Glass Unit Corner Loading—Key Parameter in Durability

ABSTRACT: Bent glass has garnered increased interest on the part of archi­tects for the realization of curved glass facades. As one method of bending of glass facade units, cold bending is an economically efficient procedure for manufacturing; however, it introduces permanent stresses in the glazing structure, especially in the corner zones of the glass units for warped designs. In a similar manner, high stresses in the corner zones are also gen­erated in general by constant surface loads acting on the panes of the glass unit, which can be explained by thin plate theory. Thus it can be expected that these unsteady loads, e. g., evoked by wind and/or snow loads, unfavor­ably interfere with the permanent stresses in the adhesives of both the struc­tural glazing sealant and the insulating glass sealant from a durability point of view. The existence of these corner loads is not adequately accounted for by the ETAG 002 guideline for structural glazing applications, which postulates a trapezoidal load distribution in the bonding with diminishing stresses in the corner zones. This paper presents numerical results of a parametric study of pressure-loaded glass units, with a focus on corner loads and stresses. The results show that the stress levels in the corner zones might be significantly higher than the design stress values used for sizing the bonding.

KEYWORDS: cold bent glass, corner load, silicone adhesive, structural glazing, warped glass

NOMENCLATURE

ETAG = European Technical Approval Guideline ETA = European Technical Approval FEA = finite element analysis SSG = structural sealant glazing

Introduction

In order to open the design space for curved glass facades and meet increased architectural demand, hot and cold bending techniques have become available for manufacturing bent glass facade units. Hot bending refers to the bending of glass under high temperature, leading to mainly plastic deformation of the glass panes, and cold bending is related to the elastic bending of the glass units while they are being bonded to the curved facade structure. The cold bending manu­facturing procedure leads to an inherent permanent stress state of the glass fa­cade, with the elastic bending of the glass panes counteracted by tension and compression loading of the bonding adhesive. Thus the adhesive transfers the glass pane bending moments to the supporting structure of the glass facade. One attempt to classify the related bending patterns from a kinematic point of view might involve sorting into warped, conical, or cylindrical displacement fields (see Fig. 1). Due to the enforced elastic deformation of the glass panes within the cold bending process, maximum adhesive loading in tensile and com­pressive directions is expected in the corner zones of the glass units subjected to warping. Cylindrical and conical bending patterns are considered less critical from a stress and durability point of view, as the maximum adhesive loading is

Cylindrical bending

(no bending)

Conical bending (partially warping)

FIG. 1—Bending patterns of potential architectural interest.

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mainly distributed along the non-curved edges. This paper focuses on the considera­tion of the mechanically challenging case of warping. Figure 2 presents as a numeri­cal example the case of a laminated glass pane (Young’s modulus = 70 000 MPa, Poisson’s ratio = 0.23) 1.5 m in length, 1.25 m in width, and 6 mm in thickness, with each pane subjected to a warping of 0.1 m. The warping in the model is applied by a bi-linear warping field established via linear displacements of the edges linked to a dedicated corner that is offset by 0.1 m in the perpendicular direction (see Fig. 3). The bonding was specified to a width of 20 mm and a height of 9 mm. The material properties of the adhesive are based on a hyperelastic material law for a

FIG. 2—Permanent tensile stress distributions in the corner zones due to warping: bi­linear warping of 10 mm (glass unit: 1.5 m in length, 1.25 m in width, 2 mm x 6 mm glass thickness; bonding: width— 20 mm, thickness — 9 mm).

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FIG. 3—Approximation of warping field as a boundary condition for the support definition.

two-component silicone adhesive typically applied for structural sealant glazing (SSG) (initial Young’s modulus ^1.5 MPa). As this load regime is of the permanent type, creep of the adhesive in the corner zones is a major topic for cold bending technologies. Thus cold bending applications favor highly flexible glass panes and bond geometries of large widths in order to reduce the permanent stress levels (a more detailed discussion of the results is given later in the context of Fig. 16).

Regarding the operating loads of the warped glass unit, typical load cases consist in distributed (“pressure type") loads acting on the surface of the glass panes, e. g., wind loads. Constant surface loads as approximations for sizing might be due to wind loads either in suction or in compression, snow weight, or glass dead loads in the case of almost horizontal glazing. Due to the complex stress and strain characteristics of structural glazing applications, design rules such as the European guideline for structural glazing ETAG 002 [1] use simpli­fying assumptions for the load and stress distributions for sizing. In the case of ETAG 002, a trapezoidal load distribution is assumed, ignoring any corner loads. Thus, ETAG 002 does not give any indication of the potential interference of the cold bending permanent stresses for short – to mid-term constant pressure load cases. This paper focuses on a review of the bond load assumptions for constant surface loads and the mechanical impact these have on cold bending warped glass facade designs from a durability point of view. The paper starts with a short review of plate theory for rectangular plates under constant pres­sure loads. After that, the outcome of plate theory is compared with parametric finite element analysis (FEA) studies.

Copyright by ASTM Int’l (all rights reserved); Tue May 6 12:07:08 EDT 2014

Downloaded/printed by

Rochester Institute Of Technology pursuant to License Agreement. No further reproductions authorized.