Building Design for
Tornadoes
William L. Coulbourne, P.E.
Applied Technology Council
bcoulbourne@atcouncil.org
Building Design for Tornadoes – OKSEA March 2013
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Agenda
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EF damage scale
2011 history of tornado damage
Design formulas for wind pressure
Illustrations of design pressures
Wind-borne missiles
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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
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Design Strategies for Tornadoes
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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
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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
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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
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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:
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Condos, apartments, townhouses
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Junior and Senior High Schools
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Elementary Schools
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Recent Events We’ve Learned From
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OK/KS 1999
Greensburg, KS 2007
Enterprise, AL 2007
Tuscaloosa, AL 2011
Joplin, MO 2011
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Joplin Tornado Path - 5/22/11
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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)
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Joplin Damage Assessment Map
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EF Damage Plotted
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Area % of EF Damage
EF level
Wind Speed
(mph)
Area on Map Percentage (%)
(acres)
0
65-85
908
22.9%
1
86-109
1179
29.8%
2
110-137
1211
30.6%
3
138-167
494
12.5%
4
168-199
166
4.2%
5
200-234
0
0.0%
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EF0 (65-85 mph)
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EF1 (86-109)
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EF2 (110-137)
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EF3 (138-167)
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EF3 (138-167)
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EF4 (168-199)
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EF5 (200-234)
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Multi-family Buildings (~2000)
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East Middle School (2009)
Gymnasium
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Auditorium
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Joplin High School
EF2
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Tuscaloosa Damage Path
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EF Damage Plotted
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Area % of EF Damage
85+%
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Housing Demographics
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EF0 (65-85 mph)
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EF1 (86-109)
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EF2 (110-137)
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EF3 (138-167)
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EF4 (168-199)
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Multi-family Building (Old)
EF1
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Multi-family Buildings (New)
EF4
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Greensburg, KS
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Well-built house, Birmingham, AL
Jan 2012 tornado
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Steel moment frame, well-built house
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Roof stays together
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Devil is in the details
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Comparison – Hurricane to Tornado
Wind Speeds
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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
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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
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Continuous Load Path Concept
Ground
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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)
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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
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Calculating MWFRS Loads Using ASCE7
 Chapter 27 ASCE 7-10
p = qGCp – qi(GCpi)
 where:
–
–
–
–
–
q = velocity pressure
G = gust effect factor
Cp = external pressure coefficient
qi = velocity pressure at mean roof height h
GCpi = internal pressure coefficient
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Changes in Calculating MWFRS Loads
 Chapter 27 ASCE 7-10
p = qh(TiGCp – GCpi)
 where:
– qh = velocity pressure at mean roof height h
– Ti = pressure coefficient increase for tornadoes
G = gust effect factor
– Cp = external pressure coefficient
– GCpi = internal pressure coefficient
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Differences for Tornado Winds
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Kd = 1.0
Kzt = 1.0
Exposure C
G = 0.90
GCpi = +/- 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 Ti factor
– 1.5 – 3.0 times
– Research suggests 1.5 increase for now
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Wind Speed Maps
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ASCE 7-10 Risk Category III/IV Structures
MRI = 1700 years
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Hurricane Safe Room Design Wind Speed Map
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Cp for MWFRS: Walls
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Cp for MWFRS: Roofs
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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
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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
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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.
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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
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Comparative Wind Pressures
Exp
C
Maximum MWFRS Pressures (psf)
Velocity
q
max roof uplift
max wall suction
ASCE 7-10 Cat II
115
26.48
-24
-21
ASCE 7-10 Cat
III/IV
120
28.83
-26
-22
ASCE 7-10 Cat II
180
64.87
-59
-50
ASCE 7-10 Cat
III/IV
190
72.28
-66
-56
EF0
85
17.02
-29
-25
EF1
110
28.50
-49
-43
EF2
135
42.93
-73
-64
EF3
165
64.13
-110
-96
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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
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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
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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
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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
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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
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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
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Summary of Construction Changes
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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
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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 P361 or ICC 500)
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Questions?
bcoulbourne@atcouncil.org
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