tropical storms and hurricanes updated 2008. tropical storms and hurricanes introduction tropical...
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Tropical Storms and Hurricanes
Updated 2008
Tropical Storms and Hurricanes
Introduction
Tropical storms and Hurricanes are formed from tropical disturbances that travel across the Atlantic ocean toward the U.S. coast line.
In 2004, Hurricanes and tropical storms were responsible for an estimated $50 billion dollars in damage. Five Hurricanes made landfall in the state of Florida. Three of them had at least 115 mph sustained winds.
In 2005, the Atlantic Hurricane season was the most active season in recorded history. The impact of the season was widespread and ruinous with at least 2,280 deaths and record damages of over $128 billion USD.
Tropical Storms and Hurricanes How Hurricanes Form
Tropical Disturbance: An area of organized convection, originating in the tropics that maintains its identify for 24 hours or more. It is often the first developmental stage of any tropical depression, tropical storm or hurricane.
Tropical Storm: A severe storm that develops offshore over tropical seas. When wind speeds reach 39 mph, the tropical disturbance becomes a tropical storm and is given an official name.
Hurricane: When a tropical storm takes a cyclonic form and has reached a constant wind speed of 74 mph or more. The eye of the storm is usually 20 – 30 miles wide and can extend over 400 miles. Strong hurricane windows can inflict moderate to severe damage to buildings and may cause flooding in coastal areas.
Tropical Storms and HurricanesSaffir – Simpson Scale
•The Saffir – Simpson Hurricane scale is a Category 1 to Category 5 rating based on the hurricane’s present intensity.
•The scale is used to give an estimate of potential property damage and flooding expected along the coast.
•Wind speed is the determining factor in the scale, as storm surge values are highly dependent on the shape of the coastline.
Tropical Storms and HurricanesProperty Damage
Flying debris is a major concern, but
research has shown that most destructive
damage occurs when the building envelope
is breached.
A breach of only 10% of the building envelope,
due to a broken window, can destroy a structure.
The increased wind pressure on the anchoring of the roof to the walls and the walls to the floor is typically what leads to building failure.
Tropical Storms and HurricanesProperty Damage Continued..
•Theory Of Destruction Debris penetrates the building envelope.Negative pressure builds up from the insideRoof lifts off allowing water to enter
•When a window is shattered or blown out, the building becomes partially enclosed and as a result, wind pressure on the roof and walls is significantly increased.
Tropical Storms and HurricanesStorm Activity in 2005
• 2005 was the most active hurricane storm season in recorded history.
• The impact was widespread and ruinous with over 2,000 deaths and record damages of over $128 billion dollars.
• A record twenty-eight tropical and sub- tropical storms formed, of which fifteen became hurricanes.
•Five became Category 4 hurricanes and four reached Category 5 strength.
Tropical Storms and HurricanesStorm Activity in 2006 & 2007
2006 was a fairly inactive storm season.
It was unusual that no hurricanes
made U.S. landfall.
2007 marked an earlier beginning to a fairly active season when Subtropical Storm Andrea formed on May 9, 2007 and Tropical Storm Olga developed on December 11th.
2007 was fairly active with 15 named storms, though their intensities did not meet predictions.
Building Codes and Wind LoadsIntroduction
• When Hurricane Andrew hit Florida in August of 1992, causing an estimated $25 billion dollars in damages, it became the “wake up call” for the construction industry.
•Andrew revealed wind zones were understated, existing standards were not being adhered to and codes were not being enforced.
•As a result, demands for missile impact tests, standards and building codes have been put in place and are subject to stringent enforcement.
Building Codes and Wind LoadsImpact of Hurricane Andrew
As a result of Hurricane Andrew, codesand standards were revised to require:
Higher Wind Loading: Coastal wind loadcharts were redefined significantly higher.
Effective March 8, 2007, Panhandle is no longer exempt from the wind-borne debrisregion.
Negative Pressures: Building componentsrequired to withstand high wind loads, greatest impact realized from negative pressure loads.
Impact and Cyclic Testing: Cladding and building components must be tested and certified for compliance.
Building Codes and Wind LoadsCode Expansion
• Codes originally developed for Florida have expanded from Texas, along the Gulf and up through the Atlantic seaboard to Maine.
•This map shows wind speeds for areas along the southeastern coast and where codes are in place.
•The contour lines on this map are wind speed lines, not design pressures. Wind speeds are converted to design pressures using ASCE-7 (American Society of Civil Engineers).
Source: Institute of Business and Home Safety
Building Codes and Wind LoadsCurrent Building Codes
Most building codes used in the United States are based on the International Building Code (IBC), www.iccsafe.org which references the American Societyof Civil Engineers (ASCE), www.asce.org, standard ASCE-7 “Minimum Design Loads for Buildings and Other Structures” for wind load design.
Even those jurisdictions that have their own statewide building code, such as Florida, Texas, and North Carolina, typically reference the same wind load provisions as those found in the IBC (with some local amendments).
For instance, in Florida, Miami Dade and Broward counties require buildings to utilize tested and approved windborne debris protection systems, such as shutters or impact windows.
Building Codes and Wind LoadsASCE 7 Development
ASCE - 7 outlines basic wind load provisions and
procedures to convert basic wind speed into
design pressures based on several variables.
The wind load provisions of ASCE - 7 have
developed over the years.
The most current versions is ASCE 7 – 05.
Section 6 of ASCE – 7 deals with wind load
calculations for “Main Wind-Force Resisting
Systems” and “Components and Cladding”
Standard Development
ASCE 7 – 88 110 MPH Maximum
ASCE 7 – 93 ANSI 58.1 Integration
ASCE 7 – 95 150 MPH Maximum
ASCE 7 – 98 150 MPH Maximum
ASCE 7 – 02 Current Standardincorporated IBC provisions
Building Codes and Wind LoadsTerminology
Design Load: Also referred to as design pressure (DP), design load is in the air pressure that a window system must be able to withstand as determined by the project architect in accordance with applicable codes. Design loads are usually expressed in both positive and negative pounds per square foot (PSF).
Wind Load: Also referred to as wind pressure, wind load is the amount of pressure exerted by the wind on a window system. Wind loads are generally expressed in pounds per square foot (PSF) and are not to be confused with basic wind speed, which is usually expressed in miles per hour (MPH).
Building Codes and Wind LoadsTerminology Cont’d…
End Zone: The end zone consist of the area within 10% of the shortest building dimension to the edge of the structure. End zones are essentially the corners of a structure and typically require higher design pressures.
Interior Zone: The interior zone consists of the area between the end zones. Interior zones typically require lower design pressures than end zones.
Opening Area: Opening area refers to the size of the window opening and is used to determine design pressures as per ASCE – 05. Large openings typically have lower design pressures than smaller openings.
Weighted Area: When an opening falls partially within an end zone, the total window area is averaged based on the square footage located in each zone.
Building Codes and Wind LoadsWhich code applies?
Georgia
State
Texas AlabamaNorth
CarolinaSouth
CarolinaFlorida
County ?SBCCI IBC IBC NCBC IBC County
Texas DepartmentOf Insurance for Coastal Counties
No HurricaneCode for OtherCounties
Florida BuildingCode (FBC) forOther Counties
Local BuildingCode for DadeAnd Broward
Building Codes and Wind LoadsWho Approves Impact Products?
There are three major product approval
Agencies for impact resistant glazing:
Texas Department of Insurance
www.tdi.state.tx.us
Florida Building Commission
www.dca.state.fl.us/fbc
Miami – Dade Product Approval
www.buildingcoeonline.com
These web sites provide information on
Approved impact glazing products.
Building Codes and Wind LoadsWho Conducts Inspections?
For standard applications, inspectionsare conducted by the local building department, as legislated by the CountyCode Compliance Office.
Special Inspectors or Consulting Engineersmay be brought in to inspect non-standardapplications.
FEMA projects are typically inspected bySpecial Inspectors, while schools are typically subject to inspection by localand state building inspectors.
Building Codes and Wind LoadsWhat’s Really Required?
Design Pressure Ratings: Elevation drawings should indicate design pressure ratings for the structure, including end zone and interior zone pressures as determined by the engineer of record in accordance with ASCE - 7.
Building Exposure: Listing of Building Exposure as defined by ASCE - 7. Structures in urban, suburban, or wooded areas are typically Exposure B, while structure in open fields or on coast lines are typically Exposure C.
Basic Wind Speed: Basic Wind Speed for the site represented in MPH. This is based on the 50-year average of the peak wind speed at a given height and exposure (typically 33’ for Exposure C) averaged over three seconds.
Building Codes and Wind LoadsWhat’s really Required?
Importance Factor: A safety factor related to the degree of hazard to human life and property. Various “essential use” facilities, such as hospitals, have an importance factor of 1.15, as compared to residential construction (1.00).
Site Specific Shop Drawings: All submissions should include supporting site specific shop drawings for all fenestration products, fenestration components and exterior cladding.
Notices of Acceptance: Submissions should also include individual product Notices of Acceptance (NOA’s) or product approvals as issued by the governing code compliance office from each participating trade.
Building Codes and Wind LoadsSummary
The codes and standards implemented in
Response to Hurricane Andrew resulted in:
Higher wind load provisions and
higher structural loading
Large and small missile impact
testing and cyclic testing
Improved code reinforcement at the
local, regional and state levels
Today’s standards help provide increased
Protection for hurricane-prone areas.
Wind Load Testing
not to code Buildings to code
Wind Load TestingIntroduction
The IBC requires fenestration in
coastal areas to be AAMA certified,
which requires windows to undergo
structural, impact and cyclic testing.
Small and large missile impact testing
helps stimulate the ability of a window
to resist hurricane driven debris.
Cyclic testing is performed after impact
testing and is designed to simulate positive
and negative pressures of hurricane-force
winds.
Building heights > 30’ – Small Missing Testing
Window Load TestingTest Standards
The standard test method for cyclic testing referenced by the IBC and AAMA, is ASTM E1886-05 “Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors and Impact Protective Systems Impacted by Missile (s) and Exposed to Cyclic Pressure Differentials”.
Small and large missile impact testing is covered by ASTM E1996-04 “Standard Specification for Performance of Exterior Windows, Curtain Walls, Doors and Impact Protective Systems Impacted by Windborne Debris in Hurricanes”
Equivalent Miami-Dade and Broward standards are TAS 201-94 “Impact Test Procedure”, TAS 202-94 “Criteria for Testing Impact and Non Impact Resistant Building Envelope Components Using Uniform Static Air Pressure”, and TAS 203-94 “Criteria for Testing Products Subject to Cyclic Wind Pressure Loading”.
Wind Load TestingLarge Missile
• Impact testing is performed on windows to simulate the ability of the frame and glazing material to resist hurricane-driven debris.
•Large missile testing is an 8’ long, 9lb, 2x4 board shot twice at a windowat a speed of 50’ feet per second.
•Window products that are large missile tested are typically used on elevations from ground up to 30’ or close to a debris generation point.
Wind Load TestingSmall Missile
Small missile testing involves 2 gram steel balls (to represent roof rock) shot at the window twice product at a speed of 130’ per second.
Windows that are small missile tested are typically used on elevations at heights over 30’.
Window products that are large missile tested can also be used at heights over 30’, however they are typically more expensive.
Building heights > 30’
Wind Load TestingCyclic Testing
• Following missile testing, products are subjected to cyclic testing designed to simulate the positive and negative pressures of hurricane-force winds.
•Windows are subjected to 9,000 cycles of positive (inward-acting) and negative (outward-acting) pressures.
•These positive and negative pressures are related to the design pressure and are applied in a prescribed sequence in cycles lasting 1 to 3 seconds.
Wind Load TestingCyclic Testing Cont’d…..
For example, given a design pressure of 100 psf, the sample would first be subjected to positive pressures from 20 psf to 50 psf for 3,500 cycles.
Most failures occur when the sample is subjected to positive pressures from 0.4 DP to 1.0 DP for 100 cycles.
Test samples are deemed to have passed cyclic testing if no openings larger than 3” in diameter, or tears or cracks longer than 5” appear.
Sequence Pressure Cycles
Positive 0.2 DP – 0.5 DP 3,500
Positive 0.0 DP – 0.6 DP 300
Positive 0.5 DP – 0.8 DP 400
Positive 0.3 DP – 1.0 DP 100
Negative 0.3 DP – 1.0 DP 50
Negative 0.5 DP – 0.8 DP 1,050
Negative 0.0 DP – 0.6 DP 50
Negative 0.2 DP – 0.5 DP 3,350
Sequence Pressure Cycles
Positive 0.2 DP – 0.5 DP 3,500
Positive 0.0 DP – 0.6 DP 300
Positive 0.5 DP – 0.8 DP 400
Positive 0.3 DP – 1.0 DP 100
Negative 0.3 DP – 1.0 DP 50
Negative 0.5 DP – 0.8 DP 1,050
Negative 0.0 DP – 0.6 DP 50
Negative 0.2 DP – 0.5 DP 3,350
Building Outside the BoxIntroduction
In some situations, the standard
procedures for specifying windows
that meet current building code
requirements may not apply.
For instance, what if a particular
window product has not been
tested in the size required for the
project?
What if the product does not have a
NOA? What if the wind loads are
unusually high? What about mid-rise
and high – raise challenges?
Building Outside the BoxProject Specific Testing
For projects that require unique
configurations of glazing that have
not been tested, it is possible to
conduct project specific testing.
In these instances, large mock ups
must be constructed in order to
undergo impact and cycle testing.
Project specific testing can be costly
and time consuming. As a result, it
is important to plan design criteria
early and involve manufacturers.
Building Outside the BoxWind Tunnel Testing
In addition to project specific testing, wind tunnel testing is an acceptable alternative to straight line, deductive calculations.
For structures with high design pressures, wind tunnel testing can reduce design pressures by 30%
Reduced loading can significantly lighten the structural performance requirements of the window and save thousands in material costs.
Building Outside the BoxComparative Analysis vs. Rational Analysis
Engineering analysis can be used to quality unique window openings, however it is
Important to distinguish between comparative analysis and rational analysis:
Comparative Analysis: Comparative analysis uses actual test criteria as a
basis for the evaluation of a product. There are allowable spans that can be
used as a known value when sizing varies. Comparative analysis is commonly
used when project requirements exceed current test sizes.
Rational Analysis: Rational analysis is not based on test criteria and essentially
consists of an engineer’s opinion regarding a particular product. Although it
can be based on logical or calculated deductions, it is not recognized by most
certifying entities as an acceptable method for analyzing impact products.
Building Outside the BoxPoints to Remember
If you plan to test, keep in mind that the largest known job size component should be tested along with any accessories that will be used with the system. Multiple or triple arrangements for fenestration products, including mullions, are extremely important. This is often overlooked!
It is often thought that rational analysis can be used to compensate for any testing insufficiencies, however this assumption is incorrect. Rational analysis
is not permitted for impact products.
Testing involves significant material costs, engineering costs, laboratory costs, and certification fees. Moreover, lead times for testing and certification often collide with critical paths. Retesting and remedial repairs, if necessary can create unexpected delays. In some cases, product approvals can take up to one year.
Windborne Debris Protection Systems
Windborne Debris Protection SystemIntroduction
In Florida, both Miami-Dade and
Broward counties require buildings
to incorporate tested and approved
windborne debris protection systems
These debris protection systems may
consist of protective shutter systems
or impact resistant window systems.
Impact resistant glazing is able to resist
wind pressures and often provides better
protection than shutter systems.
Windborne Debris Protection SystemsProtective Shutter Systems
Manufacturers offer several different
Types of protective shutter systems:
Corrugated Panels Plywood Panels Polypropylene Panels Polycarbonate Panels Accordion Panels
Ease of installation and operation are
Both important factors to consider.
Windborne Debris Protection SystemsProtective Shutter Systems Cont’d…..
Protective Shutter Systems Cont’d…
Protective shutter systems have several
Drawbacks compared to impact glazing:
Most shutter systems are difficult to
install and labor intensive to operate
Permanent shutter systems are highly
invasive and require maintenance
Temporary shutter systems tend to be
bulky and can be difficult to store
In addition, shutter systems are typically not
Practical for multi-story applications
Windborne Debris Protection SystemsImpact Resistant Glazing
Impact resistant glazing systems are specifically designed to meet strict building codes and the toughest coastal weather challenges.
Impact glazing provides 24 hour
protection that offers maximum
protection with minimum effort.
Impact resistant glazing products
offer high design pressure ratings
and are tested to withstand the most
severe hurricane-force wind loads.
Windborne Debris Protection SystemsImpact Resistant Glazing Cont’d……
Impact resistant glazing offers several
Advantages over protective shutters:
Impact glazing is a practical solution
for both new or retrofit construction
Impact glazing offers 24 hour protection
with no action required.
Impact glazing is more aesthetically
appealing alternative to shutters
The use of impact resistant glazing may
Also help reduce insurance premiums.
Windborne Debris Protection SystemsWind Speed vs. Wind Pressure
When choosing between a shutter system and impact resistant glazing, it is important to distinguish between wind speed and wind pressure.
Shutter systems can handle gusts and debris, however they cannot protect a building from wind pressure.
Wind pressure easily works it’s way around a shutter system to the glass surface. Impact glazing is the only solution that resists wind
pressure.
Impact Resistant Glazing Systems
Impact Resistant Glazing SystemsIntroduction
Impact resistant glazing is created by bonding or laminating several lites of glass together with PVB, urethane or resin innerlayer.
Insulated impact glazing has also been developed to provide improved thermal efficiencies.
When designing with impact resistant glazing, it is important to consider the configuration of the product in terms of the existing or future debris field.
Tempered, Heat Strengthened, or Annealed Typical glass thickness: 1/8” to ¼”
PVB Thickness .090”
Impact Resistant Glazing SystemsCharacteristics
Impact resistant glazing products consist of
multiple lites of glass that are bonded or
laminated together by a 0.06” to 0.10” innerlayer.
This innerlayer typically consists of PVB or in
some instances, urethane or resin.
The number of lites and the thickness of each
lite and the innerlayer can be varied in order to
achieve the required performance characteristics.
Impact Resistant Glazing SystemsDesign Flexibility
Impact glazing is typically available in
a variety of sizes and configurations,
including casement, fixed, sliding and
double and single hung windows.
Impact resistant glazing can also be
manufactured using tinted glass in a
variety of colors as required.
Reflective and low-emissivity (Low-E)
coatings can also be applied to the
glass surface in order to provide solar
and thermal control.
Impact Resistant Glazing SystemsAdvantages of Laminated Glass
In addition to impact resistance, laminated glass also provides several other
Advantages:
Increased Safety and Security: Laminated glass can be used as safety glazing
and offers increased resistance to forced entry. Depending on its thickness,
laminated glass can also be used to provide bullet and blast resistance.
Reduced Sound Transmission: Laminated glass reduces the amount of sound
that is transmitted by the glass surface. This results in higher Sound Transmission Class (STC) ratings and a quieter, more comfortable interior environment.
Increased Energy Efficiency: Laminated glass filters out more than 99% of the sun’s harmful ultraviolet rays. This helps reduce fading of interior materials and improves
energy efficiency.
Impact Resistant Glazing SystemsInsulated Impact Glazing
Impact resistant glazing was originally developed
for the warmer coastal climate of Florida, there were
no requirements for thermal values.
As impact requirements changed up along the coast,
insulated impact glazing has become a necessity.
Insulated impact glazing is similar to standard impact
glazing in terms of aesthetics. The difference is the air
space between the lites of glazing.
Impact Resistant Glazing SystemsInsulated Impact Glazing Cont’d…..
Insulated impact glazing offers impact resistance, UV protection and sound attenuation similar to that of
standard impact glazing.
Insulated impact glazing offers superior thermal performance and significantly lower U-values.
The addition of a low-emissivity coating to the second glass surface can dramatically reduce solar heat gain and increase light transmittance.
Impact Resistant Glazing SystemsDesign Considerations
In many cases, it is also important to consider the debris field that might be generated from future surrounding developments.
A new structure built next to an existing structure could significantly alter the potential debris field.
In this case, small or large missile impact glass should be considered where a potential elevated debris field exists or is expected to exist in the future.
Impact Resistant Glazing SystemsDesign Considerations Cont’d…….
Another design consideration for impact glazing is how to configure the tested products in order to accommodate larger openings.
In this case, tested products can be installed side by side, separated by mullions, to create bays of
windows.
From an engineer’s perspective, one large opening has been divided into a series of smaller openings
that meet the necessary requirements.
Impact Resistant Glazing SystemsAAMA Certification
When specifying impact resistant
glazing systems, it is important to
look for a manufacturer who is
a member of AAMA.
AAMA memberships helps to ensure
quality performance and consistency
in manufacturing.
American Architectural Manufacturers
Association (AAMA) www.aamanet.org