Morphology of Serrated Glass Facades. Repetitive and Non-Repetitive Serration. Single and Double Serrated Facades

Repetitive serration changes the perception of the facade primarily by modifying the façade’s vanishing points and through rhythmical iteration. This method is deeply anchored in the perceptive qualities of surfaces themselves. Serrated elements – also called “angled oriels” [18] – are positioned at an angle in relation to the overall façade plane, thereby altering the perception of the building and making it seem more dynamic compared to an identical building with a planar facade. The vanishing points of protruding serrated elements are in different locations than the vanishing points of the overall façade plane. This also affects the perception of the building’s scale. The iteration of façade elements makes the size appear smaller as the distance from the observer increases, which is caused by the foreshortening phenomenon. A planar façade is perceived as a polygon in perspective projection, while a serrated façade is perceived as a rhythmical sequence of polygons that are gradually reduced in size. This creates the visual effect of perspective and such a building is usually perceived as higher/taller than it actually is.

4.1.1.

Repetitive serration

Repetitive (regular) serration remains one of the most popular serration options as this geometry requires only a limited number of unique façade elements to be designed and manufactured. This is particularly true when the angle is equal to 90 degrees, in which case it is possible to use standard detailing as the internal and external corner details are usually developed as standard connections in the façade system. Thus custom solutions are not required.

Standard repetitive serration could be further geometrically developed to provide even more pleasing aesthetical results. One of the available methods is by extending one of the façade flanges beyond the point of the flange’s intersection. This was the case in Oskar von Miller Forum in Munich (arch. Herzog + Partner, 2009), where the light-transmitting flanges were artificially extended to form a rhythmical series of angled glass ribs protruding from the surface of the facade. As a result, a different geometry was formed, similar to popular timber shutter doors, with a series of narrow angled surfaces of glass panes. Fig. 4 shows the different appearance of the serrated façade depending on the viewing angle.

Figure 4.

4.1.2.

Non-repetitive serration

Non-repetitive (irregular) serration of the façade requires a different length, depth or pitch of its individual flanges. Irregular serration strengthens the perceptual phenomena described above. The vanishing points for the protruding elements not only do not match those of the whole façade, but they also differ locally with respect to their length, depth or pitch. While repetitive serration is perceived evenly along the whole façade, non-repetitive serration varies providing an even more irregular visual effect on the façade. Because irregular serration lacks rhythm it influences the perceptual qualities of the whole building not only the façade – e.g. Wakanoura Art Cube in Wakayama (arch. A.A.E., 2003).

In terms of technical feasibility this type of serration in much more complicated than its repetitive counterpart. It requires the development of a larger number of untypical junctions between elements, which translated into increased design effort and manufacturing expertise. Despite their apparent irregularity, many serrated facades are assembled from repetitive elements in an arbitrary sequence. This helps to optimize the costs and design effort. A good example of such a façade is the Osaka Fukoku Seimei Building (arch. Dominique Perrault Architecture, 2010) which features an apparently irregular serrated façade erected from standardized elements (see Fig. 5). Although such optimization offers some savings, expensive bona-fide non-repetitive serrated façades are still built e.g. Kirarito Ginza (arch. Jun Mitsui & Associates, 2014) – see Fig. 6.

Figure 5.

Figure 6.

When façade flanges are not equal in length and angle, their serration direction can be defined/decided. Serration direction is a term borrowed from mechanical science and craftsmanship and it was originally used to determine the relation between the edge of a saw’s tooth and the cutting direction. Coping saws, for instance, are designed to cut “on the pull stroke” so the edges of the teeth have to be directed toward the saw’s handle. In terms of architectural form, “serration direction” defines if the serrated façade will be oriented towards the observer with its shorter or longer flange. This aspect significantly influences the appearance of the façade because the “thinner” flanges/strips create a different rhythm than the “thicker/wider” ones. The proportions of the flange can be intentionally used by the designer to differentiate the possible visual impact of the building, e.g. by visually slimming the building from a given viewing point.

In technical terms, the change of the serration direction does not require any substantial change in the design as the connecting details are simply mirrored. This, however, doubles the number of unique façade elements to be assembled on site.

The serration direction can vary across the façade and different combinations are possible. Since one module of a facade element usually spans a single floor, the most popular choice is to change the direction of façade serration on every adjacent floor. Other solutions are also possible, such as changing the direction every second floor. In contrast to prominent vertical divisions, typical of repetitive serrated facades, buildings with a variable direction of serration on individual floors have horizontal accents. It would be difficult to rationally justify the use of variable serration e.g. by arguing that it is compensating with its orientation towards the prevailing direction of sunlight. Therefore the sculpting of the façade with different direction if serration is generally more arbitrary than in the case of single-direction serration.

One of the most prominent buildings with variable serration direction is the Music Conservatorium of Amsterdam (arch. van Dongen – Koschuch, 2005). Façade serration is characterized by a large difference in length of the flanges. As a result, either only one façade flange is visible – the longer one or two of them, but from a different perspective (with different vanishing points – see Fig. 7). Therefore, the strips/ribbons of the façade seem to “ruffle” in different directions. It should also be noticed that the shorter flange is visually doubled as it is reflected in the longer flange. This optical effect substantially influences the perception of façade’s proportions [14].

Figure 7.

A similar solution, but on a smaller scale, is featured in the building of the Centre for Virtual Engineering (arch. UNStudio, 2012) in Stuttgart. Glazed serrated façade strips are interwoven with horizontal strips of the white flat curved façade. Serration follows an arbitrary curved axial controlling line. The longer flange is glazed, while the shorter one is equipped with an opening flap for natural ventilation of office rooms. The short flanges are blue and yellow, which changes the building’s colour scheme depending on the observer’s standpoint – see Fig. 8.

Figure 8.

Variable serration direction can be used simultaneously with irregular serration. In this rare case, highly complicated façades are produced which require a large number of unique elements that have to be handled individually (see Table 2).

Technically simpler solutions feature only one serrated layer of the façade. In KfW banking group office tower in Frankfurt am Main, (arch. Sauerbruch Hutton, 2010) the external envelope is a serrated, water-tight, single gazed façade with ventilation flaps, while the internal envelope is insulated, double glazed and basically flat. Similarly, in Photonik Zentrum in Berlin Adlershof (arch. Sauerbruch Hutton Architects, 1998) the external part of the envelope is serrated, while the internal one is curved and smooth – see Fig. 9. In the Insurance Company building in Munich (arch. Baumschlager und Eberle, 2002) only the external layer is serrated (simple panes of glass), while the internal façade is a standard flat curtain wall with opening windows – see Fig. 10. A similar solution is executed at Jonsvollskvartalet in Bergen (Arkitektgruppen Cubus AS and Brandsberg-Dahl Arkitekter AS, 2015), but a different serration pattern is used that reminds ripples on the water – see Fig. 11. In general diverse venting strategies are used as the vertical gaps between the panes of glass in the external layer allow for a permanent air flow. This type of façade can be further developed as a dynamic façade, as was done in the State Museum of Egyptian Art in Munich (arch. Peter Böhm Architekten, 2011). In this building, the single-glazed external envelope of the double-skin façade can be opened depending on the weather conditions. The glass panes in the external envelope are point-fixed to steel hinged cantilevers. As the cantilevers rotate vertically the panes open and form a serrated geometry with vertical gaps for air-exchange within the plenum of the façade (see Fig. 12)

Figure 9.

Figure 10.

Figure 11.

Figure 12.

The House of Representatives (Deutscher Bundestag Office) in Berlin (arch. Lieb + Lieb Architekten BDA, 2011) features a much more complicated geometry, where both façade layers are serrated. The external layer is irregularly serrated and is vertically sealed with a ventilation opening at the bottom of the module, while the internal layer is made of IGU and is repetitively serrated – see Fig. 13. “The asymmetry of the oriels (with short and long flanges) adds a dynamic component that is strengthened by the local variation of elements to create «waves». Any onlooker walking along the building is aware of his standpoint and speed. The changing interplay of reflections on the glass surfaces, depending on the direction and view of the movement, makes the façade seem sometimes smoother and sometimes rougher as if brushed against the grain” [6]. The facade also reflects the surroundings in a very specific way. As the angle of each flange is different, a different part of the townscape is visible in the reflection: 19^th-century brickwork is juxtaposed with the reflection of other contemporary buildings. This outstanding example of a different rhythm of serration presents not only excellence in design but also a high level of engineering.

Figure 13.

ESO European Southern Observatory (arch. Auer Weber, 2008) in Garching-Forschungszentrum also features a serrated double façade, but in this case – serration follows a circular axial controlling line. The building was conceived as four large interlocking concave cylindrical single-story volumes. However, in contrast to other abovementioned examples – in ESO’s façade only one flange is a double façade with Venetian blinds, while the one other flange features a grill for external air ventilation – a similar solution to the Centre for Virtual Engineering in Stuttgart – see Fig. 14. An opening flap behind the grill allows users to regulate the air exchange according to their needs.

Figure 14.

«The main advantage of serrated geometries comes from their stiffening effect because the serration “actually makes it (glass) highly load resistant” [15]. When used properly, serration also minimizes deflections. Due to their geometry, folded/serrated facades are much more rigid and resistant to e.g. wind blows.

Moreover, the aerodynamic analysis shows that serrated façades can utilize the prevailing wind direction for the purpose of the building’s natural ventilation. This was the case in KfW Westarkade in Frankfurt am Main (Sauerbruch Hutton, 2010) [16]. “It should also be stated that segmented façades require careful water run-off management, as the multifaceted geometry can allow for easy water ingress” [17] [2].

«The fact that the façade uses more glass to cover the same area could be regarded as a disadvantage, but a detailed analysis shows that serration allows for the optimization of the transparent part of the façade in order to avoid direct solar gain» [2]. If one of the building’s façades is unfavorably oriented (for example, to an exact southern exposure) repetitive serration can be used to change that orientation, at least in the case of one of the façade’s flanges (e.g. transparent ones might face north, non-transparent – south). Because of the Sun path, only repetitive serration can be used to alleviate this problem, while irregular serration – despite having other potential benefits – does not work. Serration can also perform a selfshading function for the façade’s elements thus reducing the direct insolation and heat build-up in the rooms. As it was shown in the analysis of the 25 Ropemaker Place building in London (arch. Arup Associates, 2012) “angled window treatment is stated to reduce solar heat gain by 27% from an equivalent flat façade” [18] because of the proper arrangement of the opaque and light-transmitting parts of the serrated oriels. The application of serrated geometry also allows for deeper daylight penetration into the core of the building, as “(…) facade convolution implemented in the «deeply serrated» design may actually increase facade area, allowing light coming from side to penetrate deeper into the rooms behind the facade and thus increase the light level at the inner part of the interior spaces” [19]. The author of the paper currently researches the issues of the daylight management and the user’s visual comfort in buildings clad with serrated facades. „De Luminae” simulation software is used to produce scientifically verified results.

The sound environment in urban areas is complex as there are many sources of sound that are further influenced by a variety of acoustic propagation effects. One of the most important noise-amplifying factors are the parallel, flat and rigid surfaces of glass façades which reflect sound multiple times. In this context, the sawtooth geometry of façade serration should be recognized as large-scale surface irregularities that make the “sound wave diffusely reflected” [20]. In consequence “saw-toothed, diffusor like, solutions are causing the biggest sound reductions” [21] in urban environments. Of course, glazed serrated façades still function as a rigid sound reflecting surfaces but the sound is diffusely reflected and can be, therefore, easily dispersed.