Quasi-Static Pull-Down Experiment of a Real-Size Wooden Structure With/Without 'Wall-Of-Columns' Seismic Retrofit Implementation

Abstract eng:
During the Hyogo-ken Nanbu earthquake of 1995 we had devastating damage to structures, especially to wooden houses. It revealed that wooden houses in Japan are not vulnerable to seismic input motions with only high peak ground accelerations (PGAs) but they are vulnerable to those with both high PGAs and high peak ground velocities (PGVs). This means that severe damage will occur if structures are subject to strong ground motions with high PGA and PGV. Therefore it is our urgent need to strengthen old wooden houses in Japan. However, about 6 million old wooden houses still remain untouched because the current seismic retrofitting is very costly and troublesome in its actual implementation. So we need to develop a new retrofitting method that can be easily implemented, and yet can make a house withstand even for a severest ground motion with high PGA and PGV. For that purpose we have invented a new restraining method, the so-called “Wall of Columns”, which can fit into a wooden beam-column frame of 0.9 meter in width, the module unit of the Japanese traditional wooden house. Our basic idea is to install nine wood lumbers with 9 cm x 9 cm cross section each spanning from top beams and bottom foundation members and to connect these lumbers each other by lag-screw bolts. Once installed they behave like one loosely coupled wall as a whole to horizontal motions. To prove their performance as a retrofitting device installed only to one room at the one end, we conducted a quasistatic pull-down experiment for real-size two-storied wooden houses with and without the retrofitting walls. We applied horizontal forces on the second floor by using a 100 ton crane. The footprint of the experimental houses were 4.6 m x 8.2 m with 6.8 m in height with two stories. For a house without a reinforcement, we obtained the maximum resisting force of 27 kN, which corresponds to about 22% of the total weight. The deformation angle at this maximum resisting force was 0.07 radian and that just before the collapse was 0.15 radian. When eight “Wall-of-Columns” were installed in a room at one end, we obtained the maximum resisting force of 120 kN, which corresponds to almost the same as the total weight. The deformation angle at this maximum level was 0.13 radian at the non-reinforced side, while it was only 0.035 at the reinforced side. The angle just before the collapse was more than 0.22 radian at both sides. Thus we have proved that a single room reinforced by the “Wall-of-Columns” system can successfully sustain unbalanced horizontal seismic force up to the same amount of its weight. Finally we built numerical models to simulate results of these two pull-down experiments with a commercial FEM program. We have successfully reproduced the basic characteristics of such quasi-static behaviors found in our field experiments.

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