Wind Effects on Structures E-Book

Emil Simiu

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 785

Veröffentlichungsjahr: 2019

Beschreibung

Provides structural engineers with the knowledge and practical tools needed to perform structural designs for wind that incorporate major technological, conceptual, analytical and computational advances achieved in the last two decades. 

With clear explanations and documentation of the concepts, methods, algorithms, and software available for accounting for wind loads in structural design, it also describes the wind engineer's contributions in sufficient detail that they can be effectively scrutinized by the structural engineer in charge of the design.

Wind Effects on Structures: Modern Structural Design for Wind, 4th Edition is organized in four sections. The first covers atmospheric flows, extreme wind speeds, and bluff body aerodynamics. The second examines the design of buildings, and includes chapters on aerodynamic loads; dynamic and effective wind-induced loads; wind effects with specified MRIs; low-rise buildings; tall buildings; and more. The third part is devoted to aeroelastic effects, and covers both fundamentals and applications. The last part considers other structures and special topics such as trussed frameworks; offshore structures; and tornado effects.

Offering readers the knowledge and practical tools needed to develop structural designs for wind loadings, this book:

• Points out significant limitations in the design of buildings based on such techniques as the high-frequency force balance
• Discusses powerful algorithms, tools, and software needed for the effective design for wind, and provides numerous examples of application  
• Discusses techniques applicable to structures other than buildings, including stacks and suspended-span bridges  
• Features several appendices on Elements of Probability and Statistics; Peaks-over-Threshold Poisson-Process Procedure for Estimating Peaks; estimates of the WTC Towers’ Response to Wind and their shortcomings; and more

Wind Effects on Structures: Modern Structural Design for Wind, 4th Edition is an excellent text for structural engineers, wind engineers, and structural engineering students and faculty.

Leseprobe

Wind Effects on Structures

Modern Structural Design for Wind

Emil Simiu, P.E., Ph.D.

NIST Fellow National Institute of Standards and Technology, USA

Dong Hun Yeo, P.E., Ph.D.

Research Structural Engineer National Institute of Standards and Technology, USA

Fourth Edition

Copyright

This edition first published 2019

© 2019 John Wiley &Sons Ltd

Edition History

John Wiley & Sons (1e, 1978), John Wiley & Sons (2e, 1986), John Wiley & Sons (3e, 1996)

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.

The right of Emil Simiu and DongHun Yeo to be identified as the authors of this work has been asserted in accordance withlaw.

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Library of Congress Cataloging‐in‐Publication Data

Names: Simiu, Emil, author. | Yeo, DongHun, author.

Title: Wind effects on structures : modern structural design for wind / Emil

   Simiu, P.E., Ph.D., NIST Fellow, National Institute of Standards and

   Technology, DongHun Yeo, P.E., Ph.D., Research Structural Engineer, National

   Institute of Standards and Technology.

Description: Fourth edition. | Hoboken, NJ : John Wiley & Sons, 2019. |

   Includes bibliographical references and index. |

Identifiers: LCCN 2018038948 (print) | LCCN 2018040522 (ebook) | ISBN

   9781119375906 (Adobe PDF) | ISBN 9781119375937 (ePub) | ISBN 9781119375883

   (hardcover)

Subjects: LCSH: Wind-pressure. | Buildings–Aerodynamics. | Wind resistant

   design.

Classification: LCC TA654.5 (ebook) | LCC TA654.5 .S55 2019 (print) | DDC

   624.1/75 – dc23

LC record available at https://lccn.loc.gov/2018038948

Cover Design: Wiley

Cover Image: © Jackal Pan / Getty Images

Dedication

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

Preface to the Fourth Edition

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.

Rockville, Maryland

Emil Simiu

DongHun Yeo

Introduction

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:

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

).

Matrices of directional mean wind speeds at the site of interest, at the prototype reference height.

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.

Part IAtmospheric Flows, ExtremeWind Speeds, Bluff Body Aerodynamics

1Atmospheric Circulations

Wind, or the motion of air with respect to the surface of the Earth, is fundamentally caused by variable solar heating of the Earth's atmosphere. It is initiated, in a more immediate sense, by differences of pressure between points of equal elevation. Such differences may be brought about by thermodynamic and mechanical phenomena that occur in the atmosphere both in time and space.

The energy required for the occurrence of these phenomena is provided by the sun in the form of radiated heat. While the sun is the original source, the source of energy most directly influential upon the atmosphere is the surface of the Earth. Indeed, the atmosphere is to a large extent transparent to the solar radiation incident upon the Earth, much in the same way as the glass roof of a greenhouse. That portion of the solar radiation that is not reflected or scattered back into space may therefore be assumed to be absorbed entirely by the Earth. The Earth, upon being heated, will emit energy in the form of terrestrial radiation, the characteristic wavelengths of which are long (in the order of 10 μ) compared to those of heat radiated by the sun. The atmosphere, which is largely transparent to solar but not to terrestrial radiation, absorbs the heat radiated by the Earth and re‐emits some of it toward the ground.

1.1 Atmospheric Thermodynamics

1.1.1 Temperature of the Atmosphere

To illustrate the role of the temperature distribution in the atmosphere in the production of winds, a simplified version of model circulation will be presented. In this model the vertical variation of air temperature, of the humidity of the air, of the rotation of the Earth, and of friction are ignored, and the surface of the Earth is assumed to be uniform and smooth.

The axis of rotation of the Earth is inclined at approximately 66° 30′ to the plane of its orbit around the sun. Therefore, the average annual intensity of solar radiation and, consequently, the intensity of terrestrial radiation, is higher in the equatorial than in the polar regions. To explain the circulation pattern as a result of this temperature difference, Humphreys [1] proposed the following ideal experiment (Figure 1.1).

Figure 1.1 Circulation pattern due to temperature difference between two columns of fluid.

Source: From Ref. [1]. Copyright 1929, 1940 by W. J. Humphreys.

Assume that the tanks A and B are filled with fluid of uniform temperature up to level a, and that tubes 1 and 2 are closed. If the temperature of the fluid in A is raised while the temperature in B is maintained constant, the fluid in A will expand and reach the level b. The expansion entails no change in the total weight of the fluid contained in A. The pressure at c therefore remains unchanged, and if tube 2 were opened, there would be no flow between A and B. If tube 1 is opened, however, fluid will flow from A to B, on account of the difference of head (b – a). Consequently, at level c the pressure in A will decrease, while the pressure in B will increase. Upon opening tube 2, fluid will now flow through it from B to A. The circulation thus developed will continue as long as the temperature difference between A and B is maintained.

If tanks A and B are replaced conceptually by the column of air above the equator and above the pole, in the absence of other effects an atmospheric circulation will develop that could be represented as in Figure 1.2. In reality, the circulation of the atmosphere is vastly complicated by the factors neglected in this model. The effect of these factors will be discussed later in this chapter.

Figure 1.2 Simplified model of atmospheric circulation.

The temperature of the atmosphere is determined by the following processes:

Solar and terrestrial radiation, as discussed previously

Radiation in the atmosphere

Compression or expansion of the air

Molecular and eddy conduction

Evaporation and condensation of water vapor.

1.1.2 Radiation in the Atmosphere

As a conceptual aid, consider the action of the following model. The heat radiated by the surface of the Earth is absorbed by the layer of air immediately above the ground (or the surface of the ocean) and reradiated by this layer in two parts, one going downward and one going upward. The latter is absorbed by the next higher layer of air and again reradiated downward and upward. The transport of heat through radiation in the atmosphere, according to this conceptual model, is represented in Figure 1.3.

Figure 1.3 Transport of heat through radiation in the atmosphere.

1.1.3 Compression and Expansion. Atmospheric Stratification

Atmospheric pressure is produced by the weight of the overlying air. A small mass (or particle) of dry air moving vertically thus experiences a change of pressure to which there corresponds a change of temperature in accordance with the Poisson dry adiabatic equation

1.1

A familiar example of the effect of pressure on the temperature is the heating of compressed air in tire pump.

If, in the atmosphere, the vertical motion of an air particle is sufficiently rapid, the heat exchange of that parcel with its environment may be considered to be negligible, that is, the process being considered is adiabatic. It then follows from Poisson's equation that since ascending air experiences a pressure decrease, its temperature will also decrease. The temperature drop of adiabatically ascending dry air is known as the dry adiabatic lapse rate and is approximately 1°C/100 m in the Earth's atmosphere.

Consider a small mass of dry air at position 1 (Figure 1.4). Its elevation and temperature are denoted by h1 and T1, respectively. If the particle moves vertically upward sufficiently rapidly, its temperature change will effectively be adiabatic, regardless of the lapse rate (temperature variation with height above ground) prevailing in the atmosphere. At position 2, while the temperature of the ambient air is T2, the temperature of the element of air mass is  = T1 – (h2 – h1) γa, where γa is the adiabatic lapse rate. Since the pressure of the element and of the ambient air will be the same, it follows from the equation of state that to the difference  − T2 there corresponds a difference of density between the element of air and the ambient air. This generates a buoyancy force that, if T2 < , acts upwards and thus moves the element farther away from its initial position (superadiabatic lapse rate, as in Figure 1.4), or, if T2 > , acts downwards, thus tending to return the particle to its initial position. The stratification of the atmosphere is said to be unstable in the first case and stable in the second. If T2 = , that is, if the lapse rate prevailing in the atmosphere is adiabatic, the stratification is said to be neutral. A simple example of the stable stratification of fluids is provided by a layer of water underlying a layer of oil, while the opposite (unstable) case would have the water above the oil.

Figure 1.4 Lapse rates.

1.1.4 Molecular and Eddy Conduction

Molecular conduction is a diffusion process that effects a transfer of heat. It is achieved through the motion of individual molecules and is negligible in atmospheric processes. Eddy heat conduction involves the transfer of heat by actual movement of air in which heat is stored.

1.1.5 Condensation of Water Vapor

In the case of unsaturated moist air, as an element of air ascends and its temperature decreases, at an elevation where the temperature is sufficiently low condensation will occur and heat of condensation will be released. This is equal to the heat originally required to change the phase of water from liquid to vapor, that is, the latent heat of vaporization stored in the vapor. The temperature drop in the saturated adiabatically ascending element is therefore slower than for dry air or moist unsaturated air.

1.2 Atmospheric Hydrodynamics

The motion of an elementary air mass is determined by forces that include a vertical buoyancy force. Depending upon the temperature difference between the air mass and the ambient air, the buoyancy force acts upwards (causing an updraft), downwards, or is zero. These three cases correspond to unstable, stable, or neutral atmospheric stratification, respectively. It is shown in Section 2.3.3 that, depending upon the absence or a presence of a stably stratified air layer above the top of the atmospheric boundary layer, called capping inversion, neutrally stratified flows can be classified into truly and conventionally neutral flows.

The horizontal motion of air is determined by the following forces:

The

horizontal pressure gradient force

per unit of mass, which is due to the spatial variation of the horizontal pressures. This force is normal to the lines of constant pressure, called

isobars

, that is, it is directed from high‐pressure to low‐pressure regions (

Figure 1.5

). Let the unit vector normal to the isobars be denoted by

n

,

and consider an elemental volume of air with dimensions

dn

,

dy

,

dz

, where the coordinates

n

,

y

,

z

are mutually orthogonal. The net force per unit mass exerted by the horizontal pressure gradient along the direction of the vector

n

is

1.2

where p denotes the pressure, and ρ is the air density.

The

deviating force due to the Earth's rotation

. If defined with respect to an absolute frame of reference, the motion of a particle not subjected to the action of an external force will follow a straight line. To an observer on the rotating Earth, however, the path described by the particle will appear curved. The deviation of the particle with respect to a straight line fixed with respect to the rotating Earth may be attributed to an apparent force, the

Coriolis force

1.3

where m is the mass of the particle, f = 2ω sin ϕ is the Coriolis parameter, ω = 0.7292 × 10−4 s−1 is the angular velocity vector of the Earth, ϕ is the angle of latitude, and v is the velocity vector of the particle referenced to a coordinate system fixed with respect to the Earth. The force Fc is normal to the direction of the particle's motion, and is directed according to the vector multiplication rule.

The

friction force

. The surface of the Earth exerts upon the moving air a horizontal drag force that retards the flow. This force decreases with height and becomes negligible above a height

δ

known as

gradient height

. The atmospheric layer between the Earth's surface and the gradient height is called

the atmospheric boundary layer

(see

Chapter 2

). The wind velocity speed at height

δ

is called the

gradient velocity

,

1

and the atmosphere above this height is called the

free atmosphere

(

Figure 1.6

).

Figure 1.5 Direction of pressure gradient force.

Figure 1.6 The atmospheric boundary layer.