 groups of experimental data obtaining from different researchers are presented as well. Based on the experimental and statistical analyses, the research findings are presented as follows.

(1)The column-isolated infilled frames have exhibited the much enhanced cyclic performance.

The column-isolated infilled frames have been investigated in the experimental programme, in which the isolation gaps are only designated at column sides but still keep the contact condition with bounding beam. This special arrangement can preserve the structural stability of walls by arching mechanism, while effectively avoid the serious frame-infill interaction. The surrounding frames can be capable of reaching the highest deformation capacity, while masonry walls are able to ensure the out-of-plane stability as well.

(2)The column-panel connectors shall be tough and flexible in seismic codes.

Although the column-panel connectors have been treated as detailing requirements to resist the potential out-of-plane failure for masonry infills, it is evident that those connectors can be too strong to pose a threat to the in-plane behaviour of walls. Therefore, two types of connectors, namely “strong” and “weak” connectors, are investigated in the experimental programme, the, and they have finally led to the radically different results. Based on the tests, it is found that the weak connectors are more flexible to adjust to the deformation compatibility of structural system.

(3)Infills with openings is extremely weak and should be paid the special attention.

Besides the solid walls, the opening effect has been thoroughly investigated considering the effects from geometries, positions, and boundary conditions. It is indicated that infills with openings have seriously jeopardised the structural integrity of the wall, and dramatically changed the inherent load-transfer mechanism. The isolation gaps at sides have substantially changed the initial boundary restraint of walls and led to the diagonal shear failure at very early stage. Moreover, further analyses from hysteretic and backbone curves have both indicated the poor performance of openings with the rapid drop of strength in the post-peak behaviour.

Under a certain number of experimental investigations and statistical analyses, it is certified the prominent role of masonry infills in seismic performance of infilled frame. For solid walls, it is proved that the column-isolated infill gaps are practical, reliable, and can substantially release the constraints from masonry, and enhance the overall deformation capacity. The stability of masonry wall can be ensured by the preservation of contact condition with beam through arching action and the appropriate design of column-panel connectors. The infills with openings are not advised to use the isolation infill condition due to the congenital defect of brittleness and weakness.

Based on the view of present codes and practices, it is suggested that the efficient width of isolation gap shall be either greater than 2.5% of interstorey drift ratio of buildings or at least 100-mm wide. More importantly, special measures should be taken to facilitate the development of shear sliding mechanism at the particular region of beam-panel interface bond. The column-panel connectors shall be flexible and tough to maintain the infill stability and deformation compatibility. The structural integrity of masonry wall has played the crucial important role in infilled frames and special attention should be paid to infills with openings.