1 constructing for high-wind events the risk of severe wind in virginia characteristics of...

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Constructing for High-Wind Events

The Risk of Severe Wind in VirginiaCharacteristics of Destructive WindsHow Homes FailBuilding Wind-Resistant Homes

2Jon Ayscue June 17, 2004

I. The Risk of Severe Wind in Virginia


Assessing Risks

Jon Ayscue June 17, 2004

Tornado Activity in the US

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The combining of these two factors allows the homeowner to

determine the risk to their home

Assessing RisksFEDERAL EMERGENCY MANAGEMENT AGENCY MITIGATION DIRECTORATE


Historical Hurricane Tracks 1851 - 1999


ASCE Wind Map


Wind Speeds for Virginia Localities110 MPH 100 MPH 90 MPH

Special Wind Region

Counties of Accomack andNorthampton. City of Virginia Beach

Counties of Gloucester, Isle of Wight, Lancaster, Mathews, Middlesex, Northumberland, Southampton, Surry, York. Cities of Chesapeake, Franklin, Hampton, Newport News, Norfolk, Poquoson, Portsmouth, Suffolk.

All other Localities

Counties of Bland, Grayson, Scott, Smyth, Tazewell, Washington, Wythe


Fastest Mile vs. 3 Second Gust

• Why 3 Second Gust?

Standardization More Realistic

Measurement

• “Fastest Mile” = Hurricane “Maximum Sustained Winds”

Fastest Mile (mph) 3 Second Gust (mph)

70 85

75 90

80 100

85 105

90 110

100 120

105 125

110 130

120 140

125 145

130 150

140 160

150 170

Jon Ayscue June 17, 2004

3-Second Peak Gust

WEMITE 1, WS10

0

5

10

15

20

25

30

35

40

45

0 600 1200 1800 2400 3000 3600

time (sec)

win

d s

pe

ed

(m

/s)

Maximum gusts to 94mph (42m/s)

Maximum mean speed ~73mph (33m/s)

QuickTime™ and aGraphics decompressor

are needed to see this picture.

II. Characteristics of Destructive Winds

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Fujita Tornado Scale Saffir-Simpson Hurricane Scale

Damage and Storm Category

TAKING SHELTER FROM THE STORM: BUILDING A SAFE ROOM INSIDE YOUR HOUSE


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



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

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BUILDING PERFORMANCE ASSESSMENT TEAM


Tornado Inflow

Tornado.mov

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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

Tornadoes: Good News and Bad

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Weak (F0-F1) 40-112 mph Strong (F2-F3) 113-206

mph Violent (F4-F5) 207-318

mph

Historical Tornado Strength (United States, 1950-1994)


Historical Hurricane Strength (1900-1996 Direct Landfalls)

CategoryWind Speed

(mph)Damage

1 74-95 Minimal

2 95-110 Moderate

3 111-130 Extensive

4 131-155 Extreme

5 >155 Catastrophic

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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.

Hurricanes and Tornadoes



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


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


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


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

Gust Wind Speed = 100mph

Unmitigated building starts to lose some shingles



Gust Wind Speed = >125pm

More sheathing lost. Entire wood frame is exposed.

Gust Wind Speed = >125mph

Pieces of plywood sheathing start to be pulled away from the

frame



Mitigated Building

•Note the Secondary Water Resistance on top of plywood

sheathing

•Windows secured

Gust Wind Speed = >125mph

Whole roof failure of the unmitigated building


The Vulnerable Roof

The roof is the most vulnerable element of the structure


The Vulnerable Roof


Finding the Weak Point

Failure is Progressive

The weak point fails first

Further failure becomes more likely


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


IV. Building Wind Resistant Homes


Protecting Your Home from Wind Damage Strengthen the Roof Reinforce Gable-Ends Ensure Proper Connections Protect Openings Eliminate Debris Sources


The Continuous Load Path

Holds the envelope Together

Transmits wind forces to earth Roof connections are the most important


Construction GuidesBlue Sky guide is excellent.Developed in conjunction with Clemson UniversityConstruction techniques for up to 140-mph 3-second gust wind resistanceAvailable at:www.bluesky-foundation.net


Building Codes

Building Codes are minimum acceptable standards for building design and constructionCan be “Performance” or “Prescriptive”Building codes must be enforced in order to be effectiveSince 1994, International Building Code (IBC) predominantVirginia Uniform Statewide Building Code (USBC) based on the IBC

2000 International Residential Code for one and two-family dwellings

Does not apply to manufactured homes


Keeping the Roof On

Proper Sheathing Attachment

Gable-end Reinforcement Continuous Load Path to

Ground


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


Protecting Openings The Most Vulnerable Openings

Double-Wide Garage Doors

Patio Doors

Windows

Double Entry Doors


What’s the Weak Point?

The Garage DoorsOnly rated to 137 mph

State Farm Good Neighbor House

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