Hybrid Facade

Conventional post-and-rail designs for facades are generally carried out with the opaque materials steel or aluminum. However, such systems do not fit in well enough with the trend in modern architecture toward more transparency in the facade, even with generous areas of glazing.[23] Therefore, glass these days is used not only as an infill material, with an enclosing function, but increas­ingly as a load bearing and bracing material, e. g., in the form of glass fins. So far, facades with load bearing glass fins and beams have been exclusively cus­tom designs because currently there are no technical building regulations cover­ing the design and construction of such elements. The brittleness of glass frequently leads to uneconomical design for such applications because beside the ultimate and serviceability limit states, the issue of residual load bearing capacity must also be considered. Scientific studies [45-47] show that the resid­ual load bearing capacities of glass beams made from laminated safety glass loaded in bending are not adequately guaranteed, irrespective of the type of glass used.

Hybrid glass beams with a linear adhesive joint between the glass and the steel have therefore been developed in order to pave the way for new facade designs [48]. In this case the joint is a load bearing connection between a mini­mized steel facade section and a vertical glass fin, which together carry the wind loads. Upgrading the brittle glass with ductile (plastically deformable) steel also makes a significant contribution to improving the residual load bearing capacity and the necessary redundancy in the design. In addition, the steel elements ena­ble conventional jointing methods to be used for connecting the hybrid compo­nents to each other or to other parts of the structure.

In the future architects could therefore make use of a modular system that still permits individual designs. Up until now, similar forms of construction have been built using silicone joints. The normally used silicone adhesives need larger contact faces and because of its black color complete transparency is not ensured. (New developments show a structural silicone film adhesive, developed for point-fixed interior and exterior glazing, which combines a high transpar­ency with high tensile and shear strength, strong adhesion performance, thermal stability, and excellent weatherability [49].) Bonding with transparent light-curing acrylates therefore offers new opportunities for architects. In a sim­ilar way to reinforced concrete, we can speak of reinforced glass beams: the transparent glass is reinforced by the ductile steel. And like reinforced concrete, which although a technical breakthrough, in the end led to a whole new archi­tectural vocabulary, the hybrid beams embody great potential for a new style of architecture.

The hybrid glass beams consist of laminated safety glass with additional stainless steel elements that are connected by way of linear adhesive joints with

a transparent adhesive. The linear joints enable a continuous load transfer between the steel and the glass, and therefore avoid local stress concentrations. The essential requirements to be fulfilled by the adhesive are therefore high strength to carry the loads and at the same time adequate elasticity to compen­sate for thermal expansion and contraction. As the mechanical properties of the adhesives depend on the temperature, the given ambient conditions and the magnitude and duration of the load, the research project initially focused on vertical facade systems for interior use. One focal point of the study was the de­velopment of suitable cross-sectional geometries for hybrid glass beams that would guarantee the permanent mechanical function of the adhesive joint between the steel and the glass and permit adequate exposure to the light to ensure proper curing during production [50].

Three different cross-sections, which permit an adapter connection at a later date, were investigated (Fig. 11). In variant S1 a steel plate measuring 20 mm x 2 mm was attached to the edges of the glass. Cross-section S2 has steel side plates measuring 13 mm x 2 mm. In cross-section S2 the central pane of glass in the laminated safety glass is set back by 12 mm so that a T-section (web: 12 mm x 3 mm; flange: 20 mm x 1.5 mm) can be inserted into the ensuing groove. The nominal thickness of the layer of adhesive in all cross-sections is 2 mm. In addition, a laminated safety glass element without any steel (S0) served as a reference.

The test setup was modified for the four-point bending test according to DIN EN 1288-3 [51] in order to carry out the experimental investigations into the load bearing and residual load bearing behavior of the specimens developed, which for facade applications are primarily loaded in bending about the major axis. The hybrid glass beams investigated consisted of laminated safety glass made from three plies of 6 mm annealed glass with ground edges and inter­layers of polyvinyl butyral with a nominal thickness of 0.76 mm. The steel ele­ments were made from stainless steel grade 1.4401. The first load was applied

FIG. 11—Sections through the beams investigated (been symmetrical about centerline).

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until one pane of the laminated safety glass fractured. The load was then relieved and subsequently re-applied until all three panes of the laminated safety glass had developed at least one crack (Fig. 12). Once all three panes of glass had fractured, further load was applied in order to assess the residual load bearing behavior.

The bending stiffness of the specimen was calculated from the force and de­formation depending on the structural system. An intact glass beam with steel elements of course has a higher bending stiffness than a glass beam without steel elements. After the failure of all three panes of glass, the glass beam with­out any steel elements lost almost its entire load-carrying capacity. Compared with the hybrid cross-sections, its residual load bearing behavior was very low. But unlike the glass beam without steel elements, the hybrid beams did not col­lapse and continued to carry the loads, albeit with greater deformations. Cross­section S1 exhibited the highest bending stiffness prior to the first crack, but the highest bending stiffness for the residual load bearing capacity was shown by hybrid cross-section S3 [50]. The results clearly show the improved load bearing and residual load bearing capacities of hybrid glass beams under short-term loading. Further long-term loading tests to determine the creep and relaxation behavior are currently in progress. Investigating the effects of thermal stresses, caused by the different coefficients of thermal expansion of the materials used, was not such a priority here because in the applications considered hitherto, the adhesive joints are on the inside of the facade and the temperature fluctua­tions are minimal.

On the basis of the results obtained from this research project, a sample fa­cade was developed (Fig. 4) for the “glass technology live" exhibition at the glasstec 2010 trade fair in order to illustrate the general design principle and the appearance of hybrid glass beams in use. The current energy requirements with which facades must comply—a decisive criteria when selecting products—can

FIG. 12—Hybrid glass-steel beam in the test rig for investigating the load bearing and residual load bearing behavior.

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be satisfied by using insulating glass units with three panes of glass (triple glaz­ing). Thermal transmittance values as low as 0.78 W/m2K can be achieved. The total energy transmittance of the glazing lies between 24 % and 55 % depending on the coating.

The hybrid facade design consists of four glass posts at a spacing of 1.75 m. The glass posts 3.50 m high and 0.2 m deep consisted of laminated safety glass made from three plies of 8 mm toughened safety glass. Consequently, attractive story-high glazing is possible. Cross-section S1 with the best adhesive joint ge­ometry was selected for this facade. Stainless steel plates were attached to both edges of the glass post via linear adhesive joints using a UV – and light-curing, transparent acrylate adhesive (Fig. 13). The stainless steel plate on the inside edge measured 27 mm x 3 mm, the stainless steel plate (50 mm x 4 mm) on the edge adjacent to the insulating glass was attached as an adapter. The connec­tion between the adapter and the facade section was achieved with countersunk-head screws. This mechanical connection guarantees uncompli­cated replacement of the post should the glass break. The facade section 50 mm wide x 25 mm deep is a conventional steel section for facade systems. The linear support to the glazing was guaranteed on the outside by a very flat glazing bar 50 mm wide x 5 mm thick.

FIG. 13—Section through sample facade with hybrid steel-glass beams.

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Frame Corners

Bonded frame corners are classed as planar adhesive joints (Fig. 5). Special attention must be given to the choice of adhesive for transparent, all-glass frame corners if load bearing but also fully transparent and totally bubble-free joints are to be achieved [52]. Preliminary studies of numerous material specimens form the starting point for determining the material parameters. Small-scale specimens are tested under various boundary conditions (temperature, mois­ture, UV radiation, aging) in order to establish the strengths of a number of suit­able adhesives.

Specimen components (Fig. 14) are loaded in a testing machine in order to study the structural effect of these glued glass frame corners. The results enable digital prototypes to be designed and calibrated for numerical simulation. The verified and validated computer model should then be used to assess the distri­bution of stresses in the adhesive joint and in the glass. Numerical calculations and experimental investigations are carried out in parallel in order to optimize the geometry, load-carrying capacity, long-term reliability, and durability of the glued all-glass frame corners. The findings are incorporated in the design of the adhesive joint and help in the development and testing of an optimum form of connection with the aim of achieving a practical solution suitable for carrying loads permanently. The structural system of the all-glass enclosure was designed with redundancies so that the failure of or damage to individual ele­ments would not lead to the complete collapse of the structure. In addition to the system as planned, the failure of adhesive joints (hinges form at the corners of the frame) and the failure of the roof and rail elements (the fixed-end frame legs and the vertical enclosing elements are responsible for the stability) were analyzed numerically.

Finally, the principles for gaining approval for what was up until now a nonregulated form of construction are prepared. The goal is a fully transparent

FIG. 14—Applying a transparent adhesive to a sample component with the help of a

special injector.

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glass corner—a goal that has inspired architecture since the dawn of the Mod­ern Movement.