Title: DESIGN FOR TORNADOES
1 2Tornadoes
- 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.
3TORNADOES
4FORMATION
- 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
5FUJITA 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)
6REPORTED TORNADOES PER YEAR (SPC)
7TORNADO FREQUENCIES BY F-SCALE
TORNADO FREQUENCIES AND F-SCALE CLASSIFICATIONS
FOR 1950-1994 (NSSFC, 1995)
8TORNADO OCCURRENCES
Number of Tornadoes per 10,000 sq. mi per yr.
9MAXIMUM 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
10ATMOSPHERIC 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
11MISSILES 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
12DEGREES 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
13ACCEPTABLE 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
14TORNADO MISSILES
- Three missiles are proposed for design
- Timber plank, steel pipe, and automobile
- Missile impact speeds and height are estimated
from trajectory calculations
15TORNADO MISSILE PARAMETERS
16TORNADO MISSILE IMPACT VELOCITIES
17UNIFORM 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
18LOAD 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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23WIND OTHER CODES AND STANDARDS
24THINK BEYOND THE CODE
- Building codes and standards are minimum
requirements - Meeting minimum requirements may not absolve
design professional of legal responsibility
25BUILDING 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
26INTERNATIONAL 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
27BENEFITS 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
28ICC PUBLICATIONS
- International Plumbing Code
- International Mechanical Code
- International Fire and Building Codes
- One and Two Family Dwelling Code
29SESSION EIGHT
MULTIPROTECTON DESIGN
30MULTIPROTECTION DESIGN
Is the execution of a building design in which
every design parameter is studied for its effect
on the mitigation of defined disasters
31 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
32DESIGN PARAMETER / HAZARD RELATIONSHIPS
Earthquake
Building Design Parameters
Extreme Wind
Floods
Fire
33MPD is an integrated design, rather than an
assembled design with various hazards treated
separately
34MPD CONSIDERATIONS
- Avoid
- Extreme height/depth ratio
- Extreme length/depth ratio
- Variation of stiffness in perimeter walls
- Mass eccentrics
35UNCERTAINTIES IN BUILDING PERFORMANCE
- Building Shapes
- Complex Shapes
- U-shape plan
- L-shape plan
- T-shape plan
- Set backs
- Split level
- Multiple towers
36UNCERTAINTIES IN BUILDING PERFORMANCE
- Unusual Structural Features
- Buildings on hillsides
- Shell structures
- Cable supported structures
- Earth covered structures