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Tokyo Sky Tree

26/04/2011

In structural engineering, Tokyo Sky Tree develops the essence of advanced structural design technology to insure the highest grade of structural safety. Assumed for the structural analyses are a metropolitan epicenter type earthquake, South Kanto Earthquake, Tokai Earthquake and, in addition, a catastrophic storm with wind velocity of 70 to 80 m/s for an average of ten minutes as may be encountered once in 500 years. Various methods of structural analyses and contrivances will be introduced regarding the following subjects:

1. Starting with Research in an Unknown Research Area

The height of Tokyo Sky Tree is about 634m. The once highest tower structure in Japan, Tokyo Tower (also designed by Nikken Sekkei), which was completed in 1958, is about 333m above grade. After some 50 years have passed, we dare to design to nearly double the height of the existing tower, which nobody has yet tried, making it “a challenge to an unknown research area,” by exercising the most advanced technology. To ensure its safety, we first started research work in this unknown research area.

To study the wind conditions at 600m above ground, we flew a radiosonde balloon for meteoric measurement to obtain the data on the distribution of wind velocity levels and disturbance conditions at such an altitude.

On the other hand, in addition to the conventional subsoil investigation, a special study (micro-motion array observation), which is not usually required, was conducted to know the soil formation down to the deepest level of some three kilometers from the ground surface. By the use of the soil information thus obtained, the soil behavior during an earthquake could be simulated more precisely than otherwise to ascertain how the tower sways at this site.

The tower was structurally analyzed and designed to assure better safety than that of an ordinary super-high rise building, against unprecedented severity of earthquakes and storms through various design contrivance and verifications based on the precise investigation.

The scene of a radiosonde balloon flying

2. Tower Anchored to the Ground like a Giant Tree

As a building soars higher, its foundation is usually subjected to proportionally larger pulling-out force and pushing-down force. In the case of a slender tower like the Sky Tree tower, its foundation is particularly subjected to a larger force.
So, the foundation of the Sky Tree tower needs to be designed to resist such larger forces by making its piles nodular-wall shapes to increase the friction resistance. The nodes of those piles resemble in function “pins of spiked shoes.” Also, by being continuously connected in radial directions, the walled piles are expected to have a function like roots of a giant tree by the piles being monolithically integrated into ground.

Also, the steel beams as seen above ground are rigidly connected to fully carry the force externally applied continuously down to the in-ground piles. Conversely seen, the entire structural system can be said to be “a giant tree growing from ground.”

Outline formation of the piles (As seen from bottom to top)

3. Monolithic Composition of Structural Beams

As major structural members, the Sky Tree tower employs high-strength steel tubes, the strength of which is twice that of a standard steel channel. The steel tubes used at the foot of the tower are huge, the diameter is 2.3m and tube thickness is 10cm.

The entire tower structure is composed of “truss” elements, each of which is a combination of triangles, comprising a principal member, a lateral member and a diagonal member, each member in plural number. These members are joined to each other by branch joints, illustrated below (i.e., a branch pipe section joined by welding directly to a main pipe without using any joint plate or other members.) This type of joint is very simple in appearance and advantageous for rust prevention.

Such a joint is rarely used for a building or on-ground structure, but is used for a marine structure (for instance, oil platform jacket); therefore, the joint is designed according to the standard rules adopted for such marine construction.

4. Outline of Steel Tower Framings
 
Tripod truss: A built-up column composed of four steel columns and lateral steel members and braces. It is positioned at the top of a triangular plane and is one of the major frames to resist lateral force.
 
Lateral joint truss: Column section joining the mid-tower framing and ring trusses at every two courses (25m high). These act as load-carrying members of lateral force (in-plane) and as stiffeners to resist buckling of tripod trusses and peripheral columns.
 
Ring truss: Lateral members positioned at every course (12.5m). This truss acts as a stiffener to resist buckling of peripheral columns.

5. Vibration-controlling Structure like a Five-story Pagoda Temple

We made every effort to assure safety from swaying when the tower is subjected to an earthquake or strong wind. Finally, we arrived at the conclusion that a new vibration-controlling system should be used in which a cylindrical core of reinforced concrete at the center (center column) is structurally isolated from the peripheral steel framing, with the upper part of the core column made to function as “a balancing weight”. This control system is in principle a new application of the modern “Added Mass Control Mechanism” and can reduce the response shear force by 40% during an earthquake.

The traditional towers in Japan, “five-story pagoda temples”, have never fallen down due to any earthquake to date, which mysterious fact is ascribed to the temple’s “Shimbashira” (Center Column) built at the center of the temple.

When it came to building a super-high tower of 634m with modern construction techniques, we happened to meet with the traditional tower construction technique. We call the present vibration-controlling system “Shimbashira-Seishin” (Center Column Vibration Control).

Added Mass Control Mechanism

This mechanism is to control the swaying motions of a structure as a whole during an earthquake by providing added mass (balancing weight) so as to move in slightly delayed timing from the swaying motion in a counterbalancing way to set off the movement of the structure by the movement of the weight. Usually, steel ingots or concrete mass is used as the added mass, and sometimes equipment for building service systems, or a heat accumulator, are used for the same purpose. The present case of the core column (the staircase) used as the added mass is a world’s first attempt.

Center Column

It primarily refers to the column built at the center of the pagoda or other temple tower. In the case of the Sky Tree tower, it refers to the cylindrical shell built at its center (made of reinforced concrete and used as a staircase).

Source: NIKKEN SEKKEI

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Construction of Tokyo Sky Tree began in July 2008. This 610 m tall structure is the most advanced digital terrestrial television broadcasting tower in the world, as well as a symbol of the redevelopment of the downtown and its culture. Ties with Asakusa, a popular tourist destination where the Edo culture remains strong, is also enhanced.

Due to the constraints of the site’s triangular shape, the base of the main tower is a regular triangle with sides about 67 m in length. The observatory lobby is circular in shape and commands a 360-degree view of the Kanto district. These features give the tower a unique shape that morphs from a regular triangle at the base to a circle in the high-rise section.

The first and second observatories are approximately 350 m and approximately 450 m above ground level, respectively. The top of the main tower is approximately 500 m above ground level. A tall gain tower, to which the broadcasting antennas are attached, extends from the top of the main tower to a height of approximately 610 m above ground level.

The center of the tower is a reinforced concrete (RC) cylinder 8 m in diameter and contains a staircase. The cylinder is surrounded by a steel core that houses the elevators and equipment shafts. Around the steel core is a steel truss structure consisting of steel pipes. The planer shape of the steel truss structure varies from a regular triangle at the base to a circle in the high-rise section, as noted above. Trusses are located at the vertices of the main tower’s triangular base. Each truss is composed of four principal members connected in a horizontal plane. This steel-truss structure provides the primary resistance to earthquake and wind loads. 

The main structural frames of the tower are steel pipes with a maximum diameter of 2,300 mm, a maximum thickness of 100 mm, and a yield strength of 400-630 N/mm2. A welded branch pipe joint connects the pipes without the need for gusset plates and bolts. The welds meet the standards for strength set by the American Petroleum Institute (API), and their strength was verified by FEM analyses. The fatigue resistance of all joints to wind loads also was verified to ensure the safety of the joints.

The main tower incorporates a vibration control system. It is a center column-based vibration control system modeled after the center column of the traditional Japanese five-story wooden pagoda. In the Tokyo Sky Tree, the center column is the RC cylinder, which is separated from the steel truss structure at a height of 125 m and above. The mass of the RC cylinder in this section is used as a weight (tuned mass damper) for the vibration control system. Oil dampers are installed between the RC cylinder and steel frames to control the displacement of the cylinder and enhance the main tower’s damping performance. This staircase is called Shinchu-seishin (cylinder vibration control system) based on the central pillar of Japan’s traditional architecture, Gojunoto (five-storied pagoda). The system reduces the earthquake loads acting on the tower by up to about 40%.

The gain tower also incorporates a vibration control system. A weight and spring are installed at the top of the tower and the frequency of the system is tuned to that of the gain tower.

The foundations of the tower are rigid, cast-in-situ, diaphragm wall piles. Nodes at the tips of the piles resist the large pull-out forces acting on the foundations during typhoons and earthquakes. A full-scale, on-site test verified the resistance of the nodular piles to these pull-out forces. 

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3 comments

  1. i love tokyo and i love this tower!


  2. The best view point for Tokyo Sky Tree is from Asakusa. You can walk to Tokyo Sky Tree from Asakusa. From Asakusa, you can see it in the buildings and temples. It’s a kind of mixture view of old and modern Tokyo. But, personally, I like Tokyo Tower rather than Tokyo Sky Tree.More Photos: http://www.worldfortravel.com/2013/06/13/tokyo-sky-tree-japan/



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