Characterization of Adhesive Joints for Hybrid Steel-Glass Beams by Means of Simplified Small Scale Tests

ABSTRACT: In the recent past there have been calls for transparent and fili­gree structures in the building industry. Therefore, glass plays an increasing role not only in a classic way, as a space enclosing element, but also more and more in terms of offering load carrying functions. Glass beams for facade elements or floor girders, glass columns, or bracing facade elements are examples of this. To realize such architecturally attractive structures, bonded hybrid steel-glass elements have been developed in which each material is used in an optimized way according to its material properties. Promising examples for such bonded structures are I-beams in which steel flanges and glass webs are connected by linear adhesive bonds. The shear force is car­ried by the glass web, whereas the bending capacity of the hybrid beam is significantly increased by slender steel flanges compared to the pure glass pane. The shear forces between steel and glass are sustained only by the adhesive between them. In order to maximize the exploitation of both steel and glass, the adhesive on the one hand has to ensure an adequate stiffness but on the other hand has to be flexible enough to allow for a reduction or redistribution of local stress peaks, as well as other constraints such as ther­mal dilatation. However, the load-bearing capacity of such beams is gov­erned, besides by the mechanical and geometrical characteristics of the

adhesive joint, by aging, temperature, and creeping. In this contribution, an approach is shown for characterizing the adhesive joints for hybrid steel-glass beams by means of simplified small-scale tests. Standardized specimens (block shear and tension bulk specimens) and small-scale push-out tests are used to derive adequate mechanical values for analytical and numerical calcu­lations, allowing one to draw conclusions regarding the general load carrying behavior of large-scale hybrid steel-glass beams. The results show that full – scale hybrid steel-glass beams, especially those with butt splice bonded and U-bonded geometries, are feasible using new structural adhesives, predomi­nantly elasto-plastics such as polyurethanes or epoxy resins.

KEYWORDS: hybrid steel-glass beams, structural glazing, adhesive tech­nology, small – and large-scale tests, structural behavior

Introduction

In the recent past there have been calls for transparent and filigree structures in the building industry. Therefore, glass plays an increasing role, not only in a classic way as a space enclosing element, but also more and more in terms of offering load carrying functions. As a comparatively new development, hybrid or all-glass structures are the topic of recent research projects [1-4] in which glass forms the main supporting structure, such as glass beams for facade ele­ments and floor girders, glass columns, or bracing facade elements. The brittle glass behavior and a small tolerance toward stress peaks in contrast to the char­acteristics of plastic and ductile steel require some targeted design appropriate for both materials involved. For these contemporary transparent and load bear­ing structures, bonded hybrid steel-glass elements are suitable; in these, each material is used in an optimized way according to its material properties.

Promising examples for such bonded structures are I-beams consisting of steel flanges and glass webs that are connected by linear adhesive bonds [5]. In particular, load-bearing elasto-plastic and stiff adhesives are the focus of research, as these are considered as an enhancement to the general accepted and multiple applied silicones in facade structures.

In such beams, the shear force is carried by the glass web, whereas the bending capacity of the hybrid beam compared to that of the pure glass pane is significantly increased by slender and hardly perceptible steel flanges. The shear forces between steel and glass are sustained only by the adhesive between them. In order to maxi­mize the exploitation of steel and glass, the adhesive on the one hand has to ensure an adequate stiffness and carrying capacity but on the other hand has to be flexible enough to allow for a reduction or redistribution of stress peaks. At the same time, the mechanical characteristics of the jointcould be strongly influenced by aging, tem­perature, and creeping, governing the overall load-bearing capacity of such beams.