Performance-Based Seismic Design of Base Isolated ...

SEAOC 2011 CONVENTION PROCEEDINGS

Performance-Based Seismic Design of Base Isolated Taipei Performing Art Center Atila Zekioglu, Huseyin Darama, Simon Rees, Chas Pope, and Rory McGowan Arup-Los Angeles, California & Arup-Beijing, China Abstract

Project Description and Scope

This paper describes the performance-based seismic design of the Taipei Performing Art Center (TPAC) in Taiwan, utilizing seismic-isolation concept with friction-pendulum devices. TPAC will be the new international-standard performing arts facility for the capital of Taiwan. The design, by the Office for Metropolitan Architecture (OMA), was the winner in a two-stage international design competition held in 2008, and was selected from a total of 135 entries from 24 countries (Figure 1). TPAC is a public project for the city of Taipei and situated in a busy urban location. The site currently contains a street food market, and is adjacent to the famous Shilin Night Market as well as a mass-transit station. TPAC, at over 40,000 square meters, is believed to be the first use of friction pendulum isolators in Taiwan.

TPAC is a public project for the city of Taipei. The Client is the Department of Cultural Affairs, Taipei City Government. It is one of a series of cultural projects commissioned in Taiwan recently, including Taichung Metropolitan Opera House, designed by Toyo Ito with Arup and currently under construction, and Taipei Pop Music Centre, for which the design competition was recently won by Reiser + Umemoto again working with Arup. TPAC consists of three theatres (1,500 seat Grand Theatre and two 800-seat theatres for repertory performances), that plug into a central cube, combining the stages (and back-stages) of the three theatres in a single and efficient hall. Each theatre can be used independently or in combination, offering the possibility for theatrical experimentation and unique views through the stage into the other auditoriums.

The building’s complex geometry and irregular mass distribution led to the realization that the most-efficient and direct way to achieve the required performance in such a highly seismic area would be to base isolate the superstructure. The irregularity also led to the decision to use Friction Pendulum type isolators. These are considered to have better structural performance compared to alternatives due to higher damping and lower base shear forces, simpler connection detailing at the isolation plane, and inherent selfcentering behavior. In the proposed structural scheme, approximately 100 isolators (located at basement level) work in conjunction with the steel braced superstructure to give the most efficient, economic, and highest performance building possible for the challenging site location. The structural system was analyzed using equivalent static, linear dynamic and time history procedures, including a "beyond the code requirement” investigation of the structural members at MCE hazard with recommended acceptance criteria per the selected performance objectives. A comparative study is performed showing the effectiveness of the current code based analysis and design procedures. In overall, the seismically isolated structure met and surpassed the performance objectives while achieving a 60% reduction in the base shear, significant decrease in story drift and floor accelerations.

Figure 1. Competition model of TPAC (by OMA) The floor area is approximately 40,000 square meters and the site area is 20,750 square meters. The building centered around a Cube, approximately 53.5m by 53.5m on plan, which contains all of the stage and backstage facilities, lobbies, offices and rehearsal rooms. Floor-to-floor heights are typically 5m. The three auditoria project up to 37m from the Cube. The maximum excavation depth is 10.8 metres.

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The three theatres (Figure 2) are called the Grand Theatre (GT), Multiform Theatre (MT) and Proscenium Playhouse (PP). The auditoria project outwards from the Cube, and are elevated above the ground on columns. A key architectural feature is that the stages of the theatres can be combined in a number of alternative configurations. Parking and plant rooms are located in the single-storey basement.

The preliminary design was completed and construction documents produced in 2010-11. Construction will begin in 2012, and the centre is scheduled to open in 2014. Taipei city council expects the centre to further facilitate the development of local performing groups and add to Taipei’s image as an international cultural hub.

The site is located in the northern part of Taipei city close to Shilin Night Market and Jian Tan MRT station (Figure 3). It is bounded by Cheng De Road to the west, Ji He Road to the east, Jian Tan Road to the south, and adjacent developments to the north. The site is understood to be an old riverbed, and is predominantly flat, although is close to nearby hills (Zhang Shou Park). The site is currently occupied by a single storey food court and a car park. Arup have been commissioned by OMA on behalf of the Department of Cultural Affairs, Taipei City Government (DCA) to carry out the engineering design for the TPAC. Arup’s scope includes Structure, Building Services, Building Physics and Fire. Arup was leading the engineering design for the above disciplines for the Planning Services (PS) / Scheme Design (SD) and Preliminary Design (PD) phases, with the exception of Building Services (Scheme Design only). Arup worked with local Taiwanese architects and engineers in a collaborative way. Evergreen Ltd. was responsible for providing comments and advice (in particular Taiwan code issues) on the structural engineering in the above stages. They also carried out the PD foundation design, basement wall design, plus an independent analysis model for checking purposes. They also liaised with local authorities for approvals matters: due to its complex shape, the structural design needed to undergo a Special Structural Review, carried out by professors at the National University of Taiwan. Evergreen are responsible for the structural engineering during the Detailed Design (Construction Documents) phase.

Figure 2. Theaters’ plan view (by OMA)

Project consultants; Architect: Associate Architect: Associate Structural Engineer: Associate Services Engineers: Consulting Engineers: Facade Engineer: Scenography Consultant: Acoustician: Arup scope:

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OMA Artech Inc. Evergreen Consulting Engineering Inc. I S Lin & Associates Heng Kai Inc. ABT dUCKS Scéno DHV Structural and MEP Building Physics Fire Engineering

Figure 3. Rendered image of TPAC located at Shilin District (by OMA)

Performance Objectives It is well known that the Taiwan region is seismically active and historically experiences large earthquakes. On September 21, 1999 (Taiwan local time), a disastrous earthquake of magnitude M7.3 struck the central part of Taiwan. The epicenter was located near the town of Chi-Chi, Nautou County. Along the Chelungpu fault, a surface rupture of more than 90 km was observed. A horizontal offset about 10

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meters was observed at another site. As a direct lost of this earthquake, 2403 people were killed over 10,000 buildings collapsed or were severely damaged (Loh and Lee 2000). This is Taiwan’s worst disaster since the 1935 ShinChuTaichung earthquake, where 3325 people were lost in a magnitude M7.1 earthquake. Earthquake hazard mitigation is an important issue in Taiwan. The clear earthquake risk of the TPAC location has raised the seismic performance of the facility to the highest priority for Taipei Government, not only to protect their investments, but also to keep this crucial infrastructure operational after a major earthquake. TPAC was decided to be designed for a high level of safety, complying with the requirements of hospital building. Early in the design completion stage, possible structural solutions for this unique architectural concept were studied. Due to high seismicity of the site, building complex geometry (torsional irregularity) and high performance requirements;

while the SUPERSTRUCTURE & TRANSFER ELEMENTS are to be IMMEDIATE OCCUPANCY (Occupiable with Limited Nonlinearity) and inelastic deformation.

(a) Steel braced box

Our first decision was to create a laterally and torsionally stiff braced box around the perimeter to carry all lateral loads and free up interior for planning (Figure 4 (a)) Then base-isolate the structure to reduce the seismic forces, reduce cost, simplify detailing and achieve highest standard of performance (Figure 4 (b)). Arup initiated a performance-based design framework with the client and defined the following two major seismic performance objectives based on the client’s requirements (Figure5). 1.

(b) Seismic isolators Figure 4. Conceptual structural system of TPAC: (a) Steel braced box (Superstructure), (b) Seismic Isolators

The building will be designed for Operational Seismic Performance Level, i.e., no structural or non-structural damage for an earthquake hazard with a uniform 10% probability of exceedance in 50 years (475 years event). This earthquake hazard is commonly known as Design Basis Earthquake (DBE) or design earthquake in codes and literature. Under this design scenario, all SUPERSTRUCTURE, SUB-STRUCTURE, ISOLATOR, and TRANSFER ELEMENTS are required to be ELASTIC

2.

The building will be designed for Structural Immediate Occupancy seismic performance for an earthquake hazard with a uniform 2% probability of exceedance in 50 years, (2,475 years event). This earthquake hazard is known as Maximum Considered Earthquake (MCE) in codes and literature. Under this scenario, SUB-STRUCTURE & ISOLATORS elements are required to be ELASTIC,

Figure 5. Elements of base isolation

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Review of the Code Provisions and Seismic Hazard The structural design of TPAC (design philosophy, load and material factors, deflection limits etc.) is based on Taiwan Codes. In case if it is not well covered, such as base isolation design, reference will be made to international standards and codes of practice, in particular US standards. In this section, first a brief review of the Taiwan seismic code provisions and hazard definitions is given. Then, the site-specific PSHA study is explained and the resulting response spectrum curves are compared to the code-based response spectrum curves.

T0

S D1 / S DS

(3)

Where T is the structure’s fundamental period given in seconds. The shape of the design response spectrum is illustrated in Figure 6.

a) Taiwan Seismic Code, 2005: Based on the current seismic building code in Taiwan, the elastic seismic demand is represented by the design spectral response acceleration, SaD, corresponding to a uniform seismic hazard level of 10% probability of exceedance within 50 years. Based on the uniform hazard analysis, the mapped design 5%-damped spectral response acceleration at short periods (SSD) and at 1 second (S1D) have been tabulated for each municipal unit of village, town or city level. For simplicity, only four levels of mapped spectral response acceleration at short periods and at 1 second were defined for both the 10%/50 and the 2%/50 hazard levels as shown in Table 1. Furthermore, in order to include the local site effects, multiplying the mapped spectral response acceleration parameters by the site coefficients, one obtains the site-adjusted spectral response accelerations at short periods (SDS) and at 1.0 second (SD1): S DS

Fa

S SD

S D1

Fv

S1D

(1)

where site coefficients Fa and Fv are given in Table 2, which are also related with the mapped spectral response acceleration parameters, SSD for Fa and S1D for Fv, respectively, besides the soil type. From the above provisions (Table 2 and 3) it is evident that the non-linear amplification effects of soil layers have been considered. Based on the site-adjusted spectral response acceleration parameters SDS and SD1, the design spectral response acceleration SaD for a given structure can be developed by using the following: S DS 0.4 3T / To ; S aD

Where;

4

S DS

;

(T

0.2 T0 )

(0.2T0

T

T0 )

S D1 / T ;

(T0

T

2.5T0 )

0.4 S DS ;

(T

2.5T0 )

(2)

Figure 6. Design Response spectrum developed by site-adjusted parameters SDS and SD1 Table 1. Values of mapped spectral response acceleration parameters 10%/50 SSD (g) 0.5 0.6 0.7 0.8 D years S1 (g) 0.3 0.35 0.4 0.45 M 0.7 0.8 0.9 1 2%/50 SS (g) 0.4 0.45 0.5 0.55 years S1M (g)

Table 2. Values of site coefficients Fa Site SS = SS = SS = SS 0.5 Class 0.6 0.7 0.8 Hard Normal Soft

1.0 1.0 1.0

1.0 1.0 1.0

Table 3. Values of site coefficients Fv Site S1 = S1 = S1 = S1 0.3 Class 0.35 0.4 0.45

S1 0.5

Hard Normal Soft

1.0 1.1 1.2

1.0 1.5 1.8

1.0 1.1 1.2

1.0 1.4 1.7

1.0 1.0 1.1

SS 0.9

1.0 1.3 1.6

1.0 1.2 1.5

1.0 1.1 1.4

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The fundamental period can be determined by the following approximate expressions: 0.085 hn3 / 4 T

for Steel MF

3/ 4 n

for RC, SRC, and EBF

3/ 4 n

for others

0.07 h 0.05 h

(4)

Where hn is the height of the building above the base (in m). In addition, the fundamental period can also be estimated by a properly substantiated analysis. However, the value of estimated period shell not exceed 1.4 times of the approximate fundamental period. Due to basin effects, the corner periods noted in the response spectra associated with the earthquake data observed in Taipei Basin are generally larger than 1.0 second. This implies that the two parameters SDS and SD1 prescribed in the design response spectrum for general sites cannot be applied directly for the sites in Taipei basin. The representative values of corner period T0D between short and moderate period ranges of the design response spectrum are shown in Table 4. In addition, utilizing the uniform hazard analysis, the design spectral response acceleration SaD for a given site can be developed directly from the design spectral response accelerations at short periods SDS (=0.6g) as well as the corner period T0D for each seismic micro-zone in Taipei Basin, and can be expressed as; D

S DS 0.4 3T / To ; S aD

;

S DS D

0.4 S DS ;

(T

D

Zone 1

3.4 - 4.6

1.60

Zone 2

2.8 - 3.6

1.30

Zone 3

2.2 - 2.8

1.05

Zone 4

1.5 - 2.2

0.85

0.2 T0 )

(0.2 T0 (To

Table 4. Representation values of corner period for each micro-zone in Taipei Basin Micro-zone Range of Cv ToD or ToM (s)

D

(T

S DS To / T ;

Life Safety performance aimed. For the base isolated TPAC project, a Performance Based Design approach was employed with higher targets. TPAC is required to be elastic (R=1) in DBE event and to be Occupiable (R