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Tokyo Sky Tree

Civil Engineering Aspects in construction of tokyo sky tree

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Tokyo Sky Tree

  1. 1. 1 1.TOKYO SKY TREE Fig-1.1 Tokyo Sky Tree is a broadcasting, restaurant, and observation tower in Sumida, Tokyo, Japan. It became the tallest structure in Japan in 2010 and reached its full height of 634.0 metres (2,080 ft) in March 2011, making it the tallest tower in the world, displacing the Canton Tower and the second tallest structure in the world after the Burj Khalifa (829.8 m) The tower is the primary television and radio broadcast site for the Kanto region,the older Tokyo Tower no longer gives complete digital terrestrial television broadcasting coverage because it is surrounded by high-rise buildings. Skytree was completed on 29 February 2012, with the tower opening to the public on 22 May 2012. This was a major project because in addition to the tower itself, offices and commercial facilities, a planetarium, an art gallery, etc., are included, with a
  2. 2. 2 total area of 230,000 m². It is a major tourist facility, and in the first year after opening, the tower alone had 6.38 million visitors, and the scheme as a wholehad more than 50 million visitors. It is well known that Japan is one of the few countries in the world with severe natural environment. Large earthquakes frequently occur and typhoons occur every year between summer and autumn. In undertaking the challenge to design a building with an unprecedented height, various methods were used to set the unknown external forces. In addition, to satisfy the high target performance, it was necessary to develop mechanisms to artificially reduce the oscillations during earthquakes and strong winds; and a new concept of vibration control system was adopted that uses the mass of a central shaft. Fig-1.2
  3. 3. 3 2.BODY OF SKY TREE The tower consists of 2 different elements independently working side by side to counter the effect of natural hazards. Both of the elements are built using cutting-edge technologies and advanced materials which make it a very stable superstructure. 2.1 Steel Outer Frame The steel outer frames of Sky Tree is built using high end advanced steel beams which are huge and twice the strength of standard steel channel. This investment is important to ensure that the tower is strong and rigid to eliminate possibilities of collapsing if earthquakes or any hazards of sorts should happen. In general, the outer frame is built based on “truss” elements, each of which is a combination of triangles comprising a principal member, a lateral member and a diagonal member. This mode of framing will give highest possible stability for an ultra-tall structure like Sky Tree. Fig-2.1 2.1.1TripodTruss: A column rests on four steel columns and steel elements and side braces. It is located at the top of triangular plan, is one of the frames to withstand significant lateral forces.
  4. 4. 4 2.1.2Lateraljointtruss: Columns that combine the mid-tower with the ring truss at every two courses (between each joint 25 meters) which gives support to resist the buckling of tripod trusses and peripheral columns. 2.1.3Ringtruss: Lateral members constructed at every course in a 12.5 meters. It resists buckling of peripheral column. 2.2Coreframing: Made from reinforced concrete shell. This is the connection between the centre column and the tower body via oil dampers from height 125m to 375m (Flexible) and from bottom to 125m. These members are connected by branch joints specially invented for strength and durability purposes. The branch joints look very simple in appearance but they are in fact offering several significant advantages: a) The pipe sections are joined by welding them directly to the main pipe without using any joint plate or other member. This offers superior fatigue resistance to their bolt on counterparts. b) Due to the welded sections of pipes, rust can be prevented at greater measures as compared to using bolts and nuts which are highly subjected to rust. Fig-2.2
  5. 5. 5 3.DESIGN CRITERIA In order to achieve the high target performance, so-called "unexpected natural disasters" were set that exceeded those set by Japanese law, so that the structure was designed having allowance on the safe side. Table 1: Design criteria Level Standards of domestic law Specification of design for disturbance Structural safety limit L1 Rare Strong wind: Return period = 100 years No damage Earthquake: middle L2 Very rare Strong wind: Return period = 1350 years Virtually no damage Earthquake: Big L3 Unexpected Strong wind: Return period = 2000 years Elastic behaviour Earthquake: Hidden faults In the seismic design, the fundamental natural period of the tower was about 10 seconds, and the fundamental natural period of the ground at the site was about 8 seconds, so designing against long- period seismic motions caused by Subduction Zone Earthquake, occur at ocean trough, was essential. Directly below the project site there is about 2.5 km of sedimentary layers. To determine their condition, a micro-tremor array survey was carried out, which assisted in producing the input seismic motions used for design. Also, although there is no record of an active fault near the tower, a magnitude 7.3 earthquake whose epicenter was a nearby active fault was postulated as an unexpected earthquake, and the tower was designed for this earthquake To determine the properties of wind blowing at heights greater than 600 m, the wind speeds at high altitudes were measured using balloons, and for two years, fixed winds were measured using an ultrasonic wind gauge and wind velocity meter and a anemo-cinemograph installed on an existing 65 m high communication tower on the site. The flow of the wind design included: setting the mean wind velocity distribution in the height direction based on the high level wind measurements; directly verifying the wind force properties and the wind response properties using wind tunnel testing; producing mock wind force waveforms based on this basic material obtained;
  6. 6. 6 and performing time history response analysis using a computer in the same way as for seismic response. Baloon Figure 3.1: Balloon-launching system Figure 3.2: Observation of wind with GPS Figure 3.3: The entire wind tunnel test
  7. 7. 7 4.FOUNDATION DETAILS The foundation is a vital part of any standing structure and it is sometimes built in such a way to counter certain undesired hazards. Sky Tree’s foundation is designed and constructed specifically to counter the earthquake effects. The foundation is not built using conventional straight pole piling, instead, walls of steel-reinforced concrete is used. On these walls, there are spike-like protuberances across the surface which allows the walls to hold firm and integrate to the soil underground. This technology is called the “Knuckle Walls” and is developed by Obayashi Corporation which is the constructor of the project. This is explained further in the sub-chapter below. Figure 4.1: Foundation of the Sky Tree The steel-reinforced knuckle wall piles are driven 50m into the ground forming at 3 triangle points and at each point, multiple knuckle walls are connected radially which forms a root like shape to bind them strongly to surrounding earth. Reinforced concrete walls connect these 3 points to form a complete triangular shape driven 35m deep under the ground. Within the triangle, there are steel beams piling of 2.3m in diameter and 10cm thick rigidly connected to the tower’s structure above the ground to further enhance the rigidity of the foundation.
  8. 8. 8 Figure 3: Detailed view of Sky Tree’s foundation 4.1 Knuckle Walls To counter the forces derived from earthquakes, the walls of the foundation are equipped with nodular protuberances or “knuckles”. This method has increased the overall frictional resistance of the foundation by the quality attributes offered by the nodules which lessen any possible slips, pressure and vibrations caused by wind, seismic, uplift and compressive forces altogether. The nodules solidly anchor the piles in the ground thus substantially improve load bearing capability and due to the virtue of the shapes, Knuckle Walls are highly resistant to horizontal forces. Figure 4.2: Detailed view of Knuckle Walls
  9. 9. 9 5.SUPER STRUCTURE The site of the tower is a former railway shunting yard, with the feature that it is long in the east-west direction, but only about 80 m in the north-south direction. In order to construct a building taller than 600 m under Japan’s severe natural environment, it is difficult to provide a plan shape at the base to securely withstand the horizontal forces, when the width is not necessarily sufficient. To solve this problem, and as a result of considering various conditions, the plan shape at the base was chosen to be a triangular shape. On the other hand, as a result of considering the function of the observation deck at the upper levels, a circular plan shape is desirable. As a result, TOKYO SKYTREE has a unique shape in which the plan shape gradually changes from a triangular shape at the lower levels to a circular shape at the observation deck. This change in shape has an impressive effect on the tower’s silhouette. On the other hand, one side of the triangular shaped plan is about 70 m or only 1/9 of the height, which is a very severe proportion for resisting horizontal forces. Fig-5.1 The intention was to make the structural form as lightweight as possible, with a design that is not intimidating to the surrounding area, so a truss form was selected for the structural steel, and mainly steel tubes were adopted as the structural members.
  10. 10. 10 The steel members with the highest standard strength were the 630 MPa steel members for the base of the gain tower, which had a standard strength about double that of the steel used in a ordinary building. The members with the largest cross-section were 2.3 m diameter steel tubes used in the ridges of the tower at the lowest level, and the maximum thickness of the steel tubes was 10 cm. Table-2: Maximum size of steel pipe (high performance steel) Type Strength Maximum Diameter Maximum thickness Height (N/mm2) (mm) (mm) (m) 630N/mm2 Class 630 1200 80 500 500N/mm2 Class 500 2300 100 0 400N/mm2 Class 400 1900 60 20
  11. 11. 11 6.SUB STRUCTURE This site is located in the bank of the Sumida River and its surface layer is occupied with silt, extremely soft ground. A RC wall pile is adopted as basement (Fig.3: yellow panel), thickness is 1.2m and depth is 35m, and stands on the bearing stratum under surface soft ground. This is one of strategy for seismic design, the rigid substructure system. This system constitutes of a rigid wall pile and soft ground, and makes use of a relative displacement between the rigid substructures and soft ground to gain damping ability, the radiation damping. But this effect can’t easily grasp, and the measurement of both behaviors during the earthquake will demonstrate the effect after completed.On the other hand, the foundation was required to secure not only horizontal but also vertical rigidity, and the ability of pull-out resistance, because of the weight saving of superstructure. A comparative study in the foundation system of this tower was executed, and finally two plans were compared thoroughly. The counterweight plan uses the weight of foundation as pull-out resistance and constructs on pneumatic caisson method. Fig-6.1 The aspect ratio of Tokyo Sky Tree is about 9.0, and the top displacement is too large to broadcast if the foundation has poor vertical rigidity because of the uplift-rocking deflection of
  12. 12. 12 Foundation. For example, 10.0cm displacements of a pile convert 90.0cm displacements at the top of the tower. A SRC pile (steel reinforced concrete pile, concrete encased steel pile) was adopted as the foundation structure to consider the continuity of steel member as the steel tower. However, behaviours of SRC pile under the pull-out force depend upon the amount of cracks, the pull-out test with actual size test model indispensable for the stability and reliability of the tower during the strong wind and the earthquake. The pull-out test, the maximum load was 40,000kN, executed in the site preceded the design. Fig-6.2 Location
  13. 13. 13 7.VIBRATION CONTROL SYSTEM Two types of vibration control system are used on TOKYO SKYTREE. a) TMDs on the top (added mass control mechanism) As a tower that is used for terrestrial digital broadcasting, it is necessary to suppress the wind response of the gain tower at the top of the tower on which the broadcasting antennae are installed. Specifically, the velocity of the oscillations of the gain tower due to normal wind, which has a high frequency of occurrence, was required to be maintained less than a specified value. For this purpose two TMDs were installed on the top of the tower. The required performance could be assured with at least one device, but by providing the second device higher performance could be ensured and it can be used as a backup in case of emergency. b) The Core Column System (core column type added mass control mechanism) A vibration control system using The Core Column provided in the center of the core as added mass was newly developed. Oil damper The Core Column Movable Range plan The Core Column Movable Range Section of The Core Column Immovable Range Oil damper Fig-7.1 Notion of the response control system with The Core Column
  14. 14. 14 The Core Column is used as the evacuation stairs, and was a reinforced concrete cylindrical shaped column of diameter 8.0 m, maximum thickness 60 cm, and height 375 m. This column was connected to the main structural steel frame of the tower up to 125 m above ground, and the portion above this height was connected to oil dampers to suppress the motions of The Core Column itself, so it acted as a vibration control column that was structurally independent of the tower. This system is effective in various types of earthquake such as long period earthquake motions and epicentral earthquake motions, and compared with the case where there is no center column effect, the response accelerations are reduced by a maximum of 50% in earthquakes and a maximum of 30% in strong winds.
  15. 15. 15 8.SAFETY MEASURES The building of Sky Tree requires an extraordinary precaution in order to ensure safety of the community and also the workers. This is especially vital when it has reachmore than 400m in construction. This is due to several crucial factors: a) Workers are doing their jobs at an unprecedented height b) Workers dropping even a single bolt from a high structure can pose a serious threat to the people walking below the tower. c) The tower is being built near Narihirabashi train station in a residential area where the station platform, rails and houses are concentrated. This means that trains come and go, and nearby residents walk on roads just beneath the tower. In conclusion, the biggest difference between the construction of Tokyo Sky Tree and that of other skyscrapers, such as the 296-meter-tall Yokohama Landmark Tower, is the environment surrounding the structures as mentioned above. This has led Obayashi Corporation., the main contractor for Tokyo Sky Tree, to introduce unprecedented safety measures to ensure the community and their workers are in a safe zone Several measures are taken: a) Building huge steel frame scaffolding from the ground before lifting it into place with cranes instead of building it on the tower. This can reduce the risk of workers dropping off heavy tools which will likely to happen if scaffolding is being built on higher ground. b) Waterproof canvas to protect workers from the wind and blocking dizzying views of the ground far below c) 2 safety lines are implemented instead of 1 for all workers; if workers detach one rope to move from one scaffolding to another, the other rope stays in place. Tools and even simple ball pens are tied with cords to workers’ belts to prevent them from dropping off. d) 100m steel protective panels are set up over railway just beneath the tower to prevent collateral damage if accidents should happen on the railway when a train is passing through.
  16. 16. 16 CONCLUSION The external forces postulated during the design of TOKYO SKYTREE were strong winds with a return period of 2000 years, and an epicentral earthquake that exceed the framework of the Building Standards Law. The tower can withstand these forces with virtually elastic behavior, and is capable of continuing broadcasting without being affected by major damage. This performance against high external forces was set taking into consideration the public and social role of the tower. In order to achieve this high performance, The Core Column System was developed, which is a new concept of system for controlling vibrations using the mass of the central shaft. By verifying the effect with and without the system, it was confirmed that the acceleration response during an earthquake is reduced by a maximum of 50%, and the acceleration response during strong winds is reduced by a maximum of 30%.
  17. 17. 17 REFERENCES