Study of Load Effects on Structures Induced by Gust-Fronts

建筑环境经历了由一系列破坏性大气事件产生的自然力量，包括由雷暴相关的下降气流引起的阵风锋。准确预测结构与这些极端事件相互作用产生的荷载效应是设计和建造建筑环境并保护其居住者的关键部分。该项目的主要目标是更好地理解和量化风荷载及其在瞬态风事件（如阵风锋）下对建筑物的影响。为了实现这一目标，本文提出了一种系统的方法，着重于在瞬态流场模拟器中捕获与阵风锋相关的瞬态流动的显著特征以及伴随的负载效应，这些特征在通用建筑形状的几何比例模型上。从对美国和世界各地的雷暴数据库的分析中，可以很容易地推测出瞬态风事件及其荷载效应的重要性，这表明这些风代表了许多地方的设计风速。因此，这些风速成为在传统边界层风洞中验证的规范化设计程序的输入。然而，与对流阵风锋相关的阵风机制在运动学和动力学方面与传统湍流有很大不同，例如，它们的速度分布和短暂性。因此，人们应该质疑在代码和标准中使用基于传统分析框架的设计的适当性，这些框架通常以相同的方式处理这些根本不同的现象。此外，结构上的极端荷载对瞬态流动的影响可能很敏感，即荷载系数可能会基于阵风形式和由此产生的局部、快速的周围流动变化而提高。此外，这些流动结构的特征表明，所产生的载荷效应可能比常规流动在更大的区域上相关。这些空气动力学结果清楚地表明，比现行规范和标准所预测的更高的结构载荷的前景，要求对这些程序进行仔细检查。上述问题是本建议研究阵风锋面的主要动机。这些调查将首先从雷暴流出期间的地面风数据中收集信息，以补充现有的知识库。确定的瞬态流场的显著特征，例如均匀加速流动，然后将在由单独控制的风扇组组成的瞬态流动模拟器中进行模拟。对暴露在这些气流中的比例建筑模型所获得的实验数据进行分析，将增强对由此产生的空气动力学变化的理解并促进量化。将数值模拟的结构荷载效应与阵风锋的时频分析和建模以及压力分布和荷载的测量相结合，该研究将能够检查广泛使用的设计规范和标准的有效性，突出可能需要修改的地方。此外，与日本两个著名的空气动力/风力工程中心的合作将提供一系列令人印象深刻的经验，并提供独特的实验设施来验证本研究产生的流动模拟和伴随载荷。在美国，与暴雨和龙卷风相关的阵风锋每年造成约80人死亡，1500人受伤，财产损失约为10亿美元。这个数据正在迅速上升。基于更好地了解阵风锋对建筑物的影响而改进的规范当然有望减轻这种危害，从而使整个社会受益。因此，这项工作的更广泛影响将以减少这些损失来衡量，包括财产损失，更重要的是生命损失。同时，该项目的其他主要举措，即专业发展、教育举措等，承诺对提高公众对美国人日常生活中普遍经历的现象的认识和理解产生持久的影响。在这方面，流动模拟器将是一个有吸引力的公共教育示范工具。此外，参与拟议研究的学生将继续体现过去研究团队的种族和性别多元化形象，热情地保持大学前拓展计划的既定传统，以中小学生为目标，并涉及代表性不足的群体。最终，这项提议的研究有望为研究生、本科生和大学预科阶段受过良好训练的学生提供坚实的知识基础，为建筑和民用基础设施的抗风设计提供支持。

The built environment experiences forces of nature generated by a host of damaging atmospheric events, including gust-fronts induced by downdrafts associated with thunderstorms. An accurate prediction of load effects that result from the interaction of structures with these extreme events is a critical part of designing and constructing the built environment and protecting its occupants. The primary goal of this project is to better understand and quantify wind loads and their effects on buildings under transient wind events, e.g., gust-fronts. To accomplish this goal, a systematic approach is proposed that focuses on capturing salient characteristics of transient flows associated with gust-fronts and attendant load effects on geometrically-scaled models of generic building shapes in a transient flow field simulator. The significance of transient wind events and their load effects can be readily surmised from an analysis of thunderstorm databases both in the U.S. and around the world, which suggest that these winds represent the design wind speed for many locations. As a result, these wind speeds become the input to codified design procedures validated in traditional boundary layer wind tunnels. However, the mechanics of gusts associated with convective gust-fronts differ significantly from conventional turbulence both in its kinematics and dynamics, e.g. their contrasting velocity profile and their transient nature. Accordingly, one should question the appropriateness of using design based on conventional analysis frameworks in codes and standards, which generically treat these fundamentally different phenomena in the same manner. Further, extreme loads on structures are potentially sensitive to the influence of transient flows, i.e., the load coefficients may be enhanced based on the gust form and the resulting localized, rapid changes in the surrounding flow. In addition, the characteristics of these flow structures suggest that the resultant load effects are likely to be correlated over larger areas than in conventional flows. These aerodynamic consequences clearly point at the prospect of higher loads on structures than would be predicted by the present codes and standards, calling for a careful examination of these procedures. The issues highlighted above serve as the primary motivation to study gust-fronts in this proposal. These investigations will initiate by gleaning information from surface wind data available during thunderstorm outflows to complement the existing knowledge base. The identified salient features of transient flow fields, e.g., uniformly accelerating flows, will then be simulated in a transient flow simulator comprised of individually controlled fan banks. The analysis of experimental data garnered from scaled building models exposed to these flows will enhance understanding and facilitate quantification of the resulting aerodynamic modifications. Upon combining numerically simulated structural load effects with the time-frequency analysis and modeling of gust-fronts and the measurements of pressure distributions and loads, the study will be capable of examining the efficacy of widely used design codes and standards, highlighting the possible need for modifications. Further, the collaboration with two distinguished aerodynamic/wind engineering centers in Japan will make available an impressive array of experience and provide access to unique experimental facilities to validate flow simulations and attendant loads resulting from this study. Gust-fronts associated with downbursts and tornadoes in the U.S. result in estimated 80 deaths, 1500 injuries, and property damage in the neighborhood of a billion dollars each year . a statistic that is rapidly escalating. Improved codes that are based on better understanding of the impact of gust-fronts on structures certainly promise to mitigate this hazard, thus benefiting society at large. Therefore, the broader impacts of this work will be measured in the reduction of these losses, both in property and, more importantly, in lives. Concurrently other major initiatives of this project, i.e., professional development, educational initiatives, etc., promise to have lasting impacts in raising public awareness and understanding of a phenomenon commonly experienced in the daily lives of Americans throughout this country. The flow simulator will be an attractive demonstration tool for public education in this regard. Further, the students involved in the proposed research will continue to embody the ethnically and gender diversified profile of the past research teams, enthusiastically maintaining the established traditions of pre-college outreach programs, targeted at primary and secondary level students, and involving underrepresented groups. Ultimately, the proposed study promises to provide a strong intellectual foundation, supported by well-trained students at graduate, undergraduate and pre-college levels for wind resistant design of buildings and civil infrastructure.