Self-Mass Damper at Tokyo Swatch
Typically, mass dampers are integrated into the building by adding mass into the structure. But as one can imagine, if, say, ten percent of the total building mass is added atop the structure for seismic control purposes, it would prove to be detrimental in terms of global impact on the building. This sheds light on why mass dampers are not used more often to control seismic responses.
To overcome this obstacle, the design team contemplated ways of mobilizing existing, necessary mass to use as a mass damper.
In looking at the upper sky garden blocks and the resulting three-story voids in the superstructure, we first tried to hang the two intermediate floors, isolating them from the building structure, thus utilizing the floor plate's own mass to recreate a swinging pendulum. However, due to various issues, such as the hanger elements intruding into the office space and vertical displacements of the floors due to the rocking movement of the pendulum, alternatives were explored.
As a result, instead of being hung, it was decided that the floor plates were to be supported vertically by corbels extending from the main structure within the beam depth. These floor plates are effectively isolated laterally with special spring and damper devices connecting the floors to the main structure. Although no longer a pendulum, this system uses the weight of the structure itself to form a mass damper with significant mass.
The next step was to find a suitable device that exhibits the required characteristics to realize this system.
Development of Devices
The devices that link the mass damper floor plates to the main structure were envisaged as follows:
- To consistently support vertical loads of the floor plates;
- Bi-linear property that prevents lateral movement during small earthquakes and typhoons, but deactivates restraints under a large seismic event;
- Damping effects due to the hysteretic behavior of the bi-linear material;
- Sufficient stiffness that would restrain total lateral movement to under 300 millimeters (11.81 inches);
- Restoring force to restore original position after movement;
- A small device to fit within the available 150-millimeter (5.91-inch) height between the corbels.
At first, base-isolator bearing products were researched, but the available configurations were too large and too stiff for the intended system. Bearing devices used in isolating houses were intriguing, but difficult to implement and too soft. Realizing that a suitable product did not exist in the market, we contacted one of the leading isolator bearing manufacturers (Toyo Tire & Rubber Co., Ltd.) for their expertise.
High-damping rubbers used in typical base-isolator bearings exhibit bi-linear behavior — initially very stiff but subsequently softer beyond a certain applied shear force. In addition, this bi-linear stiffness can be adjusted accordingly by varying the height and area of the high-damping rubber material. Typically, in order to prevent crushing of the rubber material under the weight of a building, multiple steel plates are sandwiched between the layers of rubber, which also makes the bearing extremely stiff laterally.
Following several discussions, a solution of creating a device that includes only the layers of high-damping rubber and sized according to the required bi-linear property was reached — a newly developed device using familiar material. Since these rubber dampers are unable to resist vertical loads due to the exclusion of steel plates, a decision was made to support the vertical loads of the floors by a separate device all together: slider bearings.
The combination of high-damping rubbers and slider bearings were placed accordingly in plan to balance the required vertical supports and lateral stiffness. Thus the final scheme for this mass damper system was envisioned and the design moved forward.
Self-Mass Damper System
The floor plates on levels 9, 10, 12, and 13 are isolated from the main structure and used as a mass damper to reduce the seismic response of the building during a large earthquake. This new seismic control system was aptly named Self-Mass Damper to highlight the use of existing mass instead of the typical mass augmentation seen in Tuned-Mass Damper (TMD) systems. Key characteristics of this SMD system include the following:
- Each SMD floor plate is approximately 100 metric tons (110 tons), with a combined mass of the four floors equivalent to ten percent of the superstructure mass.
- A combination of slider bearings and high-damping rubber bearings are placed at the interface of the SMD floor and main structure. These devices rest upon a set of corbels extending from the main structure, and fit within the beam depth of 600 millimeters (23.6 inches).
- Each combination of bearings for each level is tuned to provide maximum damping to the overall structure while maintaining an acceptable level of lateral deformation. The SMD floor is allowed to move in all directions, thus it is also effective in all directions. Note the SMD system is not tuned to the building frequency.
- Slider bearings support the vertical loads without restraining the horizontal movement of the floors. Friction coefficient of the sliding surface is 0.013. Allowable tilt angle is one degree to absorb the construction tolerances and actual leaning of the corbel supports (and main structure) during an earthquake.
- Materials used in the rubber damper bearings are the same high-damping rubbers used in typical base-isolator devices, the key difference being that steel plates are not sandwiched between the layers of rubber. The rubber dampers were specially developed for this project by Toyo Rubber.
- Each rubber damper unit was tested to confirm characteristics, followed by a full scale push-release test on site. Measurement devices were installed into the building to verify movement following a seismic event.
Analyses and Results
Non-linear three-dimensional time history analyses were conducted to simulate the SMD system and the building's behavior during large seismic events. Although an idealized line model analysis approach is commonly used in Japan to simulate building and damping behavior, the unprecedented nature of this system necessitated analyses conducted using a full three-dimensional computer model.
A suite of seven seismic time history inputs, each with varying characteristics, were used to assess and validate the performance of the structure and damping system. This suite consisted of: a) three measured signals scaled up to meet the required PGV (peak ground velocity): El Centro NS, Taft EW, and Hachinohe NS; b) three artificial "Kokuji" signals that incorporates the soil behavior of the site: Hachinohe, Kobe, and random; and c) one site-specific wave: Kanto.
The rubber dampers were modeled as bi-linear elements with zero vertical stiffness. Slider bearings were modeled to reflect the friction coefficient of the material. Numerous parametric studies were performed to properly assess the sensitivity of the SMD system. The parameters include: a) variations of material properties to take into account differences due to temperature, workmanship, etc.; b) mass of floors; c) uneven live loading of SMD floors; and d) varying the building dynamic properties.
Although the effectiveness of the SMD system fluctuates depending upon the seismic time history input, it was established that seismic response of the structure decreased in all cases. The SMD system is most effective against earthquakes that resonate with the building's dynamic properties, resulting in a maximum base shear reduction of 37 percent (Kokuji Hachinohe signal).
On the other hand, the system is least effective under a seismic event with a strong sudden pulse, in this case El Centro NS. As such, the seismic design load was governed by the Level 2 (500-year) El Centro signal (PGV of 50 centimeters per second, or 19.7 inches per second). This figure illustrates the SMD System's projected effect upon the building when on/ off (activated/ deactivated).
The maximum displacement of the SMD floor relative to the main structure is 147 millimeters (5.79 inches) in the short direction of the building, and 215 millimeters (8.46 inches) in the long direction. Thus the allowable clearances were set respectively as 200 millimeters (7.87 inches) in the short and 265 millimeters (10.43 inches) in the long direction. This figure illustrates the time history plots of the horizontal displacements of the SMD, building, and their relative movements; note the phase difference.
Although the maximum instantaneous difference in acceleration between the SMD floors and building is approximately +200 centimeters per second-squared, or 79 inches per second-squared), the peak acceleration of the SMD floor is a mere +14 percent — demonstrating the stable behavior of the SMD system. In addition, due to the difference in dynamic properties of the SMD system compared with the building, the system will not resonate nor respond uncontrollably during any earthquake.
The daring architectural concept combined with the Swatch Group Japan's wish for a structure with high seismic resistance resulted in the implementation of a newly developed mass-damper-type passive control system, the Self-Mass Damper. Evolved from a pendulum damping scheme inspired by the movement of an antique clock, the SMD system was found to successfully reduce the seismic design load by over 30 percent, leading to a robust yet efficient structure and increasing the potential life span of the building.
Ryota Kidokoro is an associate in the Tokyo office of Arup. Born in New York City, he earned a bachelor's degree from Cornell University, and previously worked for engineering firm Thornton Tomasetti. With Arup, Kidokoro collaborates with various Japanese architects, including Shigeru Ban, Kengo Kuma, Riken Yamamoto, Itsuko Hasegawa, and Hiroshi Sambuichi.
This technical paper was presented by Ryota Kidokoro at the 14th World Conference on Earthquake Engineering in Beijing, China, in October 2008.
Japan Structural Consultants Association (2001). The Guide to Safe Buildings: JSCA Performance Menu.
Kidokoro, R. (2007). "Structural Design of the Nicolas G. Hayek Center." Kenchiku Gijutsu, No. 695, 32-35.
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