Building Design for Tornadoes – OKSEA March Building Design for Tornadoes William L. Coulbourne, P.E. Applied Technology Council

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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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:

Building Design for Tornadoes – OKSEA March Condos, apartments, townhouses

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

Building Design for Tornadoes – OKSEA March Elementary Schools

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

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

Building Design for Tornadoes – OKSEA March 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)

Building Design for Tornadoes – OKSEA March Joplin Damage Assessment Map

Building Design for Tornadoes – OKSEA March EF Damage Plotted

Building Design for Tornadoes – OKSEA March Area % of EF Damage EF levelWind Speed (mph) Area on Map (acres) Percentage (%) % % % % % %

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

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

Building Design for Tornadoes – OKSEA March EF2 ( )

Building Design for Tornadoes – OKSEA March EF3 ( )

Building Design for Tornadoes – OKSEA March EF3 ( )

Building Design for Tornadoes – OKSEA March EF4 ( )

Building Design for Tornadoes – OKSEA March EF5 ( )

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

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

Building Design for Tornadoes – OKSEA March Auditorium

Building Design for Tornadoes – OKSEA March Joplin High School EF2

Building Design for Tornadoes – OKSEA March Tuscaloosa Damage Path

Building Design for Tornadoes – OKSEA March

Building Design for Tornadoes – OKSEA March EF Damage Plotted

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

Building Design for Tornadoes – OKSEA March Housing Demographics

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

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

Building Design for Tornadoes – OKSEA March EF2 ( )

Building Design for Tornadoes – OKSEA March EF3 ( )

Building Design for Tornadoes – OKSEA March EF4 ( )

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

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

Building Design for Tornadoes – OKSEA March Greensburg, KS

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

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

Building Design for Tornadoes – OKSEA March Roof stays together

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

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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

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

Building Design for Tornadoes – OKSEA March 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 = (more than 3 times increase in vertical or uplift direction)

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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 )

Building Design for Tornadoes – OKSEA March 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 )

Building Design for Tornadoes – OKSEA March Differences for Tornado Winds  K d = 1.0  K zt = 1.0  Exposure C  G = 0.90  GC pi = +/  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

Building Design for Tornadoes – OKSEA March Wind Speed Maps

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

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

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

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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March Comparative Wind Pressures ExpCMaximum MWFRS Pressures (psf) Velocityqmax roof upliftmax wall suction ASCE 7-10 Cat II ASCE 7-10 Cat III/IV ASCE 7-10 Cat II ASCE 7-10 Cat III/IV EF EF EF EF

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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

Building Design for Tornadoes – OKSEA March 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)

Building Design for Tornadoes – OKSEA March Questions?