EARTHQUAKE RESILIENCE OF A 632- METER SUPER-TALL BUILDING WITH ENERGY DISSIPATION OUTRIGGERS

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1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska EARTHQUAKE RESILIENCE OF A 632- METER SUPER-TALL BUILDING WITH ENERGY DISSIPATION OUTRIGGERS ZHOU Ying 1, ZHANG Cuiqiang 2 and LU Xilin 3 ABSTRACT Structural systems of super-tall buildings, including RC/SRC core walls, frames, and sometimes bracings, are utilized collaboratively to resist strong earthquakes and wind loads. Outriggers are usually set to ensure the good workability of those systems. On one hand, the stiffness of outriggers contributes to the control of the displacement of the whole structure; on the other hand, the sudden alteration of the stiffness will form vulnerable stories subject to strong earthquakes. Moreover, under strong quakes, majority of energy input to super-tall buildings are dissipated by outriggers rather than coupling beams of shear walls. The idea of the paper is to propose an energy-dissipating outrigger, which will provide stiffness to the structures under frequently occurred earthquakes (63.2%/50 years) and yield under basic earthquake levels (10%/50 years). Buckling-Resistant Braces (BRBs) are included as diagonal web members of outriggers to achieve the objectives above and ensure the structural earthquake resilience. An example of the Shanghai Center, with a total height of 632m, is studied to compare the energy dissipation effect of BRB outriggers compared with traditional outriggers. The results show that the damped outrigger is more promising than the conventional outrigger system, which can bring a 6%~9% reduction in inter-story drift under rarely occurred earthquake (2%/50 years). The shear walls are effectively protected by remarkable reduction in shear forces and bending moment. The damped outrigger system will lead to a 4%~6% reinforcement saving. Earthquake resilience is also easily realized by quickly replacing BRBs after earthquakes. 1 Professor, State Key Laboratory of Disaster Reduction in Civil Engineering. Tongji University, Shanghai, China 2 PhD candidate, Research Institute of Structural Engineering and Disaster Reduction. Tongji University, Shanghai, China 3 Professor, State Key Laboratory of Disaster Reduction in Civil Engineering. Tongji University, Shanghai, China Zhou Y, Zhang CQ, Lu XL. Earthquake resilience of a 632-meter super-tall building with energy dissipation outriggers. Proceedings of the 10 th National Conference on Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 Earthquake Resilience of a 632-Meter Super-Tall Building with Energy Dissipation Outriggers ZHOU Ying 1, ZHANG Cuiqiang 2 and LU Xilin 3 ABSTRACT Structural systems of super-tall buildings, including RC/SRC core walls, frames, and sometimes bracings, are utilized collaboratively to resist strong earthquakes and wind loads. Outriggers are usually set to ensure the good workability of those systems. On one hand, the stiffness of outriggers contributes to the control of the displacement of the whole structure; on the other hand, the sudden alteration of the stiffness will form vulnerable stories subject to strong earthquakes. Moreover, under strong quakes, majority of energy input to super-tall buildings are dissipated by outriggers rather than coupling beams of shear walls. The idea of the paper is to propose an energy-dissipating outrigger, which will provide stiffness to the structures under frequently occurred earthquakes (63.2%/50 years) and yield under basic earthquake levels (10%/50 years). Buckling-Resistant Braces (BRBs) are included as diagonal web members of outriggers to achieve the objectives above and ensure the structural earthquake resilience. An example of the Shanghai Center, with a total height of 632m, is studied to compare the energy dissipation effect of BRB outriggers compared with traditional outriggers. The results show that the damped outrigger is more promising than the conventional outrigger system, which can bring a 6%~9% reduction in inter-story drift under rarely occurred earthquake (2%/50 years). The shear walls are effectively protected by remarkable reduction in shear forces and bending moment. The damped outrigger system will lead to a 4%~6% reinforcement saving. Earthquake resilience is also easily realized by quickly replacing BRBs after earthquakes.. Introduction The mega column-core wall-outrigger system is considered to be one of the most efficient structural systems to resist the lateral loading such as wind and earthquake loads. It is widely used in the high-rise buildings around world. The work theory of outrigger system is that it effectively reduces the lateral deflection and core wall moment by coupling the core wall and perimeter columns enabling buildings to utilize its entire building width as liver arm to resist the lateral load [1-5]. From this point of view, the stiffness of the outrigger is to be designed increasingly large. As far as the seismic performance is concerned, however, no abrupt change of structural stiffness is preferred. How to design the outrigger is a critical problem for engineers. Obviously, the outrigger system should be designed with appropriate stiffness. The conventional outrigger system uses steel braces with buckling behavior, and it is hard to be designed as elastic bars under major earthquakes (2%/50 years). The concept of damped 1 Professor, State Key Laboratory of Disaster Reduction in Civil Engineering. Tongji University, Shanghai, China 2 PhD candidate, Research Institute of Structural Engineering and Disaster Reduction. Tongji University, Shanghai, China 3 Professor, State Key Laboratory of Disaster Reduction in Civil Engineering. Tongji University, Shanghai, China

3 outrigger system appeared in recent years. A novel damping system by adding viscous dampers between outrigger walls and perimeter columns for a frame-core tube structure is first presented by Jeremiah [6]. The clear damped outrigger concept was given by Smith R.J. and Willford M.R. [7] and successfully applied to high-rise buildings in Philippine [8] and Korea [9]. From then on, many researchers focused on this kind of damped outrigger system [10-12]. The research above is about adding the viscous dampers to the building outriggers. The feature of this kind of damper only provides force other than stiffness with buildings. The stiffness, however, is sometimes needed if the structural deformation would be strictly limited under frequently occurred earthquakes (63.2%/50 years) like in China. This paper puts forward a damped outrigger system, which contribute stiffness to the whole structure under the frequently occurred earthquake and dissipate energy under the rarely occurred earthquake. Damped outrigger Damped outrigger system In conventional design, the outrigger system is designed as the common buckling steel members. The new outrigger presented in this paper is to replace the diagonal web member by buckling restrained brace (BRB). The parameter of the buckling restrained braces will be selected according to the seismic performance under different design levels. The force-displacement comparison of outriggers can be seen in Figure 1. Fig. 1. Comparison of traditional outrigger with BRB outrigger Working theory of the damped outrigger In China, there are three earthquake levels in the seismic design. The first level is the frequently occurred earthquake, which is of 63.2% exceeding probability in 50 years. The second level is the basic earthquake corresponding to 10% exceeding probability in 50 years. And the third level is the rarely occurred earthquake, which is 2% exceeding probability in 50 years. In practical engineering, the first level will be used to determine the cross section and reinforcement of members. Elastic forces and inter-story drifts will be checked. The second level

4 is usually realized by the seismic measures. The elasto-plastic inter-story drifts will be checked under the third level to prevent building collapse. Accordingly, various demands are set on the damped outriggers under three levels. A BRB outrigger should provide stiffness at the first level and change the stiffness and dissipate earthquake energy under the second and third levels. The mechanism is shown by a super-tall building below. Information of the building Building analysis Fig. 2 The elevation of Fig. 3 The plan layout of Fig. 4 Typical outrigger configuration the Shanghai Center the Shanghai Center of the Shanghai Center Shanghai Center is a super-tall building located in Shanghai, with a total height of 632 meter. The structural height is 580 meter and it has 124 stories. The building is divided into 9 zones by 8 strengthened stories, seen in Figure 2. The main structural members are the mega composite columns, concrete core walls, outrigger systems and belt trusses. The typical floor and outrigger system can be seen in Figure 3 and Figure 4, respectively. The member section and material is listed in Table 1. Figure 5 shows the analytical model of Shanghai Center in SAP2000 and Perform-3D. It is noted that the outrigger system in Perform-3D is shown in Figure 5(c). Belt truss system and mega columns are defined as mega frame, shown in Figure 5(d).

5 Table 1. Member information of Shanghai Center. No. of zone Mega column* Corner column* Concrete of column Thickness of outer wall * Thickness of inner wall * C C C C C C C C C C C C C C C C60 * unit in meter Concrete of wall (a) SAP2000 model (b) Perform-3D model (c) Outrigger (d) Mega-frame Fig. 5 Analytical model of Shanghai Center Periods of the building A comparison of periods between SAP2000 model and Perform-3D (PF3D) model is shown in the Table 2. Table. 2. Comparison of periods No. SAP2000 PF3D Error No. SAP2000 PF3D Error % % % % % % % %

6 % % % % % % % % % % % % % % % % % % % % % % Ground motion selection Dynamic time-history analysis is commonly used in performance-based earthquake engineering to predict the response of a structure subjected to earthquake ground motions. The selection of ground motions is very important to dynamic results. In Chinese code, based on the site and soil classification, the requirement for the selection of ground motions is 1) the average response spectrum of the selected ground motions should be statistically match the design response spectrum; and 2) the structural base shear force of each time history analysis should be in the range of 65%~135% of that of the response spectrum analysis, and the average base shear force of all time history analysis should be 80%~120% of that of the response spectrum analysis. Based on the matching degree between the design spectrum and the ground motion response spectra, ground motions is selected under earthquake levels. The peak ground acceleration is 0.035g and 0.2g under the frequently and rarely occurred earthquake, respectively. Fig. 6 Ground motions under frequently occurred earthquake Fig. 7 Ground motions under rarely occurred earthquake Comparison of the analytical results Inter-storey drift ratio The inter-story drifts of the building with traditional and BRB outriggers are compared in Figure 8. It can be illustrated that the new type outrigger replaced by buckling restrained braces

7 provides the same stiffness as the conventional outrigger under the frequently occurred earthquake. So the inter-storey drift of the whole structure is the same as the structure without damped outriggers. From the nonlinear time history analysis under rarely occurred earthquake (Figure 9), the inter-storey drift of the building with damped outrigger is smaller than that of the common building with conventional outrigger system. From the comparison of mean value, the inter-story drift is reduced by 6% and 9% in X and Y direction, respectively. This denotes that the damped outrigger system effectively control the inter-storey drift under rarely occurred earthquake. (a) X direction (X:Y=1:0.85) (b) Y direction (Y:X=1:0.85) Fig. 8 Inter-storey drift ratio (PGA=0.035g) (a) X direction (X:Y=1:0.85) (b) Y direction (Y:X=1:0.85) Fig. 9 Inter-storey drift ratio (PGA=0.2g) Shear force envelop of the concrete core wall The shear force envelop along the height of the concrete core wall can be seen in Figure 10~11. The reduction of damped outrigger to the shear force envelop is 10% and 20% in X and Y direction, respectively. The damped outrigger can smooth the shear abruption along the height of the concrete core wall. It can thus protect the core wall under the rarely occurred earthquake from suffering severe damages.

8 (a) X direction (X:Y=1:0.85) (b) Y direction (Y:X=1:0.85) Fig. 10 Shear force envelop (PGA=0.035g) (a) X direction (X:Y=1:0.85) (b) Y direction (Y:X=1:0.85) Fig. 11 Shear force envelop (PGA=0.2g) Moment envelop of the concrete core wall Figure 12 and 13 give the moment envelop of the concrete core wall under frequently and rarely occurred earthquakes, respectively. Using the damped outrigger, it can be calculated that the moment envelop of shear wall can be reduced by 7% in X direction and 4% in Y direction. (a) X direction (X:Y=1:0.85) (b) Y direction (Y:X=1:0.85) Fig. 12 Moment envelop (PGA=0.035g)

9 (a) X direction (X:Y=1:0.85) (b) Y direction (Y:X=1:0.85) Fig. 13 Moment envelop (PGA=0.2g) Additional damping ratio The energy dissipated by BRB elements can be calculated through the force-deformation hysteretic curves. The additional damping ratio is derived from the Eq. 1. Ed ξa = (1) 4π E s in which, E d : The energy dissipated by all the BRB elements; E s : The strain energy of the whole structure. ξ a : Additional damping to the whole structure by the BRB elements. The additional damping ratio by the BRB elements at different time histories can be shown in the Figure 18. (a) X:Y=1:0.85 (b) Y:X=1:0.85 Fig.18 Additional damping ratio at different time histories under the rarely occurred earthquakes Economic analysis In practical engineering, shear force is usually taken by horizontal reinforcement, whereas the moment is resisted by the vertical reinforcement. So from the envelops of shear force and moment, the saving ratio can be calculated and seen in the Figure 19. The saving ratio was calculated in the following equation: ( ANon ABRB ) Ω= 100% (2) A Non

10 in which, A Non is the area envelop of the building without damped outriggers; A BRB is the area envelop of the building with damped outriggers. It can be concluded from Figure 19 that the saving ratio to both horizontal and vertical reinforcement will reach 4%~6%. The damped outrigger is a cost-effective system. (a) Ratio=3.84% (X:Y=1:0.85) (b) Ratio=5.73% (Y:X=1:0.85) (c) Ratio=4.71% (X:Y=1:0.85) (d) Ratio=5.73% (Y:X=1:0.85) Fig. 19 The saving reinforcement ratio under rarely earthquake Conclusions The average height of top 10 super-tall buildings in the world has reached 600m. Most of them introduce outriggers to coordinate the lateral systems to resist horizontal earthquake and wind loads. The stiffness of the outriggers is needed to control the drift while the vulnerable stories around outriggers are easily formed under earthquakes. Thus, a damped outrigger that replaces the diagonal web members by buckling restrained braces is proposed and studied on Shanghai Center Tower. From the comparison of the damped outrigger structure to the traditional one, the earthquake resilience of the system can be realized as follows. (1) The damped outriggers can be set on the basis of building demands under earthquake levels. For example, they can contribute stiffness to the whole structure under the frequently occurred earthquake and dissipate energy under the rarely occurred earthquake. (2) With the damped outrigger, there is no much difference in the building inter-story drift under frequently occurred earthquake but a 6%~9% reduction under rarely occurred earthquake. The global displacement is generally controlled and smoothed.

11 (3) The shear force and bending moment for the shear walls are also remarkable reduced under strong earthquakes using damped outrigger. The shear walls are effectively protected. (4) Through the economic analysis, the damped outrigger system will lead to a 4%~6% reinforcement saving. (5) The damped outriggers can be quickly replaced by new BRBs after earthquakes, so the building is easy to retrofit to realize the earthquake resilience. Acknowledgments The authors are grateful for the financial support in part from the National Natural Science Foundation of China (Grant No , ) and Shanghai Rising-Star Program (Grant No. 13QA ). References 1. Hoenderkamp JCD. Shear wall with outrigger trusses on wall and column foundations. The Structural Design of Tall and Special Buildings, 2004; 13: Stafford SB, Salim I. Parameter study of outrigger-braced tall building structures. Journal of the Structural Division, 1981; 107: Hoenderkamp JCD, Bakker MCM. Analysis of high-rise braced frames with outriggers. The Structural Design of Tall and Special Buildings, 2003; 12: Coull A, Who L. Analysis of multi-outrigger-braced tall building structures. Journal of Structural Engineering, 1989; 115(7): Moudares FR. Outrigger-braced coupled shear walls. Journal of Structural Engineering, 1984; 110(12): Jeremiah C. Application of damping in high-rise building. Massachusetts Institute of Technology, Boston, USA Smith RJ, Willford MR. The damped outrigger concept for tall buildings. The Structural Design of Tall and Special Buildings, 2007; 16: Willford M, Smith R, Scott D and Jackson M. Viscous dampers come of age. Structure Magazine, 2008; 6: Park K, Kim D, Yang D. A Comparison Study of conventional construction methods and outrigger damper system for the compensation of differential column shortening in high-rise buildings. International Journal of Steel Structures, 2010; 10(4): Wang ZH, Chang CM, Spencer BF and Chen ZQ. Controllable outrigger damping system for high rise building with MR dampers. Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, 2010; 7647:76473Z Z Chen Y, McFarland DM, Wang Z, Spencer BF, and Bergman LA. Analysis of tall buildings with damped outriggers. Journal of Structural Engineering, 2010; 136(11): Zhou Y, Lu XL and Zhang CQ. Seismic performance of a super-tall building with energy dissipation outriggers. Vibration and shock 2011; 30(11): (in Chinese)