For Devra, SueYeun,
Zohar,Nitzan, Abigail, and Arin

The quarter of a century that elapsed since the publication of the third edition of Wind Effects on Structures has seen a number of significant developments in micrometeorology, extreme wind climatology, aerodynamic pressure measurement technology, uncertainty quantification, the optimal integration of wind and structural engineering tasks, and the use of “big data” for determining and combining effectively multiple directionality‐dependent time series of wind effects of interest. Also, following a 2004 landmark report by Skidmore Owings and Merrill LLP on large differences between independent estimates of wind effects on the World Trade Center towers, it has increasingly been recognized that transparency and traceability are essential to the credibility of structural designs for wind. A main objective of the fourth edition of Wind Effects on Structures is to reflect these developments and their consequences from a design viewpoint. Progress in the developing Computational Wind Engineering field is also reflected in the book.

Modern pressure measurements by scanners, and the recording and use of aerodynamic pressure time series, have brought about a significant shift in the division of tasks between wind and structural engineers. In particular, the practice of splitting the dynamic analysis task between wind and structural engineers has become obsolete; performing dynamic analyses is henceforth a task assigned exclusively to the structural engineering analyst, as has long been the case in seismic design. This eliminates the unwieldy, time‐consuming back‐and‐forth between wind and structural engineers, which typically discourages the beneficial practice of iterative design. The book provides the full details of the wind and structural engineers' tasks in the design process, and up‐to‐date, user‐friendly software developed for practical use in structural design offices. In addition, new material in the book concerns the determination of wind load factors, or of design mean recurrence intervals of wind effects, determined by accounting for wind directionality.

The first author contributed Chapters 1–3; portions of Chapter 4; Chapters 5, 7, and 8; Sections 9.1 and 9.3; Chapters 10–12 and 15; portions of Chapter 17 and Part III; Part IV; and Appendices A, B, D, and E. The second author contributed Chapter 6; Section 9.2; and Section 23.5. The authors jointly contributed Chapters 13, 14, 16, and 18. They reviewed and are responsible for the entire book. Professor Robert H. Scanlan contributed parts of Chapter 4 and of Part III. Appendix F, authored by Skidmore Owings and Merrill LLP, is part of the National Institute of Standards and Technology World Trade Center investigation. Chapter 17 is based on a doctoral thesis by Dr. F. Habte supervised by the first author and Professor A. Gan Chowdhury. Dr. Sejun Park made major contributions to Chapters 14 and 18 and developed the attendant software. Appendix C is based on a paper by A. L. Pintar, D. Duthinh, and E. Simiu.

We wish to pay a warm tribute to the memory of Professor Robert H. Scanlan (1914–2001) and Dr. Richard D. Marshall (1934–2001), whose contributions to aeroelasticity and building aerodynamics have profoundly influenced these fields. The authors have learned much over the years from Dr. Nicholas Isyumov's work, an example of competence and integrity. We are grateful to Professor B. Blocken of the Eindhoven University of Technology and KU Leuven, Dr. A. Ricci of the Eindhoven University of Technology, and Dr. T. Nandi of the National Institute of Standards and Technology for their thorough and most helpful reviews of Chapter 6. We thank Professor D. Zuo of Texas Tech University for useful comments on cable‐stayed‐bridge cable vibrations. We are indebted to many other colleagues and institutions for their permission to reproduce materials included in the book.

The references to the authors' National Institute of Standards and Technology affiliation are for purposes of identification only. The book is not a U.S. Government publication, and the views expressed therein do not necessarily represent those of the U.S. Government or any of its agencies.

The design of buildings and structures for wind depends upon the wind environment, the aerodynamic effects induced by the wind environment in the structural system, the response of the structural system to those effects, and safety requirements based on uncertainty analyses and expressed in terms of wind load factors or design mean recurrence intervals of the response. For certain types of flexible structure (slender structures, suspended‐span bridges) aeroelastic effects must be considered in design.

1.1 The Wind Environment and Its Aerodynamic Effects

For structural design purposes the wind environment must be described: (i) in meteorological terms, by specifying the type or types of storm in the region of interest (e.g., large‐scale extratropical storms, hurricanes, thunderstorms, tornadoes); (ii) in micrometeorological terms (i.e., dependence of wind speeds upon averaging time, dependence of wind speeds and turbulent flow fluctuations on surface roughness and height above the surface); and in extreme wind climatological terms (directional extreme wind speed data at the structure's site, probabilistic modeling based on such data). Such descriptions are provided in Chapters 1–3, respectively.

The description of the wind flows' micrometeorological features is needed for three main reasons. First, those features directly affect the structure's aerodynamic and dynamic response. For example, the fact that wind speeds increase with height above the surface means that wind loads are larger at higher elevations than near the ground. Second, turbulent flow fluctuations strongly influence aerodynamic pressures, and produce in flexible structures fluctuating motions that may be amplified by resonance effects. Third, micrometeorological considerations are required to transform measured or simulated wind speed data at meteorological stations or other reference sites into wind speed data at the site of interest.

Micrometeorological features are explicitly considered by the structural designer if wind pressures or forces acting on the structure are determined by formulas specified in code provisions. However, for designs based on wind‐tunnel testing this is no longer the case. Rather, the structural designer makes use of records of non‐dimensional aerodynamic pressure data and of measured or simulated directional extreme wind speeds at the site of interest, in the development of which micrometeorological features were taken into account by the wind engineer and are implicit in those records. However, the integrity of the design process requires that the relevant micrometeorological features on which those records are based be fully documented and accounted for.

To perform a design based on aerodynamic data obtained in wind‐tunnel tests (or in numerical simulations) the structural engineer needs the following three products:

1. Time series of pressures at large numbers of taps, non‐dimensionalized with respect to the wind tunnel (or numerical simulation) mean wind speed at the reference height (commonly the elevation of the building roof) (Chapters 4–6).
2. Matrices of directional mean wind speeds at the site of interest, at the prototype reference height.
3. Estimates of uncertainties in items (1) and (2) (Chapter 7).

These products, and the supporting documentation consistent with Building Information Modeling (BIM) requirements to allow effective scrutiny, must be delivered by the wind engineering laboratory to the structural engineer in charge of the design. The wind engineer's involvement in the structural design process ends once those products are delivered. The design is then fully controlled by the structural engineer. In particular, as was noted in the Preface, dynamic analyses need no longer be performed partly by the structural engineer and partly by the wind engineer, but are performed solely, and more effectively, by the structural engineer. This eliminates unwieldy, time‐consuming back‐and‐forth between the wind engineering laboratory and the structural design office, which typically discourages the beneficial practice of iterative design. Chapters 1–7 constitute Part I of the book.

1.2 Structural Response to Aerodynamic Excitation

The structural designer uses software that transforms the wind engineering data into applied aerodynamic loads. This transformation entails simple weighted summations performed automatically by using a software subroutine. Given a preliminary design, the structural engineer performs the requisite dynamic analyses to obtain the inertial forces produced by the applied aerodynamic loads. The effective wind loads (i.e., the sums of applied aerodynamic and inertial loads) are then used to calculate demand‐to‐capacity indexes (DCIs), inter‐story drift, and building accelerations with specified mean recurrence intervals. This is achieved by accounting rigorously and transparently for (i) directionality effects, (ii) combinations of gravity effects and wind effects along the principal axes of the structure and in torsion, and (iii) combinations of weighted bending moments and axial forces inherent in DCI expressions. Typically, to yield a satisfactory design (e.g., one in which the DCIs are not significantly different from unity), successive iterations are required. All iterations use the same applied aerodynamic loads but different structural members sizes. Part II of the book presents details on of the operations just described, software for performing them, and examples of its use supported by a detailed user's manual and a tutorial. Also included in Part II is a critique of the high‐frequency force balance technique, commonly used in wind engineering laboratories before the development of multi‐channel pressure scanners, material on wind‐induced discomfort in and around buildings, tuned mass dampers, and requisite wind load factors and design mean recurrence intervals of wind effects.

Part III presents fundamentals and applications related to aeroelastic phenomena: vortex‐induced vibrations, galloping, torsional divergence, flutter, and aeroelastic response of slender towers, chimneys and suspended‐span bridges. Part IV contains material on trussed frameworks and plate girders, offshore structures, tensile membrane structures, tornado wind and atmospheric pressure change effects, and tornado‐ and hurricane‐borne missile speeds.

Appendices A–E present elements of probability and statistics, elements of the theory of random processes, the description of a modern peaks‐over‐threshold procedure that yields estimates of stationary time series peaks and confidence bounds for those estimates, elements of structural dynamics based on a frequency‐domain approach still used in suspended‐span bridge applications, and elements of structural reliability that provide an engineering perspective on the extent to which the theory is, or is not, useful in practice. The final Appendix F is a highly instructive Skidmore Owings and Merrill report on the estimation of the World Trade Center towers response to wind loads.