Building Design for Tornadoes - Oklahoma Structural Engineer`s
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Transcript Building Design for Tornadoes - Oklahoma Structural Engineer`s
Building Design for
Tornadoes
William L. Coulbourne, P.E.
Applied Technology Council
[email protected]
Building Design for Tornadoes – OKSEA March 2013
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Agenda
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
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
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
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
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?
[email protected]
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