What are the models of disaster resilient structure?

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Takenaka has developed mid-level isolation system installed while the buildings are still being used. This new method entails improving and classifying the columns on intermediate floors of an existing building into flexible columns that incorporate rubber bearings (base isolation systems) and rigid columns which have been wrapped in steel plates to add to their toughness. A combination of these two types of columns is then used to improve the earthquake-resistant performance of the building as a whole. On this occasion this method was applied to a project to improve the earthquake resistance of the head office of Himeji Shinkin Bank (Himeji City). This is the first method of improving earthquake resistance in Japan that classifies the columns on the same floor as flexible columns and rigid columns, and it is the first case in west Japan (the Kansai region) of attaching rubber bearings by cutting columns on the intermediate floors of an existing building.

The enactment of the Law for Promoting Improvements to Increase Earthquake Resistance of Buildings (1995) and the need to use existing buildings for longer periods of time due to the low economic growth have resulted in an increase in earthquake resistance strengthening work in the construction industry. Recently in particular, there have been increasing numbers of improvements that do not disrupt tenants in computer office buildings, banks, hospitals and collective housing. This method involves improving earthquake resistance while the buildings are still being used as normal, without affecting their normal operations or residential environment.

The client's demands for improving the earthquake resistance of the head office of Himeji Shinkin Bank built in 1972 (one floor below ground, eight floors above ground, and steel framed reinforced concrete structure) were as follows:

The improvement to earthquake resistance is not carried out on the sales floor (2F) and above, and operations can be carried out normally during construction. The building will not be destroyed by a major earthquake of six to seven on the Japanese seismic intensity scale. Costs of construction shall be kept as low as possible.

To meet these demands, originally we reviewed adopting the base isolation method which reduces the seismic force transmitted from the ground to the building. There are three types of base isolation systems, depending on the location where rubber bearings are incorporated: "pile head isolation," "foundation isolation" and "mid-level isolation." On this occasion, with the former two the gap between the underground exterior walls and the site boundary is narrow, making installation of base isolation systems difficult, so mid-level isolation was adopted. The planning was carried out to base isolate the first floor which is mainly a car park and concentrate deformation on this floor in the event of an earthquake. However, conventional earthquake resistance strengthening methods for intermediate floors had the following technical problems. By cutting horizontally all columns and walls on a specific intermediate floor (the first floor on this occasion) and installing rubber bearings in the columns that have been cut, that floor becomes extremely flexible, and the building will sway horizontally with the large sway amplitude of 40-50 centimeters under maximum level earthquakes. It therefore becomes possible that the finishing materials, piping and existing elevators may not be able to keep pace with the deformations and break, perhaps resulting in their protruding from the site of the building.

In the head office of Himeji Shinkin Bank, columns with rubber bearings incorporated in them to allow them to move flexibly and rigid columns which were made tougher by wrapping steel plate were placed effectively, thereby suppressing horizontal deformation and improving the earthquake resistance of the building as a whole. Specifically, firstly 28 of the 44 columns on the first floor were cut off by a thickness of 50 centimeters, and rubber bearings were inserted into the gaps. The remaining 16 columns were covered with steel plate with a thickness of 9-22 milimeters (13 columns with nine milimeters, and three columns with 22 milimeters), thereby adding a toughness to the existing rigidity. A total of six vibration control units incorporating viscous materials with high energy absorption performance were installed in walls, to play the role of dampers. This reduced the swaying of the building.

The earthquake resistant performance in the event of a major earthquake that occurs after improvements have been completed using these measures is as follows, assuming seismic motion on a par with the 1995 Great Hanshin Earthquake (Hyogoken Nambu Earthquake):

The acceleration that occurs on the second floor and floors above that is reduced to approximately 40 percent of the figure before improvements. The damage to columns on the second floor and higher is limited to cracks, with the concrete not dropping and steel bars were not exposed. The maximum horizontal deformation for the first floor was kept to around 13 centimeters which the elevator could keep pace with, indicating that the columns wrapped in steel plates and the columns to which rubber bearings have been fitted are able to safely support the building.

Up until now Takenaka has been building a track record in improving the earthquake resistance of buildings while the buildings are still being used as normal. We intend to add this new "mid-level isolation system while the buildings are still being used" as a new selection on our menu, and promote it throughout the country.

Artist's Impression of Improvement Work While Buildings Are Still Being Used (Improvement Work on the First Floor, Second Floor and Above Are Business as Usual)

Overview of Himeji Shinkin Bank Building Location 105 Junishomae-cho, Himeji City, Hyogo Prefecture Client Himeji Shinkin Bank (Himeji Credit Bank) Structure Steel-framed reinforced concrete (partly steel structure) Number of floors 1 fl. below and 8 floors above ground, 3 penthouse floors Use Offices (bank) Total floor space 12,601.20 m2 Building area 1,806.03 m2 Completion November 1972 Design Yamashita Sekkei Construction Takenaka Corporation

Earthquake Resistance Strengthening and Renewal Work Design Takenaka Corporation Construction Takenaka Corporation Construction schedule May 1999 to April 2000

External Appearance of Head Office of Himeji Shinkin Bank (Under Construction of Improvement Work on the First Floor)