Takeuchi 2011
This paper presents case studies of structures using energy-dissipating and self-centering systems: the new library building and the secretariat building retrofit on Tokyo Tech’s O-okayama campus. These structures were designed to withstand level-2 earthquakes with a maximum acceleration response of 1000cm/sec2 at their elastic limit and they were expected to be usable after such an earthquake. The structural systems of the buildings are modeled analytically to see that they can withstand an earthquake.
System Concept
The New Library: The old library at Tokyo Tech was insufficient for withstanding seismic forces. The building could not adopt retrofit designs because of deterioration of the concrete materials. The authors proposed an underground library, which maintained an unobstructed view along the axis from the main gate to the main building and kept valuable documents away from sunlight. An above-ground building was added to this design, named the Glass House.
The Glass House is composed of three triangular stories. V-shaped diagonal columns support each story. Perimeter frames are composed of perimeter beams and plate columns at the second and third floors and form a vierendeel truss. The toe of the plan is a 15 m cantilever and is supported by a pair of diagonal columns. The EV shaft, composed of truss frames with concrete walls, receives about 50% of the horizontal force.
The Secretariat Building: The Secretariat Building is located on the O-okayama campus. Its seismic performance was judged to be insufficient, and it was decided that retrofit was necessary. In order to reinforce the longitudinal frames, additional concrete walls or braces along the windows of office blocks and between the front pilotis columns were considered. However, this concept had some problems. Designers wanted to avoid additional braces between the front columns since they could change the appearance of the building. Moreover, in the office floors, the spaces between the columns were occupied by various facilities; therefore additional braces between the columns were not desirable; and concrete walls that block the windows were not acceptable.
An integrated façade concept was applied. This concept aims to design a new façade that can improve the architectural features, the seismic performance, and the energy efficiency of the building. This façade was composed of seismic energy-dissipating braces, horizontal louvers and glass. The authors also developed diagonal energy-dissipating louvers with the diagonal parallel louvers acting as buckling-restrained braces. These hysteretic dampers are used as energy dissipation braces. They are also effective at controlling sun light.
Seismic Performance of the Structure
The New Library: To analyze the dynamic characteristics of the new library, eigenvalue analyses were carried out; and the seismic performance of the structure was evaluated by a time-history analysis. The first mode of the structure was found to be a twisting mode. The second mode dominated the vibration in the y direction, and third mode in the x direction. The fourth mode primarily consisted of vibration of the vierendeel truss in z direction. All these vibration modes had similar natural periods, and the seismic response of this structure was expected to include a combination of these modes. Analysis results show that members did not exceed their ultimate limits, and thus the structure remained within its elastic limit.
The Secretariat Building: The retrofitted structure could generally withstand an earthquake with a response of 1000cm/sec2 up to the elastic limit of the existing frames.
Reference
Takeuchi, T. (2011). “Seismic Design of Recent Projects on Tokyo Tech O-okayama Campus,” Proceedings of the 8th International Conference on Urban Earthquake Engineering, Tokyo Institute of Technology, Tokyo, Japan, March 7-8.