抄録
On October 15, 2013, Bohol Island locating in the central region of the Philippines experienced an earthquake whose magnitude was 7.1, and a number of reinforced concrete (RC) buildings were seriously damaged by the earthquake. This study focuses on an RC building which was damaged by the earthquake because of no hoops in the beam-column joints. The damage was observed to the exterior beam-column joints. Such RC buildings with poor joints are likely to exist worldwide especially in developing countries; thus, practical strengthening methods need to be developed.
This study proposes a method to strengthen and rehabilitate exterior beam-column joints by installing RC wing walls, and presents the design procedure for wing walls installed to prevent the beam-column joint failure. A series of experiments using three specimens was conducted to verify the effectiveness of the proposed method.
Three 1/2.5-scale exterior beam-column joint specimens representing the earthquake-damaged building in Bohol Island were prepared, and the seismic performance and behavior of the specimens were evaluated under reversed cyclic loads. One of the specimens without the strengthening was the control specimen (J3) to observe the behavior of the existing frame. Another specimen was strengthened by installing wing walls before the test (J3-WI). The other specimen (J3A) was subjected to preliminary loads up to a deformation at the maximum strength before the strengthening, and then it was rehabilitated by installing wing walls (J3A-WI) and loaded in the same way as the other two specimens. The wing walls were designed to prevent joint failure by reducing the moment applied to the joint; the joint moment was reduced by tension/compression from the pulled/pushed wing wall to the adjacent beam.
The un-strengthened control specimen J3 failed at the joint and a story yield mechanism was formed in the second story. The observed behavior was brittle and the damage was similar to that of the focused building. The strengthened specimen J3-WI showed ductile behavior forming a beam yielding mechanism. The rehabilitated specimen J3A-WI also behaved in a similar manner, however, the strength deteriorated under cyclic loads beyond a drift angle of 6% rad. This was caused by a loss of bond between concrete and beam longitudinal bars in the existing beam-column joint where the preliminary damage was applied as mentioned above.
The experimental results verified that the strengthening method of installing wing walls beside existing columns was effective to upgrade the seismic performance of existing beam-column joints without hoops by shifting the failure modes from brittle joint failure to ductile beam yielding. However, when wing walls are installed into damaged frames, they should be designed considering deterioration of bond between concrete and beam longitudinal bars in the existing joint.
