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1、外文文献翻译 英文Uniform-risk-targeted seismic design for collapse safety of building structuresSHI Wei, LU XinZheng & YE LiePing*Key Laboratory of Civil Engineering Safety and Durability of Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing 100084, ChinaReceived June 8 , 2011
2、; accepted January 6, 2012; published online March 28, 2012SeismicAbstract :Seismic design should quantitatively evaluate and control the risk of earthquake-induced collapse that a building structure may experience during its design service life. This requires taking into consideration both the coll
3、apse resistant capacity of the building and the earthquake ground motion demand. The fundamental concept of uniform-risk-targeted seismic design and its relevant assessment process are presented in this paper. The risks of earthquake-induced collapse for buildings located in three seismic regions wi
4、th the same prescribed seismic fortification intensity but different actual seismic hazards are analyzed to illustrate the engineering significance of uniform-risk-targeted seismic design. The results show that with Chinas current seismic design method, the risk of earthquake-induced collapse of bui
5、ldings varies greatly from site to site. Additional research is needed to further develop and implement the uniform-risk-targeted seismic design approach proposed in this paper. Keywords building structure, collapse safety, collapse resistant capacity, seismic hazard, uniform-risk-targeted seismic d
6、esignCitation: Shi W, Lu X Z, Ye L P. Uniform-risk-targeted seismic design for collapse safety of building structures. Sci China Tech Sci, 2012, 55: 1481-1488, doi: 10.1007/s11431-012-4808-71 IntroductionInvestigations on earthquake damage show that the collapse of building structures is the primary
7、 source of casualties and property losses during and after a severe earthquake. Therefore, ensuring the collapse safety of building structures is a critical objective of earthquake engineering 1, 2. In recent years, collapse fragility analysis (CFA) 35 based on incremental dynamic analysis (IDA) 6 h
8、as been widely used due to its excellent capacity in quantitatively evaluating resistance to earthquake-induced collapse. Through combining the results of a collapse fragility analysis with information on the seismic hazard of the site where the building is located, the risk of earthquake-induced co
9、llapse of the investigated building during its entire design service life can be quantitatively evaluated and controlled. This is the fundamental concept of uniform-risk-targeted seismic design 7, 8. The risk of earthquake-induced collapse is measured by the total probability that the building may c
10、ollapse due to earthquakes in Y years (e.g., 50 years). The objective of uniform-risk-targeted seismic design is to ensure that all buildings throughout a nation should have a uniform acceptable level of risk of earthquake-induced collapse, although they may be located in different seismic regions w
11、ith different seismic hazards.Seismic design of building structures in China is performed according to the seismic zonation map of China. As a simplified expression of the seismic hazard, the seismic zonation map identifies the seismic fortification intensity for each seismic region as well as the c
12、orresponding ground motion values for design purposes. The current Chinese seismic zonation map is defined on the basis of earthquakes with a 10% probability of being exceeded in 50 years (referred to as the fortification-level earthquake) 9, 10, while the collapse-prevention-level ground motion val
13、ues are extrapolated from the fortification-level values 9, 10. Although current Chinese seismic design based on the fortification-level earthquake already takes into account the safety requirements to prevent the earthquake-induced collapse, it sometimes fails to comprehensively account for the var
14、iation in seismic hazard due to the complex nature of earthquake mechanisms. Specifically, it is likely that in some regions, the extrapolated collapse-prevention-level ground motion values cannot satisfy the exceedance probability of 2% in 50 years as required in the Chinese Code for Seismic Design
15、 of Buildings 9. Consequently, buildings in some regions that are designed in accordance with the code 9 may have a high risk of earthquake-induced collapse. Furthermore, even if the ground motion values for collapseprevention-level earthquakes were accurately defined using a seismic hazard analysis
16、, the collapse risk of building structures still would not be geographically uniform throughout the nation because the hazard of a mega-earthquake (i.e., an earthquake more intensive than the collapse-prevention-level earthquake) 1 is different for different regions.Before 1997, the seismic zonation
17、 of the United States was mapped based on the earthquakes with an exceedance probability of 10% in 50 years. Since 1997, the maximum considered earthquake (MCE), corresponding to an exceedance probability of 2% in 50 years, has been defined as the baseline for the seismic zonation of the United Stat
18、es. Recently, some adjustments have been applied to MCE ground motions to achieve a uniform collapse risk of 1% collapse probability in 50 years 7. The development of the current seismic zonation map of the United States provides a good model for future seismic zonation mapping of China.This paper f
19、irst presents the basic concept of IDA-based collapse fragility analysis. Second, three cities with the same seismic fortification intensity but different seismic hazards are selected for analysis. The collapse risks of a given reinforced concrete (RC) frame structure located in each of the three ci
20、ties are predicted and compared. The result shows that though the three cities have the same seismic fortification intensity, the collapse risks of the same building in the three cities are quite different. Additional research is required to develop the uniform-risk-targetedseismic design approach p
21、roposed in this paper.2 Collapse fragility analysis based on IDA2.1 Basic processCollapse fragility analysis 35 based on IDA 6 involves the following four steps: (a) subjecting a structural model to a set of Ntotal earthquake ground motion records, (b) increasingly scaling each ground motion to mult
22、iple levels of intensity, (c) implementing nonlinear time-history analyses, and (d) obtaining the collapse probabilities versus intensity levels for further statistical analysis. At a certain intensity level, the number of ground motions that will result in structural collapse is referred to as Ncol
23、lapse. The collapse probability at this intensity level is estimated as Pcollapse=Ncollapse/Ntotal. By step-by-step scaling of the intensity level, the complete sequence of the buildings structural behavior can be investigated, from elasticity to yielding to collapse. Meanwhile, the serial collapse
24、probabilities versus incremental intensity levels are obtained (see the data points shown in Figure 1(a). By assuming a rational probability distribution (e.g., a lognormal distribution 4, 5), the cumulative distribution function of the selected intensity measure (IM) corresponding to structural col
25、lapse, referred to as the collapse fragility curve as shown in Figure 1(a), is obtained by statistical methods. The collapse fragility curve is a rational representation of the structures collapse resistant capacity, and its reliability depends on the selection and the total number of the ground mot
26、ion records adopted in the IDA 11. The FEMA 695 report proposes that the number of ground motion records should be larger than 20 to reflect the random nature of earthquakes 11.Figure 1 Collapse fragility curves. (a) From the perspective of conditional probability of structural collapse; (b) from th
27、e perspective of the cumulative probability function of collapse resistant capacity.2.2 Probabilistic significance of the collapse fragilitycurveThe collapse fragility curve represents the conditional collapse probability at given values of IM, denoted as P (collapse|IM) (see Figure 1(a) 12. For a g
28、iven ground motion record, the intensity of the ground motion is increased step by step until it reaches the threshold intensity IMcritical at which the structure is collapsed by the ground motion. This IMcritical is defined as the collapse resistant capacity (denoted as CRC) of that structure subje
29、cted to that ground motion. The value of CRC for a given structure signifies the maximum intensity level IMcritical of ground motion that the structure is able to resist. CRC has the same unit as the corresponding IM. Because of the record-to-record uncertainty of different ground motions, CRC is al
30、so a random variable. Hence, the collapse fragility curve also represents the cumulative distribution function of CRC.From the above discussion, the collapse fragility curve can be understood from the perspective of the conditional probability of collapse at a given intensity level (Figure1(a) and i
31、t can also be understood from the perspective of the cumulative probability distribution of CRC (Figure 1(b). For example, point A(IM*, P(collapse|IM*) on the curve in Figure 1(a) represents the conditional collapse probability at a given intensity equal to IM*, while the same point A(CRC*, P(CRCCRC
32、) represents the probability that earthquakes with intensity levels higher than CRC hit the building site (i.e., exceedance probability corresponding to the intensity level of CRC), obtained by implementing integration operation to P(IM).4 Example4.1 Structural layoutThe reinforced concrete frame st
33、ructure shown in Figure 2 is selected to illustrate the collapse risk assessment process. The six-story building is 22.6 m in height, with a seismic fortification category of C, and it is located on a site with a seismic fortification degree of seven (for which the corresponding peak ground accelera
34、tion, PGA, corresponding to a 10% exceedance probability in 50 years, is 0.10 g) and a site classification of II. The structure is designed in accordance with the Chinese Code for Seismic Design of Buildings (GB50010-2010) 9. A detailed description of its seismic design can be found in the study by
35、Shi et al. 5.4.2 Collapse fragility analysisNonlinear finite element modeling and IDA are performed using TECS (Tsinghua Earthquake Collapse Simulation) 1418, a program developed based on the general-purpose finite element software MSC. MARC for collapse simulation of complex structures. The collaps
36、e fragility analysis adopts the 22 far-field ground motion records proposed in the FEMA 695 report 11. The spectral acceleration at the fundamental period Sa(T1) is selected as the intensity measure 4, 5, 19. The collapse criterion is defined as the struc ture losing its vertical bearing capacity su
37、ch that it is unable to maintain enough space for life safety 5, 19.Figure 2 Structural layout (unit: mm). (a) Plane; (b) elevation.Figure 3 Collapse fragility curve of the RC frame.The serial collapse probabilities at discrete intensity levels are obtained via IDA and are denoted as “IDA points”, a
38、s shown in Figure 3. If the collapse fragility curve shown in Figure 3 is assumed to follow a lognormal distribution, the characteristic values of the probability distribution can be estimated based on the IDA results, i.e., =1.0257 and = 0.2596, where is the mean value of ln(CRC) and is the standar
39、d deviation of ln(CRC). Therefore, the cumulative probability distribution function (Figure 3) and the probability density function of CRC (Figure 4) are derived via the following equations, respectively 12.Figure 4 Probability density function of CRC.， （3），（4）4.3 Seismic hazard analysisThree locati
40、ons, Puxian City in Shanxi Province, Rizhao City in Shandong Province and Mojiang City in Yunan Province, are selected to compare the risks of earthquakeinduced collapse of the structure described above in different regions. The three regions have the same seismic fortification degree of seven, but
41、their actual seismic hazards are different. The ground motion PGA values corresponding to the serviceable-level earthquake, fortification-level earthquake and collapse-prevention-level earthquake for the three regions, according to Chinas seismic hazard characteristic zonation 20, are listed in Tabl
42、e 1. The three intensity levels are defined as the exceedance probabilities of 63% in 50 years, 10% in 50 years and 5% in 50 years, respectively. Table 1 shows that the three regions have the same PGA values for the fortification-level earthquake but different PGA values for the serviceable-level and collapse-prevention-level earthquakes.Because the spectral acceleration at the structural fundamental period Sa(T1) is used as the ground motion intensity measure in this analysis, the PGA values in Table 1 should be converted to correspond