A COMPARISON OF THE SEISMIC DESIGN OF TALL RC FRAME-CORE .

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10NCEETenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake EngineeringJuly 21-25, 2014Anchorage, AlaskaA COMPARISON OF THE SEISMIC DESIGNOF TALL RC FRAME-CORE TUBESTRUCTURES IN CHINA AND THEUNITED STATESXinzheng Lu1 , Mengke Li2, Xiao Lu3 and Lieping Ye1ABSTRACTTo evaluate the main differences in the structural seismic design codes of China and the UnitedStates from a structural system viewpoint, a comparative evaluation was conducted on a typicaltall reinforced concrete (RC) frame-core tube building, a widely used structural form in bothcountries. The building, for which the original design information was provided by the PacificEarthquake Engineering Research Center (PEER), was first redesigned according to Chineseseismic design codes. Next, the component dimensions, dynamic characteristics, seismic designforce and construction material consumption in these two buildings were compared in detail.Subsequently, nonlinear finite element models of the two buildings were constructed to evaluatetheir seismic performance at the Maximal Considered Earthquake (MCE) level. This study findsthat there are clear differences between the Chinese and U.S. seismic design methods. For thistall building, the seismic design force determined by the Chinese response spectrum is largerthan the force determined by the U.S. spectrum at the same seismic hazard level, and the Chinesecodes specify a stricter inter-story drift ratio requirement, leading to a larger seismic design forceand a correspondingly higher amount of material consumption. However, the two buildingsexhibit roughly similar performances at the MCE level. These outcomes suggest usefulinformation for the further optimization of the design of tall buildings in China.1Professor, Dept. of Civil Engineering, Tsinghua University, Beijing, 100084, ChinaGraduate Student, Dept. of Civil Engineering, Tsinghua University, Beijing, 100084, China3Assistant Professor, School of Civil Engineering, Beijing Jiaotong University, Beijing, 100044, China2Lu XZ, Li MK, Lu X and Ye LP. A comparison of the seismic design of tall RC frame-core tube structures in chinaand the united states. Proceedings of the 10th National Conference in Earthquake Engineering, EarthquakeEngineering Research Institute, Anchorage, AK, 2014.

10NCEETenth U.S. National Conference on Earthquake EngineeringFrontiers of Earthquake EngineeringJuly 21-25, 2014Anchorage, AlaskaA Comparison Of The Seismic Design Of Tall RC Frame-Core TubeStructures In China And The United StatesXinzheng Lu1 , Mengke Li2, Xiao Lu3 and Lieping Ye1ABSTRACTTo evaluate the main differences in the structural seismic design codes of China and the UnitedStates from a structural system viewpoint, a comparative evaluation was conducted on a typicaltall reinforced concrete (RC) frame-core tube building, a widely used structural form in bothcountries. The building, for which the original design information was provided by the PacificEarthquake Engineering Research Center (PEER), was first redesigned according to Chineseseismic design codes. Next, the component dimensions, dynamic characteristics, seismic designforce and construction material consumption in these two buildings were compared in detail.Subsequently, nonlinear finite element models of the two buildings were constructed to evaluatetheir seismic performance at the Maximal Considered Earthquake (MCE) level. This study findsthat there are clear differences between the Chinese and U.S. seismic design methods. For this tallbuilding, the seismic design force determined by the Chinese response spectrum is larger than theforce determined by the U.S. spectrum at the same seismic hazard level, and the Chinese codesspecify a stricter inter-story drift ratio requirement, leading to a larger seismic design force and acorrespondingly higher amount of material consumption. However, the two buildings exhibitroughly similar performances at the MCE level. These outcomes suggest useful information forthe further optimization of the design of tall buildings in China.IntroductionTall buildings have rapidly become popular in China over the past several decades. However,China is also an earthquake-prone country located at the intersection of the Pacific and Eurasianseismic belts. Thus, the seismic safety of China’s tall buildings is a critically important issue.Although considerable progress has been made in the latest Code for the Seismic Design ofBuildings GB50011-2010 [1] and Technical Specification for Concrete Structures of TallBuilding JGJ3-2010 [2], which are the major seismic design codes in China, none of the tallbuildings in China has undergone a truly strong earthquake. The lack of exposure to such strongearthquakes limits the information that may be used to improve the design philosophies of tallbuildings in China. Therefore, it is important to study the efforts made in countries withsubstantial experience in effective seismic design for tall buildings.The United States (U.S.) and Japan both have a long history of tall building construction1Professor, Dept. of Civil Engineering, Tsinghua University, Beijing, 100084, ChinaGraduate Student, Dept. of Civil Engineering, Tsinghua University, Beijing, 100084, China3Assistant Professor, School of Civil Engineering, Beijing Jiaotong University, Beijing, 100044, China2Lu XZ, Li MK, Lu X and Ye LP. A comparison of the seismic design of tall RC frame-core tube structures in chinaand the united states. Proceedings of the 10th National Conference in Earthquake Engineering, EarthquakeEngineering Research Institute, Anchorage, AK, 2014.

and as a result have developed comprehensive seismic design philosophies. The tall buildings inboth countries have exhibited good seismic performance during earthquakes. Many detailedcomparisons of the seismic design codes of the U.S., Japan and China have been performed byvarious researchers. However, the actual seismic safety of the structures in each country isensured by the entire system of structural design specifications; therefore, a simple comparisonof an isolated provision or coefficient may not be sufficient to fully assess the designphilosophies and margin of safety in the respective codes. Hence, an effective researchmethodology would involve selecting a building with a specified seismic design objective,designing it based on the design specifications of the different countries, and then comparing theperformances of these buildings. Because such a comparative study entails an enormousworkload and is difficult to implement, few studies using this methodology have been reported.Therefore, based on a typical reinforced concrete (RC) frame-core tube tall building,which is a widely used structural form in both China and the U.S., a comparative evaluation wasconducted to identify the differences between the seismic design practices in the two countriesfrom a structural system viewpoint. First, the building was redesigned according to the Chineseseismic design codes based on the original building information provided by the PacificEarthquake Engineering Research Center (PEER). Next, the component dimensions, seismicdesign force, dynamic characteristics and construction material consumption of the two buildingsdesigned according to the Chinese and U.S. codes were compared in detail. Nonlinear finiteelement models of the two buildings were then constructed to evaluate their seismic performanceat the MCE level, providing useful information to enable further improvement of the designphilosophies for tall buildings in China.Background of the StudyTo evaluate and improve the performance-based seismic design of tall buildings, PEER launchedthe Tall Buildings Initiative (TBI) research program in 2006. As part of the TBI program, a casestudy project on tall buildings was conducted under Task 12 of the program, and the final report,entitled Case Studies of the Seismic Performance of Tall Buildings Designed by AlternativeMeans [3], was subsequently released. One of the buildings in this report, Building 2, is an RCframe-core tube structure and the detailed design information of this building is given, thusproviding a representative benchmark for our comparative study of the seismic design of tallbuildings in China and the U.S.Building 2 is a 42-story residential building that includes a 6.1-m tall penthouse and fourstories below ground. It is located in Los Angeles. This building is an RC frame-core tubestructure with a total height of 141.8 m above ground. Fig. 1a shows the three-dimensional viewand typical floor plan of the prototype Building 2 presented in the case study report [3]. In thisreport, Building 2 was designed according to the three U.S. design codes, with the three differentdesigns designated as Building 2A, Building 2B and Building 2C, respectively. Building 2A wasdesigned according to the International Building Code [4], which requires the use of ASCE 7-05[5] and ACI 318-08 [6]. As the IBC 2006 [4] is one of the most widely used seismic designcodes, the following discussion will focus on Building 2A.Based on the design information on Building 2A, this building was redesigned according

to the Chinese building design codes, mainly including GB50011-2010 [1], JGJ3-2010 [2] andthe Code for Design of Concrete Structures GB50010-2010 [7]. PKPM design software wasemployed, which is a product of the China Academy of Building Research (CABR). In thefollowing discussion, the building designed according to the Chinese codes is referred to asBuilding 2N, for which the three-dimensional view and typical floor plan are shown in Fig. 1b.In the design procedure of Building 2N, the vertical design loads (superimposed dead loads andlive loads), the site conditions, the seismic hazard level and the geometry, including the overalldimensions of the structure, the position and dimensions of the core tube, the column grid arrayand the story height are the same as those of Building 2A. Note that the effect of the basementwas not taken into account, which means that the structure was fixed at the ground 447010AEE70107010YFFColumn 1Column 2Column 3Beam 1X(a) Building 2A(b) Building 2NFigure 1. Three-dimensional view and typical floor plan of Building 2A and 2N (mm).Seismic Design LoadThere are some differences between the seismic design methods in the Chinese and the U.S.codes. In the Chinese codes, the fortification level earthquake (i.e. 10% probability ofexceedance in 50 years) is used to define the Seismic Ground Motion Parameter Zonation Mapof China [8]. The Chinese code for the structural seismic design of buildings adopts a two-stagedesign method. The first design stage is an elastic design procedure under frequent earthquake(i.e. 63% probability of exceedance in 50 years). In this stage, the design seismic forces arecalculated with the acceleration spectrum at the level of frequent earthquake, so the load carryingcapacity and the elastic deformation are evaluated with these corresponding seismic forces. Thesecond design stage is an inelastic deformation check procedure under severe earthquakes (i.e.2 3% probability of exceedance in 50 years). In this stage, seismic inelastic deformation needsto be assessed to avoid serious damage or collapse.The seismic design method in IBC 2006 [4] is an inelastic design procedure under thedesign earthquake, which means a structure can be economically designed according to thereduced elastic seismic design forces, while elements are detailed to reliably exhibit ductilebehavior in order to maintain the basic life safety performance objective. IBC 2006 [4] utilizesMaximum Considered Earthquake (MCE, 2% probability of exceedance in 50 years) ground

motion maps to define the earthquake intensity in different regions in the conterminous UnitedStates. The design procedure is as follows. The MCE spectrum is first calculated according to themapped acceleration parameters and site coefficients, and the corresponding design spectrum is2/3 times that of the MCE spectrum. The design spectrum is then reduced by the responsemodification coefficient R for the calculation of the seismic design lateral force or base shear,which is used in the following elastic structural analysis. Through the elastic analysis, theinternal forces in components can be obtained. The design lateral force-induced drift from theelastic analysis should thus be multiplied by a deflection amplification factor, Cd to estimate themaximum inelastic drift.This study focuses on the difference in seismic performance between the two structuresdesigned according to the Chinese and U.S. codes. Building 2A is located on an NEHRP siteclass C, with an equivalent shear-wave velocity of 30 m soil (VS30), equal to 360 m/s. Thecharacteristic period of the site is 0.455 s. This site condition is approximately equal to Site-classII and the 3rd Group in GB50011-2010 [1]. A key problem in this study is to determine theproper intensity of earthquake for the seismic design of Building 2N that ensured an identicalseismic hazard level between Buildings 2N and 2A. As the exceedance probability of MCEdefined in the U.S. design code is approximately equivalent to that of a severe earthquake asdefined in the Chinese design code, the response spectra for a severe earthquake in an 8.5 degreeseismic intensity zone and 9 degree seismic intensity zone in China are plotted in Fig. 2 incontrast to the site-specific MCE spectrum. It should be noted that the corresponding peakground acceleration (PGA) value of the fortification level earthquake (i.e. 10% probability ofexceedance in 50 years) is 300 cm/s2 in the 8.5 degree seismic intensity zone and 400 cm/s2 inthe 9 degree seismic intensity zone.1.61.41.2Acceleration (g)1Spectrum for 8.5 degree zoneSite-specific MCE spectrumSpectrum for 9 degree zone0.80.60.40.200123T (s)456Figure 2. Comparison between the site-specific MCE spectrum and the two Chinese responsespectra.Fig. 2 shows an approximate match between the site-specific MCE spectrum and the twoChinese response spectra. It is obvious that the response spectrum for the 9 degree seismicintensity zone better matches the site-specific MCE spectrum for short periods; the responsespectrum for the 8.5 degree seismic intensity zone better matches the site-specific MCE spectrumfor moderate periods (approximately 2.5 s) and the values of the two Chinese response spectraare both greater than the value of the site-specific MCE spectrum for long periods (beyond 2.5 s).Therefore, the 8.5 degree seismic intensity specified in the Chinese seismic code was selected as

the design intensity for Building 2N and used for the seismic design of Building 2N for thefollowing reasons. (1) As specified in the Chinese code JGJ3-2010 [2], the height limit for RCframe-core tube structures, such as Building 2N, is rigorously strict in zones of 9 degree seismicintensity (no more than 60 m). Therefore, a 9 degree seismic intensity is not suitable for thedesign intensity requirement of Building 2N. (2) According to the empirical formula of thefundamental period for the RC frame-core tube structures in China, the estimated fundamentalperiod of Building 2N is approximately 2.52 to 5.04 s. Fig. 2 shows that the response spectrumfor a severe earthquake in the 8.5 degree seismic intensity zone is closer to the site-specific MCEspectrum in this period range.Comparison of the Design ResultsEffective Seismic Weight and the Design PeriodThe effective seismic weight and the design periods of the two buildings are compared in Table 1.The seismic weight of Building 2N is the sum of the self-weight of the structure plus 0.5 timesthe live load, in accordance with the provisions of 5.1.3 of the Code for Seismic Design ofBuildings GB50011-2010 [1]. The effective seismic weight of Building 2A includes the totaldead load and other loads required by Section 12.7.2 in ASCE 7-05 [5].Table 1.The effective seismic weight and design periods.Effective seismicweight (ton)Period (s)T1T2T3Building 2NBuilding anslation mode in the X directionTranslation mode in the Y directionTorsion modeAs the Chinese codes adopt an elastic design procedure under the frequent earthquake,the design periods of Building 2N are calculated using the elastic stiffness provided by PKPMsoftware. However, IBC 2006 [4] adopts an inelastic design procedure under the designearthquake in which effective component stiffness values (e.g. 0.7EIg for columns and 0.35EIgfor beams) are used when developing the analysis model for design. The case study report [3]provides the stiffness assumption used in the design of Building 2A. Note that the basement isincluded in the analysis model of Building 2A, which makes the design periods longer.Materials and Dimensions of the Main ComponentsThe layouts of Building 2A and Building 2N are shown in Fig. 1. The materials and dimensionsof the main components in the two buildings are compared in Table 2. It is evident that Building2N has larger columns and more internal walls in the core tube than Building 2A. This differenceis mainly because the seismic design force determined by the Chinese response spectrum islarger than the force determined by the U.S. spectrum at the same seismic hazard level, and theChinese codes specify a stricter inter-story drift ratio requirement, leading to a larger seismicdesign force and a higher structure stiffness.

Table 2.The materials and dimensions of the main components.Building 2ABuilding 2Nfc’ 5 ksi ( 34.5 MPa)C40Beams762 914250 500, 450 900’fc 5, 6, 8, 10 ksimaterialC60, C50, C40( 34.5, 41.4, 55.2, 69.0 MPa)Columnsdimension (mm)1170 1170 - 915 9151500 1500 - 800 800materialfc’ 5, 6 ksi ( 34.5, 41.4 MPa)C60, C50, C40Shear wallsthickness (mm)610, 460600-400Note: The standard compressive strengths for C40, C50, and C60 concrete prismsare 26.8 MPa, 32.4 MPa, and 38.5 MPa, respectively.materialdimension (mm)Design Lateral Force and Inter-Story Drift RatioThe design seismic forces of Building 2N are calculated with the acceleration spectrum at thefrequent earthquake, so the load carrying capacity and the elastic deformation are evaluated withthese corresponding seismic forces. The design seismic forces of Building 2A are calculated withthe reduced design acceleration spectrum by the response modification coefficient R; and thensubsequently, from an elastic analysis, the internal forces in components can be calculated, andthe drift at the design lateral force may be obtained (should be multiplied by Cd).The seismic design forces along the building height of Building 2A and 2N are displayedin Fig. 3a (in which the response modification coefficient R is already considered). Clearly, theseismic base shear force of Building 2N is 1.47 times that of Building 2A. The seismic responsecoefficients in the Chinese and U.S design codes are shown in Fig. 3b. The design informationon Building 2A in the case study report [3] indicates that the design base shear force Vt,determined directly by modal response spectrum analysis, is smaller than 85 % of the base shearforce V, calculated by the equivalent lateral force (ELF) procedure. However, Section 12.9.4 inASCE 7-05 [5] clearly specifies that if the modal base shear force Vt is less than 0.85V, thedesign modal base shear force shall be scaled with 0.85V/Vt. The seismic response coefficients ofBuilding 2A, as well as the upper limit (Eq.12.8-3 in ASCE 7-05) and lower limit (Eq.12.8-5 inASCE 7-05), are also shown in Fig. 3b. The base shear force of Building 2A determined by theELF procedure is controlled by the lower limit of Eq.12.8-5 in ASCE 7-05 (the red line in Fig.3b), while the equivalent seismic response coefficient to the 0.85V is shown as the blue line.Thus, the comparison in Fig. 3b indicates that the seismic re

the Tall Buildings Initiative (TBI) research program in 2006. As part of the TBI program, a case study project on tall buildings was conducted under Task 12 of the program, and the final report, entitled Case Studies of the Seismic Performance of Tall Buildings Designed by Alternative Means [3], was subsequently released.

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