1. Building Design for Tornadoes – OKSEA March 20131 Building Design for Tornadoes William L. Coulbourne, P.E. Applied Technology Council bcoulbourne@atcouncil.org

2. Building Design for Tornadoes – OKSEA March 20132 Agenda  EF damage scale  2011 history of tornado damage  Design formulas for wind pressure  Illustrations of design pressures  Wind-borne missiles

3. Building Design for Tornadoes – OKSEA March 20133 Why Design for Tornadoes?  Low probability but high consequence event  Property damage can be extreme  Loss of life is real threat  As professionals we should not assume there is nothing we can do  We can use existing technology

4. Building Design for Tornadoes – OKSEA March 20134 Design Strategies for Tornadoes  Use ASCE 7 wind load provisions  Modify some of the factors  Use higher wind speeds than ASCE 7  Understand the limitations of what we don’t know

5. Building Design for Tornadoes – OKSEA March 20135 Tornado Damage Scale  EF scale is based on observed damage  Scale goes from Category EF0 – EF5 with corresponding wind speeds from 65 mph to 200 mph  Primary reference is from Texas Tech Univ. titled: A Recommendation for an ENHANCED FUJITA SCALE (EF-Scale) 2006

6. Building Design for Tornadoes – OKSEA March 20136 Tornado Damage Scale  28 Damage Indicators used – structure or use types (e.g.): – One or two family residences –Apartments, condos or townhouses –Large shopping mall –Junior or Senior high school –Warehouse building –Free standing towers

7. Building Design for Tornadoes – OKSEA March 20137 Tornado Damage Scale  Each Damage Indicator has a range of wind speeds associated with degrees of damage for that structure type – for one and two family residences:

8. Building Design for Tornadoes – OKSEA March 20138 Condos, apartments, townhouses

9. Building Design for Tornadoes – OKSEA March 20139 Junior and Senior High Schools

10. Building Design for Tornadoes – OKSEA March 201310 Elementary Schools

11. Building Design for Tornadoes – OKSEA March 201311 Recent Events We’ve Learned From  OK/KS 1999  Greensburg, KS 2007  Enterprise, AL 2007  Tuscaloosa, AL 2011  Joplin, MO 2011

12. Building Design for Tornadoes – OKSEA March 201312 Joplin Tornado Path - 5/22/11

13. Building Design for Tornadoes – OKSEA March 201313 Joplin, MO Tornado – 5/22/11  Joplin, MO info –Located in SW corner of Missouri –Population of ~50,000 –Established in 1873 –Area of 31.5 sq. miles –Previously had tornado impact town in 1971, killing one  Evaluations for 5/22 tornado by NWS classify it as an EF-5  Fatality count ~ 160  Over 8000 buildings damaged (path crossed through major commercial and residential areas)

14. Building Design for Tornadoes – OKSEA March 201314 Joplin Damage Assessment Map

15. Building Design for Tornadoes – OKSEA March 201315 EF Damage Plotted

16. Building Design for Tornadoes – OKSEA March 201316 Area % of EF Damage EF levelWind Speed (mph) Area on Map (acres) Percentage (%) 065-8590822.9% 186-109117929.8% 2110-137121130.6% 3138-16749412.5% 4168-1991664.2% 5200-23400.0%

17. Building Design for Tornadoes – OKSEA March 201317 EF0 (65-85 mph)

18. Building Design for Tornadoes – OKSEA March 201318 EF1 (86-109)

19. Building Design for Tornadoes – OKSEA March 201319 EF2 (110-137)

20. Building Design for Tornadoes – OKSEA March 201320 EF3 (138-167)

21. Building Design for Tornadoes – OKSEA March 201321 EF3 (138-167)

22. Building Design for Tornadoes – OKSEA March 201322 EF4 (168-199)

23. Building Design for Tornadoes – OKSEA March 201323 EF5 (200-234)

24. Building Design for Tornadoes – OKSEA March 201324 Multi-family Buildings (~2000)

25. Building Design for Tornadoes – OKSEA March 201325 Gymnasium East Middle School (2009)

26. Building Design for Tornadoes – OKSEA March 201326 Auditorium

27. Building Design for Tornadoes – OKSEA March 201327 Joplin High School EF2

28. Building Design for Tornadoes – OKSEA March 201328 Tuscaloosa Damage Path

29. Building Design for Tornadoes – OKSEA March 201329 29

30. Building Design for Tornadoes – OKSEA March 201330 EF Damage Plotted

31. Building Design for Tornadoes – OKSEA March 201331 Area % of EF Damage 85+%

32. Building Design for Tornadoes – OKSEA March 201332 Housing Demographics

33. Building Design for Tornadoes – OKSEA March 201333 EF0 (65-85 mph)

34. Building Design for Tornadoes – OKSEA March 201334 EF1 (86-109)

35. Building Design for Tornadoes – OKSEA March 201335 EF2 (110-137)

36. Building Design for Tornadoes – OKSEA March 201336 EF3 (138-167)

37. Building Design for Tornadoes – OKSEA March 201337 EF4 (168-199)

38. Building Design for Tornadoes – OKSEA March 201338 Multi-family Building (Old) EF1

39. Building Design for Tornadoes – OKSEA March 201339 Multi-family Buildings (New) EF4

40. Building Design for Tornadoes – OKSEA March 201340 Greensburg, KS

41. Building Design for Tornadoes – OKSEA March 201341 Well-built house, Birmingham, AL Jan 2012 tornado

42. Building Design for Tornadoes – OKSEA March 201342 Steel moment frame, well-built house

43. Building Design for Tornadoes – OKSEA March 201343 Roof stays together

44. Building Design for Tornadoes – OKSEA March 201344 Devil is in the details

45. Building Design for Tornadoes – OKSEA March 201345 Comparison – Hurricane to Tornado Wind Speeds

46. Building Design for Tornadoes – OKSEA March 201346 What We Know How To Do  Maintain load path continuity  Maintain roof-to-wall connections  Maintain wall-to-floor and foundation connections  Keep walls standing

47. Building Design for Tornadoes – OKSEA March 201347 Suggested Tornado Design Premise  Strengthen building like we do for hurricanes  Do not try and protect for wind-borne debris  Do design so interior walls stay in place  Keep exterior corners together  Maybe consider a way to ‘vent’ the upper portion of the building

48. Building Design for Tornadoes – OKSEA March 201348 Continuous Load Path Concept Ground

49. Building Design for Tornadoes – OKSEA March 201349 Research - Increase in Uplift Pressures  Reference: Tornado-Induced Wind Loads on a Low-Rise Building, Dr. Partha Sarkar, Dr. Fred Haan, Journal of Structural Engineering 2010  Tornado simulator used to determine pressure coefficient differences with ASCE 7-05 standard  Results were: –Cx = 1.0 (no increase in lateral direction) –Cy = 1.5 (50% increase in wind parallel direction) –Cz = 1.5-3.2 (more than 3 times increase in vertical or uplift direction)

50. Building Design for Tornadoes – OKSEA March 201350 ASCE 7-16 Commentary Proposed Changes  Modify standard wind pressure equation for differences in tornado wind structure  Discuss differences so practitioners have a basis for design  Use wind speed maps from ICC and FEMA  Provide rationale and references

51. Building Design for Tornadoes – OKSEA March 201351 Calculating MWFRS Loads Using ASCE7  Chapter 27 ASCE 7-10  where: –q = velocity pressure –G = gust effect factor –C p = external pressure coefficient –q i = velocity pressure at mean roof height h –GC pi = internal pressure coefficient p = qGC p – q i (GC pi )

52. Building Design for Tornadoes – OKSEA March 201352 Changes in Calculating MWFRS Loads  Chapter 27 ASCE 7-10  where: –q h = velocity pressure at mean roof height h –T i = pressure coefficient increase for tornadoes G = gust effect factor –C p = external pressure coefficient –GC pi = internal pressure coefficient p = q h (T i GC p – GC pi )

53. Building Design for Tornadoes – OKSEA March 201353 Differences for Tornado Winds  K d = 1.0  K zt = 1.0  Exposure C  G = 0.90  GC pi = +/- 0.55  Consider using q at mean roof height h for all pressures  Wind speeds – FEMA 361 or ICC 500 or EF Category wind speed  Uplift increase ? How much ? Call it T i factor –1.5 – 3.0 times –Research suggests 1.5 increase for now

54. Building Design for Tornadoes – OKSEA March 201354 Wind Speed Maps

55. Building Design for Tornadoes – OKSEA March 201355 MRI = 1700 years ASCE 7-10 Risk Category III/IV Structures

56. Building Design for Tornadoes – OKSEA March 201356 Hurricane Safe Room Design Wind Speed Map

57. Building Design for Tornadoes – OKSEA March 201357 C p for MWFRS: Walls

58. Building Design for Tornadoes – OKSEA March 201358 C p for MWFRS: Roofs

59. Building Design for Tornadoes – OKSEA March 201359 Testing the Theory  Calculated wind pressures for 7 building sizes  Evaluated results for 65 to 165 mph  Sizes from 10’x20’ to 45’x50’, 1 and 2 stories, roof pitch 4:12, overall areas range from 200 sf to 4500 sf  Attempt here was to try and determine at what building size and shape are loads critical to failure

60. Building Design for Tornadoes – OKSEA March 201360 Preliminary Results  Used weight to resist uplift, sliding and overturning forces  Evaluated anchor bolt spacing required to resist sliding, uplift forces for just the roof and then entire building  Searching for those design conditions for which we believe we have solutions

61. Building Design for Tornadoes – OKSEA March 201361 Some Limiting Design Speeds  Roof lifts off with toe-nailed connection @ approx. 105 mph  Uplift pressure exceeds weight of house @ approx. 125-135 mph  Wall studs can be broken @ 105 mph  Houses can slide @ approx. 105 mph when A.B. exceed 6 ft o.c.

62. Building Design for Tornadoes – OKSEA March 201362 MWFRS Calculations  Assuming certain building sizes, we can determine loads: –Net sliding force per foot of perimeter –Anchor bolt spacing required to resist sliding –Net uplift force on roof per foot of perimeter –Outward force on exterior walls at connections –Outward force on exterior wall corners

63. Building Design for Tornadoes – OKSEA March 201363 Comparative Wind Pressures ExpCMaximum MWFRS Pressures (psf) Velocityqmax roof upliftmax wall suction ASCE 7-10 Cat II11526.48-24-21 ASCE 7-10 Cat III/IV12028.83-26-22 ASCE 7-10 Cat II18064.87-59-50 ASCE 7-10 Cat III/IV19072.28-66-56 EF08517.02-29-25 EF111028.50-49-43 EF213542.93-73-64 EF316564.13-110-96

64. Building Design for Tornadoes – OKSEA March 201364 Example Building  For 2 story building, 1500 sf in total size  Assume design wind speed is top end of Category EF2 = 135 mph  Roof uplift = 500 lbs/ft around perimeter  For 10 ft tall walls, lateral force outward at wall-floor intersection = 321 lbs/ft  For 10 ft tall wall corner, lateral force outward = 96 lbs/ft vertically

65. Building Design for Tornadoes – OKSEA March 201365 Example Solutions  Roof to wall connection in uplift – for truss spacing of 2 ft., connector must resist 1000 lbs., use SST – 2-H10-2  Wall to roof connection for lateral load – for 2 ft spacing, connector must resist 640 lbs, use SST - 2-H10-2  Wall to floor connection – use 3-16d box nails per foot  Wall corner connections – use SST – 3-A23 along 10 ft tall wall

66. Building Design for Tornadoes – OKSEA March 201366 Other Important Considerations  Glazing – allow to break?  Improve connections between top and bottom of interior walls to structure  Lack of interior wall collapse improves survivability if inside building during storm  Floor to foundation connection  Reinforced foundation

67. Building Design for Tornadoes – OKSEA March 201367 Components  Do components matter?  Loss of components won’t allow the building to collapse  Loss of components won’t allow the roof to blow off  Loss of components won’t allow the walls to bulge or won’t move the house off the foundation

68. Building Design for Tornadoes – OKSEA March 201368 Wind-borne Debris – Tornado Missiles  Building components are physically tested to determine their debris resistance  For 250 mph – the test “missile” is: –A 15-pound 2x4 –Shot from a cannon at 100 mph horizontally, 67 mph vertically

69. Building Design for Tornadoes – OKSEA March 201369 Test of URM Wall Wall penetrated by a 15-pound 2x4 at 100 mph –Could have killed or injured occupant –Safe room failure –Wall fails to resist 9-lb missile traveling at 34 mph

70. Building Design for Tornadoes – OKSEA March 201370

71. Building Design for Tornadoes – OKSEA March 201371 Missile Resistance  Requires a material section with sufficient EI to resist the energy  If we choose to limit deflection of the material struck by the missile, we can design a material section with sufficient EI to resist the impact

72. Building Design for Tornadoes – OKSEA March 201372 Summary of Construction Changes  Select a design wind speed (up to 135 mph)  Nail roof sheathing for high winds  Add roof-to-wall connectors  Either add connectors or insure sheathing is nailed to resist uplift through load path  Add wall-to-sill connectors (nails)  Add corner strengthening  Bolt sill plate with 3”x3” steel washers min. 6 ft. on enter – consider 4 ft. on center and within 1 ft. of every corner

73. Building Design for Tornadoes – OKSEA March 201373 Conclusions  Significant commentary will be added to ASCE 7 on tornado loads  Should continue to pursue ways to mitigate effects from Category EF2 and lower wind speeds  Should study if there are ways to mitigate effects from Category EF3  Should encourage installation of safe rooms/shelters to improve life safety in Category EF4-5 events (use FEMA P-320 or P- 361 or ICC 500)

74. Building Design for Tornadoes – OKSEA March 201374 Questions? bcoulbourne@atcouncil.org