Volume 45, Issue 7 - July 2010

feature
Allowable Strength of Glass
How to Design Safely with Glass for Today’s Threats
by Rick De La Guardia


Before we design with glass, we must first understand the material with which we are trying to design. Glass is a very strong, yet very unpredictable material. The smallest imperfection on the surface or edge of the glass can greatly affect its performance. There can be great variations in glass strength based on test results of supposedly identical specimens due to this fact. Materials such as concrete or steel generally are more reliable and predictable with respect to test results designed to determine their strength. They also have clear guidelines as to their allowable stress. With glass, the allowable strength varies depending on its application. The allowable strength of glass is based on a probability of breakage in its intended use and safety concerns associated with its failure. According to American Society of Testing and Materials’ (ASTM) reference standard E1300, Standard Practice for Determining Load Resistance of Glass in Buildings, the generally acceptable failure rate for architectural glass in non-hazardous locations or applications is eight broken glass lites in one thousand combined with a factor of safety of between two and three.

However, that probability of failure and factor of safety can change based on the risk involved in a particular application and its intended use. A large aquarium window or glass railing, for example, should be designed to have a lower probability of failure and higher factor of safety associated with its design than, say, a large picture window on the first floor of an office building.

"There is a misconception that impact-resistant glass is not supposed to break under loading or impact. Impact glass is designed to withstand large uniform loads without breakage but it also is designed to take an impact and break—but not allow the wind-borne debris or missile to penetrate the envelope."

 

Flood Mitigation
Designing glass and glazing systems for flood mitigation is barely in its infancy, if not still in the conceptual stage. Protection is limited to only a few feet of water. Most designs for flood mitigation incorporate rather bulky and unattractive barriers or submarine type-metal doors. The Federal Emergency Management Agency (FEMA), through its publications FEMA 55, Coastal Construction Manual: Principles and Practices of Planning, Siting, Designing, Constructing, and Maintaining Residential Buildings in Coastal Areas, and FEMA Technical Bulletin 3-93, Non-Residential Floodproofing-Requirements and Certification, offers the designer a wealth of information regarding the design loads required to properly design for flood mitigation. The loadings include:
• hydrostatic pressure—the force applied to the structure from standing flood waters using the base flood elevation as the design height, which depends on where the structure is located. This information can be obtained from local flood insurance rate maps (FIRM);
• hydrodynamic pressure—the force applied on the structure by the moving flood waters using the velocity of the flood waters as obtained from FIRM;
• wave action—loading due to the crashing of the flood waters on the structures near the coast; and
• impact—loading based on a downed tree log 1-foot in diameter weighing 500 to 1,000 pounds, carried by the flood waters and hitting the structure at the flood velocity.
Once again, the resources for the design of glazing for this application are extremely limited and it is up to the designer to use his or her engineering judgment.

Reading the References
Safely designing with glass depends not only on understanding the material itself but the reference standards, building codes, intended use, selection process, product specifications, design review and installation and inspection processes and procedures. The generally accepted reference standard specified in most building codes for the design of glass is ASTM E1300. This standard provides the maximum uniformly distributed load that a glass specimen can safely withstand based on a laundry list of parameters, including glass type, thickness, its aspect ratio (long dimension divided by short dimension), load duration, intended use, boundary or support conditions and probability of failure. Compare this design method to steel or concrete design, with respective code-referenced standards American Institute of Steel Construction (AISC) and American Concrete Institute (ACI). These standards provide an allowable stress to compare to the stress imposed on the member or material based solely on rational analysis.
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As you can see, designing safely with glass is much more complicated than designing with the other building materials in the market today. Now throw in the fact that ASTM E1300 has its limits and that it is up to the designer to understand those limitations. Designers must piece together and interpret other resources and references, of which, in my opinion, very few quality ones exist—with the exception of the Glass Association of North America’s (GANA) Glazing Manual and Laminated Glazing Reference Manual, among others. Now you begin to understand why glass is so misunderstood and complicated to work with outside of normal parameters.

ASTM E1300 is intended for use in uniform loading conditions involving wind, snow and dead load applications of glass only. Section 1.2 of E1300 reads:

This practice applies to vertical and sloped glazing in buildings for which the specified design loads consist of wind load, snow load and self-weight with a total combined magnitude less than or equal to 10 kPa (210 psf). This practice shall not apply to other applications including, but not limited to, balustrades, glass floor panels, aquariums, structural glass members and glass shelves.

If you look closely at the partial list of applications that are not covered by the standard, you’ll see that designers basically are on their own when it comes to dealing with glass in the most hazardous conditions—which are precisely the areas that require the most guidance. Some publications, notably GANA’s Glazing Manual, do provide an allowable stress for these conditions. Still, this usually is not a precise value, but rather a stress range depending on the particular application and consequence of failure.

The Right Application
The intended use of glass and application type will affect the glass selection process greatly. The glass products that protect our buildings and homes are a very important link in the overall protection of the building envelope. The building envelope is the first line of defense in a hazardous event, and glass often is the first material that is put to the test during a storm. The glass must be strong enough to preserve the integrity of the building envelope and transfer the forces exerted on it by the design loads to the products that support it—all the way to the main building structure. Any failure in that link can have a catastrophic affect on the building’s inhabitants and contents. We have seen many examples of this failure in the past few years, especially when it deals with windloads and wind-borne debris.

Today’s threats are ever-increasing in scope and magnitude and come in the form of natural and manmade hazards. Hurricanes are a frequent threat to glass and the building envelope, and therefore you will see more literature and focus on this type of threat than other hazards. However, with the increasing threat of terrorism, a different form of threat—ballistics and blast—is gaining considerable attention. We also have been reminded, by the catastrophe of Hurricane Katrina and, more recently, by the events in Nashville, that flood loads cannot be ignored.

Safety glass also is not limited to protecting building inhabitants or its contents. There are municipalities on the west coast of Florida that require “turtle friendly” glass that is not allowed to reflect too much light so that turtle hatchings don’t migrate toward the street instead of toward the moonlight and ocean.

Hurricane Safety
Safety glazing, as it pertains to hurricane mitigation, has come a long way. We have advanced greatly in the design of safety glass to protect us and our property from damage due to wind and wind-borne debris.

Impact-resistant glass is actually a combination of the three basic glass types and a laminate film or resin (interlayer) that bonds them together. The three basic glass types are annealed, heat strengthened (twice as strong as annealed) and tempered (four times as strong as annealed). Impact-resistant glass is created when any combination of the three basic glass types are bonded together with an interlayer that keeps the glass in place should it break. There is a misconception that impact-resistant glass is not supposed to break under loading or impact. While it is true that impact-resistant glass is designed to withstand large uniform loads without breakage, it also is designed to take an impact and break—but not allow the wind-borne debris or missile to penetrate the envelope.

In Miami-Dade County, Fla., home of arguably the strictest glass and glazing codes in the country, there exist three test protocols by which test procedures are based (TAS 201, 202 and 203). These test protocols are designed to subject the building products, including the glass, to uniform load, impact, cyclic, water and air infiltration tests. A product cannot be sold in the state of Florida without first passing these rigorous tests and obtaining a notice of acceptance. One of the impact test criteria (large missile) actually is a 9-foot-long, 9 pound 2-by-4 wood specimen launched out of a laser-guided canon at a speed of 50 feet per second to impact the glass or product three times. The glass or product must still be operable and protect the building envelope after impact and subsequent windload cycling.

Resources
Looking for more information? Check out the following references:
• ASTME1300, Standard Practice for Determining Load Resistance of Glass in Buildings – standard with general information for glass loads;
• ASTM F1642, Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loadings – standard for designing for blast mitigation;
• ANSI Z97.1, Glazing Materials Used in Buildings, Safety Performance Specifications and Methods of Test – test method and safety performance specification for designing with glass in hazardous locations;
• 16 CFR 1201, Safety Standard for Architectural Glazing Materials – test method and federal standard for designing with glass in hazardous locations;
• FEMA 55, Coastal Construction Manual: Principles and Practices of Planning, Siting, Designing, Constructing, and Maintaining Residential Buildings in Coastal Areas and FEMA Technical Bulletin 3-93, Non-Residential Floodproofing-Requirements and Certification – technical information for designing for flood mitigation;
• Glass Association of North America’s Glazing Manual – resource with general and specific glazing guidelines; and
• Glass Association of North America’s Laminated Glazing Reference Manual – resource on laminated glass with technical data and installation guidelines.

Blast Resistance
The design of glass for blast mitigation has gained considerable attention lately from storefront and window wall manufacturers. The standard for design of glazing systems for this type of threat is the ASTM standard F1642, Standard Test Method for Glazing and Glazing Systems Subject to Airblast Loadings. The primary design criteria for blast mitigation is per formance of a proper threat assessment and determination of the blast protection zone for a particular threat level or type of threat. It is obvious that for certain blast magnitudes there is very little that can be done to protect the structure itself, much less the glass.

Reflection Issues
Human and building content safety are not the only consideration one must adhere to when endeavoring to safely design with glass. As development along the along the west coast of Florida has increased, it was observed that sea turtle hatchlings, who instinctively follow a light source into the sea, were distracted from the reflection of the moon and stars on the surface of the sea. Instead, the hatchlings were confused by the street lights and transmission of the business lights through storefront windows and condominiums along the shore and would mistakenly make their way to the street, facing almost certain death. Once this situation was discovered, a law was implemented that led to the Florida Model Lighting Ordinance for Marine Turtle Protection whose purpose and intent is:

… to implement Section 161.163, Florida Statutes, which requires the department to designate coastal areas utilized, or likely to be utilized, by sea turtles for nesting, and to establish guidelines for local government regulations that control beachfront lighting to protect hatching sea turtles. This rule is intended to guide local governments in developing ordinances which will protect hatchling marine turtles from the adverse effects of artificial lighting, provide overall improvement in nesting habitat degraded by light pollution, and increase successful nesting activity and production of hatchlings.

The ordinance requires that:
Tinted glass shall be installed on all windows and glass doors of single or multi-story structures within line-of-sight of the beach.

The ordinance further defines tinted glass as:
… any glass treated to achieve an industry-approved, inside-to-outside light transmittance value of 45% or less. Such transmittance is limited to the visible spectrum (400 to 700 nanometers) and is measured as the percentage of light that is transmitted through the glass.

Hazardous Locations
Non-building envelope glazing products also must be considered when designing glass with safety in mind. Aside from protection from natural or manmade hazards, one must consider the use of glass located in hazardous locations (glass in guards, overhead applications, doors, windows with a significant drop on the other side, etc.). The impact loads associated with such products are usually of a human kind. The safety glass standard that covers this type of design is published by the American National Safety Institute (ANSI) reference standard Z97.1, Glazing Materials Used in Buildings, Safety Performance Specifications and Methods of Test (see June 2010 USGlass, page 42 for more information). ANSI Z97.1 actually is based on an energetic teenager weighing 100 pounds running into a glass panel at full speed. The equivalent force of that impact is determined to be 400 foot-pounds, which must be resisted by the glass. There is another safety glass standard published by the Consumer Product Safety Commission (CPSC), reference standard 16 CFR 1201, Safety Standard for Architectural Glazing Materials. You can learn more about the two documents from the GANA glass informational bulletins GANA 03-0609 titled “Differences Between Safety Glazing Standards,” available at www.glasswebsite.com. It is interesting to note that the design of this type of glass is not covered under the glass standard ASTM E1300.

For information on glass in balustrades, aquarium glass and structural glass, refer to the technical resources of professional associations and manuals such as the GANA Glazing Manual.

Designing for Safety
So what are the first steps to safely design with glass for today’s ever-increasing threats, both natural and man-made? Ask yourself the following questions, the answers to which will lead you in the right direction:
• Know Your Product: What type of glass are you working with? What are the strengths associated with the type you have chosen (annealed, heat strengthened, tempered, laminated, insulating, laminated-insulating)?
• Know Your Application: Are you designing for a hazardous location? What are the consequences associated with failure? What are the accepted probabilities of breakage and factors of safety for that particular application? What hazard are you designing for?
• Know The Code, Standards And Resources: What edition of the standard does the code specify? Does the standard apply to the type of application you are proposing?
• Know Your Location: Are you in an area that requires design for wind borne debris? Are there any local ordinances that affect the design or selection of glass?


Rick De La Guardia is president of DLG Engineering Inc. Mr. De La Guardia’s opinions are solely his own and not necessarily those of this magazine.

 


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