The Impact Of Hurricanes On Structures

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Wind Effects
Structural
Issues
Prepared by: Daniel Comarazamy
Grupo de Estabilidad Estructural (Ge2/INTEC)
Santo Domingo, Dominican Republic, 2005
Reviewed by: Tony Gibbs

Pan American Health Organization, 2005
A publication of the Area on Emergency Preparedness and Disaster Relief, PAHO/WHO.
The views expressed, the recommendations formulated, and the designations
employed in this publication do not necessarily reflect the current policies or opinions
of the Pan American Health Organization or of its Member States
The Pan American Health Organization welcomes requests for permission to reproduce
or translate, in part or in full, this publication. Applications and inquiries should be
addressed to the Area on Emergency Preparedness and Disaster Relief, Pan American
Health Organization, 525 Twenty-third Street, N.W., Washington, D.C. 20037, USA;
fax: (202) 775-4578; e-mail: [email protected]
The production of this publication was made possible thanks to the financial support of
the Disaster Preparedness Program of the European Commission Humanitarian Aid
Office (DIPECHO-III). In addition, the following agencies provided assistance for the
production and development of this material: the Division of Humanitarian Assistance,
Peace and Security of the Canadian International Development Agency (HAPS/CIDA),
the Office of Foreign Disaster Assistance of the U.S. Agency for International
Development (OFDA/USAID), and the Department for International Development
(DFID) of the United Kingdom.
TABLE OF CONTENTS
OBJECTIVES ........................................................................................ 5
INTRODUCTION
................................................................................. 5
STRUCTURAL VULNERABILITY .............................................................. 5
NONSTRUCTURAL VULNERABILITY ........................................................ 5
ADMINISTRATIVE AND FUNCTIONAL VULNERABILITY .............................. 6
THE PROBLEM .................................................................................... 6
HURRICANES
..................................................................................... 7
THE IMPACT OF HURRICANES ............................................................... 7
HOW HURRICANES ............................................................................. 8
CLASSIFICATION OF HURRICANES ........................................................ 8
FACTORS AFFECTING WIND IMPACT
............................................... 8
LOCATION AND OBSTRUCTIONS ........................................................... 8
TERRAIN ROUGHNESS ....................................................................... 10
BUILDING HEIGHT ............................................................................ 10
SURROUNDING TOPOGRAPHY ............................................................ 11
IMPORTANCE OF THE STRUCTURE ....................................................... 11
WIND DIRECTIONALITY ..................................................................... 11
WIND SPEED .................................................................................... 11
TURBULENCE ................................................................................... 11
MODIFIED BASIC PRESSURE .............................................................. 12
WALL OPENINGS .............................................................................. 12
THE STRENGTH OF STRUCTURAL ELEMENTS . .............................. 13
INTERNAL FORCES AFFECTING STRUCTURAL ELEMENTS ........................ 13
DESIGN PRESSURE ........................................................................... 13
TYPES OF HURRICANE DAMAGE .......................................................... 14
THE IMPACT OF HURRICANES ON STRUCTURES ......................... 15
WIND-RESISTANT DESIGN STANDARDS .............................................. 15
STRUCTURAL VULNERABILITY DUE TO ERRONEOUS DESIGN .................. 15
HURRICANE-RESISTANT DESIGN PROCESS ................................ 16
BASIC STUDIES ................................................................................
DEMAND ..........................................................................................
DESIGN ...........................................................................................
VERIFICATION ..................................................................................
VULNERABILITY OF EXISTING HEALTH FACILITIES
16
17
17
17
....................... 18
WIND VULNERABILITY ASSESSMENT METHODOLOGY ............................
QUALITATIVE METHODS ......................................................................
QUANTITATIVE METHODS ....................................................................
RETROFITTING EXISTING STRUCTURES ...............................................
COMMONLY EMPLOYED RETROFITTING TECHNIQUES ............................
18
18
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Disaster Mitigation in Health Facilities
OBJECTIVES
The main objective of this material is to assist health infrastructure planners
(including hospital administrators, engineers, architects, technicians, etc.) from
hurricane-prone Latin American and Caribbean countries (Slide No. 2) in learning
about the impact that extreme winds can have on buildings. It also strives to convey
the basic principles of mitigating hurricane damage in health facilities.
This training material focuses on reducing the vulnerability of structural elements:
namely, those parts of a building—such as foundations, columns, beams, walls, and
slabs—that withstand gravitational and lateral loads to ensure the stability of the
structure.
Examples are drawn from the partial or total failure of hospitals in the region as a
result of hurricane damage, and guidelines and recommendations suggest how to
reduce the structural vulnerability of existing and planned health facilities.
This material aims to make the hospital community and the health sector in general
aware of those factors that render health facilities vulnerable to hurricanes.
INTRODUCTION
Immediately following a hurricane, all health facilities must continue operating in
order to provide effective treatment for the medical emergencies that commonly
occur. It is imperative for hospital administrators to carry out integrated vulnerability
assessments that evaluate the structural, nonstructural, and operational aspects of
the services provided.
Although this material is limited to the impact of strong winds, it is important to note
that in many Latin American and Caribbean countries hospitals must be designed to
withstand multiple natural hazards such as earthquakes, floods, volcanic eruptions,
and landslides. For this reason, it is important to locate health facilities in areas that
are the least likely to be impacted by these phenomena.
STRUCTURAL VULNERABILITY
The terms structural or primary systems refer to the parts of a building that bear its
gravitational and lateral loads, transferring these loads to the ground and keeping
the structure stable. They include foundations, columns, load-bearing walls, beams,
and diaphragms.
NONSTRUCTURAL VULNERABILITY
Nonstructural or secondary systems fall into three categories: (1) architectural
elements; (2) basic (mechanical and electrical) service infrastructure; and (3)
equipment. The first category includes those building components—windows, doors,
roofs, suspended ceilings, partitions, and non–load-bearing walls—that are attached
to the structural elements but do not play a primary role in resisting wind or other
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loads. The second category comprises those elements that are essential to the
operation of the building, such as piping or electrical wiring and heating, ventilation,
and air conditioning (HVAC) systems. The third category includes the physical
contents of the building such as medical equipment and supplies, furniture,
machinery, etc.
In health facilities, nonstructural components, particularly medical equipment, are in
many cases more expensive than the structure itself. They account, on average, for
more than 80% of the total cost of building and equipping a hospital.
ADMINISTRATIVE AND FUNCTIONAL VULNERABILITY
Functional and administrative vulnerability depends on several factors:
(1) The distribution of architectural spaces and their relationship to the medical
and support services provided inside the hospital;
(2) The impact of this distribution on administrative processes such as
contracting, procurement, maintenance, case-management, and internal and
external communications; and
(3) The physical and functional interdependence that link the different areas of a
hospital.
The appropriate distribution and articulation of the spaces that make up a health
facility can guarantee the continuity of the hospital’s operations not only in normal
times but also during emergencies and disasters, when demand for health care
services is likely to peak.
THE PROBLEM
The diverse natural disasters that have afflicted Latin America and the Caribbean
bear proof that health facilities are particularly vulnerable to such events—
particularly hurricanes, earthquakes, and floods.
Several factors aggravate this situation:

A marked increase in the population density of hurricane-prone areas;

The quality of construction materials, particularly away from major cities, is
often defective and often they are not used appropriately to withstand strong
wind pressures;

Modern wind-resistant construction methods are seldom applied.
Hospitals are critical facilities that must continue to serve the community both during
and immediately after a severe hurricane. It is essential to assess their vulnerability
and implement mitigation measures in order to ensure operational continuity in
emergency situations.
A hurricane can damage health facilities in one or more of the following ways:
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
Strong winds exert positive (inward) and negative (outward) pressures on the
envelope, or exterior, of the structure and on its connections.

The wind’s uplift force may detach objects or hurl those that are already loose
at speeds of 100 km/h or more. Effectively, this turns them into windborne
missiles that can rupture a building’s envelope, causing injuries and severe
damage.

The rainfall that accompanies hurricanes can infiltrate the interior of a
building that has lost roofing, doors, or windows, resulting in damage to
costly equipment, furnishings, or other assets.

Rainfall can be so heavy during a hurricane that it causes floods, erosion, and
landslides which can lead to major damage.

The storm surges that sometimes occur, especially when hurricanes coincide
with high tides, can affect health facilities built near the coast, damaging
structures while letting water infiltrate the buildings.
In short, the operations of a hospital during and after a hurricane can be interrupted
due to damage to both structural and nonstructural elements.
Major hurricanes such as Georges (Slide No. 3) and Mitch (Slide No. 4), both of
which took place in 1998, have caused millions of dollars in damage and a large
number of deaths and injuries, primarily due to secondary effects of the hurricanes,
such as landslides or floods (Slide No. 5).
HURRICANES
THE IMPACT OF HURRICANES
Over the last two decades, adverse natural phenomena have affected over 800
million people worldwide, killing many and causing economic losses worth more than
US$50 billion.
According the data from the Pan American Health Organization, between 1981 and
2001 more than 100 hospitals and 650 healthcare centers were severely damaged by
the effects of natural hazards (Slide No. 6). The Economic Commission for Latin
America and the Caribbean (ECLAC) reported direct economic losses of US$ 3,120
million.
From a financial as well as social perspective, the vulnerability of health facilities to
such phenomena is more critical than for other types of structures (Slide No. 7).
HOW HURRICANES FORM
Hurricanes arise when several conditions occur simultaneously (Slide No. 8). They
are defined as low-pressure weather systems that get their energy from the latent
heat that results from the condensation of vapor over warm tropical ocean waters.
For a hurricane to form, the temperature of the surface ocean waters to a depth of at
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least 60 meters must remain at 26°C or higher over several days. A low-pressure
area, with air-mass convergence in the lower part of the atmosphere, is also
required. In this low-pressure area, the winds must be hot and humid.
The hurricanes that travel through the islands of the Caribbean start out over West
Africa’s coastal waters as a tropical disturbance. If the conditions are propitious, they
increase in intensity until they become tropical depressions (winds of up to 63
km/h), then tropical storms (sustained winds of between 63 and 118 km/h), and
finally—and most destructively—hurricanes, with sustained wind speeds greater than
119 km/h (Slide No. 9).
The conditions most likely to breed hurricanes in the Atlantic and Caribbean are
prevalent each year from 1 June to 30 November—the hurricane season.
Meteorologists employ anemographs along a series of hydrometric stations in order
to measure and record wind velocity during hurricanes (Slide No. 10).
CLASSIFICATION OF HURRICANES
Hurricanes are classified according to their intensity, which is measured in two ways:
their minimum central barometric pressure, or their maximum wind velocity. A direct
relationship between the two forms of measurement can be determined through
mathematical models based on theoretical and empirical information.
In the early 1970s, Herbert Saffir and Robert Simpson compiled a table classifying
hurricanes according to those parameters and specifying the damage caused by each
category (Slide No. 11). The Saffir-Simpson scale, which identifies five increasingly
destructive levels of hurricanes from Category 1 to Category 5, has been adopted in
the Americas to portray hurricane intensity. Based on data collected, annual
comparisons have been made regarding the number and categories of hurricanes
that have affected the Caribbean and North Atlantic over the past 50 years (Slide
No. 12).
FACTORS AFFECTING WIND IMPACT
LOCATION AND OBSTRUCTIONS
The location of a building vis-à-vis the impact of strong winds (Slide No. 13), as
well as the presence of other buildings of smaller size (Slide No. 14), can cause
increases in wind speed and turbulence affecting both the front (windward wall) and
the back (leeward wall) of the facility, significantly increasing the wind’s basic
pressure. Other conditions, such as openings in the lower part of buildings, cause an
unusual increase in wind velocity (Slide No. 15) as well as turbulence behind the
buildings.
The same is true for buildings with gable roofs, where the wind induces turbulence
affecting the rear and side walls as well as the leeward part of the roof (Slide No.
16).
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The pressure exerted by strong winds on the structural system is a function of the
dynamic part of Bernoulli’s equation (Slide No. 17), known as basic pressure, which
is modified by the following factors:

Roughness of the terrain

Height of the building

Surrounding topography

Importance of the structure

Wind directionality

Wind speed

Turbulence

Openings in building envelopes
Different International Standards (Slide No. 18) show several ways to measure the
basic and design pressure (Slide No. 19) and also the different types of buildings
and structures under study (Slide No. 20).
Noting that the trend is to follow ASCE-7 and International Building Code (IBC)
philosophy, we introduce, as an example, the formulas, tables, and notations of both
standards (Slide No. 21).
The factors or coefficients affecting the basic pressure are shown in the following
table (Slide No. 22 and Slide No. 23):
DEFINITION
FACTOR
Directionality
Kd
Importance
I
Exposure
Kz
Topography
Kzt
3-sec gust
G
External pressure
coefficient
Internal pressure
coefficient
Cp
Cpi
Design pressure
p
Design force
F
MEANING
Takes into account the probability of the maximum
wind coming from the same direction that produces the
maximum pressure
Converts from a 50-year return period wind speed to a
100-year return period recommended for hospitals
Represents the wind velocity at any ‘z’ height above
the ground
Takes into account the fact that the structure might be
located on an isolated hill or escarpment with higher
wind speeds than on level ground
Represents the turbulence-structure interaction and the
dynamic amplification of the wind
Estimates the wind pressure on the exterior of the
building
Reflects the internal pressure of the building dependent
on the number and size of wall openings
Represents the design pressure that is no greater than
the modified basic pressure
Net force on special structures and open buildings
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TERRAIN ROUGHNESS
Terrain roughness exerts its own effects on wind velocity and turbulence: the
rougher the surface, the lower the speed but the greater the turbulence.
The size and density of surface objects such as buildings and trees affect roughness.
Roughness length indicates the extent of influence of terrain roughness on wind
speed and longitudinal turbulence: the more uneven the terrain, the longer the
extent of roughness, increasing the effect of friction on wind speed retardation
(Slide No. 24).
BUILDING HEIGHT
When a fluid in motion interacts with a solid surface, the resultant viscosity
generates shearing forces in the opposite direction to that of the fluid’s motion.
A similar effect occurs in the interface between the surface of the earth and the wind
running over it: close to the surface of the soil, viscosity reduces the speed of the air
almost down to zero.
As altitude increases, wind speed increases too, until it reaches a constant speed
called gradient speed, which is independent of the roughness of the terrain. This
variation in wind speed as a function of altitude can be predicted mathematically with
a logarithmic equation. In practice, however, a much simpler model is used: the
power-law method for extrapolating wind speed from one altitude to another (Slide
No. 25).
Some building codes in the Caribbean include tables specifying the roughness
coefficients that affect basic pressure (Slide No. 26) in terms of both exponential
windward variability and the basic uniform pressure on leeward walls.
SURROUNDING TOPOGRAPHY
The topography of a region can significantly affect wind behavior. Broadly speaking,
hills and escarpments generate sudden accelerations in wind velocity that can
sometimes increase wind loads by up to 80%. These increments depend on several
factors, such as the location of the structure or horizontal and vertical attenuation
effects (Slide No. 27).
Studies must also be carried out to assess potential curbs on wind velocity due to the
protection of leeward valleys and ravines (Slide No. 28).
IMPORTANCE OF THE STRUCTURE
A differential factor affecting basic wind pressure is the importance of the structure’s
use, which is a function of a quadratic relation between the basic wind speed
associated with an average recurrence interval and that of a 50-year recurrence
interval. An importance factor is established depending on the structural system
used. In the case of hospitals it is recommended to assume a 100-year return
period.
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WIND DIRECTIONALITY
Basic pressure is affected by the uncertainty effect caused by the likelihood of the
wind hitting the structure from any given direction. This parameter, known as the
directionality effect, must be taken into account when assessing design pressure.
WIND SPEED
Wind speed is the single most important factor when trying to determine basic
pressure. The various wind-resistant building standards in the Caribbean and Central
America express this parameter in dissimilar ways (Slide No. 29), but the most
common is to measure three-second gusts.
More sophisticated studies (Slide No. 30) and recent computer simulations make it
possible to determine the various local levels of risk and even to establish risk-zoning
criteria.
TURBULENCE
Wind motion is turbulent, and it is difficult to give a concise mathematical definition
of turbulence. However, it is known that wind turbulence exists due to the lower
viscosity of air in comparison with water. Any air motion faster than 4 km/h is
turbulent; i.e., air particles move erratically in all directions.
For structural engineering purposes, it can be said that wind speed has two
components: average velocity (which increases as a function of altitude) and
fluctuations due to turbulence.
MODIFIED BASIC PRESSURE
Basic pressure, as modified by the parameters mentioned above, is known as
Modified Basic Pressure, as per ASCE-7, which will vary depending on the conditions
of each region (Slide No. 31).
ρ
Density of the air mass given regional conditions of pressure and temperature
Κz
Factor that varies with height and terrain roughness
Κzt
Factor that depends on the topography of the terrain and several attenuation
factors
Κd
Wind directionality component
Ι
Importance of the structure for a 50-year return period
V
Design wind speed measured according to the standards in use in the region.
The trend is toward three-second gust measurements
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WALL OPENINGS
Openings in walls exposed to the action of the wind are highly significant when
defining pressure coefficients. They are the holes in the structure’s façade and, in the
case of hospitals, can be critical (Slide No. 32).
All windows, doors, and other apertures will be considered openings unless they and
their connectors have been detailed and designed to withstand wind loads and windborne missiles. Based on this criterion, structures fall into three categories:

Open structures: Systems through which the wind flows freely or that have
wall openings making up over 80% of the total envelope area;

Closed structures: Those that are not defined as open or partially closed
structures;

Partially closed structures: Those that meet the following conditions:
o
The total area of openings in the wall exposed to positive or inward
pressure is greater than 1.1 times the sum of the openings in the rest
of the outer structure, including the leeward and lateral walls, the
roofing, etc.
o
The total open area of the wall exposed to positive pressure exceeds
0.4m2 or 1% of the entire wall—whichever figure is smaller—while the
ratio of the total area of openings to the total area does not exceed
20% in the rest of the outer structure—i.e., excluding the windward
wall.
THE STRENGTH OF STRUCTURAL ELEMENTS
INTERNAL FORCES AFFECTING STRUCTURAL ELEMENTS
Structural elements are affected by internal forces; among them axial load, shear
force, bending or flexural moment, and torsional moment (Slide No. 33).

The axial load is defined as a compression or traction force that can shorten
or expand the length of an element. In the case of reinforced concrete
elements, their capacity is linked to the concrete’s resistance to compression,
their confinement using transversal reinforcement and their longitudinal
reinforcement. Failures are caused by deficiencies in one or more of these
characteristics.

Shear force involves the action of two parallel forces in opposite directions on
an element, tending to cause a displacement or slippage of one part of the
element with regard to another. The failure caused by this type of force is
sudden and unexpected. When it affects vertical elements, it can compromise
the stability of the entire structure.

Flexural moment is defined as the tendency of a force to induce rotation
around a given axis. A sufficiently severe rotation can cause an element to
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fail. Within limits, however, it helps to dissipate energy. This type of force,
like the axial load, makes elements expand or contract.

Torsional moment is a force that tends to cause non-coplanar rotation in the
element with regard to its longitudinal axis. Failure due to this cause tends to
be as dangerous as failure due to shear force, since both forces cause
distortion or tangential stresses on the structural system. It can cause the
partial or total collapse of the structure (Slide No. 34).
DESIGN PRESSURE
The action of the wind exerts a basic pressure that, modified by all the factors
mentioned above, is known as design pressure. Design pressure is a function of
various pressure coefficients that are determined experimentally, gust coefficients,
and the internal pressures that occur during a hurricane.
When designing the architectural configuration of a multistory hospital, it must be
taken into account that, buildings dynamically sensitive to the action of the wind,
i.e., flexible buildings, are generally subjected to higher wind forces than those
buildings that are not dynamically sensitive, i.e., those that are rigid.
This condition depends mainly on the natural frequency of the structure: a building is
considered flexible when its natural frequency is lower than 1 Hz. Hence, when
determining design pressure, the gust coefficient will be different depending on
whether the structure is rigid or flexible (Slide No. 35).
As noted earlier, design pressure increases exponentially with altitude on the
windward side and is uniform on side walls, leeward walls and roofs, both in the case
of tall buildings (Slide No. 36) and systems with gabled roofs (Slide No. 37).
TYPES OF HURRICANE DAMAGE
The correct location of health facilities is vitally important in hurricane-prone areas,
as was brutally demonstrated when Hurricane Gilbert tore the roof off Princess
Margaret Hospital in a matter of minutes as it tore through Jamaica in 1988 (Slide
No. 38).
Extreme winds can sometimes uproot the very foundations of a structure (Slide No.
39). The designers of a health facility must factor in that hurricanes can overturn
inadequately anchored buildings, particularly if their structure is light.
When not designed correctly, steel frames can also fail under strong wind pressure,
as when Hurricane David tore through Dominica in 1979, leaving much of the
population homeless (Slide No. 40). Generally, this type of failure takes place at
the connections.
Masonry structures are not immune to the action of the wind. Although they are
considered comparatively safe, the loss of the roof makes masonry walls perform as
if they were structurally unattached, causing their partial or complete failure.
The design of concrete structures is generally guided by seismic load considerations.
When this is not the case, the designer must make sure that the hospital can
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withstand the wind load in order to prevent a catastrophe in the event of a
hurricane.
Timber structures are inherently the least safe (Slide No. 41). Moreover, due to its
growing scarcity and cost, wood is not necessarily more affordable than concrete and
steel.
It is worth noting, however, that wooden health facilities can be designed and built
to withstand hurricanes, so long as the timber chosen has the right mechanical
characteristics for structural use, the correct connections are employed, and effective
maintenance routines protect the wood from humidity and insects.
The proper attachment of roofing support systems is of crucial importance for health
facilities (Slide No. 42).
THE IMPACT OF HURRICANES ON STRUCTURES
WIND-RESISTANT DESIGN STANDARDS
The philosophy of most wind-resistant design standards is simple—ensuring that the
design and construction of structures allow them to withstand likely wind loads
without damage, and exceptional wind loads with the least damage possible. The
most important of these standards include ASCE-7, which is used in the United
States, Australia’s AS1170.2, the Caribbean’s CUBiC, Europe’s ENV 1991-2-4, and
Japan’s AIJ. Although there are some differences among these standards, they all
follow a similar procedure for calculating wind loads.
The theory behind such standards starts out from the fact that basic pressure is
proportional to wind speed squared. Wind speed varies from place to place
depending on the climatic characteristics of the area in standard conditions (speed
measured at an elevation of 10 m above ground level over open terrain without
obstructions, with a recurrence period of 50 years).
Once the basic design wind-speed of the region has been determined, modifications
are introduced based on the local topography, terrain roughness, and altitude.
Inappropriate location and design can wreck even expensive structures, particularly
when they are close to the coast (Slide No. 43).
STRUCTURAL VULNERABILITY DUE TO ERRONEOUS DESIGN
Floor plan
A structure’s floor plan determines largely how the building will respond to wind
loads. It is advisable that the configuration be symmetric, in order to ensure a
balanced distribution of the wind loads; if a non-symmetrical configuration has been
chosen, the designer of the health facility must be sure that the structure will not be
affected by torsion (Slide No. 44). In addition to a carefully thought-out structural
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design, strict attention must be paid to controlling the quality of the building
materials, their use, and the construction methods employed.
Elevation plan
All other things being equal, the height of a building affects its response to wind
loads. If the design introduces sudden changes in configuration or size from one
story to another, high pressures will be generated, particularly at the corners and on
protruding elements, with negative effects on the structure.
Roofing
The type of roof shape that best interacts with wind loads is a hipped roof (the kind
that slopes in all four directions from a rectangular plan), particularly if the angle of
the slope is between 20 and 30 degrees. The next best type of roof—at least in
comparison with mono-sloped or flat roofs—is the gable roof (one with two slopes),
so long as it slopes at a similar 20-30 degree angle (Slide No. 45). It is also
advisable that eaves or overhangs be eliminated or kept short in order to prevent
extreme wind loads from causing roof structural failure, since long eaves tend to
generate high uplift pressures, particularly in the case of low-slope or flat roofs
(Slide No. 46). Experience shows that the local pressures caused by strong winds
are greatest at the corners and on the ridges of a roof.
Reinforced concrete roofs generally perform satisfactorily when confronted with
lateral wind loads. However, attention must be paid to the strength and suitability of
the connections between the concrete roof and masonry walls or tie beams, given
the negative, outward pressures that can be generated when the breaching of an
entry door or window due to collision with windborne debris creates an opening in
the building envelope.
The influence of nearby buildings
The concentration of buildings in a particular area may have positive or negative
effects on the wind resistance of a health facility. If the buildings were designed and
built following up-to-date wind-resistant standards, windward buildings will protect
the other structures and the action of the wind will have few negative effects (Slide
No. 47).
By contrast, if the windward buildings have not been designed and sited properly,
they may contribute to the turbulence that affects nearby structures (Slide No. 48).
Other effects
Hurricanes have other devastating effects that can affect a health facility indirectly.
For instance, the heavy rains associated with hurricanes can increase the volume of
water flowing through rivers and brooks, eroding the base of a bridge’s pier (Slide
No. 49) or causing a landslide that cuts off a stretch of road or breaks off lifelines
(Slide No. 50), isolating the facility or disrupting key utilities.
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HURRICANE-RESISTANT DESIGN PROCESS
The process of designing health facilities to withstand extreme wind loads involves a
series of steps that must be guided by the existing risk demand where the structure
is to be sited.
BASIC STUDIES
It is necessary to know the basic design wind speed of the region where the health
facility is to be located. The basic design speed has to do with the likelihood of
maximum annual wind speeds. Depending on where the structure is located, these
speeds will be classified as hurricane- or non-hurricane wind speeds.
In the case of nonhurricane-force winds, the design wind speeds will be based on
the records for the region. In the case of hurricane-force winds, records may be too
scarce or inaccurate to reflect the actual design wind speed. If the latter is the case,
simulations must be carried out in order to assign the correct design wind speed to
the hurricane-susceptible area.
In addition, those sites that are prone to flooding, landslides, or storm surges must
be identified, since such phenomena—common byproducts of hurricanes—can
significantly compromise the structural integrity of a hospital.
DEMAND
Most of the problems that affect the vulnerability of health facilities to hurricanes and
other disasters are caused by lack of communication and coordination among the
various stakeholders entrusted with the design of the facilities. A joint effort is
required of all those involved—medical personnel, administrators, engineers, and
architects—so that the ultimate design has near-zero tolerance for wind-induced
damage. For instance, any breach of the building’s envelope during a hurricane, such
as parts of the roof flying off or the collapse of doors or windows, can lead to the
functional failure of the entire hospital.
DESIGN
Once the basic design wind speed has been determined, wind-engineering principles
must be applied in order to determine the wind loads that the health facility will
actually have to endure. Given likely wind pressures, the structural engineer can
design the facility so that its primary structural system can withstand the wind loads
borne by the structure with little or no significant damage.
Structural analysis must take into consideration both positive and negative internal
pressure. In both cases, assessments must be made of the pressures perpendicular
to the roof ridge (Slide No. 51) and parallel to it (Slide No. 52).
It is also important to pay attention to the design of the nonstructural elements,
such as components and cladding, since such elements may be subjected to
significant pressure because of their limited effective areas.
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Disaster Mitigation in Health Facilities
VERIFICATION
The displacements suffered by a structure’s elements because of the pressures
exerted by extreme winds can cause both structural and nonstructural damage. It is
essential to verify that the maximum absolute and relative displacements that may
occur fall within the parameters set by the most up-to-date local construction
standards.
For example, health facilities designed as flat-slab structural systems, supported
directly by columns without caps or drop panels able to absorb piercing forces (Slide
No. 53) have been shown to be inefficient and difficult to restore after a hurricane.
A simple evaluation of the plans will rule them out altogether.
It is also important to make sure that the wind loads affecting the structure will be
transferred by design from the roof down to the foundations through a continuous
load-transference path (Slide No. 54). If the design does not take this into account,
the connections must be redesigned correctly to prevent the collapse of the structure
(Slide No. 55).
VULNERABILITY OF EXISTING HEALTH FACILITIES
Structural vulnerability assessments are carried out in order to evaluate the safety of
existing or planned health facilities in the face of extreme winds (Slide No. 56).
Current wind-resistant hospital design standards demand that the structure be able
to withstand:
o
The design hurricane event without damage; and
o
Hurricanes greater then the design event with only minor nonstructural
damage that can be repaired easily.
Such assessment is extremely important in the case of existing health facilities, since
it identifies weak structural or nonstructural elements that require retrofitting in
order to ensure functional continuity in the aftermath of a hurricane.
A hospital vulnerability assessment is aimed at evaluating the susceptibility of the
structure to hurricane-caused damage, as well as to cataloguing the kinds of damage
that may occur (Slide No. 57).
WIND VULNERABILITY ASSESSMENT METHODOLOGY
Wind vulnerability assessment procedures fall into two categories:
a) Qualitative methods
b) Quantitative methods
Qualitative methods
Qualitative methods (Slide No. 58) are employed to carry out a quick and
straightforward assessment of the structural safety of the health facility in question.
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Disaster Mitigation in Health Facilities
The structure is classified according to such factors as the age of the building, its
state of conservation and maintenance, the quality and characteristics of the
materials used, the number of stories, the architectural configuration, and the
structural systems.
One of the key aspects to bear in mind, when applying such methods, is the
likelihood of elements to buckle. It is one of the most common problems found in
structures subjected to strong wind loads.
Other factors that influence qualitative vulnerability assessments are the topography
of the site, its exposure, its location in an urban or rural setting, and the design wind
speed for the area where the health facility is located.
If the qualitative assessment uncovers deficiencies in the building’s ability to
withstand strong winds, it is necessary to carry out more detailed quantitative
assessments.
Quantitative methods
Quantitative methods evaluate the resilience of an existing health facility’s primary
structure, also known in this context as the main wind-force resisting system (Slide
No. 59).
Broadly speaking, the procedure is similar to that employed when assessing the
design of new structures. The main difference, of course, is that variables such as
the actual site, structural system, the quality of the construction materials, and use
of façades and other nonstructural elements cannot be chosen from scratch, guided
by the most up-to-date wind engineering standards. Instead, these variables must
be accepted as a given and analyzed in order to determine their degree of
vulnerability to extreme wind conditions, as well as what corrective measures may
be taken.
Quantitative methods, if they are to be effective, call for large amounts of data—in
fact, as much pertinent data as can practically be collected. This makes them more
precise than qualitative assessments in predicting the likely types of failure that may
occur, both overall and in the case of specific elements, and hence more reliable
when evaluating the resistance of a building to extreme winds. On the other hand,
they are costlier and more time consuming for the assessment team.
Regardless of the method of analysis, if the structure does not meet the performance
objectives required, retrofitting is required to reduce its vulnerability to strong winds.
RETROFITTING EXISTING STRUCTURES
When vulnerability assessments uncover structural deficiencies, existing structures
must be retrofitted. Retrofitting, however, should not affect the functionality of the
health facility or the quality of the health care provided, nor should it imply
temporarily vacating a critical facility that by definition should operate year-round.
In determining which elements to reinforce, attention should be paid to the specific
hazards associated with extreme wind conditions. Some elements might need to be
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Disaster Mitigation in Health Facilities
added; others should perhaps be discarded or replaced. Nonstructural elements must
also be taken into account. Do they augment the structural response of the facility to
hurricanes (Slide No. 60), or degrade it?
The design of new elements must be as rigorous, in terms of wind-resistance, as that
produced when a new health facility is being planned. Special attention must be paid
to the detailing of the connections between the new elements and the existing
structure. Broadly speaking, by transferring wind and other loads to the new
structural elements, the retrofitting should relieve the existing structure of those
deformations and stresses that formerly increased its vulnerability.
It is worth noting that the structural retrofitting of a hospital to improve wind
performance is significantly simpler and less expensive than structural retrofitting
aimed at reducing seismic vulnerability.
COMMONLY EMPLOYED RETROFITTING TECHNIQUES
Structural retrofitting strives to improve a building’s capacity to withstand the high
pressures associated with hurricane-force winds. It also seeks to reinforce those
nonstructural elements that, should they fail, would allow the force of the wind to
breach the building’s envelope, generating internal pressures that could threaten the
structural integrity of the facility as a whole. The penetration of wind and water
would doubtless affect the functionality of the hospital during and after a hurricane
as well as putting at risk the medical and support equipment.
When retrofitting a hospital, the following measures will improve hurricane
performance:

Ensure that wooden or steel columns are properly anchored to the foundation
system (Slide No. 61).

Use galvanized connectors to guarantee a good connection between wooden
beams and columns (Slide No. 62).

Make sure that the roof is waterproof—for instance by heeding appropriate
building standards when attaching asphalt membranes or corrugated iron
sheets to the roof bracing system.

Provide additional lateral reinforcement by introducing shear walls or bracing
elements.

Strengthen the rigidity of external masonry walls by incorporating additional
concrete columns.

Use galvanized straps to ensure a good connection between the primary and
secondary beams, particularly in sloped roofs (Slide No. 63).

Ensure a good connection between wooden beams and preformed concrete
beams by using special clamp irons (Slide No. 64).

Carry ouy an effective maintenance routine, such as the protection of metal
structures from corrosion.
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Disaster Mitigation in Health Facilities

Provide adequate anchorage of metal sheet roofing and steel joists to the
main masonry structure (Slide No. 65).

Connections between structural and nonstructural elements, such as masonry
partitions, must be made in such a way to ensure that lateral loads be carried
out by the primary system (Slide No. 66) and that deformations should not
induce failure of the wall (Slide No. 67).

Retrofitting of interior or exterior walls of a hospital should comply with local
regulations in accordance with the recommendations of vulnerability
assessments (Slide No. 68).

Additional structural retrofitting details are shown in the following slides:
(Slide No. 69), (Slide No. 70), (Slide No. 71), (Slide No. 72), and
(Slide No. 73).
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Disaster Mitigation in Health Facilities
TEXT, SLIDE, AND IMAGE CREDITS

D. Comarazamy—Text and Slides

T. Gibbs—Photos and text review

C. Compañy—Photos

J. Vermeiren, OAS—Images and Graphs

A. Comarazamy—CAD drawings

F. Sanchez—Graphic design and editing

NOAA—Images of hurricanes

PAHO, Disaster Mitigation in Health Facilities
Prepared by:
Grupo de Estabilidad Estructural (Ge2) / INTEC
Ave Los Próceres, Galá
Apdo 349-2
Santo Domingo, Dominican Republic
www.intec.edu.do
Wind Effects - Structural Issues
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