Advanced Structural Wind Engineering

The time-dependent aerodynamic forces experienced by a structure immersed in the air flow relate to the wind properties directly. Understanding of the strong wind characteristics is very important for safe and serviceable design. This chapter focuses on strong winds and their characteristics. Wind climates that may bring strong wind, for instance monsoons, tropical cyclones, thunderstorms and so on, are briefly described together with the underlying hydrodynamics. Variation of wind speed with height above ground and turbulent structure inside the atmospheric boundary layer, including turbulence intensity and scale, gust factor, peak factor, decay factor of the coherence function, probability distribution function, power spectrum, and their variations with wind speed, are discussed in detail. In addition, Typhoon Maemi is referred as an example of strong wind event to exhibit the wind characteristics of strong winds.

The modern approach to the specification of the design wind speed favours the explicit format which directly presents the design value of the wind speed instead of hiding the value behind the product of the characteristic wind speed and a partial factor. This approach leads to better reliability consistency in case of highly variable geographic conditions and it avoids underestimations for systems with unfavourable non-linear characteristics. Some leading codes therefore have adopted this concept. This chapter presents tentative target values of the exceedance probability of the design wind speed for the ultimate limit state with reference to the design working life considering six importance classes for buildings and structures. Additionally, tentative target values are given for the serviceability limit state with reference to a single year. For the practical application, strategies are discussed in regard to the quality check of data, the sampling of the appropriate ensembles and the estimation of the design value considering an appropriate confidence interval. Since statistical uncertainties remain large even for several decades of observations, a new approach is presented which leads to consistent values of the design wind speed. Additionally, the complex topic of directionality is addressed on the example of storms induced by strong frontal depressions.

The aerodynamic characteristics of bluff bodies differ substantially from streamlined bodies, and an understanding of bluff body aerodynamics is essential to make progress in understanding wind engineering. Streamlined bodies like aircraft wings have a rounded nose, a thin profile, and a sharp trailing edge. Their wakes are small and for small angles of attack, the lift force developed is considerably greater than the drag force. On the other hand, bluff bodies have a large separated wake, with unsteady flow, and the drag force is comparable with the lift force. It is necessary to understand the size and nature of these forces to ensure that engineered designs are fit for purpose under wind action.

In this Chapter, the formulation of aerodynamic forces of bridge decks is introduced with static force components, quasi-steady and unsteady aerodynamic force, and transient forces, and followed by various types of wind induced responses of bridges, including aerodynamic instability of flutter and galloping, buffeting vibration and vortex induced oscillation.

This chapter examines the wind-induced vibrations of buildings and structures. In particular, the alongwind and the crosswind excitation mechanisms and response processes are discussed. Turbulence buffeting is the dominant excitation mechanism in the alongwind direction. In the crosswind direction, excitations associated with incident turbulence, vortex shedding and motion dependent excitations are the prominent mechanisms causing crosswind response. Many advanced wind codes and standards have adopted statistical and spectral analysis processes to predict wind loads and responses caused by these alongwind and crosswind excitations. The potential impact of interference excitations of tall buildings and beneficial effects of aerodynamic modifications of building shape are also reviewed in this chapter. Interference effects on the alongwind, crosswind and torsional responses of tall buildings from either an upstream or a downstream building are outlined. The beneficial effects and the economic perspectives of aerodynamic modifications of building shape in reducing the wind induced load and response of tall buildings are discussed.

This chapter describes a relatively well developed method for determining the along-wind response of tall buildings to wind excitation using a theoretical approach. It has been found to give good predictions, and is now the basis of along-wind response predictions in some wind loading standards and codes. It is based on the premise that the along-wind motion of a relatively slender building is driven primarily by the onset turbulence, which excites the building as a single degree of freedom system with low damping.

The gust factor approach described in this chapter relies heavily on the previous work of researchers such as Vickery, Davenport and others, who developed this theory around 50 years ago.

Building components and cladding are important to the overall performance of buildings during extreme windstorms. Failure of even relatively small components can lead to internal pressurization, which increases the overall net loads on other components, or to significant rain water infiltration, which increases losses. In this chapter, we focus on the nature of wind loads on low-rise buildings and test methods used to define the ultimate capacity of building products and components. Wind loads are shown to have significant spatial and temporal variations, which are greatly simplified in building codes and standard test methods. Most building codes specify component loads as single peak values that have only a few different values over the various building surfaces for a small range of buildings. Many building components have complex and redundant load paths; the chapter discusses how load sharing can be handled in such systems. The design of some building component and cladding systems depend on the storm duration and the numbers of load cycles, which is also discussed.

This chapter considers the trajectories of compact, rod-type and plate-type windborne debris in horizontal winds, using a combination of experimental and numerical studies. These studies indicate that the ratio of horizontal debris speed to wind gust speed is primarily a function of the horizontal distance traveled by the debris. Empirical expressions to approximate the horizontal speed of these debris as a function of travel distance and time, are developed, and may be used to establish rational debris impact criteria.

This chapter is a collection of notes describing the progress made in the last few decades on the assessment of wind loads on buildings through various codes of practice. This is necessary to understand the current code provisions for estimating wind-induced pressures on buildings, which in turn can help improve these provisions. Background studies carried out for the development of standard wind provisions almost exclusively used boundary layer wind tunnel experiments, with very limited attempts to capitalize on computational approaches. Topics presented in this chapter include: wind structure; wind speed models; wind pressure provisions for gable roof buildings with intermediate roof angles; pressure coefficients for hipped roofs; and internal pressures. It was generally observed that the current codes must incorporate the effects of upstream terrain roughness, different roof shapes and other architectural features. Advancements in Computational Wind Engineering (CFD and ANN approaches) have been discussed. Recent innovative approaches, such as wind load paths and Database-Assisted Design (DAD) using time histories of wind pressure coefficients for specific building geometries are expected to be used for the provisions of future codes of practice.

Modern structural systems are becoming increasingly complex and numerical simulation of the potential loads with which they will be affected is critical for analysis, design, and optimization of safe and reliable structures. Monte Carlo analysis approaches are often used, which involve the input of loads into a structural model and the output of responses. Besides being necessary for numerical analysis, digitally simulated data is also necessary to drive computer controlled test facilities. Both approaches necessitate an ensemble of input signals that accurately represents what the structure may expect to experience during its lifetime. Therefore, simulation of time histories of wind velocity, pressure, and force fluctuations are necessary, in addition to simulation of structural response, which allows assessment of attendant functionality and safety under service and design loads, respectively. Random processes simulated for analysis purposes are often assumed to be Gaussian and stationary for simplicity. Many wind events, however, are characterized by non-stationarity and non-Gaussianity. Therefore, simulation methodologies are necessary for univariate and multivariate processes, uni-dimensional and multi-dimensional fields, Gaussian and non-Gaussian data, stationary and non-stationary processes, and conditional and unconditional cases. In order to accomplish this task, methods based on the time, frequency, and time-frequency domains are employed. This paper summarizes a historical perspective, recent developments, and future challenges for simulation. Also included in the discussion are computational tools employed for data and response analysis. Examples are presented to illustrate some of the topics discussed.

Computational Fluid Dynamics (CFD) is basically a numerical approach to simulating or predicting phenomena and quantities of a flow by solving the equations of motion of the fluid at a discrete set of points. CFD has greatly improved recently and has become an application tool for wind engineering problems. This paper first explains the characteristics of NS equations, and then describes in detail the discretization schemes, turbulence models, inflow turbulence and numerical procedures. Finally, it provides an example of calculation of strong wind in hilly area by utilizing Large Eddy Simulation.

Wind-induced building motion can interfere with building occupants’ daily activities and general well-being. However, human perception of motion and tolerance of wind-induced tall building vibration are essentially a subjective assessment. Hence, there is currently no single internationally accepted occupant comfort serviceability criteria which set a design standard for satisfactory levels of wind-induced vibration in tall buildings. This chapter reviews past studies on human perception of motion and tolerance thresholds of wind-induced tall building motions. Building vibration acceptability, occupant comfort serviceability criteria and assessment methodologies that have been commonly adopted for the assessment of occupant comfort in wind-excited tall buildings are outlined. Occupant comfort evaluation that utilises motion simulator to provide building developers/owners and design professionals an experience of different levels of simulated wind-induced building vibrations is described, thus facilitating an assessment of the acceptability of the building vibration and the need to adopt vibration mitigation measures. The relevance of two motion sickness theories: Sensory Conflict Theory and Postural Instability Theory on human response to wind-induced building motion is discussed, which may lead to design strategy that aims to minimize the adverse effects of building motion on occupant comfort and well-being and potential degradation of manual task and cognitive performance.

This chapter first discusses physical causes of damping in buildings and dynamic properties of buildings, especially amplitude dependency of damping. It is emphasized that there is no evidence of increasing damping ratio in the very high amplitude range within the elastic range of main frames, unless there is damage to secondary members or cladding. Then, damping predictors are introduced based on the Japanese Damping Database, and design values for damping ratio are recommended. Next, damping estimation techniques are introduced and some points to note in damping estimation are discussed. The feasibility and efficiency of two simple and user-friendly but accurate damping estimation techniques are discussed. One is the Frequency Domain Decomposition technique, which uses Singular Value Decomposition of the Power Spectral Density matrix of multiple outputs, and the other is the Multi-mode Random Decrement technique. Both can be applied to ambient excitations such as micro-tremors, enabling easy handling of closely-spaced and even repeated modes. Some full-scale examples demonstrating the damping estimation efficiency of both techniques are also shown.

In recent years, a drive towards the construction of buildings of increasing heights and bridges of increasing span lengths has generated the most fascinating challenges in the civil engineering field. Indeed, the particular sensitivity to wind loads of such extreme structures has to be faced when designing structural systems that have to possess adequate stiffness and damping characteristics to ensure acceptable performance for survivability, serviceability and habitability. In order to meet these needs, several routes can be taken that involve different design aspects. In addition to the modification of the structural system, which is the first and foremost option for the structural designers, these can be divided in the adoption of shape tailoring measures, aimed at improving the aerodynamics/aeroelasticity of the structure, and the introduction of auxiliary motion control devices. In this chapter these two aspects are treated, with the intent of providing the reader with a general overview of the engineering solutions that may be considered for the control of the wind induced response of structures. In particular, discussion on the aerodynamic/aeroelastic shape tailoring is provided, alongside with meaningful examples of application of this strategy to real structures. The possibility of adopting motion control devices is also investigated. The most important typologies of devices for the control of wind induced vibrations are classified according to the principle on which they are based and their main characteristics are illustrated.