19 April 2007


Background On Testing & Standards

Today, many products are available that have been tested for windborne debris impact and cyclic wind loading. Several test standards exist including: 1]ASTM E1996 Standard Specification for Performance of Exterior Windows, Glazed Curtain Walls, Doors and Storm Shutters Impacted by Wind-borne Debris in Hurricanes (written by the American Society of Testing and Materials) 2] PA 201 Impact Test Procedures (written by Miami-Dade County Building Code Compliance Office) 3] SSTD 12 TEST Standard for Determining Impact Resistance from Wind-borne Debris (written by the Southern Building Code Congress International, Inc.

Each of these test standards is based on similar testing with slight variations. The tests consist of two parts: impact testing and cyclic wind loading.

The impact test consists of shooting objects at a series of specimens. If the product is going to be installed above 30 feet, it has small objects fired at the product simulating roof ballast rock and other small debris that would be picked up and carried by the wind higher than 30 feet above the ground. If the product is going to be installed below 30 feet, it has a 2x4 fired at the product simulating larger objects that would be picked up and carried by the wind along and close to the ground, but too heavy to get high into the air.

Following the impact test, the series of impacted specimens are put through cyclic wind pressure loading, both positive and negative pressures. The cyclic wind pressure loading is intended to simulate wind pressures created by a hurricane from approach through departure.

The testing results in two designations: Impact Resistance — Yes or No

Design Pressure Rating — in pounds per square foot (both negative and positive) If a window product is not intended to pass the impact portion of the test, it will still be tested with cyclic wind pressure loading and be assigned a Design Pressure Rating (positive and negative pressures). Because it is not deemed to be impact resistant, it will still require the protection of a hurricane shutter or storm panel.

A common misconception is that a window or door product can be designated to withstand a specified wind speed. The products are given a design pressure rating not a “wind speed rating”. A Category 5 hurricane creates different pressures, both positive and negative, on different surfaces of a structure. So, depending on where the window or door is placed, it may experience higher or lower wind pressures under the same wind speed conditions. The higher the Design Pressure Rating a product receives, the higher the cyclic wind pressures to which it was tested.

Whether impact resistant windows and doors, shutter systems, or temporary storm panels are selected for hurricane protection, it is imperative to select products that have been tested according to a code-approved standard with tests that are independently verified. Installing untested products or products that have been tested without independent verification may result in a product that does not provide the needed protection.

American Society of Civil Engineers (ASCE) 7-98: ASCE 7-98 is the 1998 edition of the American Society of Civil Engineers Minimum Design Loads for Buildings and Other Structures. This is the national standard to which building codes refer when specifying design load requirements for: dead, live, soil, flood, wind, snow, rain, ice, and earthquake loads on buildings. The wind loading section of this standard is considered to contain the most up-to-date requirements available. ASCE 7-98 only contains methods to calculate loads on structures. An engineer or architect must take the wind pressures specified in the Standard, and apply them to the building being designed. These wind pressures will determine how the building must be constructed.
ASCE 7-98 has special requirements for buildings sited in areas called “wind-borne debris regions.” Wind-borne debris regions are defined as areas along the Atlantic and Gulf Coasts where the design wind speed is 120 mph or above, or within one mile of the coastal mean high water line where the wind speed is 110 mph or above. In these regions, the standard requires the designer to assume that the windows will be broken by flying debris unless the windows are either impact-resistant or protected by impact protection (shutters or impact-resistant glass). When windows are considered to be broken, the wind pressures trying to lift off the roof are greatly increased, so construction has to be significantly strengthened.

ASCE 7-98 recognizes the option of designing for higher internal pressure as an alternative to designing with impact protection (shutters or impact-resistant glass). Blueprint for Safety recommends designing with impact protection because while designing for internal pressure may protect the overall structure, it will not prevent flying debris from breaking windows, causing building damage or allowing significant water and wind infiltration into the house. Blueprint recommends impact protection as the optimal way to protect lives and property.
Blueprint recommends that whenever structural components are built, added or modified, they should be designed using rational analysis based on wind loads calculated according to ASCE 7-98 using a basic wind speed defined by ASCE 7-98 but no less than 120 mph. The architectural details in the manual are engineered to 140 mph for illustration purposes, but will require modifications to meet the 150 mph requirements in certain coastal sections of Miami-Dade or Monroe County.

Construction Details: Construction details are provided in this guide for the purpose of improving the structural performance of residential homes. The prescriptive requirements contained in the details are based on the latest engineering knowledge reflected in the ASCE Standard 7-98, SBCCI Standard for Hurricane Resistant Residential Construction SSTD10-99 and the new unified Florida Building Code. The latest engineering knowledge contained in this guide is intended to provide a higher level of structural integrity within the limitations in building geometry, materials and wind climate specified within the minimum requirements of building codes.

It is recognized that a large number of alternatives are available to a designer or builder for providing wind resistance. The Blueprint guidelines are not intended to prevent the use of such alternative materials or methods, permitted in the statewide Florida Building Code. The methods presented in this guide provide wind-resistant designs and construction details for one- and two-story residential homes of conventional wood-framed and masonry construction typical in Florida.

The wind loads indicated in the details apply specifically to a building having a width of 30 feet and a length of 60 feet with story heights of 10 feet. The structure has gable end walls with a roof pitch of 4:12; having a slope of 18 degrees. In addition, the design parameters used in conjunction with ASCE 7-98 for residential design are: Design wind speed of 140 mph.

Exposure B — for urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. The unified Florida Building Code specifies the use of Exposure B for all residential construction except seaward of the coastal construction line.

Building is rectangular and regular in shape.
Building is enclosed.
Building is a low-rise building.
Building has simple diaphragms that transmit wind loads through floor and roof diaphragms to the vertical main wind force resisting system.

Visit http://www.blueprintforsafety.org for additional information. BE SAFE!

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