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Advanced Building Skins
values are characteristic values. The resistance (glass strength) is reduced by a global safety factor
(Table 2).
Table 2: Allowable bending stress according to TRLV [6].
Type of Glass
- 4 -
zul [N/mm 2 ]
overhead vertical
Annealed float 12 18
Laminated (Annealed) 15 22.5
FTG 50 50
4 Comparison of the Safety Levels According to IEC 61646 and TRLV or DIN 18008
The IEC 61646 contains only the performance of one mechanical load test with a pressure or suction
load of 2.4 kN/m². The standard explains in the comments the origin of this value with a wind pressure
of 0.8 kN/m 2 , corresponding to a wind speed of 130 km/h and a safety factor of γ = 3.
This relationship between wind velocity and pressure can be confirmed by DIN 1055-4 [7]. However,
a characteristic wind pressure of 0.8 kN/m 2 is achieved already for a building in wind zone 2 at a
building height of 10 m to 18 m. This does not yet include the cpe-factor, which takes into account the
geometry of the building, the mounting position in the building and the size of the loaded area.
Which actions (wind, dead weight, snow and ice loads, and combinations thereof) are associated with
the increased pressure load of 5.4 kN/m 2 and which application should be covered with this load is not
described in IEC 61646.
However, the specified global safety factor of γ = 3 for the wind pressure of 0.8 kN/m 2 is too low for a
design based on only one test. A comparison with the safety level of the TRLV demonstrates this. For
horizontally mounted laminated glass made of annealed glass, the global safety factor is determined
by:
N
45
f k
mm²
global
3.
0
(2)
N
zul 15
mm²
This global safety factor is based on the characteristic value (5%-fractile) of the glass strength.
According to IEC 61646, the safety factor is, however, based on a mechanical load test with only one
test specimen. The (high) scattering of the glass strength is therefore not considered. If, in addition to
the 5%-fractile value, the mean value of naturally aged annealed glass with fm = 80 N/mm² [8] and the
95%-fractile are estimated, this results in the global safety factors according to Table 3.
Table 3: Safety factors according to TRLV for horizontal mounted LSG made of annealed float glass.
Strength of the tested
glass [N/mm 2 ]
5%-fractile value mean value 95%-fractile value
45 80 115
Global safety factor 3.0 5.3 7.7
Comparison to IEC 61646 100% 178% 256%