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3
Wind-Engineering Research Needs
INTRODUCTION
Wind-related problems cover a wide range of topics including damage
to both low-rise structures (housing and small commercial and industrial
buildings) and mid- and high-rise buildings (residential and office), bridges,
towers, stacks, and power lines. In addition, a number of non-w~nd-Ioad-
related issues deserve research attention. Reentrainment of exhaust gases
from laboratory building fume hood exhausts into building ventilation systems
and local dispersion of toxic gas releases or of toxic fiances from accidental
spills represent health and safety issues to which wind engineering can make
major contributions to eliminate or reduce the hazard. These are areas that
do not have adequate design methodologies at present. Other issues that
affect the economic success of a project include pedestrian acceptability of
wind and blowing dust or snow near building entrances and in plaza areas.
Wind-engineering research can also result in the reduction of soil
erosion caused by wind and the proper modeling of iceberg movement and
of] spills, both of which are important issues related to offshore oil
operations. Moreover, wind energy potentially represents approximately 10
percent of electric system installations in the United States.
The United States currently sustains several billion dollars per year in
property and economic Tosses due to windstorms, along with significant loss
of life. In September 1989, Hurricane Hugo caused $4 billion to $5 billion in
insured Tosses, yielding a total loss value of $7 billion to $S billion to the U.S.
mainland alone. Yet even with this large actual Toss record and tremendous
potential loss waiting to occur, the United States spends no more than about
$4 million per year on wind mitigation, most of which is allocated to storm
warning capability. Less than $] million is spent each year in wind-eng~neenng
mitigation research (National Research Council, 1989~.
Indeed, even the annual loss figures presented above suffer from a high
degree of uncertainty due to lack of funding for research into windstorm
effects The United States has not commissioned a study of wind damage loss
assessments since the mid-1970s.
In spite of the lack of funding, engineers have explored the reasons for
the heavy financial Tosses in the United States. A task committee of the
American Society of Civil Engineers (ASCE) published a series of nine,
coordinated papers in 1989, which detailed many of the reasons for large
losses and outlined a number of solutions (American Society of Civil
Engineers, 1989~. The insurance industry also published a document in 1989
demonstrating that strengthening housing units during construction to resist
_
so
OCR for page 56
56 Bred and the Built Environment
hurricane winds would result in a minimal additional cost in the range of I.5
to 4.0 percent (All-Industry Research Advisory Council, 1989~.
Although much is known about how to mitigate wind damage, much is
yet to be learned. Sustained research is needed In the areas of defining wind
loads, determining more economical ways to resist these wind loads In both
new and existing structures, and finding ways to implement solutions In the
construction environment, where training of construction workers is limited
and inspection is a cost that local communities would prefer not to incur
In this chapter, various areas of wind eng~neenng are identified and the
issues related to research are presented.
RESEARCH MEI]IODOLOGY
Various methodologies have been developed to advance our knowledge
base in wind-engineenug research and practices. These methodologies include
physical modeling using atmosphenc, boundary-layer wind tunnels; numerical
modeling taking advantage of powerful computing capability; and full-scale
field measurements to vend predictions made by physical and/or numerical
modeling. In addition, innovative experimental approaches, such as tornado
simulation, are required to address particular problems (Lund and Snow,
1991~. Recent developments in the area of probabilistic methods and
statistical inference provide powerful tools to facilitate implementation of
uncertainties arising from the complex nature of wind problems. Post-wind-
storm disaster investigations also provide the best and most direct way to
assess the current state of w~nd-hazard mitigation practices and can contnbute
significantly to our efforts to reduce the threat of hazards.
Physical Modeling
That portion of the atmosphere within the first 1000 to 2000 ft of the
earth's surface comprises the atmosphenc boundary layer (ABL,J. Physical
modeling in wind tunnels of atmospheric winds within the ABE has matured
over the past 30 years. This modeling capability permits engineers to address
the effects of winds on the built environment on a routine, en~neering-
design basis. Windflow about buildings and over complex terrain, wind loads
on structures, dispersion of pollutants, wind ejects on pedestrians, snow or
sand deposition and drift, and heat transfer from structures are some of the
problems that can be addressed through wind tunnel modeling.
BoundaIy-layer wind tunnels are designed specifically to mode} the
variation in mean wind speed with height above ground and the vertical
distribution of turbulence, or gustiness, in the ABL. Figure 3-1 shows how the
wind speed varies with height and how that variation changes with the
character of roughness on the ground surface. Boundary-layer wind tunnels
differ from aeronautical wind tunnels in that they contain long test sections
to allow the development of appropriate velocity and turbulence profiles.
OCR for page 57
Wnd-Engyneenng Research Needs 57
M.u, Spoed Promo
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FIGURE 3-1 Mean wind velocity over level terrains of differing roughness.
Source: Davenport, ~ G. 1967. Gust Loading Factors. Journal of Structural
Division. Reproduced by permission of American Society of Civil Engineers.
Figure 3-2 shows the first major boundary-layer wind tunnel built at Colorado
State University in 1961. Scale models of terrain, buildings, bridges, power
plants, or other features of interest can be installed in the wind tunnel for
study. Figure 3-3 shows a Apical wind tunnel study for wind loads on a
structure, in which a mode] under study, along with a mode} of the
surrounding city, has been installed in a boundary-layer wind tunnel.
The thermal structure of the atmosphere is important in many
applications, such as dispersion of pollutants. Boundary-layer wind tunnels can
be constructed to include the temperature variation of the atmosphere in the
mode} wind simulation. Stably stratified winds (when the ground is colder
than the air above, such as night conditions or winds over water or snow) or
unstably stratified winds (when the ground is warmer than the air above, such
as winds on a hot, Sumner day) can be modeled by appropriate cooling and
heating of the model ground surface or air.
Modeling technology is sufficiently developed that several consulting
engineering firms are now routinely testing for wind loads on buildings,
dispersion of pollutants, pedestrian comfort in wind, and snow deposition and
drifting. These studies are inexpensive for major development projects, often
accounting for less than 0.1 percent of the total project cost. In some cases,
cost savings of 5 to 10 percent or more may be obtained as a result of these
tests.
A wide variety of wind tunnel tests are available on a commercial or
research basis to define wind effects on structures. A plastic model of a
OCR for page 58
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Encore under saw can be ins1~mented a1 hundreds of location 10
measure local Ouch pressures far use in design of 1be sl~cmre~s
daddy. For End loads scam on ~ lamer panes groups of pressure sensors
can be combined 10 he ares-=eraged pressure forces. ~1ema~e~, ~ model
~ he s1mcm~e can be moused on ~ balance 10 medium he i~l=1=eou~
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remove lose of He sincere.
For over ~pUc~io~, tracer amen such ~ 1~-conce~rsdon
~~oc~o~ or c^on diadem can be released Tom points of poDudon
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concerto ~ dad pOiD1S of interest gab a gas cbroma10gr~b. In
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If ibis plume contained Sac gases, reen1rsi~eD1 iD10 Dewy buDding-~r
bakes could pose bomb conceal. Correcdon of ibis he of problem her
he budding is placed info seance can be veal heave Ad peyote m
be selected 10 herons leek of 10~C ma1edal~ he same measurement
Coach can be used iD emerging response plying far sccidenlal spins or
g~ releases or 10 design deuces 10 ~~( Con1HiD, 0[ dell those releases.
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60 Hued and the Built Environment
FIGURE 34 Building exhaust downwash model under test in wind tunnel.
Source: Cermak Peterka Peterson, Inc.
Wind tunnel tests can also be used to measure wind fluctuations in
pedestrian areas near structures so that pedestrian wind exposur~and
consequent comfort levels can be gauged. In addition, winds in approach
areas to helicopter pads can be measured to identity high turbulence or wind
shear that could endanger helicopter operations on windy days. Wind-speed
measurements in wind tunnels can also be used to locate anemometers to
prevent shielding from upwind obstacles or to find correction factors for data
from shielded anemometers.
Transport of particulate material is a highly complex process that, in
many cases, can be modeled satisfactorily in a wind tunnel. Snow deposition
and drift in the presence of wind or snow drifting are important for designing
building entrances, highway geometries, parking lots, or pedestrian walkways
to avoid drift buildup, and accompanying maintenance costs. Depths of snow
drifts on roofs can be estimated to aid in design of roof structures for snow
load.
For small projects such as single- or multiple-family dwellings, small
commercial structures, or industrial buildings, wind turmel tests are often too
costly relative to the overall project cost. For these structures, which make up
the overwhelming bulk of construction in the United States, reliance must be
placed on building codes to define wind loads. Because of the high cost of
full-scale tests, wind tunnel data, obtained on a research basis, have been
used to develop the w~nd-loading provisions of these codes.
OCR for page 61
~nd-Engyneenng Research Needls 61
Unfortunately, funds have been insufficient to develop, through research
and testing, the level of understanding of wind loads that would provide
maximum economy of construction. Currently, no private organizations
possess the resources or mission to fund the appropriate level of research
needed for code development. Neither has the National Science Foundation
nor any other federal agency found sufficient funds to support such an effort.
Thus, research funds are vitally needed for better definition of wind loads in
a codified form.
Numerical Modeling
Numerical modeling of turbulent flows around complex boundaries is
difficult and demands the use of very large computers. Consequently, the
calculation of wind flow around structures, of the dispersion of pollutants over
buildings or complex terrain, and of wind loads on structures using numerical
solutions of the governing differential equations is still in its infancy. These
calculations have the potential to predict wind flow, pollution dispersion, and
wind loads with great accuracy.
To date, the mean concentrations of pollutants far downwind of
buildings and over some complex terrain features and the mean pressures on
simple cubical buildings have been predicted with some skill. Fluctuating
concentrations or pressures are still not within the range of calculation
capability, nor can mean values be calculated for other than very simple
geometnes. The availability of inexpensive but powerful computers will make
possible significant progress in numerical calculation capability in the near
future. Still needed are improved numerical algorithms, protocols for easy
implementation of parallel processing, and improved graphical display
software.
For many years, empirical mathematical models have been used for the
prediction of pollutant dispersion by winds. However, in many instances, these
empirical models are subject to large errors. Numerical solutions to the
governing equations could significantly increase the accurac y of such
predictions. Two major areas in which research is needed are fast algorithms
arid turbulence modeling for winds in which temperature gradients are
important.
The development of numerical modeling warrants special attention.
Initially, the requirements wall be for groups with special expertise working
in cooperation with others interested in computational fluid dynamics and
with access to powerful computers. Further development should then lead to
the broader usage of these techniques.
Field Measurements
Field measurements of wind and its effects are necessary to validate
predictions made by physical or numerical modeling. Complexities in field
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62 Wnd and die Built Environment
measurements arise from the surrounding topography, the wind climate, the
unpredictable nature of wind, the complicated structural system of buildings,
and the unavailability of off-the-shelf instrumentation. In addition, perhaps
the most basic, intrinsic difficulty of field measurements is their
nonrepeatability. The natural wind has too many large variations in intensity,
direction, and turbulence levels, as well as in meteorological aspects, to
elect it to repeat itself precisely. These complexities make field
measurements expensive and time consuming.
New technologies in instrumentation and data acquisition systems
provide opportunities in field measurements that were not available
previously. Solid-state electronics, remote sensing devices, and computerized
data acquisition systems permit reliable and detailed measurements. Data on
the response of high-r~se buildings, window glass, and other structures or
elements can be obtained using these technologies.
To reduce the cost of field measurements as well as to gain meaningful
results, field measurements should be targeted for specific objectives. A few,
carefully plaT,ned, long-term experiments should be conducted in the field to
provide baseline data. These experiments should be supplemented by well-
targeted, short-term experiments using mobile equipment, perhaps in the
region of hurricane landfalls.
The site for an experiment should be topographically clean with
frequent high winds. Complex topography should be avoided. A fairly
extensive array of meteorological instruments must be deployed to assess gust
sizes and directions, air densities, and storm characteristics. It is necessar, to
have at least one tall meteorological tower and a few short ones at each site.
The structures for which responses are measured should be structurally
simple: transmission lines, long-span bridges, or free-standing towers are good
candidates for measuring responses. International, cooperative projects with
other windstorm-prone countries can help to reduce manpower costs and
speed up data collection.
Field measurements have the potential of providing significant benefits
In the future. It is recommended that a few, long-term field experiments be
established to provide baseline data. Along with these expenments, physical
modeling in the wind tunnel and numencal modeling should be planned to
develop improved modeling technologies.
Innovative Experimental Approaches
Although boundaIy-layer wind tunnels provide a major too} for exploring
wind loads on structures, they cannot provide all necessary experimental
capability. These facilities do not, for example, directly simulate tornadoes,
thunderstorm outflows, or hurricane eyewall winds. The extent to which these
phenomena are different in wind-Ioading characteristics from the straight-
line winds associated with extratropical windstorms or frontal passages, which
boundary-layer wind tunnels simulate well, is unknown. Investigation into
unique physical facilities that could mode! these phenomena should be
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~nd-Eng~neenng Research Needs 63
undertaken. The ability of numerical models to perform this type of modeling
should also be investigated.
Some laboratory simulators have been used to mode! tornadoes on
small scales. These facilities provide great insight into the structure of
tornadoes, but they are not funded at a level sufficient to exploit their full
potential. A combination of laboratory measurements, numerical simulations,
and fuB-scale testing is the most likely route to fuller understanding of
tornado flows. Larger laboratory facilities than currently exist would benefit
this investigation. Improvements in Doppler radar might provide field
measurements of tornado velocity fields for comparison to laboratory and
numerical simulations. Additional improvements in radars or other
instruments capable of measuring tornado velocity fields are also needed.
The resistance of small structures, such as wood frame buildings, small
office or business buildings, and masonry buildings to wind loads is currently
difficult to predict. Model versions of these structures cannot duplicate their
complicated failure modes. Physical modeling facilities are needed in which
simulated wind loads can be applied to full scale models of such smaller
structures. Otherwise, a very large wind tunnel can impose the wind loads by
winds occurring in nature. Research is needed to determine the most effective
method for testing these structures and to design and build such facilities. A
numencal component should accompany this research effort so that the
results from numerical studies can be used to reduce the extent of the needed
physical studies. Facilities to mode} wind-wave action on structures are also
needed to better analyze and quanta y the complex, fluid-structure interactions
expenenced by offshore platforms.
Structural Safety and Reliability
Traditionally, structural safety in the design process is ensured by
including appropriate safety factors to account for shortcomings stemming
from a lack of full knowledge. from insufficient data. or from inherent
.
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vanan~ty In tne pro Stem s parameters. Untortunately, the factor ot safety
concept does not provide a quantitative measure of stn~ctural safety or
reliability. Probabilistic assessment of structural safety is receiving increased
attention and acceptance with the emergence of probability-based design
formats such as the load and resistance factor design.
Recent developments in the area of probabilistic methods and statistical
Inference offer a convenient mathematical framework to cope with
uncertamnes arising trom a variety of sources. Research is needed to quantii y
the uncertainties associated with venous problem parameters, to examine
propagation of these uncertainties, and to assess the influence of uncertainties
on the design process.
· ~
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64 Wind And the Buill Environment
Damage Investigations
Post-wind-sto~m disaster investigations offer the best opportunity to
assess how successfully current knowledge and technology are being applied
to reduce the Impacts of extreme wind hazards. The examination of darr aged
structures and lifeline systems allows wind engineers to identity future needs
in wind-engineenng research and practices, and to develop better ways to
effectively implement these technologies once they are developed.
SEVERE WIND FORCES
J
Windflow over bluff bodies, such as buildings or bridges, is different in
many respects from flow over streamlined bodies such as wings or airfoils.
Such bodies are immersed in the atmospheric, boundary-layer flow in which
the mean velocity vanes rapidly with height above ground and the turbulence
in the approaching wind is much higher than that which typically impinges on
an airfoil in flight. The turbulent nature of the wind has been illustrated in
Figure 2-16, which shows that wind is composed of a mean with turbulent
fluctuations (gusts). The gustiness in the wind and its variation with height
above ground (Figure 3-~) are the main features that distinguish it from
aeronautical flows characterized by smooth flow.
Bluff body aerodynamics control wind loads on such diverse structures
as buildings, bridges, tall solid towers, trussed towers, stacks, cooling towers,
and cables. The basic w~nd-Ioad mechanisms are buffeting caused by
turbulence (gustiness) in the approaching wind, wake excitation caused by
turbulence generated in the region immediately downwind of the structure,
wake excitation caused by vortex shedding from the structure (periodic
shedding from alternate sides of the body of packets of fluid that have rolled
up into a rotating mass), aeroelastic effects in which the wind loads are
altered by the motion of the body, and galloping excitation caused when
aerodynamic damping (resistance to motion in a fluid caused by fluid
viscosity) and mechanical damping (resistance to motion caused by internal
friction within the structure itself are overcome by aeroelastic effects.
Local cladding loads on the surface of buildings are a significant design
issue because water leakage through unintentional wall and roof cracks is a
major cause of building damage. The largest fluctuating pressures usually act
outward from the building surface and can cause cladding to fall during a
storm. Although w~nd-Ioad codes specify local wind pressures, wind tunnel
tests often find significant variations from that loading. One of the largest
local pressures on a building is frequently found near a corner of the roof.
The flow mechanism responsible for this phenomenon is called roof vortex,
shown in Figure 3-5. Additional research into local pressure fluctuations has
the potential to improve empirical Dredictinns of Thin nr~cc,~r~c for ,,~-
in building codes.
Wind loads on structures fluctuate randomly in time in response to
random changes in wind speed and direction and to random pressure
.__ of -v A- _- r~01~O lV1 Urn
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~nd-Engyneenng Research Needs 65
I. ~ ~ ~~ ~ . ~ ~ .... ....
FIGURE 3-5 Roof vortex in wind tunnel mode! leading to high roof wind
loads. Source: Fluid Mechanics and Wind Engineering Program at Colorado
State University.
fluctuations generated by the wind flow about the structure. Wind-Ioading
mechanisms have not been satisfactorily descnbed mathematically because of
the extreme complexity of the turbulent flows responsible for w~nd-Ioading
variability. As a result, wind loads are now largely expressed empirically. New
developments in chaos theory provide possible avenues for understanding the
nature of turbulence and wind loading in the presence of turbulence. An
analytical component of wind-Ioad research emphasizing chaos theory might
provide advances in understanding these complex w~nd-Ioad mechanisms.
Low Buildings
A few research projects have been funded by the metal buildings
industry to study design loads appropriate for small buildings. As a result, a
revised w~nd-Ioad code has been produced for use in the design of
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68 Wnd arid die Buill Environment
Damping ant! Stn~cture Control
To decrease the effects of fatigue, structure defections can be reduced
by increasing the structure stiffness or by increasing its damping ability.
Prediction of damping in many structures, including buildings, is relatively
primitive. The uncertainty in wind loading on a building can easily be 50 to
100 percent due to uncertainty about damping behavior. Increasing the
damping In structures could significantly decrease the cost of construction of
buildings, bridges, towers, and other flexible structures. Recent research and
practice indicate that design for control of damping in structures is within
reach if adequate resources can be applied to research into damping methods.
Current damping practice includes the use of tuned mass dampers
(massive weights near a building top that are attached to the building frame
through spnngs), nscoelastic dampers (thousands of small devices placed
throughout a building to dissipate kinetic energy in the structured, or
aerodynamic fanugs (changes in the structure shape to reduce the wind loads
causing the motion technique used on bridges but not practical for
buildings). These approaches to limiting motion are relatively expensive.
Active control of structure motion is a promising research area for
reducing the cost of many structures. It involves the sensing of structure
motion with a control system that activates motion reduction dences (such as
a tuned mass damper). Once activated, the system tends to reduce the
structure motion. Active control devices are common on aircraft and have the
potential to significantly reduce the cost of engineered structures susceptible
to w~nd-induced motion. Considerable research is required before these
devices could be considered to control wind-induced motion of buildings,
bridges, or towers. Similar active systems, properly designed and installed, can
also pronde potential benefits to the structure's resistance to earthquake
loads.
Sloshing of fluid in a tank is another method for dissipating energy in
a structure. Limited applications have been used in the United States to
restrict motion in water towers. Recent research demonstrates potential
application for damping of buildings and bridges by using fluid sloshing.
Research in the United States is needed to develop fluid-lioshing damping
technology to a practical design level.
Bridges
Conventional bridges are not very sensitive to the dynamic effects of
wind because of their relatively high stiffness. On the contrary, suspended-
span bridges, which include suspension and cable-stayed bridges, are very
sensitive to wind effects. In addition to buffeting effects of wind, they are
susceptible to aeroelastic effects, which to a great extent caused the Tacoma
Narrows disaster. These suspended-span bridges are often even more sensitive
to wind during various construction phases than they are after completion.
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Wnd-Eng~neenng Research Needs 69
The aerodynamic stability of a bndge is governed by the bridge
geometry, its spectrum of natural frequencies, and its damping. Information
on the aerodynamic behavior of bndges is determined through physical
modeling in wind tunnels. A wide range of mode} tests are available, such as
section models, taut strip models, and full-bridge models.
Section model tests, which are currently the primary investigative tool,
help to determine aerodynamic characteristics of the bridge section that are
then utilized In an analytical mode! to determine the bridge's overall dynamic
behavior. However, an analysis based only on section mode! test results
conducted in smooth flow often fails to describe the three-dimensional bridge
behavior in natural wind conditions. Improvements have been suggested in
this regard, such as appropriate modeling of turbulence in the approach flow.
Full-br~dge models, if they are both structurally and aerodynamically accurate,
can provide information on the overall dynamic behavior of the bridge and
offer the convenience of modeling the surrounding terrain to accurately
sunulate the approach flow conditions.
Motion reduction dewces can help to improve the aeroelastic stability
of a bridge. These can be considered during the design phase or can be
incorporated once the bridge is built. The behavior of freestanding bridge
towers during construction deserves special attention and, often, motion
reduction devices are needed to control their motion.
Motion Perception
Many structures move sufficiently in the wind that occupants can sense
the motion and may object to its magnitude. These structures include office
buildings, residential buildings, offshore platforms, airport control towers,
bridges, and other flexible structures. Very little research has been performed
on the levels of motion that are acceptable for various uses. The design of
many buildings is governed by the acceptability of motion to its occupants;
the current level is based on only a few, uncontrolled studies. However, the
constriction costs of some structures might be significantly reduced if these
levels were relaxed on the basis of more solid research into acceptable levels
of motion.
Offshore Winds and Their Effects
Wind-related issues concerning offshore drilling activities may be
divided into two general areas: the design and analysis of offshore structures,
and offshore exploration and operation.
Wind speeds at various return intervals are the essential input for the
design of conventional fixed structures. For these relatively stiff structures,
ondy the steady wind effects are of interest and they typically contribute less
than 10 percent of the total environmental Toads. However, for exploration
deep water, where conventional platforms may not be appropriate due to
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70 Wind and the Built Environment
their sensitivity to the dynamic wave load effects, attention has been focused
on the development of innovative structural systems to explore the frontiers
of offshore reserves. The most promising of these systems are the so-called
compliant structures, such as the tension leg platform.
Recent, preliminary studies have suggested that, under certain
conditions, wind loads are comparable to wave loading at low frequencies,
which emphasizes the need to improve our understanding of the wind-loading
mechanisms and to quantiiN,r their effects. A limited amount of data from wind
measurements taken over the ocean exhibit considerable variability. The
problem is essentially due to the difficulty of taking measurements, and it is
compounded by the variable nature of the sea surface, which continuously
translates and deforms. If the wind flow field and its characteristics are not
much influenced by the exact form of the surface, but rather by the energy
loss and rate of momentum transfer due to surface friction, then the
relationship established for the wind characteristics over land may be
applicable over the sea surface.
The steady wind loads are expressed in terms of wind velocity and
aerodynamic force coefficients. The overall-platform aerodynamic force
coefficient is determined by synthesizing force coefficients of the several
components and substructures of the platform, based on a projected area
approach utilizing code recommended values. Generally, these values are
conservative due to complex structural configurations and the influence of
interference and shielding. There is a need for better quantification of the
interference and shielding effects to develop a procedure for more accurate
assessment of steady, aerodynamic load effects on platforms.
Wind intensity significantly affects offshore exploration activities,
especially the operation of floating rigs and drilling ships. Windstorm
information is vital for the planning of drilling operations. The operation of
platform cranes, the transfer of personnel, and offshore helicopter flight
operation are also affected by wind conditions. Wind forecasts and real-time
data are crucial for the smooth operation and safety of these activities.
Fatigue Problems
Fatigue occurs when a structure fails after a large number of cycles of
oscillation or vibration at a stress level well below that which we cause
failure after only a small number of cycles. Fatigue is a major problem for
bridges and for many other structures or portions of structures.
The fluctuating nature of wind causes cyclic loadings on roof and wall
panels of buildings. In a siow-mov~ng hurricane, such cyclic loadings can cause
fatigue failures of these panels, resulting in their removal and consequent
wind and water damage to the building interior. For example, widespread
damage to residential units in Darwin, Australia, during Cyclone Tracy in
December 1974 was attributed to fatigue failure in roof panels. In addition,
damage in the Caribbean during Hurricanes Gilbert (September 1988) and
OCR for page 71
Wnd-Eng~neenng Research Needs 71
Hugo (September 1989) suggests that metal roof panels in these areas failed
due to the fatigue of materials.
In the design and field operation of a wind turbine, fatigue failure of the
rotor has been the dominant concern. The rotor can be subjected to as many
as ~ x 108 c~vcles of ultrahigh stress level during its life span.
Fatigue failure of many engineering materials has been studied through
C-YCiiC load testing ~ which the load changes from positive to negative within
each cycle, or from zero to positive within each cycle. Wind loading, however,
involves cyclic loading in the presence of a significant mean load that can
change sign from stolen to storm. It is not clear that the fatigue load models
developed without the presence of a mean load can satisfactonlY Predict
fatigue loading due to wind.
Further research in load history and cyclic loading should be pursued
to improve understanding of fatigue problem in windstorms. Future use of
higher-strength materials, entailing lighter components, is likely to increase
fatigue-related problems, thus making such research even more critical.
CODES AND STANDARDS
Building codes in each locality control design and construction of
buildings and structures in that locality. Most communities in the United
States adopt, in large part, one of three mode} building codes, namely, the
National Building Code of the Building Officials and Code Administrators
International, the Standard Building Code, or the Uniform Building Code.
Wind-Ioad provisions in these mode} building codes are patterned after the
ASCE Standard on Minimum Design L,oads for Buildings and Other
Structures, ANSI/ASCE 7-~.
ANSI/ASCE 7-~S is the only consensus w~nd-Ioad standard currently
available in the United States. All three mode] codes utilize the basic w~nd-
speed map of the ANSI/ASCE 7-~. However, the si~rnIar~cy in w~nd-Ioad
provisions between model building codes and the ANSI/ASCE 7-~S stops
with the w~nd-speed map.
The factors that influence the magnitude of And loads on a building,
addition to wind speed, are the terrain surrounding the building, the shape
of the building, arid the desired safety of the building frame and components.
The mode} building codes use some of these factors from the ANSI/ASCE
7-~8, modify some factors based on experience, or ignore some of the factors
as a part of tradition. In addition, some factors in mode] building codes are
adopted from industry manuals.
The use, adoption, and modification of w~nd-Ioad factors by the mode}
building codes result from an attempt to simplify the w~nd-Ioad provisions,
but also represent the lobbying efforts of industries and special interest
groups. Even with these modifications, final wind Toads for most buildings are
fairly consistent in all mode} building codes, though anomalies exist and, in
some cases, the wind loads between mode! building codes differ by 50 percent
or more. All three mode] building codes provide the use of ANSI/ASCE 7-~8
OCR for page 72
72 Wnd and the Built Environment
as art alternative to be applied at the discretion of the designer. The nation
would ~ ^—^~^ ~~ ~ 1 . ~ 1 . ~ . . . . .
a;; Immensely or one w~na-ioac provisions in the mode! building
cones were the same as those found in the ANSI/ASCE 7-~S standard.
The current version of the ANSI/ASCE standard, which was crafted by
a volunteer group, represents an outstanding effort but does not represent the
best that could be produced by a funded development effort. Needed
improvements to the ANSI/ASCE 7-~8 include better wind-speed definition;
improved wind directionality; improved gust factor models; inclusion of
torsional wind loads (twisting about the vertical axis of buildings); improved
local pressure prediction and overall frame loads through a fancily of loading
coefficients for various building shapes; improved alongwind dynamic loading
prediction model; inclusion of a workable across-wind mode] of wind loading;
w~nd-Ioad prediction for structures during construction; fatigue-Ioading
prediction; inclusion of a risk-based design procedure; and improved standard
construction details.
Improvements in code provisions could save billions of dollars each year
in reduced construction costs and windstorm damage. This benefit can be
realized only through an extensive research program directed specifically at
the various needed improvements.
RETROFIT REROOFING
with
, . . ~ . .
Retrofit, as used in this report. refers to the rovf~rina of an "Y;C:~;~= -~'
~ _ ~ ~ . ~ ,
. _,< _ _ _~ ~ v ·~ ~^—_ _V ~—A—1Aj5 AL All ~1;~ ~111~ 1 ~1
llCW row malenals anchor structure to Drovide increa.cer1
on_—~^ ~~ ~ ~ ~ ~ ~~
~~!LllsIlLIlO~s us Mu Prove uralnage. In lYdd, approximately 250,000,000
sq ft of metal retrofit roofing systems were installed in the United States. This
market is growing at the rate of about 15 to 20 percent per year.
In its simplest form' retrofit reroofing is the adding of another roof over
an existing roof, in the same shape and form as the existing one. In most
instances, however, retrofit reroofing systems consist of building a structure
with a new roof elevated above an existing roof. Retrofit substructures are
designed for both function and economy. The normal function is to provide
slope and direction to the new roof for proper drainage. The economical
challenge is to design lightweight members while minimizing the number of
pieces and the impact of additional wind loads on the existing structure.
In the northern part of the United States, the primary loading for
reroofing systems is snow load. In the southern part, the lightweight reroofing
systems are extremely sensitive to wind. The new geometric configuration of
the roofing system may affect the wind load that is imposed on the system.
For instance, if the onginal system was flat and the new system had a ridge
line, additional loads would be imposed in the area of the ridge. The existing
structure must accommodate these increased wind loads. The accurate
assessment of wind loads is a critical factor to be considered in most
reroofing systems. At present, however, the installation of new reroofs
receives little engineering attention.
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Mnd-Engyneenng Research Needs 73
AI)DITIONAL RESEARCH TOPICS
In addition to the research needs described above relating to the impact
of severe wind forces on the built environment, a number of other research
topics deserve the attention of the w~nd-engineering community. Some of
these have been noticed only recently by the public because of the
demonstrated adverse impacts on the human environment of occurrences such
as of} spills and soil erosion. Some of them, such as wind flow issues in the
urban environment, pollutant dispersion, and ventilation, are highlighted
because of the demand for more comfortable and healthier living. The
development of new technology and the use of new materials also offer the
opportunity to reexamine the use of wind as an alternative ener~v source. the
research needs of which are described briefly in this section.
Bind Flow in the Urban Environment
O. — — ,
Acceptability for human comfort of any development includes both the
aesthetic values of the space and the physical environment designed for
project occupants. A plaza intended for relaxation watt not be used if the
space is noisy, dirty, chilly, windy, or dangerous in some way. One of the
major factors influencing the intended use of pedestrian areas is the physical
comfort of the area, including wind forces on, and thermal comfort of, the
individual. Wind in a pedestrian area can cause the space to be underutilized,
especially if the temperature is low. A pedestrian's thermal balance is
influenced by air temperature, humidity, wind speed, the presence of sunlight
or shade, and the amount of clothing worn. To date, little research has been
performed to guide the development of better models for pedestrian
acceptability in the presence of wind, temperature, humidity, etc.
Several cities, including Boston, San Francisco, and Pittsburgh, require
that wind tunnel tests be performed for all projects in which the wind speeds
are likely to cause pedestnan comfort problems. However, even where cities
have w~nd-speed requirements, no attempts have been made to account for
thermal comfort. Most cities have no requirements at all for wind comfort
and leave decisions on pedestrian acceptability to developers. Since
developers have an uneven interest in the ultimate quality of the developed
space, quite different acceptability criteria are frequently applied to similar
projects in the same city. Implementation of accurate and economical
prediction methods for pedestrian environments could lead to improved
productivity of new and existing projects. Research is thus needed to provide
realistic guidelines for human comfort.
Pollutant Dispersion
Wind eng~neenug has made major contributions to the understanding
and treatment of air pollution problems. However, many challenges remain.
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74 Hued arid the Built Environment
Atmospheric dispersion problems have historically been addressed with
emp~ncal equations based on a Gaussian dispersion model. These equations
work reasonably wed In situations that do not involve terrain or large, nearby
structures. On instances where complicated geometries come into play, the
models may be in error by factors of 10, 100, or more.
Dispersion problems can be divided into two categories: extraordinary
events that occur rarely, and regular or daily events. An example of the
former would be industrial spills occurring infrequently but with immediate
threat to human life. Reingestion of fume hood or laboratory exhausts that
could induce long-term health effects through In,w-]eve~1 ren~t~1 -~rnr~cilr~
would be an examD]e of the. latter
~ < _ _ ~ _ _ _~ vim ~ ~ ~
Emergengy response to toxic spills is now often based on little or no
advance planning. In industrial settings, likely spill locations and magnitudes
can often be anticipated and planned for. Wind tunnel studies can be
performed ahead of time and the results stored on a Commuter fr)r olli~lr
reference. The computer can
rid _ then be connected to meteorological
instrumentation to develop a real-time prediction of toxic cloud extent.
Although some simple systems of this type do exist, additional research is
needed to optimize the design.
Laboratory or fume hood exhausts from buildings are frequently
reingested into the air intake system, thereby exposing occupants to dangerous
levels of chemicals on a regular basis. Wind tunnel modeling of these cases
can readily identifier solutions. However, research is needed in both numerical
and wind tunnel modeling to develop methods of prediction that are more
economical than current wind tunnel testing methods.
New pollution sources are required to show that national, ambient air
quality standards are being met using modeling procedures approved by the
Environmental Protection Agengy. New air quality regulations will require a
demonstration that health and safety thresholds are not exceeded or that the
cancer risk is insignificant. However, present Gaussian-Wpe or other empincal
models are inadequate to describe pollution dispersion in a number of
situations, and wind tunnel or numerical modeling is required to adequately
address these situations, which include dispersion in winds about buildings,
complex terrain, nonuniform roughness (urban or industrial settings), blowing
dust or particulates, area or volume sources, mobile sources, mountain valley
wind systems (thermally driven flows), and land or sea breezes.
Although wind tunnel modeling of pollution dispersal is relatively well
developed and numerical modeling is developing, there are a number of
research areas in which work wall enhance our ability to quantitatively
evaluate pollution levels. These include Reynolds number effects, dense gas
effects, plume buoyancy effects, dispersion in stable or unstable atmospheric
stratification, and hybrid modeling in which physical and numerical modeling
are combined.
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~nd-Engineen~g Research Needs 75
Ventilation
Increasing energy efficiency has become a major goal in the United
States. Concern about the increase of carbon dioxide production and acid rain
effects from the burning of fossil fuels has led to the consensus that the
nation's per capita energy usage must drop.
Wind engineering is one of the bases of ventilation technology, since
Grind is the driving force for infiltration of air through building skins and also
causes direct heat transfer from the building exterior surface. Improved
models of inf;Itration and heat transfer can thus improve energy efficiency in
buildings. Use of wind speeds to control natural ventilation could have
significant energy benefits.
Band Energy
Nationwide, more than 10,000 wind turbines generating about 700 MW
of electrical capacity were in operation by the end of 1984. According to the
Office of Technology Assessment (1985), wind energy as an alternative energy
source could provide 2l,000-MW capacity, representing about 10 percent of
electric system installations.
The trend in research and development has been toward larger rotors
to achieve economies of scale. A wind turbine with a capacity of 4 MW
requires a rotor diameter approaching 400 ft. In wind energy systems of all
size ranges, the rotor is the part most vulnerable to structural failure. The
rotor is stressed by a variety of forces: gravity-induced stress reversals,
centrifugal forces, w~nd-induced thrusts, and wind turbulence. Of these forces,
the last two are directly related to the properties of wind.
As mentioned above, fluctuating wind gusts can cause fatigue problems.
Research in time and spatial variations of wind gusts and their dynamic effect
on turbine rotors should be pursued vigorously to understand cyclic loading
on the rotor structure and to mitigate fatigue failures.
Soil Erosion
Soil erosion by wind is a global problem that induces both on-site and
off-site damage. The on-site damage includes sandblasting of plants, exposure
of plant roots, loss of plant nutrients, and Toss of agricultural productivity. The
off-site damage can be in varied forms: air and water pollution, sand
deposition on highways, dust damage to households, automobile damage, and
landscape damage. Off-site wind erosion costs in the western United States
are estimated at between $3.8 billion and $12 billion per year (Piper, 1988~.
These costs are much higher than the on-site costs. For example, in New
Mexico, on-site costs of wind erosion are estimated to be $10 million annually
(Dens and Condra, 1989), whereas off-site costs are about $466 Bullion per
year (Huszar and Piper, 1986~.
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76 Wnd and He Built Environment
Reducing dam age due to wind erosion wall require improvements in the
technology of wind erosion prediction as well as erosion control. Near-ground
wind characteristics, dust transport models, the erodibility of soil, and
instrumentation of a soil sampler are all fruitful] topics for research in
mitigating wind erosion.
Modeling of Iceberg Movement and Oil Spills
Wind plays an important role in iceberg drift. The potential threat of
an iceberg Impact with an offshore instalIation is of great concern in some
northern, onshore of] fields. The overall ice management task involves
projecting iceberg position in view of the environmental conditions
determined from weather forecasts.
Winds and surface currents are also important driving forces for the
movement of of! spills. Forecasts of actual trajectories of of] slicks provide
essential input to spill mon~tonng and control activities. Research to refine
models for both iceberg and of! slick movement would thus directly aid in
addressing the environmental and safety risks associated with the production
and transport of oil.
RECOMMENDATIONS
A number of observations, conclusions, and recommendations can be
drawn from the discussion presented in this chapter:
I. A strong research effort is needed to better define wind loads in
a codified fob for small projects, such as housing for single or multiple
families, small commercial structures, or industrial buildings, which comprise
the overwhelming bulk of construction in the United States. For these types
of structures, wind tunnel tests are often too costly to consider; thus reliance
must be placed on building codes to define wind loads. Because of the high
cost of full-scale tests, wind tunnel data obtained on a research basis have
been used to develop the w~nd-loading provisions of current codes.
Unfortunately, funds have been insufficient to attain the level of
understanding of wind loads that would provide maximum economy of
construction. No private organizations have the resources or mission to fund
the level of research needed for better code development. Neither the
National Science Foundation nor any other federal agency has found
sufficient funds for this task.
2. The availability of inexpensive but powerful computers will make
possible significant progress in numerical calculation capabilities in the near
future. Still needed are improved numerical algorithms, development of
turbulence modeling (especially considering the temperature gradients and
protocols for easy implementation of parallel processing), and improved
graphical display software.
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Wnd-Er~gyneenng Research Needs 77
3. It is recognized that full-scale field experiments are costly.
Nevertheless, field measurements can provide significant, future benefits. It
is recommended that a few, long-term field experiments be conducted to yield
baseline data, provided that a National Wind Science and Engineering
Program can be established to secure sustained funding for the eypenments.
In tandem with these experiments, physical modeling in the wind tunnel and
numerical modeling should be planned to develop improved modeling
technologies.
4. A combination of laboratory measurements, numerical simulation,
and full-scale testing represents the quickest path to a more complete
understanding of tornado flows. Larger laboratory facilities than currently
exist would benefit this investigation. Additional improvements In radar or
other instn~ments capable of measuring tornado velocity fields are also
needed.
5. Closely spaced w~nd-obse~v~ng stations along the coastline are
needed to obtain better definition of hurricane winds. One option is to
develop a deployable w~nd-recording station that can be placed in the path
of an approaching hurricane.
6. Additional research into the phenomenon of w~ndflow around
buildings should be conducted to improve the empincal prediction of cladding
pressures.
7. New developments in chaos theory provide possible avenues for
understanding the nature of turbulence and wind loading in the presence of
turbulence. An analytical component of w~nd-Ioad research emphasizing the
chaos theory might provide advances in understanding the complex wind-
load mechanisms.
8. A major research effort is required to improve knowledge of
dynamic loading effects on flexible structures, so that relatively simple
procedures can be employed by a designer to account for dynamic load
effects.
9. Active control devices may be effective in controlling w~nd-induced
motions of structures. However, considerable research on these devices is
required to ensure that they perform as designed after construction.
10. Sloshing of fluid in a tank could be used for damping the motion
~ . ~ . ~ . . . . .
to be demonstrated
of buildings and bridges, but its effective use wait have
through further research.
Il. Additional research is needed to better define the levels of motion
that are acceptable for various uses in flexible structures. Findings from this
research might substantially reduce the construction costs of some structures.
12. Aerodynamic force coefficients used for offshore platform design
are conservative in general due to complex, structural configurations and the
influence of interference and shielding. Better quantification of the
interference and shielding effects to develop a procedure for more accurate
assessment of steady, aerodynamic load effects on these platforms is strongly
needed. The motion of compliant offshore structures subject to strong winds
needs to be investigated by both computational and experimental methods to
better understand and quantify the dynamic effect of wind.
, ., ~
· . .
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78 Wind Id He Buill Environment
13. Further research in load history and Cyclic loading should be
pursued to improve understanding of the fatigue problem in windstorms. Use
of higher-strength materials, entailing lighter components, is likely to increase
fatigue-related problems, thus making such research even more critical.
14. Most cities have no requirements for wind comfort and leave
decisions on pedestrian acceptability to developers. As a result, quite different
acceptability criteria are frequently applied to similar projects in the same
city. Research in this area is needed to provide realistic and uniform
guidelines for human comfort.
15. Research is needed to develop better numerical models and more
economical wind tunnel testing methods for prediction of fume hood exhausts
to ensure that the public is not endangered by the reingestion of exhausts into
air intake systems.
16. Fluctuating wind gusts can cause fatigue problems on rotors of wind
turbines. Research in time and spatial variations of wind gusts and their
dynamic effect on turbine rotors should be pursued vigorously to understand
cyclic loading on the rotor structure and to mitigate fatigue failures.
17. To improve the technology of wind erosion prediction and control,
research is needed on near-ground wind characteristics, instrumentation of a
soil sampler, dust transport models, and erodibiTicy of soil.
Representative terms from entire chapter:
wind tunnel
