Figuring strength of glass

Ever increasing in popularity, minimally supported glass, such as glass canopies and balustrades, find their way into many commercial construction projects. These applications require designers to confirm that the glass meets the appropriate design loads. Whatever the situation, strength of glass plays an important role in choosing the best glass solution for a project. There are many references and tools to aid in this process. The ASTM International’s ASTM E 1300 Standard Practice for Determining Load Resistance of Glass in Buildings, was developed to help people design with glass. It describes procedures to determine load resistance of different types of glass for a specified probability of breakage. This standard includes design charts for monolithic, laminated and insulating glass constructions with one-, two-, three- and four-sided support conditions.

Computer software programs have been developed to help designers confirm the proper selection of glass based on specific design-load requirements. For example, in the United States, Standards Design Group Inc.’s Window Glass Design, 2004-Deluxe Version, represents one such program. For more information, visit www.standardsdesign.com/WGD/2004/Default.htm.

The DuPont Strength of Glass Calculator, another software option, can be viewed and used at www.dupont.com/safetyglass. DuPont researchers developed this calculator “to move beyond the existing capability in standards and allow the most efficient use of laminated glass, especially when using DuPont’s structural interlayer, SentryGlas Plus,” says Steve Bennison, a research associate with DuPont in Wilmington, Del.

This interactive tool allows users to evaluate glass strength and deflection based on different project scenarios. The main purpose of the glass calculator is to assist designers in the proper selection of glass thickness to resist a specified load or load condition. This tool grows in importance because of a growing interest among designers in incorporating into their projects windows, façades, canopies, balustrades and floors not supported continuously on four edges.

Glass retention represents an important requirement that leads designers to the selection of laminated glass. As such, the calculator proves an easy-to-use tool available free of charge to aid in the design of laminated glass for one- and two-sided supported projects.

Begin here
When you view the calculator on the site, the first screen asks you to choose a case. Each case represents how your glass will be supported and how the load will be applied. This calculator can be used for projects where the glass will be supported either continuously along one edge of the glass (one-sided) or continuously along two opposite edges of the glass (two-sided) as shown in the diagram at left.

Two different loading scenarios are possible: uniform load and line load. They can be applied separately or in combination, as shown below.

Building codes dictate some loads and oftentimes designers arrive at other loads through discussions with building owners when they seek to understand the conditions that they expect for certain locations and applications. Uniform loads have pressures that span across the entire surface of the glass piece, such as wind or snow loads. Some loads only exert pressure in certain areas. A line load has pressure applied across a particular line of the glass to emulate someone leaning against the glass.

Choose an appropriate case and click on the “Choose Beam Parameters” button, as shown on the screen below, and accept the disclaimer. The “beam” in this calculator is the piece of laminated glass under evaluation.

You will come to another screen where the upper portion offers places to type in  your parameters and the lower part will show the results. In the upper portion of the screen, users detail parameters such as temperature, time, length, glass type and edge finish, and the design-load values. The lower portion will show results of the calculation, where the strength, deflection, and effective thickness values will appear after the “calculate” button is pushed.

Next, type in the following  parameters:

•  Temperature, typically, internal applications can use 30 degrees Centigrade and outdoor applications can use 50 C.
• Time. Glass will “feel” the pressure of a load for different amounts of time, depending on the type of load. Glass strength requirements will be higher for longer load durations than for shorter load durations. Typically, wind gusts are considered short-term, three-second uniform load. Snow could accumulate for much longer, such as a month. Permanent loads, including self-weight, can be long-term load durations, greater than one year.
•  Length of glass that is unsupported.
•  Glass type such as annealed, heat-strengthened or tempered.
• Edge finish of the glass such as clean cut, seamed or polished.
•  Load or magnitude of the relevant design load.

Note that you do not need to specify the glass or interlayer thickness at this time; this can be done using the slide bar at the bottom of the page.

Click on the “Calculate” button.

The lower portion of the screen will now show the results of the calculation, with the thinnest glass construction automatically selected. The slider button at the bottom can be moved to see the changes when thicker glass constructions are used. The information highlighted in the grey section includes the glass stress and a statement about whether the laminate meets or fails the ASTM strength recommendation based on ASTM E 1300 and laminate deflection. The two graphs represent the glass stress and deflection, as they vary with glass thickness. Two different interlayers are evaluated, SentryGlas Plus and polyvinyl butyral. SentryGlas Plus is an advanced polymer interlayer that offers 100 times the stiffness and five times the tear resistance of traditional PVB laminates. Different glass stress and deflection can be obtained with using one or the other interlayer.

Take a canopy, for instance
In this canopy example, the glass is supported continuously along two opposite edges for two-sided support and a uniform snow load is expected. Once the calculator is viewed, choose Case 4—uniform pressure, P (kilopascals), two-sided support—and click the “Choose beam parameters” button. Next, “Accept” the disclaimer. For this example, the following parameters are used:

• Temperature: 50 C for this external application
• Time: One month, the time estimated for a snow load duration
• Length, unsupported: 2.5 meters
• Edge finish: seamed
• Glass type: heat-strengthened
• Load: 1.44 kilopascals, equivalent to 30 pounds per square foot.

Click the “Calculate” button. The initial construction comes up as a laminate built of two lites of 2.5-millimeter glass; however, the laminate constructions fail the ASTM strength recommendation, as shown at left.

Moving the slider bar—at the bottom of the section—to 12 millimeters shows the SentryGlas Plus laminate meets the ASTM strength recommendation and would exhibit 10-millimeter deflection; while the PVB laminate fails the ASTM strength recommendation at 12 millimeters, with an estimated deflection of 31 millimeters, as shown below.

Consider a balustrade
In this balustrade example, we consider glass supported along the bottom edge only. A line load across the top would emulate many people leaning against the railing. Here we choose Case 1—Line load (kilonewton per meter), one-sided support—and click the “Choose beam parameters” button. Next, “Accept” the disclaimer. For this example, the following parameters are used:

• Temperature: 50 C for this external application
• Time: One minute, estimated for someone leaning temporarily against the top
• Length, unsupported: 2 meters
• Edge finish: seamed
• Glass type: tempered
• Load: 1 kilonewton per meter.

Click the “Calculate” button. The initial construction comes up as a laminate built of two lites of 2.5-millimeter glass. However, the laminate constructions fail the ASTM strength recommendation. Both laminates meet the ASTM strength recommendations when 8-millimeter glass lites are chosen (slider bar at the bottom section). However, the estimated deflection is quite significant (SentryGlas Plus: 59 millimeter or ~2.3 inch; PVB: 144 millimeter or ~5.7 inch. Therefore, a thicker glass construction might be chosen to reduce the amount of deflection seen with this configuration.

Differentiating products
The type of interlayer can make a difference in glass strength and overall deflection for a laminate construction. Deflection is the amount that the glass will bow under the specified load. Typically, a low deflection is desirable; the acceptable magnitude depends on the application and relevant building codes. The DuPont SentryGlas Plus interlayer is a stiffer interlayer than traditional PVB interlayers. In many cases, especially if the construction is bending dominated where the aspect ratio is two to one or greater, a laminate with SentryGlas Plus is stronger and shows significantly less deflection than the corresponding PVB laminate. This can be seen in our example cases above—in the canopy example, the laminate with 12-millimeter lites and SentryGlas Plus met the strength requirements, whereas it would take a thicker construction with PVB to meet the same load requirements. In both examples, the deflection was estimated to be significantly lower with the SentryGlas Plus laminate.

Design assistance
While this calculator helps users such as architects, engineers, designers and specifiers consider the relative influence of common laminated glass design variables, it considers only simply supported constructions and a few loading conditions. For more complicated systems, other analysis methods might be employed. Design professionals should consider working with engineering professionals to determine appropriate configurations. Actual proof testing is recommended. In addition, design loads are often dictated by relevant building codes. The examples in this article are not meant to represent actual design-load requirements for all canopies and balustrades.

Different scenarios, different needs
As interest in the use of minimally supported glass grows, it is critical to remember that these installations will require engineering to confirm that specified loads have been met. The calculator serves as one effective tool that can help designers evaluate different scenarios to assist in choosing the best glass for these innovative and attractive uses.