DESIGN FOR TORNADOES

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• DESIGN FOR TORNADOES

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Tornadoes

• A vortex of air (of the order of 1000 ft in
  diameter) which develops within a severe
  thunderstorm.
• Powerful explosive forces may be caused by the
  difference between the pressure within the
  structure and the lower pressure prevailing
  within the tornado funnel.

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TORNADOES
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FORMATION

• Vorticity is concentrated in a limited column
  beneath the thunderstorm
• Concentration is believed to be produced by
  converging winds at low levels stimulated by
  rapid vertical motion in the thunderstorm
• Conditions favorable for thunderstorm and hence
  tornado formation
• Cool dry air from west overruns
• Warm moist air from Gulf of Mexico and the
  presence of
• A triggering mechanism

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FUJITA SCALE FOR TORNADO INTENSITY

• F0 Light Damage (40-72 mph)
• F1 Moderate Damage (73-112 mph)
• F2 Considerable Damage (113-157 mph)
• F3 Severe Damage (158-206 mph)
• F4 Devastating Damage (207-260 mph)
• F5 Incredible Damage (261-318 mph)

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REPORTED TORNADOES PER YEAR (SPC)
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TORNADO FREQUENCIES BY F-SCALE
TORNADO FREQUENCIES AND F-SCALE CLASSIFICATIONS
FOR 1950-1994 (NSSFC, 1995)
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TORNADO OCCURRENCES
Number of Tornadoes per 10,000 sq. mi per yr.
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MAXIMUM TORNADO WIND SPEEDS

• Cannot be measured
• Must use indirect methods
• Once thought to be 400-500 mph
• Most intense tornadoes observed have wind speed
  in the 250-300 mph range

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ATMOSPHERIC PRESSURE CHANGE

• Rotating winds create low pressure near center of
  storm
• Difficult to measure
• Theoretical value can be calculated
• Maximum APC is less than 3 psi in the most
  intense tornado

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MISSILES AND DEBRIS

• Tornado-generated missiles
• Roof gravel, tree limbs, sheet metal
• Timber planks, plastic pipes
• Steel pipes and wide-flange sections
• Storage tanks, automobiles, railroad cars

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DEGREES OF TORNADO PROTECTION

• Near Absolute
• Nuclear power plants
• Protection of Function
• Hospitals, fire stations, emergency operating
  center
• Containment
• Nuclear or highly toxic materials or very
  valuable materials (should maintain the building
  integrity)
• Occupant Protection
• Above ground storm shelters to protect people
  during the passage of tornado

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ACCEPTABLE LEVEL OF RISK

• Is a management decision
• Should be consistent over different geographical
  regions
• Should be consistent with risk associated with
  other natural phenomena
• Is a trade-off between risk and economics

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TORNADO MISSILES

• Three missiles are proposed for design
• Timber plank, steel pipe, and automobile
• Missile impact speeds and height are estimated
  from trajectory calculations

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TORNADO MISSILE PARAMETERS
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TORNADO MISSILE IMPACT VELOCITIES
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UNIFORM APPROACH TO WIND DESIGN

• ASCE 7-98 is used for design for straight winds
  and hurricanes
• With slight modification, the ASCE 7-98 methods
  can also be used for tornado design
• Thus, a uniform approach treats three different
  wind storms the same

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LOAD COMBINATIONS

• WT Wq T Tornado loading q wind loading
• WT Wa a APC loading
• WT WM M Missile loading
• WT Wq 0.5Wa

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WIND OTHER CODES AND STANDARDS
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THINK BEYOND THE CODE

• Building codes and standards are minimum
  requirements
• Meeting minimum requirements may not absolve
  design professional of legal responsibility

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BUILDING CODES AND STANDARDS

• National Standards
• ASCE 7-98, ACI, AISC, ASTM,.
• U.S. Model Codes
• Standard Building Code (SE), Uniform Building
  Code (W), BOCA (NE)
• International Codes

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INTERNATIONAL CODE COUNCIL (ICC)

• Developing a single code for U.S.
• Founders are ICBO, SBCCI and BOCA
• Eliminate technical disparities in existing
  codes
• Without regional limitations, it will be known as
  an international code

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BENEFITS OF ICC

• Consistent set of requirements
• Manufacturers products must meet a single set of
  standards
• Will encourage states and localities to adopt
  International Code
• Consistent code enforcement
• Unified positions on issues and policies

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ICC PUBLICATIONS

• International Plumbing Code
• International Mechanical Code
• International Fire and Building Codes
• One and Two Family Dwelling Code

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SESSION EIGHT
MULTIPROTECTON DESIGN
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MULTIPROTECTION DESIGN
Is the execution of a building design in which
every design parameter is studied for its effect
on the mitigation of defined disasters
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MPD SHOULD BE A
COMMUNITY-WIDE PROGRAM

• Requires cooperation between
• Architect and Engineering Communit
• must practice disaster mitigation
• Code/Standard enforcing agencies
• must develop effective land use program
• Insurance industry
• must offer incentives for better construction
• Research and development agencies
• must continue

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DESIGN PARAMETER / HAZARD RELATIONSHIPS
Earthquake
Building Design Parameters
Extreme Wind
Floods
Fire
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MPD is an integrated design, rather than an
assembled design with various hazards treated
separately
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MPD CONSIDERATIONS

• Avoid
• Extreme height/depth ratio
• Extreme length/depth ratio
• Variation of stiffness in perimeter walls
• Mass eccentrics

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UNCERTAINTIES IN BUILDING PERFORMANCE

• Building Shapes
• Complex Shapes
• U-shape plan
• L-shape plan
• T-shape plan
• Set backs
• Split level
• Multiple towers

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UNCERTAINTIES IN BUILDING PERFORMANCE

• Unusual Structural Features
• Buildings on hillsides
• Shell structures
• Cable supported structures
• Earth covered structures