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MATERIALS 2017 SHOW PREVIEW
Understanding laminated glass
Structural applications are one of the glazing areas offering the greatest added value for glass processors, but it is important to avoid improvisation and risks according to Michel Palumbo and Maria Teresa Mazzucchelli, glass safety service designers for Vetrostrutturale.
the detail of how laminated glass behaves, especially in a moment of catastrophe. This is partly why the latest technical standards and the most recent technical documents require understanding of the behaviour of laminated glass after a partial or total glass catastrophe. For many years, the role of polymer inter-
S
layer in the overall behaviour of laminated glass was not taken into account given polyvinyl butyral’s minimum or negligible shear transfer between panes of glass. PVB is without doubt the most popular plastic inter- layer in the production of laminated glass for structural and non structural applications. Today, however, there are extremely inter-
esting laminate interlayers on the market with substantially more sheer behaviour than that offered by traditional PVB. These polymers primarily include the so called structural PVBs (DG41 by Solutia and Trosifol ES by Kuraray) as well as Kuraray’s Sentry Glass Plus and Evasafe by Bridgestone.
Compatible
What makes these polymers worthy of note is not only their greater rigidity to shear stress but also the stability and retention of this prerogative in the medium and long term, and in temperature ranges that are totally compat- ible with the standard requirements for creating laminated glass walk-on glass, roofs, steps, railings and walls/shop windows. Correct design must globally consider the
aspects influencing the mechanical behaviour of glass for structural applications and take into account the behaviour of the “laminated glass and relative securing systems” not only in conditions of ordinary functioning (SLS) or overload conditions (ULS) but also in cases of partial (CLS) or total (PR) breakage of the laminated glass in the least favourable environmental conditions. The temperature of the load application is
a key parameter in determining the behaviour of laminated glass. As the temperature increases, the rigidity of the laminated glass diminishes because of the reduced rigidity of the plastic interlayer. All plastic interlayers alter their behaviour according to temperature, some considerably (standard PVB and structural PVB), others in a far less
ometimes, glass designers focus too heavily on the aesthetic and formal aspects of projects without going into
obvious manner (Evasafe and DG41). At temperatures above 20ºC with medium
and long duration loads, the behaviour of Evasafe is especially interesting since its stiff- ness remains virtually constant whereas that of PVBs (both ordinary and structural) con- tinue to decrease with time and temperature, falling rapidly below the values of Evasafe. If the behaviour of the polymer varies substantially according to temperature, the overall behaviour of the laminated glass will also vary. At the design phase, it is necessary to consider the most serious conditions for the laminated (glass plus plastic interlayer) in standard conditions. Another parameter that impacts strongly
on the behaviour of plastic interlayers (and consequently that of the laminate containing them) is the duration of the load application. For applications envisaging the long-term
application of loads, and having to size the laminated glass in the most demanding dura- tion and temperature conditions, it may prove effective, in order to reduce the glass thickness in play, to use a polymer with reduced variability in stiffness over time and/or with temperature, rather than a plastic that performs well in ordinary conditions but is far less stiff in the more extreme cases. A third extremely important design aspect
when choosing a plastic interlayer consists in assessing so-called environmental robustness, which is a polymer’s ability to withstand environmental attacks, expressed essentially by assessing resistance to water, humidity and UV rays over time. Another important aspect of the design
stage is the behaviour of the laminate glass in the event of partial (one sheet in the laminate) or total breakage of the laminate glass. In the case of structural applications, this aspect clearly cannot be overlooked. Safety criteria in the event of a glass catastrophe can be schematically defined as follows:
• Collapse Limit State – the need to guarantee structural safety by foreseeing that every glass element for structural applications can sustain the external design loads, even with a broken pane.
• Post Breakage behaviour – “the need to limit the risk of immediate post-breakage collapse”. This aim, obvious to all skilled designers, must be achieved by defining
suitable combinations of glass and plastic interlayer, in keeping with the chosen securing system.
Design awareness
The role of alternative plastic interlayers to the PVB ones available on the market, and the availability of technical design documents published in Europe in recent years, means that it is today possible to design laminate glass with formerly unthinkable awareness. This new design awareness calls for the com- bination of the need for resistance with design loads and post breakage safety criterion. The introduction of the Collapse Limit
State and Post Breakage concepts have rooted the awareness that laminated glass is far more than a combination of simple panes of glass and that its global behaviour depends onthe securing systems as well as the plastic interlayer. In order to improve the behaviour of
laminated glass, or to increase the sizes but with the same thickness (a crucial aspect in the architectural sphere), it is possible to con- sider interlayer polymers that make a major contribution to the transfer of shear stress between panes subjected to external actions. The intrinsic features of these polymers for structural applications are also very interest- ing in terms of contribution to the Post Breakage behaviour they are able to provide to the laminate as a whole. Today’s designers have the option of using
several types of laminated glass thanks to the presence on the market of various types of interlayer with diverse specifications that may be more or less suited to a specific situation.
028 3025 0409
www.mgwglassservices.com MATERIALS 2017 EXHIBITOR
WWW.ARCHITECTSDATAFILE.CO.UK ADF APRIL 2017
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