土木毕业英文翻译.pdf

Seismic Assessment of Buildings Considering Post Earthquake Safety Karl Telleen Rutherford all rights reserved Tag The researchers worked with practicing structural engineers who tested the procedure on several example structures R all rights reserved Post earthquake occupancy status is typically determined after an earthquake by an inspector performing a visual screening of the damage and assigning a Green Yellow or Red Tag to the building to indicate full access limited access or no access permitted For different building types ATC 20 indicates particular damage observations that can be associated with each tag color In qualitative terms ATC 20 states that to receive a Green Tag the structure must be capable of withstanding at least a repetition of the event that caused the initial damage without collapse p 26 The Advanced Seismic Assessment Guidelines are not a replacement for ATC 20 s rapid post earthquake evaluation procedures Instead the Advanced Seismic Assessment method defines quantitative performance criteria for use in pre earthquake seismic assessments so that they relate to the ATC 20 framework Expected post earthquake tagging classifications are used because they are more specific and more quantifiable alternatives to traditional performance classifications such as Life Safety The method leads to retrofit recommendations that are directly related to building accessibility Also for buildings that have been assessed according to the Advanced Seismic Assessment Guidelines prior to an earthquake descriptions of expected visible damage of the type that can be drawn from the method will assist post earthquake inspectors in making rational tagging decisions when an earthquake does occur Applicability of the Method The Guidelines were developed for the seismic assessment of utility buildings but they are applicable to most types of structures Rutherford all rights reserved Step 1 Nonlinear Analysis of the Intact Structure Figure 1 Produce an incremental dynamic analysis IDA plot of peak spectral acceleration versus peak roof drift for the intact structure Typically this is achieved by performing a nonlinear static pushover analysis of the intact structure and then using the spreadsheet tool SPO2IDA Vamvatsikos 2002 to compute IDA curves based on the structure s pushover curve and fundamental period of vibration For most low and mid rise buildings where higher mode effects typically do not significantly influence nonlinear dynamic behavior the SPO2IDA program provides a consistent and practical means for approximating IDA results quickly Define damage states at roof drifts where significant losses in seismic force resisting capacity occur A damage state is a hypothetical condition that a building could be left in after an earthquake For example a building with concrete walls could have minimal cracks it could have small cracks or it could have large cracks and buckled reinforcing bars presumably the earthquake resisting capacity of the damaged walls is reduced from what it was before the first earthquake For practical purposes analysis of two to four representative damage states is typically sufficient Damage states should be chosen to capture the range of damage severity or roof drifts that will affect structural performance in aftershocks so that the resulting data points in Figure 3a provide a sufficient basis for interpolating the capacity curves shown in that figure If residual drift is likely to be significant estimate the expected residual drift at each damage state Maffei 2008 provides guidance for estimating residual drift and reducing a building s earthquake resisting displacement capacity accordingly 0 500 1000 1500 2000 2500 0 1 2 3 4 5 6 7 8 DS1 DS2 DS3 DS4 0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 0 1 2 3 4 5 6 7 8 Sa g IDA 16 IDA 50 IDA 84 Step 1 Nonlinear Analysis of the Intact Structure a Nonlinear static force displacement response pushover curve for the intact structure with damage states DS1 DS2 DS3 and DS4 defined at roof drifts where significant losses in seismic force resisting capacity occur b Incremental dynamic analysis IDA plot for the intact structure obtained using the four segment simplification of the static response from a and the spreadsheet tool SPO2IDA The SPO2IDA program infers nonlinear dynamic behavior of the structure using empirical relationships derived from incremental dynamic analyses of thousands of single degree of freedom systems with different force displacement response characteristics and subjected to different ground motions The program accounts for variability in nonlinear dynamic response by plotting three different confidence levels of maximum roof drift For example for a given spectral acceleration the IDA 84 curve indicates 84 confidence that the corresponding roof drift will not be exceeded In the elastic range the IDA curves are the same as the static response at greater roof drifts there is more variability in nonlinear dynamic response so the IDA curves spread further apart 430 ATC all rights reserved 完整结构响应 损坏状态 四段简化 基底剪力 k i p s V 屋顶漂移 静态响应 谱加速度 容量 屋顶漂移 Step 2 Nonlinear Analysis of the Damaged Structure Figure 2 Produce IDA plots of the damaged structure This is achieved using similar methods to those in Step 1 However strength and stiffness degradation of damaged elements will cause the damaged structure to have properties that are different from those of the intact structure i e The engineer must modify element properties in the structural analysis model Repeat this analysis for each damage state defined in Step 1 The engineer should appropriately model strength degradation If residual drift is expected the deformation capacity of the damaged structure should be reduced Maffei 2008 offers recommendations for modeling strength degradation and accounting for residual drift 0 500 1000 1500 2000 2500 0 1 2 3 4 5 6 7 8 V kips DS1 DS2 DS3 DS4 0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 4 0 Sa g IDA 16 IDA 50 IDA 84 Step 2 Nonlinear Analysis of the Damaged Structure a Nonlinear static force displacement response pushover curve for damaged structure in Damage State DS3 Similarly pushover and IDA curves must also be determined for damage states DS2 and DS4 though for brevity they are not shown here b Incremental dynamic analysis IDA plot for DS3 obtained using the four segment simplification of the static response from a and the spreadsheet tool SPO2IDA Step 3 Occupancy Limit States Figure 3 Plot the main shock spectral acceleration Sa to cause each damage state from Step 1 versus the aftershock acceleration to collapse the damaged structure from Step 2 and interpolate a curve between data points Identify main shock spectral accelerations a S at which occupancy limit states are expected to occur Occupancy limit states refer to the boundaries between the performance levels Green Tag and Yellow Tag or between Yellow Tag and Red Tag In Figure 3a the ordinates of Point A define a S for the limit state Onset of Yellow Tag and the ordinates of Point B define Onset of Red Tag The concept shown in Figure 3a is central to the Advanced Seismic Assessment method because it relates the performance of the building in an earthquake main shock to the safety of occupants following the earthquake defined in terms of collapse potential in the event of an aftershock In this example the limit state Onset of Yellow Tag is defined as having 84 confidence of no collapse in an aftershock equal to the main shock IDA 84 capacities from Step 2 are used Onset of Red Tag indicates 50 confidence IDA 50 capacities Based on these boundaries better than 84 confidence represents Green Tag performance between 50 and 84 confidence is Yellow Tag and less than 50 confidence is Red Tag See Maffei 2008 for further discussion of these tagging criteria their development and adaptability 431 ATC all rights reserved 完整的结构响应 D S 3 状态下损伤结构的响 四段简化 损伤状态 基底剪力 谱加速度 容量 静态响应 屋顶漂移 0 1 2 3 4 5 6 7 8 屋顶漂移 B A 00 5 1 1 5 22 5 Sa g 0 4 Sa g Step 3 Occupancy Limit States a For a given spectral acceleration demand the building s expected seismic performance level Green Tag Yellow Tag or Red Tag is based on the capacity of the damaged structure after an earthquake to survive an aftershock without collapse Point A marks the limit state Onset of Yellow Tag point B marks Onset of Red Tag Intersection with the horizontal axis marks Collapse in the main shock b One method for comparing earthquake hazard to structural performance uniform hazard spectra for the building site with expected main shock spectral acceleration to cause each limit state the ordinates of points A and B in Figure 3a indicated at the building s fundamental period of vibration Step 4 Fragility Curves Figure 4 Using the intact structure incremental dynamic analysis from Step 1 at the spectral accelerations to cause each limit state a S from Step 3 determine the uncertainty value associated with each limit state is the square root sum of squares of aleatory uncertainty R which measures the randomness of nonlinear response to ground motions and epistemic uncertainty U which measures uncertainty of structural capacity The Guidelines provide recommended default values of U for certain types of structures The SPO2IDA program outputs values of R associated with each spectral acceleration value based on the variability or spread of the IDA curves in Figure 1b Produce a fragility curve spectral acceleration Sa versus probability p of exceeding the limit state for each limit state Given a S and a fragility curve can be calculated according to the relation x aa eSpS 1 where p normal cumulative distribution x If desired fragility curves can also be produced for other limit states of interest such as Onset of Damage or exceedance of critical floor accelerations or story drifts Of these only the occupancy limit states Onset of Yellow Tag and Onset of Red Tag require analysis of the damaged structure as described in Steps 2 and 3 Producing fragility curves for Onset of Damage Collapse or floor accelerations or story drifts only requires analysis of the intact structure to identify spectral accelerations a S at which these limit states occur 432 ATC all rights reserved 引起塌陷的余震加速度反应谱 g 0 0 51 22 5 谱加速度 0 0 5 1 1 5 2 2 2 5 2 5 0 5 0 1 1 5 绿色标签红色标签黄色标签 绿 黄 红 在5 0 年内2 在5 0 年内5 在5 0 年内1 0 在5 0 年内5 0 极限状态性能 1 53 建设期 s 主震加速度反应谱 余震后未塌陷自信度5 0 余震后未塌陷自信度8 4 主震 余震 0 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 1 0 0123 Sa g FIGURE 4 Step 4 Fragility Curves Median p 50 values of spectral acceleration for each limit state correspond to the ordinates of Figure 3a Tails of each curve are extrapolated based on the uncertainty Steep fragility curves indicate less uncertainty flatter curves indicate more uncertainty Earthquake return periods of interest are labeled at their corresponding spectral acceleration values based on hazard data for the building site 0 10 0 08 0 06 0 04 0 02 0 00 0 02 0 04 0 06 0 08 0 10 020406080100120 FIGURE 5 Direct Incremental Dynamic Analysis IDA Method Nonlinear response history for a selected main shock to cause damage state DS4 followed by an incrementally scaled aftershock Similarly response histories must also be run for the intact structure and for damage states DS2 and DS3 though for brevity they are not shown here Peak spectral accelerations and maximum drift ratios are recorded for each scaling of the aftershock and plotted as in Figure 6 Figure reprinted from R all rights reserved 500yrs 1000yrs 2500yrs 5000yrs 超过极限状态的概率 谱加速度 损伤发病 黄色标签的发病 红色标签的发病 塌陷 主震 余震 屋顶漂移 时间 s e c 负方向余震的2 5 倍比例因子 负方向余震的1 2 5 倍比例因子 负方向余震的2 倍比例因子 主震的损伤 正方向余震的1 2 5 倍比例因子 正方向余震的1 5 倍比例因子 屋顶漂移 谱加速度 记录A记录B记录C记录D中位数 当前梁毁坏 当前梁毁坏 1 0 Analysis Methods For most structures particularly low and mid rise buildings a nonlinear static pushover analysis is used as described in Steps 1 and 2 above For such structures higher mode effects typically do not significantly influence the nonlinear dynamic behavior Based on the structure s static force displacement response pushover curve and the fundamental period of vibration the spreadsheet program SPO2IDA computes IDA curves Figures 1 and 2 The SPO2IDA program infers nonlinear dynamic behavior of the structure using empirical relationships derived from incremental dynamic analyses of thousands of single degree of freedom systems having different force displacement response characteristics and subjected to different ground motions For structures in which higher modes of vibration influence dynamic behavior the relationships from single degree of freedom systems in SPO2IDA may be less representative of the building s response For such structures a direct IDA procedure is necessary to produce IDA curves The direct IDA procedure involves running nonlinear response history analyses of both the intact and the damaged structure incrementally scaling up the ground motion records and recording peak drifts and spectral accelerations Figures 5 and 6 Several issues such as the number of ground motion records to run and the method for defining damage states make the direct IDA procedure less straightforward to implement than the inferred procedure using SPO2IDA EXAMPLE APPLICATIONS Rutherford all rights reserved For each building our seismic assessment and design for seismic retrofitting if necessary focus on key objectives Ensure adequate lateral displacement capacity of the gravity load resisting system particularly columns Gravity load resisting elements must be able to deform laterally with the building while supporting vertical loads The lateral displacement capacity of concrete columns for example can be limited by inadequate shear reinforcement ties If lateral deformations are expected to be larger than the deformation capacity of existing columns we can increase the columns deformation capacity such as by fiber wrapping or we can provide an alternate path for gravity loads such as supplemental columns placed adjacent to the vulnerable columns Ensure that the structure s governing mechanism of nonlinear seismic response is desirable For all but the simplest buildings there are several potential ways that the building could respond to a strong earthquake and the seismic assessment should consider each of these potential mechanisms For example in a moment frame building an undesirable story mechanism a concentration of lateral deformations in a single story is usually more likely if beams are strong compared to columns In a building with concrete walls a story mechanism is usually more likely if walls are weaker in shear than they are in flexure or if walls are not continuous to foundations To ensure a desirable mechanism of response we first identify the governing response of the existing structure If the existing structure s response is inadequate to achieve the desired seismic performance for the building we provide retrofit items that change the governing response to a more desirable mechanism for example providing additional shear strength to concrete walls so that they will yield in flexure in a major earthquake and will not exceed their shear strength capacity Both identification of the existing structure s response and design of seismic retrofit items are based on the principles of capacity design Paulay and Priestley 1992 For evaluation of existing structures this means determining the expected strengths of structural components and then performing a plastic analysis a nonlinear analysis or a comparison of relative strengths in order to establish which component actions e g flexure in walls shear in walls flexure in columns flexure in beams etc reach their capacities under the expected pattern of seismic lateral forces Maffei 2000 For designing retrofit items this means designating the desired component actions that will experience nonlinear behavior usually material yielding and providing all other actions with sufficient strength to ensure essentially elastic behavior little to no yielding when the designated yielding actions reach their capacities For both evaluation and retrofit we consider the post yield behavior of all elements expected to reach their yield strength in order to estimate the overall building response Provide ductile detailing and adequate connections To ensure desirable post yield behavior and avoid strength degradation we provide special structural details at members that we expect to yield in a major earthquake Ductile detailing means for example providing closely spaced horizontal shear reinforcement in concrete walls and columns to promote flexure governed behavior and to restrain buckling of vertical reinforcement under cyclic loading To ensure a complete force path for carrying earthquake forces to the building s foundations we check connections between elements of the seismic force resisting system in the existing structure as well as in the seismic retrofit design If connections represent weak links in the seismic force path we provide strengthened connections and or ensure that yielding connections have ductile detailing 435 ATC all rights reserved Providing the type of ductile behavior described above tends to make structures resilient to earthquakes such that they can sustain some damage in a large main shock and still provide adequate safety in the event of an aftershock The following examples describe seismic assessment and retrofit projects for selected buildings in Figure 7 Each building has unique structural characteristics and seismic performance goals and in all cases the Advanced Seismic Assessment method offers a sound approach for quantifying performance related to post earthquake safety San Francisco Central Services Garage The San Francisco Central Services Garage is a three story 140 000 square foot building built in the early 1930s Figure 8 The gravity load resisting system consists of reinforced concrete slabs beams columns and spread footings A concrete pier and spandrel system around the building perimeter provides the majority of the building s strength an