Short and long duration glass loads in ASTM E1300

Why duration matters

Annealed glass breaks from tiny surface flaws that grow under stress, and they grow more the longer the stress lasts. E1300 bases its charts on a load of 3 seconds or less and reduces the resistance for longer loads. A wind gust is short. A load that stays on the glass for weeks, such as snow, is long. The same lite therefore has a lower load resistance under a long load than under a short one, and the design load you enter should match the duration you choose.

The two durations

• Short duration: a load of 3 seconds or less, such as a wind gust.
• Long duration: a load of about 30 days.

E1300 also gives factors for durations in between, in an appendix. GlassLoad.com offers the two durations above and checks one load at a time. It does not combine loads. Which duration fits a load on your project, and how to combine loads, is a decision for the engineer of record.

Glass type factors for a single lite

Load resistance is LR = NFL x GTF. Table 1 of E1300 gives the factor GTF.

• Annealed glass: 1.0 for a short load and 0.43 for a long load.
• Heat strengthened glass: 2.0 for a short load and 1.3 for a long load.
• Fully tempered glass: 4.0 for a short load and 3.0 for a long load.

The factors for heat treated glass fall less with duration than the factor for annealed glass.

Example

Take a lite 1200 mm by 1500 mm and 6 mm thick, the lite of E1300 Example 1. Its NFL is about 2.5 kPa. Annealed, the load resistance is about 2.5 kPa for a short load and about 1.1 kPa for a long load. Fully tempered, it is about 10 kPa for a short load and about 7.5 kPa for a long load. The 120 in by 60 in lite at 6 mm has an NFL of 1.05 kPa, so annealed glass resists about 0.45 kPa for a long load and fully tempered glass 3.15 kPa.

Probability of breakage for a long load

Table 1 applies the long duration factor to the load. Annex A2 of E1300 24 applies the duration to the glass stress. The two do not give the same answer, most of all when the glass deflects a lot. So an annealed lite can pass at its load resistance for a long load and still show a probability of breakage above 8 per 1000. GlassLoad.com shows a warning when this happens. The pass or fail result follows Table 1.

In GlassLoad.com

Choose short or long duration in the calculator, and the report shows the factor used. The same factors apply in E1300 09a, 12a, 16 and 24 for a single lite. Open the calculator, read how to calculate load resistance or see the questions and answers. Results are to be verified by a qualified professional.