Optimizing methodologies for cold bending of glass

 

Modern free form glass architecture has become increasingly popular in the structural sealant glazing application, as demonstrated by many iconic building projects, such as Allianz tower in Milan. Amongst the different technologies to manufacture such glass, cold bending is one of the most cost efficient and aesthetically pleasing solutions. In this process, flat glass panels are elastically deformed to follow the façade contours by bonding to a metal frame with a silicone sealant. This operation imposes a permanent bending force onto the silicone as the bent glass strives to return to its initial flat shape. This can lead to silicone creep and tear failure if the joint is not properly dimensioned. Previous research by the authors has evidenced through a combination of experimental testing and finite element analysis, how principal strain in the sealant needs to be limited to ensure durability and avoid tearing. This current research proposes an optimized methodology for cold bent glass and SSG design through a simulation-based DOE study for different glass and metal frame form factors as well as silicone sealant parameters. In addition to the level of cold bend glass deflection, the metal frame and sealant thickness have the most significant effect on the peak strain in the sealant during the cold bent glass operation. Two solutions are proposed to improve the silicone sealant durability and reduce creep deformation.

 

 

Silicones – an important enabler of sustainable design

 

Recently, a global effort of the construction industry has been observed to achieve more sustainability and circularity in design, aligned with legislative requirements such as the ESPR or the EU Green Deal. This presentation will review the impact of silicones at various levels of a sustainable design. It is well known that the use of silicone sealants strongly contributes toward a durable air and water tight building envelope and enables energy saving multi pane glazing. Both applications significantly contribute to improving energy efficiency and hence reducing the operational carbon of a façade. The benefit during the lifetime of the building outweighs several times the carbon emissions required during the production of these silicones, especially when using the DOWSIL™ Carbon-Neutral Silicones Service for façade applications. Furthermore, silicones can also contribute at other levels of a sustainable design, such as material efficiency. Silicone bonding can help reduce resource use, such as aluminium used for frames. Silicones as bonding system are easier to disassemble at end of life compared to more rigid assembly methods and allow reuse of valuable materials.

 

 

Advanced engineering methods unlock higher permitted stresses for structural glazing designs

 

Bonding of glass onto aluminium frames, known as Structural Silicone Glazing (SSG), has been active for more than 50 years on facades. Traditionally the silicone bite is calculated using a simplified equation described in guidelines such as ETAG002 or ASTM C1401, which assumes a homogenous stress distribution along the sealant bite. Although this approach will remain a valid method for which there is proven performance, recent research has indicated that joint dimensioning can be optimized for excessively large joint bites such as projects that are being designed to perform in high velocity wind zones.  As joint bite dimensions increase, the distribution of stress in the structural silicone sealant diverges from homogeneity in mostly tension, under negative windload. This divergence is due to the rigidity of the sealant as a function of glass rotation, where mixed modes of stresses can be observed in compression, tension and shear along the width of the silicone bond. Alternative methods of compliance to the conventional design techniques are important as they allow a greater design freedom without sacrificing long term durability. Physical prototypes from small scale H-pieces to full scale panels measuring 914mm by 1524mm were tested with different joint aspect ratios varying between 2:1 to 4:1 width to depth ratios.  Finite element analysis of the tested units also provided insight into the behaviour of sealant distribution and potential failure identification.  Based on the performed testing, new methods of compliance for dimension joints are proposed beyond normal conventional methods outlined in ETAG002 and ASTM C1401.

 

 

New model for performance of silicone bonded facades during seismic events

 

Some of the world’s largest metropolitan areas are located within highly active seismic regions. Earthquakes can lead to major architectural and human impact, especially when glazed curtain walls are involved. Silicone structurally glazed façades are a durable assembly system, known to optimize the envelope’s weatherproofing performance and aesthetics, (removed  but) which can also potentially contribute to a better seismic resistance of the building’s façade.

This paper presents the seismic study of silicone bonded glass curtain wall systems, combining both finite element analysis and experimental validation. Applying finite element analysis to silicone requires appropriate material models which suitably mimic the typical loading undergone during specific events. In the case of earthquakes, repetitive loading cycles occur, which can highlight the stress relaxation behavior of silicone elastomers. The material model developed was validated using both test results obtained inhouse on mid-scale build-ups and results of the experimental campaign carried out at the laboratory of Permasteelisa Group on full-scale unitized curtain walls. These tests considered varied silicone types (Young modulus) and joint dimensions (width and thickness or aspect ratio) and allowed identifying criteria for failure prediction and suitable design strength values as well as joint dimensioning guidelines optimizing a response to seismic events.

 

 

Glass fins: Benefits of Three-Sided Adhesion

 

Three-sided adhesion refers to the instance where a sealant will attach to each side of the substrate forming the joint as well as the back of the joint.  When this occurs, it limits the degrees of freedom that the sealant can operate and create limitations in its movement or can generate excessive stresses that are detrimental to the long term sealant performance.  Historically, this has been viewed as a universal concern and applied to sealant applications outside of weatherproofing.  But when you consider the use of sealants in structural applications, three sided adhesion may provide some benefit when the application requires a sufficient stiffness to prevent movement such as the secondary sealant on an insulation glass unit.  Finite element analysis is used to explore sealant behavior in glass fin application including the potential for failure to provide guidelines in optimizing modeling procedures to achieve the best design outcomes.