tornado design

1Tornado-Resistant Construction Workshop

NCSEA Webinar - Building Design for Tornadoes 1

Building Design for Tornadoes

William L. Coulbourne, P.E.Applied Technology [email protected]


Agenda

EF damage scale 2011 history of tornado damage Design formulas for wind pressure Illustrations of design pressures ASCE 7 Commentary



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


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

dont know



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



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




Each Damage Indicator has a range of wind speeds associated with degrees of damage for that structure type for one and two family residences:


Condos, apartments, townhouses



Junior and Senior High Schools


Elementary Schools



Recent Events Weve Learned From

OK/KS 1999 Greensburg, KS 2007 Enterprise, AL 2007 Tuscaloosa, AL 2011 Joplin, MO 2011


Joplin Tornado Path - 5/22/11



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)


Joplin Damage Assessment Map



EF Damage Plotted


Area % of EF DamageEFlevel WindSpeed

(mph)AreaonMap

(acres)Percentage(%)

0 6585 908 22.9%1 86109 1179 29.8%2 110137 1211 30.6%3 138167 494 12.5%4 168199 166 4.2%5 200234 0 0.0%



EF0 (65-85 mph)


EF1 (86-109)

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Tornado-Resistant Construction Workshop


EF2 (110-137)


EF3 (138-167)

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EF3 (138-167)


EF4 (168-199)

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EF5 (200-234)


Multi-family Buildings (~2000)

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Gymnasium

East Middle School (2009)


Auditorium

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Joplin High SchoolEF2


Tuscaloosa Damage Path

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EF Damage Plotted

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Area % of EF Damage

85+%


Housing Demographics

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EF0 (65-85 mph)


EF1 (86-109)

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EF2 (110-137)


EF3 (138-167)

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EF4 (168-199)


Multi-family Building (Old)

EF1

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Multi-family Buildings (New)

EF4


Greensburg, KS

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Well-built house, Birmingham, AL Jan 2012 tornado


Steel moment frame, well-built house

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Roof stays together


Devil is in the details

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Comparison Hurricane to Tornado Wind Speeds


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 except for critical facilities

Do design so interior walls stay in place Keep exterior corners together Maybe consider a way to vent the upper

portion of the building


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)


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 or upper end of EF scale wind speed range

Provide rationale and references

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Calculating MWFRS Loads Using ASCE7

Chapter 27 ASCE 7-10

where: q = velocity pressure G = gust effect factor Cp = external pressure coefficient qi = velocity pressure at mean roof height h GCpi = internal pressure coefficient

p = qGCp qi(GCpi)


Changes in Calculating MWFRS Loads


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

p = qh(TiGCp GCpi)

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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 Ti factor = 1.5


Wind Speed Maps

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NCSEA Webinar - Building Design for Tornadoes 55MRI = 1700 years

ASCE 7-10 Risk Category III/IV Structures


Hurricane Safe Room Design Wind Speed Map

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Cp for MWFRS: Walls


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 10x20 to 45x50, 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


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.


Wind Pressure ComparisonsWind

Speed (mph)

ASCE 7 Pressures Directionality Method

Tornado Design Pressures

Roof Uplift (psf)

Wall Suction (psf)

Roof Uplift (psf)

Wall Suction (psf)

115 -30 -22 -52 -38120 -33 -24 -58 -42130 -38 -28 -66 -49140 -45 -33 -79 -58150 -51 -38 -89 -66160 -58 -43 -102 -75170 -66 -49 -116 -86180 -74 -55 -130 -96190 -82 -61 -144 -107200 -91 -67 -159 -117

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Alternative Method


where TF = tornado increase factor

p = [qGCp qi(GCpi)]TF


Alternative: Tornado Factor

Original Exposure Category

B C

MWFRS 2.5 1.75

C&C 2.0 1.5

Global lateral loads 2.0 1.5

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Example

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


Example 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


Components

Do components matter? Loss of components wont allow the

building to collapse Loss of components wont allow the roof

to blow off Loss of components wont allow the walls

to bulge or wont 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


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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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

See TTU Wind Science & Engineering or NSSA site for missile testing results

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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 3x3 steel washers min. 6 ft.

on enter consider 4 ft. on center and within 1 ft. of every corner


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)

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[email protected]

tornado design

Documents

design strategies

tornado damage scale

observed damage scale

mo tornado

agenda ef damage scale

degrees of damage

damage indicators

joplin tornado path