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May 15, 2000
Development of a New Method for Seismically Strengthening
Existing Steel-Frame Structures and the Acquisition of Official Certification
Easy to do in a short time and at low cost, the new Bolt Adjustment-Type Buckling Strengthening Brace Method is winning acclaim as an extremely
reliable and effective earthquake-resistant method.
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Tokyo Electric Power Company
Tokyo Electric Power Service Co., Ltd.
Takenaka Corporation
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Over the course of about a year and a half, three companies, Tokyo Electric
Power Company, Tokyo Electric Power Service Co., Ltd., and Takenaka
Corporation, developed the Bolt Adjustment-Type Buckling Strengthening
Brace Method1) (patent pending), a new method
for seismically strengthening existing steel-framed structures that
has recently received official certification from the Building Center
of Japan. This certification has facilitated the patent approval process,
and is expected to help bring this new method into widespread use.
Conventional methods have included
1) replacing existing braces with more durable ones,
2) adding new braces, and
3) constructing new steel plate walls.
Unfortunately, these methods have been plagued with such problems as
the complexity of their work (large-scale work), and the way they increase
costs and construction time. However, the new method is a relatively
simple one in which steel pipe buckling stiffeners are wrapped around
existing braces, which can be up to about 10 meters in length. This
new method can also be applied to nearly the entire spectrum of steel-frame
structures, including electric power facilities, large shopping centers
and office buildings. Other merits of this new method include:
1) The steel pipe buckling stiffeners are lightweight and easy to install;
such simplicity helps to reduce construction time.
2) Since this method uses commercially available steel materials, it
has been designed with cost reduction in mind.
3) Since new braces or steel plate walls are not required, the area
needed for construction and installation can be reduced.
4) Existing braces are utilized, helping to make more efficient use
of resources.
It should be noted that this method had already been used for seismic
strengthening at three of Tokyo Electric Power Company's thermal power
plants (Oi, Yokosuga, Kashima), including the main building of the Number
2 Unit of Oi Thermal Power Plant. Compared with previous methods (for
example, replacing existing braces with more durable ones), the new
method has reduced the cost of strengthening brace sections by half,
and has also halved the construction period. The continued use of the
new method is expected to save Tokyo Electric Power Company 50 million
yen annually in equipment costs.
Note 1):
Braces are designed to prevent the deformation and collapse of buildings
resulting from earthquakes and typhoons; they are reinforcements that
are installed at angles around openings and other places. There are
reports that during the Great Hanshin Earthquake of 1995, there were
clearly distinguishable differences not only due to the presence or
lack of braces, but also due to the strength. When an earthquake causes
braces to warp, the seismic force is conveyed directly to columns and
beams, making them highly susceptible to collapse. Therefore, strengthening
of braces that are highly resistant to warping helps to significantly
improve earthquake resistance.
Brief Description of the New Method
The occurrence of an earthquake adds tremendous compressive force to
braces of steel-framed structures2), so buckling
occurs and resistance rapidly declines. However, with the new method,
bolts and steel pipes are installed at set intervals in existing braces,
which suppresses buckling and keeps resistance from declining.
Note 2):
Buckling occurs when bars, rods, etc., receive force in the axial (vertical)
direction, resulting in horizontal deformation after a certain threshold
is reached.
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Example of Application
<Tokyo Electric Power Company's Thermal Power Plants>
The occurrence of a severe earthquake can disrupt
the electrical power supply. Therefore, in the aftermath of the Great
Hanshin Earthquake, Tokyo Electric Power Company in 1996 decided to systematically
strengthen thermal power plants built in or before 1981, which have low
earthquake resistance.
The interior of the buildings of thermal power plants is a congested array
of pipes and cable; therefore, it is not easy to install new reinforcements.
In addition, to prevent vibrations and dust from occurring during installation,
the strengthening work must be completed in a short time during a periodic
inspection. The new Bolt Adjustment-Type Buckling Strengthening Brace
Method can shorten installation time and provide effective earthquake
resistance for existing structures.
Tokyo Electric Power Company has achieved great success in its use of
this new method at three of its thermal power plants. Plans are also being
made to use it at the main building of the Number 5 Unit of Hirano Thermal
Power Plant, which is currently under construction.
<Brief outline of the work done at the Oi Thermal Power Plant>
Name of project: Seismic strengthening of the Number 2
Unit of the Oi Thermal Power Plant
Location: Shinagawa-ku, Tokyo
Structure and number of the floors: Steel-framed structure, 4 above-ground
floors
Total floor space: ca. 8,500m2
Design: Tokyo Electric Power Company, Tokyo Electric Power Service
Co., Ltd.
Construction: Takenaka Corporation
Project schedule: October 1999- February 2000
<Other applications>
This new method can be used in many other types
of applications, from large-scale shopping centers to office buildings.
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