Constructing for High-Wind Events
The Risk of Severe Wind in Virginia
Characteristics of Destructive Winds
How Homes Fail
Building Wind-Resistant Homes
1
I. The Risk of Severe Wind in
Virginia
Jon Ayscue
June 17, 2004
2
Assessing Risks
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June 17, 2004
3
Tornado Activity in the US
Jon Ayscue
June 17, 2004
FEDERAL EMERGENCY MANAGEMENT AGENCY MITIGATION DIRECTORATE
Assessing Risks

The combining of these two factors allows the homeowner to
determine the risk to their home
5
Historical Hurricane Tracks 1851 1999
Jon Ayscue
June 17, 2004
6
ASCE Wind Map
Jon Ayscue
June 17, 2004
7
Wind Speeds for Virginia Localities
110 MPH
100 MPH
90 MPH
Counties of
Accomack and
Northampton. City
of Virginia Beach
Counties of
All other
Gloucester, Isle of
Localities
Wight, Lancaster,
Mathews, Middlesex,
Northumberland,
Southampton, Surry,
York.
Cities of
Chesapeake,
Franklin, Hampton,
Newport News,
Norfolk, Poquoson,
Portsmouth, Suffolk.
Jon Ayscue
Special Wind
Region
Counties of
Bland, Grayson,
Scott, Smyth,
Tazewell,
Washington,
Wythe
June 17, 2004
8
Fastest Mile vs. 3 Second Gust
•
•
Fastest Mile (mph)
3 Second Gust (mph)
70
85
Why 3 Second Gust?
 Standardization
75
90
80
100
 More Realistic
Measurement
85
105
90
110
100
120
105
125
110
130
120
140
125
145
130
150
140
160
150
170
“Fastest Mile” =
Hurricane “Maximum
Sustained Winds”
Jon Ayscue
June 17, 2004
9
3-Second Peak Gust
Maximum gusts to
94mph (42m/s)
WEMITE 1, WS10
45
Maximum mean
speed ~73mph
(33m/s)
40
wind speed (m/s)
35
30
25
20
15
10
5
0
0
600
1200
1800
2400
3000
3600
time (sec)
Jon Ayscue
June 17, 2004
II. Characteristics of Destructive Winds
Damage and Storm Category
Fujita Tornado Scale
Saffir-Simpson Hurricane Scale
TAKING SHELTER FROM THE STORM: BUILDING A SAFE ROOM INSIDE YOUR HOUSE
12
Hurricane-Force Winds
 …are more turbulent than most other wind storms (tens of
thousands of gusts)
 …are sustained for longer periods of time (hours)
 …change slowly in direction, seeking out the most critical
angle of attack
 …carry large amounts of debris
Jon Ayscue
June 17, 2004 13
Tornadic Winds
…are relatively brief, minutes rather that hours
…are damaging outside the main funnel area due to inflow
…can occur with little if any warning, unlike hurricanes
…like hurricanes, can carry large amounts of debris which
can be as or more damaging than the wind itself
BUILDING PERFORMANCE ASSESSMENT TEAM
15
Tornado Inflow
Jon Ayscue
June 17, 2004 16
Tornadoes: Good News and Bad
 The Good News - most tornadoes are no stronger than
minor hurricanes
 The Bad News - a tornado doesn't have to strike your
home to destroy it because of inflow
 More Bad News - nothing can protect your home from a
direct hit by a F4-F5 tornado
17
Historical Tornado Strength (United
States, 1950-1994)
 Weak (F0-F1) 40-112 mph
 Strong (F2-F3) 113-206
mph
 Violent (F4-F5) 207-318
mph
F3
7%
F2
21%
F4-F5
4%
F0-F1
68%
18
Historical Hurricane Strength (19001996 Direct Landfalls)
Category
Wind Speed
(mph)
Damage
1
74-95
Minimal
2
3
95-110
111-130
Cat 5
1.3%
Cat 4
9.5%
Cat 2
23.4%
Moderate
Extensive
4
131-155
Extreme
5
>155
Catastrophic
Cat 1
36.1%
Cat 3
29.7%
Jon Ayscue
June 17, 2004 19
Hurricanes and Tornadoes
Homes can be built to withstand all but the strongest
hurricane winds
 Mitigation during Construction
 Post-construction Retrofitting
Homes cannot withstand strong tornadoes
 Saferooms provide protection > 250 m.p.h.
20
III. How Homes Fail
Windborne Debris
 Debris causes a large
fraction of damage
 Size increases with wind
speed
 Windows and doors are
especially vulnerable
Courtesy of Texas Tech University
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June 17, 2004 22
Protecting from Windborne Debris
 Eliminate Sources of Debris
 Secure out-buildings
 Remove unsecured items from around the house
 Remove trees within ‘falldown” distance
 Protect Openings
 Especially garage and patio doors
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June 17, 2004 23
Anatomy of a Destroyed Home
•The roof fails or gable-ends
collapse
•Lost windows and doors
contribute to failure
•With the roof lost, walls
collapse
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June 17, 2004 24
Gust Wind Speed = 100mph
Unmitigated building starts to
lose some shingles
Courtesy North Carolina Blue Sky Foundation and Applied Research Associates
Gust Wind Speed = 125pm
Due to increased internal pressure
resulting from a large missile hit,
this unmitigated building losses
one window and one sliding glass
door
Jon Ayscue
June 17, 2004 25
Gust Wind Speed = >125mph
Pieces of plywood sheathing
start to be pulled away from the
frame
Courtesy North Carolina Blue Sky Foundation and Applied Research Associates
Gust Wind Speed = >125pm
More sheathing lost. Entire wood
frame is exposed.
Jon Ayscue
June 17, 2004 26
Gust Wind Speed = >125mph
Whole roof failure of the
unmitigated building
Courtesy North Carolina Blue Sky Foundation and Applied Research Associates
Mitigated Building
•Note the Secondary Water
Resistance on top of plywood
sheathing
•Windows secured
Jon Ayscue
June 17, 2004 27
The Vulnerable Roof
 The roof is the most
vulnerable element of the
structure
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June 17, 2004 28
The Vulnerable Roof
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June 17, 2004 29
The Vulnerable Roof
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June 17, 2004 30
Finding the Weak Point
Failure is Progressive
The weak point fails first
Further failure becomes
more likely
Jon Ayscue
June 17, 2004 31
Every House has a Weak Point
 Design Goal - 156 mph
Wind Survival
 As Built - 137 mph
Survival
 What’s the Weak Point?
State Farm Good Neighbor House
Jon Ayscue
June 17, 2004 32
IV. Building Wind Resistant Homes
Jon Ayscue
June 17, 2004 33
Protecting Your Home from Wind
Damage





Strengthen the Roof
Reinforce Gable-Ends
Ensure Proper Connections
Protect Openings
Eliminate Debris Sources
Jon Ayscue
June 17, 2004 34
The Continuous Load Path
 Holds the envelope
Together
 Transmits wind forces
 to earth
 Roof connections are
 the most important
Jon Ayscue
June 17, 2004 35
Construction Guides
Blue Sky guide is excellent.
Developed in conjunction with
Clemson University
Construction techniques for up to
140-mph 3-second gust wind
resistance
Available at:
www.bluesky-foundation.net
Jon Ayscue
June 17, 2004 36
Building Codes
Building Codes are minimum acceptable standards for
building design and construction
Can be “Performance” or “Prescriptive”
Building codes must be enforced in order to be effective
Since 1994, International Building Code (IBC) predominant
Virginia Uniform Statewide Building Code (USBC) based
on the IBC
 2000 International Residential Code for one and twofamily dwellings
 Does not apply to manufactured homes
Jon Ayscue
June 17, 2004 37
Keeping the Roof On
 Proper Sheathing
Attachment
 Gable-end Reinforcement
 Continuous Load Path to
Ground
Jon Ayscue
June 17, 2004 38
Gable-End Bracing

failure can lead to roof
failure
 Gable-ends can be easily
reinforced
 Gable-end collapse was
the most common failure
during Hurricane Andrew
Jon Ayscue
June 17, 2004 39
Protecting Openings
 The Most Vulnerable Openings
 Double-Wide Garage Doors
 Patio Doors
 Windows
 Double Entry Doors
Jon Ayscue
June 17, 2004 40
What’s the Weak Point?
The Garage Doors
Only rated to 137 mph
State Farm Good Neighbor House
Jon Ayscue
June 17, 2004 41