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1、Engineering Seismology (3),Main Characteristics and Attenuation of Ground Motion,http:/www.cen.bris.ac.uk/civil/students/eqteach97/glossary.htm,http:/www.cen.bris.ac.uk/civil/students/eqteach97/earth4.htm,强地震动相当复杂，只能选择其主要的特征，地震动三要素： 幅值(amplitude)，最大值PGA、PGV、PGD，有效值EPA、EPV，均方根值Arms、Vrms、Drms，SI等； 频谱特
2、征：富氏谱(Fourier spectrum)、反应谱(response spectrum)和功率谱(power spectrum)等； 持续时间(duration)，绝对持时(0.05g或0.1g)、相对持时(5%-95%)等。,地震动幅值的统计平均变化规律,同样距离处，地震动幅值随着震级的增大而增大，在大震级段加速度幅值会产生饱和现象。 同震级，地震动幅值随着距离的增大而减小，大地震在近场也会产生饱和现象。 土层场地，尤其是软弱场地上，地震动幅值一般比基岩场地上大。在震中区或地震断裂附近，基岩场地上的地震动幅值有可能比土层场地上大。,The strongest earthquake mot
3、ions that have been recorded to date have peak accelerations between 1 g and 3 g, where 1 g (= 980 cm/sec2) is the acceleration of the Earths gravity field, although records with such large peak values are rare. It is less clear what threshold of ground motion needs to be exceeded to be considered “
4、strong motion”. Probably a logical level to choose would be about 10 cm/sec2, as the older strong-motion instruments (accelerographs) that traditionally defined the field are not able to resolve ground accelerations with amplitudes smaller than this. Modern digital accelerographs are much more sensi
5、tive, able to resolve peak accelerations to 0.1 cm/sec2 or smaller. People at rest are able to feel motions as small as 1 cm/sec2. In moderate magnitude earthquakes, damage to structures that are not designed for earthquake resistance appears at accelerations of about 100 cm/sec2.,PGA、PGV、PGD,这类参数因为
6、具有简单、直观的物理意义及 计算简便等优点，所以被人较早的认识和接受。 人们从静力的观点看待地震动，直观地认为最大加速度可以作为地震动强弱的标志，因为由此产生的侧向惯性力可代表地震动对结构的破坏作用；这以后又发展用地震动的峰值速度和峰值位移作为地震动强弱的标志，认为地震动的峰值速度与地震动的能量有关，峰值位移与变形有关。,最大增量速度（IV）、最大增量位移（ID）,增量速度：加速度脉冲下的面积，实际上代表速度变化的增量，它与质量的乘积代表结构的动量或者相当于地震左右的冲量作用 增量位移：速度脉冲下的面积,可用来刻画近断层区域的地震动破坏势,RMSA 、RMSV、RMSD,均方根加速度、均方根速
7、度和均方根位移定义为: 其中：x(t)为地震动的加速度、速度或位移， Td为地震动的持时。,对于量度地震动的能量方面更为有效一些,AI,阿里亚斯（Arias）烈度,谱烈度(Spectrum Intensity),Sv为对应阻尼比为的相对速度反应谱，T为周期，常取为0或0.2，是一个客观的物理量，并不涉及任何宏观现象。 谱烈度也是一个从能量的角度表征地震动潜在破坏势的参数，因为Sv反映了弹性单自由度体系的能量需要，但谱烈度一个明显的缺点就是它没有考虑持时的影响，而持时对结构的累积损伤是很重要的。,地震动反应谱(response spectrum),单自由度弹性系统在地震动作用下最大反应的绝对值与
8、体系的自振特征(自振周期或频率和阻尼比)之间的函数关系,（日）大崎顺彦著，吕敏申、谢礼立译，地震动的谱分析入门，地震出版社，1980 反应谱的算法,Response spectra describe peak time-domain response of a suite of single-degree of freedom oscillators to the seismic excitation. Response spectra play an important role in the development of engineering designs. The pseudo-re
9、lative velocity, PSV, is obtained from Sd by PSV = (2/ To) Sd. The pseudo-relative acceleration, PSA, is obtained from Sd by PSA = (2/ To) 2 Sd. In general, PSA Sa and PSV Sv, although these different spectra can have different asymptotic properties at high and low frequencies. The spectra are usual
10、ly computed for a range of damping, from h = 0% (undamped) to h = 20% of critical. This range is used because most manmade structures are similarly lightly damped. A damping of h = 5% is the most likely to be reported.,拟反应谱（pseudo response spectrum）,三联 反应谱,标准反应谱(normal response spectrum),动力放大系数 、 和
11、标准化形式的设计谱（规范）,由于加速度峰值往往不能很好地反映地震动的破坏作用，特别在高频分量较多时，可使加速度峰值很大，但高频分量对大多数结构物的反应或破坏并不起关键的作用，为了克服以上缺点，人们提出了有效峰值的概念，认为从抗震结构观点看，只有对结构反应有明显影响的量才是重要的，其中最常用的参数有：有效峰值加速度（EPA）、有效峰值速度（EPV）。,将阻尼比为5的加速度反应谱在周期0.10.5秒之间平均为一常值Sa，将阻尼比为5%的速度反应谱在周期1秒附近平均为一常值Sv，则有效峰值加速度、有效峰值速度的定义如下： EPA=Sa/2.5, EPV=Sv/2.5 这样定义的有效峰值与真实峰值相关
12、，但一般来说，并不等于、甚至不比例于真实的峰值。常数2.5是一个经验系数，其物理意义相当于大量地震动加速度反应谱的平均放大倍数。 峰值加速度与地震动过程中结构的最大内力无法直接联系，而有效峰值加速度则弥补了这个缺点，,Duration,Definition bracket duration（relative or absolute） Energy duration Engineering duration,Effect to seismic response of structures,The simplest of these is peak acceleration, which is e
13、asily obtained from the accelerogram. After records are digitized, it is common to obtain several additional parameters. Time domain parameters often include peak velocity and peak displacement. Duration of the strong shaking is generally considered important, but there is not a unique definition of
14、 duration. One simple approach is to measure the interval between the times when the peak acceleration first and last exceeds some threshold, usually 0.05g. The alternative approach namely to define the amount of time in which 90% of the integral of the acceleration-squared takes place. These two de
15、finitions lead to opposite results as distance increases. At large distances the peak ground motions decrease so that the interval duration goes to zero even though the ground was moving. On the other hand, the energy becomes dispersed, resulting in an increase in the time interval over which 90% of
16、 the total energy in the seismogram arrives. In the frequency domain, the Fourier amplitude spectrum and a class of spectra known as response spectra are generally determined.,其它参数,位移延性 能量（输入能量、滞回能量）,讨 论 如何用地震动的参数合理的表征地震动的潜在破坏势（或对结构的破坏能力）？ 评判的标准是什么？,地震动的衰减 (GM attenuation),最简单的预测地震动的经验关系，其中震源的影响用震级表示，地震波从震源到场地的传播影响用距离来描写，场地的影响用场地分类来描写；将距离项分解为几何扩散和非弹性衰减两项。 预测地震动的三种方法？,The peak acceleration and other peak parameters can be described by a .ground moti
