Constructing for High-Wind Events

1. Constructing for High-Wind Events • The Risk of Severe Wind in Virginia • Characteristics of Destructive Winds • How Homes Fail • Building Wind-Resistant Homes

2. I. The Risk of Severe Wind in Virginia

3. Assessing Risks

4. Tornado Activity in the US

5. Assessing Risks FEDERAL EMERGENCY MANAGEMENT AGENCY MITIGATION DIRECTORATE • The combining of these two factors allows the homeowner to determine the risk to their home

6. Historical Hurricane Tracks 1851 - 1999

7. ASCE Wind Map

8. Wind Speeds for Virginia Localities

9. Fastest Mile vs. 3 Second Gust • Why 3 Second Gust? • Standardization • More Realistic Measurement • “Fastest Mile” = Hurricane “Maximum Sustained Winds”

10. 3-Second Peak Gust Maximum gusts to 94mph (42m/s) Maximum mean speed ~73mph (33m/s)

11. II. Characteristics of Destructive Winds

12. Damage and Storm Category TAKING SHELTER FROM THE STORM:BUILDING A SAFE ROOM INSIDE YOUR HOUSE Fujita Tornado Scale Saffir-Simpson Hurricane Scale

13. 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

14. 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

15. BUILDING PERFORMANCE ASSESSMENT TEAM

16. Tornado Inflow

17. 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

18. 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

19. Historical Hurricane Strength (1900-1996 Direct Landfalls)

20. 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.

21. III. How Homes Fail

22. Windborne Debris Courtesy of Texas Tech University • Debris causes a large fraction of damage • Size increases with wind speed • Windows and doors are especially vulnerable

23. 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

24. 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

25. 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

26. 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.

27. 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

28. The Vulnerable Roof • The roof is the most vulnerable element of the structure

29. The Vulnerable Roof

30. The Vulnerable Roof

31. Finding the Weak Point • Failure is Progressive • The weak point fails first • Further failure becomes more likely

32. Every House has a Weak Point State Farm Good Neighbor House • Design Goal - 156 mph Wind Survival • As Built - 137 mph Survival • What’s the Weak Point?

33. IV. Building Wind Resistant Homes

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

35. The Continuous Load Path • Holds the envelope Together • Transmits wind forces • to earth • Roof connections are • the most important

36. 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

37. 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 two-family dwellings • Does not apply to manufactured homes

38. Keeping the Roof On • Proper Sheathing Attachment • Gable-end Reinforcement • Continuous Load Path to Ground

39. 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

40. Protecting Openings • The Most Vulnerable Openings • Double-Wide Garage Doors • Patio Doors • Windows • Double Entry Doors

41. What’s the Weak Point? State Farm Good Neighbor House • The Garage Doors • Only rated to 137 mph