设定地震场地相关设计反应谱的研究

我国现行国家标准《水工建筑物抗震设计标准》(GB 51247—2018)规定,场地相关设计反应谱宜采用与基于《工程场地地震安全性评价》(GB 17741—2005)的设计地震动峰值加速度相应的“设定地震”确定。为便于工程技术人员理解和应用,本文进行了确定设定地震内涵和步骤的高级科普性阐述,将按《〈水电工程水工建筑物抗震设计规范〉(NB 35047—2015)设定地震确定场地相关设计反应谱补充说明》,以“最大发生概率原则”的确定方法加以说明。我国缺乏直接由强震记录统计的加速度反应谱衰减关系,权且采用美国“下一代衰减关系(NGA)”,其中涉及到震中距和断层距的转换,本文对转换过程进行了进一步论述,以期形成共识。此外,考虑到不同“NGA”衰减关系给出的加速度反应谱差异较大,提出确定设定地震反应谱的新思路。建议汇集已有岩基上较近距离的实测强震记录的反应谱资料,对其按震级和距离进行适当分档,然后插值确定设定地震的场地相关设计反应谱,这样既可以免除当前我国缺乏反应谱衰减关系的困难,还能使确定场地相关设计反应谱的过程大为简化和合理。

基于设定地震确定重大水电工程场地相关设计反应谱

重大水电工程设计反应谱的合理确定是工程抗震设计的基本前提。本文提出基于场地地震安全性评价给定的输入参数、衰减关系和设计地震动加速度值,依据发生概率最大的原则和潜在震源中主要发震构造来确定设定地震的震级和震中距的方法。由设定地震计算场地相关设计反应谱时,建议采用不经烈度转换的反应谱衰减关系。实例分析表明,本文方法所确定的设定地震及场地相关设计反应谱较为合理,能满足重大水电工程抗震设计需求。

Seismic safety of high concrete dams

China is a country of high seismicity with many hydropower resources. Recently,a series of high arch dams have either been completed or are being constructed in seismic regions,of which most are concrete dams. The evaluation of seismic safety often becomes a critical problem in dam design. In this paper,a brief introduction to major progress in the research on seismic aspects of large concrete dams,conducted mainly at the Institute of Water Resources and Hydropower Research(IWHR) during the past 60 years,is presented. The dam site-specific ground motion input,improved response analysis,dynamic model test verification,field experiment investigations,dynamic behavior of dam concrete,and seismic monitoring and observation are described. Methods to prevent collapse of high concrete dams under maximum credible earthquakes are discussed.

Seismic safety analysis of tall concrete dams, investigation and insights on critical challenges

This paper discusses critical and potentially controversial issues related to the seismic safety of tall concrete dams. These include the seismic input at a dam site, the effective treatment of the damage-rupture process, and the consideration of compressibility of reservoir water for hydrodynamic pressure. Major challenges to currently popular but questionable treatments of these critical problems are presented. Insights and additional research on these critical challenges are emphasized and explained based on prior published works of the author. More reasonable alternatives to dealing with these potentially controversial problems are provided in light of engineering practice in China. First, the design seismic input at depth as deconvoluted from an arbitrarily selected recorded accelerogram at a control point of an artificially developed free-field surface with the elevation of the dam crest is difficult for engineering projects to accept as appropriate. It may be more reasonable to use the design seismic incident motions as half of the ground surface motions from seismic safety analyses obtained from deterministic or probability approaches conducted by seismologists according to approved standards or guidelines. Second, since seismic damage to the dam must be estimated separately following uniaxial tensile and compressive experimental damage evolution rules, a simplified and realistic nonlinear elastic model is proposed as an alternative to the plastic-damage coupling model, which is very complex and includes assumptions based on a number of uncertainties. Finally, the effect of the reflection coefficient for compressibility of reservoir water on hydrodynamic pressures is very sensitive. The notion that the applied unified reflection coefficient at the reservoir bottom could be frequency-dependent and exhibit a significant variability in space as confirmed by field tests is questionable. To neglect the compressibility of reservoir water it may be closer to engineering practice at present.

Lessons learned from Wenchuan earthquake for seismic safety of large dams

This paper describes some special features of the Wenchuan earthquake that affected dam safety. Damage and performance of dams, primarily for four dams over 100 m high located in the affected earthquake area, are briefly described. Lessons learned related to dam safety from this devastating earthquake are preliminarily drawn. As the seismic safety of high dams during strong earthquakes has gained more attention around the world, some critical issues related to dam construction in China are considered and extensively discussed. Questions such as ＂Why is dam construction necessary in earthquake prone countries such as China？＂, ＂Can we accurately evaluate the seismic safety of high dams in China？＂, ＂Did reservoir impounding of the Zipingpu and Three Gorges Projects trigger the Wenchuan Earthquake in some way？＂ and ＂What is the strategic priority of dam safety for large dams in China？＂ are discussed. Finally, the corresponding tactics with response to the challenge are suggested and recent preliminary progress mainly achieved in IWHR is briefly introduced.

On the obstacles and way to assess the seismic catastrophe for high arch dams

To prevent possible seismic catastrophe, naturally, its assessment is deeply concerned over in China as a series of arch dams of about 300 m high will be constructed in the severe seismic regions. In this paper the major obstacles to the seismic catastrophe assessment of high arch dams which focused on clearly defining the Maximum Credible Earthquake (MCE) and reasonably selecting its site-specific seismic input parameters as well as quantitatively evaluating the limit state of dam-breach for designers are emphasized. Some breakthrough progress with pending problems is presented, such as to adopt more reasonable seismic input parameters based on seismic hazard evaluation of dam site; to develop model and method more fit in with the reality for non-linear seismic analysis of dam-foundation-reservoir system. The ideals of further improvement both in evaluating the MCE and defining the quantitative index of its performance objective are discussed, including how to use semi-empirical method of simulating strong ground motion near fault, how to solve the long-standing problem of stress singularity at dam heel, and how to investigate dynamic behaviors of fully-graded dam concrete through dynamic tests and 3-dimensional meso-mechanics analysis checked by CT technique.