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200 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

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“We require from buildings two kinds of goodness: first, the doing their practical duty well: then that they be graceful and pleasing in doing it.”

John Ruskin

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A large part of the latest architectural trends aim at producing geometrically complex glass envelopes, characterized by new shapes and innovative products. Architects have always taken inspiration from nature and one of the main style in contemporary architecture is known as non-standard architecture or blob architecture, in which envelopes have an organic, bulging shape (Fig. 1 and 2) that cannot be described analytically, i.e. free form architecture [5]. “Blobs” have become common since the construction of the Guggenheim Museum in Bilbao by Frank O. Gehry. At the same time, architects have developed a strong interest in transparency and in structural glass [9]. Such characteristics are not only applied to standard Cartesian forms, but also to simple or double curved surfaces.

The feasibility of free-form geometries, especially if we consider glass components, relies mainly on the recent developments both in digital technology and in manufacturing techniques, that allows to produce single or double curved elements.

Complex shapes in glass architecture are an area of great engineering challenges, since several issues are involved, such as shape and architectural components, structure, feasible segmentation into discrete panels, functionality, materials, and cost. The quest for optimise

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201 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

complex glass envelopes becomes even more significant if we consider key-issues such as

sustainable design and “buildability”.

Geometric computing has recently found a new field of applications, namely the various geometric problems which lie at the heart of rationalization and construction-aware design processes of freeform architecture [6]. The current façade design and construction process presents several weaknesses and is often unable to manage with such complexities: thereby risk minimization, cost pressure and complexity of the design process leads to a poor innovation in this sector.

Fig. 1 Ufa Cinema Center - Coop Himmelb(l)au, 1998. Fig. 2 Kunsthaus, Graz - Peter Cook & Colin Fournier, 2003.

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The traditional methods of design are basically changing in recent years, due to several reasons: changes in the cultural context, new frontiers in technology, renewed possibilities and meanings for the architectural project. Such factors produce a new level of freedom for architects and engineers, and allow to experiment ever more complex forms using traditional materials (glass, timber, steel) and building systems in a novel and quite unusual way.

The forms of contemporary architecture appear nowadays more complex, and such complexity cannot be reduced in a simple way, but can be subdivided into different complexities [72]. Such issues consist of conceptual, spatial, technological complexities and require a deep and critical analysis to understand and organize the multiplicity of

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202 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

knowledge, entities, relationships that are expanding continuously. The complexity of the architectural forms today indicates that "[...] something very complex, consisting of many parts interacting with each other in a non-trivial manner, so that all the parties have a degree of autonomy one from the other, but depend on each other” (Sala & Cappellato, 2004). In the present study the term "complex shapes" can be referred to projects with at least three kinds of complexity [72].

A first type of complexity can be recognized in the so-called fluid forms, i.e. surfaces that are not based on Euclidean geometry, but on sections of spherical solids, to compose volumes with different degrees of curvature [89]. In this context, some examples can refer to projects and design concepts proposed by architects such as Gehry Associates, Bernard Tschumi or Metamorph.

Fig. 3 a, b. Norman Fosters Architects, 30 St. Mary Axe Tower, London.

A second declination includes projects with complexity that involves the building system level or a component, both from a technical and an architectural point of view. Such issue can be often solved with the help of specialized knowledge. One example is the 30 St. Mary Axe Tower (informally known as “the Gherkin”), built in London by Norman Foster and Arup engineers, which presents a fully glazed double skin facade, consisting on diamond cells (Fig. 3 a, b). The tower is aerodynamically designed so as to reduce the wind load on the structure; the shape of the envelope, composed of 5,500 glass panels, allows for natural ventilation and contains solar shading devices.

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203 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

Also the Prada building designed by Herzog & De Meuron in Tokyo (Fig. 4 a, b), presents a high complex envelope with a combination of convex, flat and concave glass panels over a rhomboid-shaped grid. The shape of the panels, the gaskets and the additional components are designed to work as a membrane: this contributes to a stiffer hyper-static structural behaviour, due to the specific seismic condition of Japan.

Fig. 4 a, b. Herzog & De Meuron, Prada building, Tokyo.

A third aspect relates to complex shapes made through a relationship between the architectural model and the properties of material behaviour. In these projects, geometric computing and information technologies allow the design concept to become a real product, by methods of mathematical calculation. On this front, we can refer to several work studies, such as Kas Oosterhuis and Zaha Hadid, leading the development of digital technologies to the so-called "F2F" (File To Factory), i.e. from design to production, through the use of a common language [59].

Even if the debate about the language of complex forms is strong, the issue about their actual “buildability” is less clear and discussed, since such skins require a significantly different approach from the traditional constructive logics. Thinking about the buildability of free-form shapes means to associate the freedom of the creative design to a deep attention at the most appropriate technology to produce each element. This is really important because in these projects, more than in simpler ones, the constructability of the forms refers

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to structures, materials, technologies at the same time. The properties of materials strongly influence the different kind of design approach, since each building material usually has limitations in structure and form [59]. Regarding to glass envelopes, two main issues can be identified, related to process and products.

First, an important part of the realization process of free-form skins is their decomposition into smaller parts (panels) such that the entire cost of manufacturing and handling is as small as possible, and such that the numerous side-conditions concerning dimensions, overall smoothness, and so on are satisfied. In addition, any resolution of the given design into panels must not visibly deviate from the original architect’s design. The panelling problem requires a deep process of optimization, involving several aspect of design. The complex geometry of some contemporary envelopes requires to adopt design strategies, best practices and novel technologies to optimise the project since the early stages of design (Fig. 5 a, b). Optimisation can be achieved from several points of view, such as materials, elements and technologies: such approaches shall be deeper outlined in the following paragraph.

Fig. 5 a, b. Strasbourg Train Station, NCF DAAB RFR. This surfaces is not a freeform surface in the strict sense since it is modeled from basic geometric primitives like tori and spheres.

Secondly, glass sheets are produced on the float line and do not naturally adapt themselves to curved and free-form shapes. In these architectural applications, more flexible materials such as ETFE seem to be more suitable. In spite of these disadvantages there have been several developments which have made feasible the use of glass on free-form envelopes, even if these application are not cost-effective [89]. Curved geometries stand out two major issues for glass: first of all, the curvature of the glass may be discretised into a mesh of planar elements, often triangular shaped. A triangular mesh is not always

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205 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

acceptable, for aesthetics and manufacturing limits: in such cases it is necessary to adopt the more expensive option of producing curved sheets of glass. The other problem is that the panel size required to build up a curved surface may vary significantly, increasing manufacturing difficulties and costs.

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The realization of free-form and complex surfaces, consisting of hundreds of different panels, produces new challenges to all stages of the design and construction process, from the concept step to the detailing and production [5]. The optimisation and rationalisation can be applied in each phase of design, related to the kind of goals to be achieved. The level of approximation can vary also according to the different kind of building systems and the properties of components as gaskets, fixing and cladding systems etc. (Fig. 6).

Fig. 6 Relations among architecture, research, education

One of the first aspect can affect the optimisation of the geometric properties of the “skin”: the aim is often to reduce complexity in design concept and then produce a similar idea with flat or single-curved surfaces; thereby ordinary construction elements can be used,

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with lower costs and more efficient buildability. But sometimes the study of geometric model involves the design and the architectural language that characterizes the envelope; nowadays recent advances in architectural geometry provide a new design technical support for architectural free-form structures.

The research field architectural geometry has already successfully proposed new ways to mathematically describe and manage geometric limitations in non-standard architecture, in particular for the important problem of panelling complex surfaces (Fig. 7 and 8). It must be also underlined that the construction of the model is strongly affected by the production method that you want to adopt and materials that you will use. Such choices are determined by the morphological characteristics of the project, the scale and size of detail, as well as by the characteristics construction elements. On the basis of these constraints, architectural geometry tools can provide new representations including mathematical formulations of the limitations related to material, construction, and even cost. Then designers can explore the remaining degrees of freedom and adapt the architectural skin in an efficient way.

Fig. 7 Discretization into planar segments - Triangulation.

BMW Welt, München Coop Himmelb(l)au, 2007. Fig. 8 Discretization into curved segments - Free-form. Lentille du metro de Saint Lazare, Paris, RFR, 2003.

BIM technologies are becoming a more and more important tool to support design and optimisation process. NURBS surfaces are nowadays implemented in most of CAD systems and are commonly used for architectural free-form design. Such parametric surfaces, however, were originally invented for different industries, like the car and the design industry, and are poorly suited to address the challenges specific to bringing an architectural freeform design to production. The panelling optimisation is usually decoupled from design and it is not possible to obtain direct feedback about the panelling during the early design of

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a free-form surface. The new techniques and representations from architectural geometry go far beyond what is currently available in commercial CAD systems. As an example, the recently introduced Evolute Tools for Rhino (data from www.evolutetools.com) is a step towards more interaction, at least for panelling with planar and single curved panels. The aim is to include in a single model different aspects of geometry, such as the structure supporting the glass panels, that is often realized with single curved steel beams.

The computational framework can also help to optimise the glass skin in terms of

calculations. This second aspect of optimisation requires structural, thermal and acoustic

information that shall be included in the model. In traditional project drawings, those data are separated and therefore it is very difficult to find out the weaknesses of a shape or a component. The envelope, since the early stages of design, requires approximate verification of performances that allows designers to make further decisions [72]. This is becoming more and more necessary in each phase of design, since different professional roles are involved in the organization of the construction process, particularly with regard to the contribution of specialists in matters of structural, thermal and environmental issues.

A third significant aspect of optimisation strategies involves the industrial production

and the manufacturing processes of complex forms. In many industry sectors, such as the

design industry, designers collaborate with the manufacturer since the early phases, in order to check the consistency of their processing and to assess the available technical solutions; in the construction industry, such synergistic approach is quite uncommon, and the verification often occurs in an executive step, just before the installation. Today, however, the relationship with the manufacturing sector is becoming more and more required, especially to deal with complex shapes. The renewed potential of the industry, which provides not only products but a deep know-how and a flexible production systems through tailor-made systems, and the practice of the mass-customization, seem to provide technical solutions suited to envelopes that shall have, at the same time, complex shapes and high performances. The relationship with the industry mainly is due to the introduction of computing tools, which allow a more effective development of the project contents and a direct intervention in the production chain.

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Novel technologies are often developed with the help of consultants, who support the designer in the resolution of project specifications. Thus, the different actors involved - designers, specialists, specialized industries for a system or a component - spread the articulation of the design and construction process. Thereby, the risk of fragmentation must be avoided and a need for clarity is required, not only in the design stage but also and especially in the construction phase. Taking early the aspects of the construction is a critical issue in complex envelopes for an effective verification of the correctness of the interpretation contents of the project, in order to control the technical quality and to test the innovative technologies. The parametric detail is the core of a building process that takes the architect’s data and produces it directly, a process that can be called file to factory. In such way, advanced geometric tools can enable the cost-efficient realization of complex architectural designs with glass.

As examples of optimisation approaches, it is interesting to shortly analyse two different projects, both concerning the introduction of complex glass envelopes in historic context, using digital technologies to improve the development of the design and construction process.

An innovative computational method has been applied for the geometric optimization of the Pavilion that will be built on the first platform of the Eiffel Tower in Paris, designed by the architects Moatti et Rivière (Fig. 9 a). The project was then carried out by RFR and the geometric optimisation was developed in collaboration with Evolute, a geometry consulting company that has implemented a general framework to make all the recent results from research available to the industry. In the project for the front façade of the Eiffel Pavilion, such tool was used as a support during the design phase, in order to choose the most effective panelisation and the most suitable type of glass for the transparent free-form surface, using flat, single curved, and double curved panels. Also the steel structure, supporting the façade, was efficiently optimised.

To realize the design, the reference surface, given by the architect, is first decomposed into glasses of manageable size, around 70 panels, and then the curve network is composed. The curve network defines how the panel boundaries will be cut. The panelling problem is to

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209 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

approximate the reference surface and reproduce the curve network with a collection of panels (Fig. 9 b and c). The quality of a panelisation is mainly determined by the divergences and the angles between neighbouring panels. Minimizing cost means preferring simple panel shapes like planes or cylinders as well as using the same shapes in different parts of the envelope. Various combinations of planar (triangular and quadrilateral pattern) and curved (single and double) panels have been carried out, depending on the desired budget and “smoothness” of the surface. Thus, the global fitting, the reflection and refraction pattern for the surface and the cost of a panelling solution can be assessed since the design phase, in order to solve most of the detailing problem at an early phase and to have a reliable overview on costs (data from www.evolutetools.com).

Fig. 9 a, b, c. Pavilion of the Eiffel Tower (courtesy of Evolute).

Another reference project is the development of glass and steel envelope for Les Halles in Paris. The architectural concept was carried out by Patrick Berger and Jacques Anziutti, as the engineering development was technically analysed by Ingérop. The complexity of the glass roofing and the multiplicity of roles and professionals involved, lead to the need of a unique model in which all the information can be available. The French company DECODE (DEvelopment COmplex DEsign) was commissioned to collect all the 2D data, provided by different consultants, to select them and to finally produce a single three-dimensional digital model; through such computational tool, an assessment on the consistency to the different technical specifications can be made, once placed in relation to each other.

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The construction of a single model (Fig. 10), with a degree of detail that varies according to the needs, is a first step to verify all the problems or the anomalies within the project, and report them to the specialists who provide the changes for the resolution of inconsistencies. The complexity of Les Halles project has its worst issues in detailing, when the constraints of materials and building systems have to cope the characteristics of a complex morphology. Therefore the design process develops a real and constant evolution, as all the data are updated and adapted by a multidisciplinary point of view in the synthesis model.

Secondly, the management and the extraction of the data from the 3D model consist of a basis for the development of the tender documentation (Fig. 11). The model can also produce short movies, over traditional drawings, thereby the different firms can explore the updated model and make their proposal, though a deeper comprehension of the project. Once the tender is carried out, the general contractor is provided with all the basic document to develop the construction (Fig. 12). Another significant advantage of this procedure is the improved communication and exchange of information among clients, designers, specialists, manufacturers and contractors.

Fig. 10 BIM model for the canopy of Les Halles (courtesy of DECODE).

The architectural demand for free-form glass surfaces will increase in the future decades. The main issue for producing curved glass components is to understand the relationship between the design and the buildability of such complex envelopes. In addition, the combinations of the manufacturing and installation processes and the consequent constrains must be deeply understood, related not only to glass panes but also to the interface between glass and other building elements. Besides, new challenges involve the glass design and glass industry, from manufacturing to the production control and

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buildability, since all glasses are sophisticated as products. For instance, cold bent glass is a very recent technology, therefore its long term performance shall be assessed carefully, especially regarding to the behaviour of interlayer under long term longitudinal shear strain.

Fig. 11 Canopy rendering (courtesy of DECODE). Fig. 12 Canopy detail on building site (courtesy of DECODE).

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In recent decades there has been an increasing use of glass as a structural material for façades and generally for envelopes. The trend towards dematerialization and transparency, essential aspects of contemporary architecture, has meant that glass took on also the function of load transmission. Over thirty years, the use of glass has spread widely from small infill elements in curtain walls to large panels that can play a primary role in structural behaviour. The supporting elements of the façade have been replaced by thin steel cables and glass fins. More recently, the improvement of bonding techniques has made possible the increase of the percentage of glass in façade [40].

Such development was made possible through a joint effort between industry and research. Industry has provided more reliable materials and novel technologies and industrial improvements, concerning better performing and larger glass panes, have allowed the use of glass as a load-bearing material in facades, roofs, canopies, staircases, beams and floors [37]. Research field has deepened the knowledge of the material and this has allowed the designers to create new challenges and applications.

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The transparency of glass associated to its good structural performance has been exploited also in interesting renovation and restoration projects, or for the expansion of existing facilities since the use of glass can be useful to mitigate the impact of such intervention.

However, until now glass as a material has not been sufficiently studied, and many of the standards and regulations remain below the level of technique or insufficient. For this reason, innovative uses mainly require very expensive test to confirm previous calculations. In Italy, the National Research Council has recently developed a design guideline for the use of glass as a structural material [24]. The standard aims at collecting the available knowledge on the structural glass, keeping the vision proposed by the Euro Codes. However it will have to be tested by the design practice in real-world cases.

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Concerning glass façades structural behaviour, there are different types of classification, based on the role of the glass element, that can represent the external “coating” or can also have a supporting function. Each element of glass inserted in a building system has a structural role, since, in addition to support the dead loads, it must also withstand live loads, such as wind pressure, snow load, seismic loads, thermal loads etc. [88]. Secondary elements in a frame structure are usually neglected in the static assessment, but this simplification cannot be easily made for glass elements, due to their inner fragility.

Thereby, the term “structural glass” can be potentially misused. Strictly, such term was coined to describe those particular façade systems that were designed to have a uniform and continuous feature with frameless glass panels visible from the outside. In practice, these panels are not placed within a framework that supports them on four sides, as in the traditional façades, but they are glued over a metal structure [84]. In this way the structure is not visible from the outside, and the external surface is perceived as a single glass surface, interrupted only by the silicone joints between a panel and the other. The adjective “structural” derives from the fact that the silicone has the function of retaining the glass panels and not only of a sealant.

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Today glass is also used for structural elements, such as beams, columns, membranes [42], generally made with traditional building materials such as steel. In addition, glass has many advantages, but generally it is not considered a "good" structural material, as it presents several negative characteristics.

GLASS AS A STRUCTURAL MATERIAL:

ADVANTAGES DRAWBACKS

TRANSPARENCY

CHEMICAL STABILITY -DURABILITY

THERMAL STABILITY

MECHANICAL STABILITY (NO VISCOSITY)

MECHANICAL RESISTANCE

BRITTLENESS

LOW TENSILE STRENGTH (VARIABILITY

LOW FIRE RESISTANCE

From a structural point of view it is mainly important to understand the flow of loads and stresses in each structural glass member [78]. Designing with glass implies a different approach to the post-breakage behaviour, since glass can break for various unexpected reasons: safety and robustness considerations are essential in glass design, since glass is a fragile material. The inherently fragility of glass means that it is unable to optimize local and global behaviour, using plasticity resources, as other materials, like steel, concrete, wood: such types of materials can thus to redistribute tension peaks to widespread areas, reducing them to acceptable values; then, substantial increases of deformation level or presence of lesions can reveal that the material is reaching the limit of its resistance, before it collapses. Glass, however, presents a completely different behaviour.

Annealed glass (see Chapter 4) is characterized by a relatively low tensile strength and breaks in large fragments; heat or chemical treatments like tempering increase the tensile strength of glass and modifies the fracture patterns to small pieces; fully-tempered glass shatters into many small rounded pieces but presents a total absence of residual load-bearing capacity. Therefore, the use of heat-strengthened glass has increased, since it has a considerable tensile strength, but at the same time, in case of breakage, it breaks into sufficiently large fragments and still guarantees a residual load-bearing capacity (see Chapter 4).

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Fig. 13 From post-breakage to "Damage tolerance", Structural Lab, Gent University.

Fig. 14 Adhesion tests, Structural Lab, Gent University.

In order to improve the post-breakage resistance, the use of laminated glass panes can produce a “safety” product (Fig. 13, 14): laminated glass allows to assemble panes of various thickness (usually from 6 to 19 mm) and of various types (annealed, fully tempered, heat-strengthened glass) by combining their advantages in a range of different solutions (Chapter 4). The overall tensile resistance is thus greater than the sum of the resistances of the individual glass lites, due to the adhesion between the layers (Fig. 15); then, when laminated glass gets broken, the interlayer tends to hold the glass shards in place and reduces the injury.

Fig. 15 Tensile behaviour.

Recently, the glazing industry developed a wide range of novel products and materials, in order to improve the quality of glass itself and to overcome its post-fracture limitations. The principal innovations in this field can be summarized as follows:

 The development of heat strengthened glass and chemically strengthened glass, also combining these two different processes.

 The stronger and stiffer interlayers which are able to enhance the glass post-breakage resistance.

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 The improved knowledge of interlayer behaviour under short and long term conditions, blast and fire loading.

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The use of glass in buildings stands out several challenges from structural design issues. In general, a glass product cannot be addressed as “safety glass” itself: the use of laminated glass does not guarantee a proper post-breakage performance of the entire glass structure (see Chapter 4). It is essential to define the boundary conditions and foresee the alternative load paths: the glass façade shall be designed as a system [78]. This concern implies a innovative approach towards glass structures. When dealing with traditional materials, design methods often prefer to simplify calculations and not to go into detail of the local structural behaviour, relying on capability of such materials to absorb and redistribute unforeseen actions. Glass lacks such resources, therefore it is extremely important to adopt some design criteria which tend to give globally ductility to a glass structure, due to the lack of ductility of the material itself. There are two key-criteria for the glass design approach: hierarchy and redundancy [42].

Hierarchy assigns a precise role to each structural component (primary element,

secondary, etc.); secondary elements may, in exceptional circumstances, while collapsing the primary elements must be protected or multiplied.

On a system level, then, it is essential that redundancy doubles the key structural elements and creates alternative load paths shall to ensure that the failure of one glass element does not cause disproportionate collapse of the remaining parts of the structure. As a result, a glass structure may be deemed safe if it ensures adequate residual post-strength and stability for normal actions and in addition it provides safe failure or adequate residual post-fracture capacity thereby minimising the risk of injury [66].

The final aim is to meet safety requirements, despite the possible local failures or breakages. Design criteria such as redundancy and hierarchy provide a fail-safe design (Fig. 16 and 17) for glass structures: the structural behaviour of glass is such that the breakage risk can only be determined using statistical theory. The strength of any given pieces of glass

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depends on the glass quality, i.e. cracks, defects, surface and edge condition, microscopic flaws, etc., as well as the thickness, area, size, length to width ratio, fabrication, support conditions, and type and rate of applied loading. Statistic considerations become then essential and can indicate how to foresee several breakage scenarios and how to identify alternative load paths.

Fig. 16 RFR, Serres, La Villette, Paris Fig. 17 Alternative load path, sketch by Peter Rice

The principal innovation in glass design are:

 The development of edge retention and enhanced connections that provide a fail-safe system.

 The adoption of design approaches that ensure that there are alternative load paths in the glass structure.

The correct evaluation of all loads acting on glass is extremely important to proper glass selection. In order to establish a meaningful design load it is essential to not only evaluate the various types of loads acting on glass but also how to combine these loads (dead loads, live loads, snow loads, wind loads, seismic loads). It is essential to assess the behaviour of glass structures under extreme loads and in particular its post-fracture performance [66].

Glass is also subject to static fatigue, therefore its ability to resist an applied load decreases with the duration of the load [9]. Most manufacturers and industry literature is based on a one-minute load duration. Strength values obtained from industry literature

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must be therefore multiplied by a load duration factor to obtain the appropriate glass strength for long-term loads, such as snow loads and dead loads.

Glass deflection depends on the applied load, the glass thickness, the aspect ratio

(length/width) and the glass support conditions (two, three or four-sided). For two-sided support glass conditions, the length of the unsupported span is the critical glass dimension [9]. In general, insulating glass units should not be used in two-sided support conditions because the large deflections occurring under relatively low loads will place excessive stresses on the edge seal, which could then lead to early seal failure.

Thermally induced edge stresses are usually the outcome of the warmer centre portion

of a glass pane exposed to solar energy expanding more than the cooler edges that are covered and cooled by the glazing system or building elements. Several factors influence the amount of thermal stress in glass, including glass edge quality, size, thickness and shape, building orientation, shading patterns, external structures and framing system.

Tempering or heat-strengthening increases glass edge strength and thus decrease the chance of thermal breakage. The effect of thermal stress must be considered early in the design stage of a façade project to minimize glass breakage problems. Unusual breakage can occur in very small insulated glass units due to loads induced by temperature extremes owing to large IGU air-space volume to glass size ratio and high relative stiffness of small glass lites. To assess the thermal risk of an existing façade, the following information are fundamental:

 Location of the building  Orientation

 Type of glass being used including details of the insulating glass units  Size of building overhang if present

 Size of mullion and transom caps if present  Details of any internal or external blind/louvres

 Details of any back up i.e. where a panel makes up a level to a floor or ceiling behind the glass allowing hot air to be trapped and reflecting back at the glass.

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 The window size and if it opens. I.e. changes the angle to the sun.  Details of internal heating systems.

 Any other details like other buildings or trees casting a shadow onto the glass.

During the year the sun changes its path through the sky. If the glass in a building is subject to excess thermal stress it would be expected to have problems in its first year of use [9]. The most challenging periods are in spring and autumn when the sun angles are low and the nights are cold. In mid-summer, the edges of the glass will be warmer anyway and the sun's path is more directly overhead putting less direct heat into the glass.

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The static design of a glass façade, both in new build or in renovations projects, must follow a sequence of independent stages:

1. Acquisition of project data: geometry, location, characteristics of the materials; 2. Load analysis and definition of load combinations;

3. Definition of the structural scheme; 4. Structural analysis;

5. Analysis and verification of anchoring systems;

6. Assessments of local static behaviour and of alternative load paths.

The wind acts on a façade creating a variety of different forces through direct action, down draughts, vortices and funnelling. When the wind meets a building in its path, it either slows down at the face of the building or deflects and accelerates around the sides: at the face the wind slows down and kinetic energy is converted to pressure and where the wind is deflected over and around the building, the pressure is reduced and can become negative behind the building, eddies are created causing a drop in pressure.The design wind load is derived from site wind speed. This may be taken from a map of the specific country (Italy, UK etc.) drawn by the Meteorological Office showing hourly mean wind speed at 10m above sea level. The site wind speed is then corrected for altitude, local topography, seasonal

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exposure, direction, nature of surrounding terrain, height of the building, building shape and dimensions.

The specific value of the pressure on each surface depends upon the angle at which the wind approaches, so the orientation needs to be known (Fig. 18). The shape of the building, or slenderness ratio, the position of the installation and details of any features affecting the distribution of wind or snow also need to be considered. Neighbouring buildings can affect wind pressure, so their proximity and height need careful consideration.

Pressure is exerted by the wind internally, depending on the position and size of openings connecting to the outside of the building, and the porosity of the envelope. Positive internal pressures will add to external suction forces acting on the cladding, as well as having an effect on internal elements.

Façade elements are not designed to take the building’s structural loads. They must support however, their own weight (dead loads). In glass façades, the requirements of stiffness rather than strength govern the design of curt ain wall systems. Vertical loads are relatively light if compared to lateral wind forces and design considerations for suction forces due to difference in building and external pressures cause panels to blow out rather than blow in, especially at building corners.

Then, there is always relative movement between the curtain wall and the building frame to which it is attached, due to temperature changes, wind action, gravity forces and displacements in the building frame. Larger the units or greater the length of the metal frame larger is the movement that needs to be accommodated. Such movements are accommodated for at the joints, vertically and horizontally, with the help of “slip joints”.

The larger the height and width of a bay the greater the forces of stress developed at the joints due to movements induced by wind and structure of the wall relative to the

Fig. 18 Wind pressures and depressions (Courtesy of

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220 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

building. The width of the bay is normally kept between 1m to 1.8m with a height ranging from 3m to 4m.

Glass installations, such as bolted systems and curtain walling, support dead loads and resist wind loads, but these loads must be transferred back to the building’s structural frame via brackets and anchor points. The system is normally located in front of the structural frame, generally concrete floor slabs or structural steel work. The set-off distance is the depth of the bolt and bracket in structural glass assemblies or the mullion depth in curtain walling (Fig. 19). The brackets and anchor points must themselves accommodate:

 building movement  thermal movement  construction tolerances  self-weight of the system

 applied loads such as wind action.

Anchors that tie the installation back to the building structure can be either cast in place, i.e., positioned in the formwork before the concrete is cast, or post installed. The availability and nature of the structural support needs to be known. In the UK, the support and fixing of external façade cladding is covered by Building Regulations Approved Document A Section 2. It provides guidance regarding the support and fixing of external façade cladding which, by reason of weight or height, would present a hazard if it became detached from the building. Façade cladding will meet the requirement if it is capable of safely sustaining and transmitting to the supporting structure of the building, all dead, imposed and wind loads.

The service life of even the most durable glazing may be shorter than that of durable adjacent wall claddings such as stone or brick masonry. Therefore, the design of the curtain wall and perimeter construction should permit curtain wall removal and replacement without removing adjacent wall components that will remain.

Fig. 19 Set-off distance (Courtesy of Pilkigton).

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221 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

The service life expectancy of components that are mated with the curtain wall into an assembly should match the service life expectancy of the curtain wall itself. Require durable flashing materials, non-corroding attachment hardware and fasteners, and moisture resistant materials in regions subject to wetting.

I

SSUE

C.

E

NERGY

-P

ERFORMANCE

D

ESIGN

7.8 T

OWARDS

S

USTAINABLE

G

LASS

E

NVELOPES

Sustainability is increasingly on the global agenda [79]; there is also a growing awareness that buildings are responsible for a large proportion of CO2 emissions,

approximately for 40% of the total European energy consumption and a third of CO2

emissions (Energy Efficient Building European Initiative - E2B EI). In the construction industry, standards are imposed by legislation such as the EU Energy Performance of Buildings Directive (2002/91/EC). The main key points of the Directive are:

 a common methodology for calculating the integrated energy performance of buildings;  minimum standards on the energy performance of new buildings and existing buildings

that are subject to major renovation;

 systems for the energy certification of new and existing buildings and, for public buildings, prominent display of this certification and other relevant information. Certificates must be less than five years old;

 regular inspection of boilers and central air-conditioning systems in buildings and in addition an assessment of heating installations in which the boilers are more than 15 years old.

Then Directive 2009/28/EC promotes energy efficiency, energy consumption from renewable sources, the improvement of energy supply, and establishes a common framework in order to limit greenhouse gas emissions and to promote cleaner transport. To this end, national action plans are defined, as are procedures for the use of biofuels. Each

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222 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

Member State has a target calculated according to the share of energy from renewable sources in its gross final consumption for 2020.

This target is in line with the overall “20-20-20” goal for the Community. On a wider view, the concept of a Net Zero Energy Building (NZEB) has rapidly spreading; the U.S. Department of Energy (DOE) and the National Renewable Energy Laboratory (NREL) have presented several definitions for "net zero energy", and they encourage building designers, owners, and operators to select the metric that best fits their project.

Using advanced glazing solutions can significantly reduce the need for heating and cooling in buildings, thereby reducing energy consumption and associated CO2 emissions.

Independent studies [93] show that savings of more than 100 million tons of CO2 could be

achieved annually if all Europe's buildings were fitted with advanced energy saving glass. On the basis of these findings, the EU could achieve around one third of the energy saving targets for buildings identified in the 2006 "Action Plan for Energy Efficiency" simply by promoting the use of energy saving glazing. A recent study on glazing type distribution in the

Fig. 20 Glazing type distribution in Europe.

EU building stock [91] reveals that (Fig.20):

 44% of the windows in Europe's buildings are still single glazed.

 Less than 15% of Europe's windows contain energy-saving glass whereas these solutions have been available on the market for over 20 years.

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223 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

 Early uncoated double glazing is still used in a vast number of buildings. Although their energy performance is limited compared to solutions available nowadays, they are too often regarded as efficient by poorly informed property owners.

The thermal control of a glass envelope is a crucial point for the assessment of the envelope as a whole [33]. Basically, two different kinds of problems can be addressed:

1. Expansion and contraction of materials (glazing, steel or aluminium components, timber elements, gaskets etc.).

2. Passage of heat.

The first issue is strongly related to structural design. Failure can be caused by a temperature difference between the centre and the an edge of a glass panes. Thermal deformation results in the expansion of the centre of the pane which provokes tensile stresses within the edges and causes failure. Such deformations can be reduced with a the introduction of types of joint seals and the wall anchorage.

An increase in glazed areas potentially leads to a greater passage of heat and a reduction in the thermal performance of the building’s façade. The reduction in overall U-value of the glazed wall, consisting of the glass panes and the frame or the supporting structure, is recommended. This can be accomplished by minimizing the portion of metal framing members exposed to the outside, thermal breaks, using double rather than single glazing and providing good insulation at the large opaque areas of the wall [21].

The use of high-performance glazing types (see Chapter 4) and products can offer additional improvements in the window's insulating properties. These technologies offer additional benefits in terms of energy efficiency (Fig. 21).

Studies [93] also show that significant additional energy savings can be achieved in the central parts of Europe with cold winters such as the UK, Poland, France, Benelux, etc. thanks to the installation of triple glazing on the most exposed orientations of buildings, in combination with Low-E double glazing on the South façades.

Even in the warmest regions of Europe, heating in winter is often necessary. It is true however that preventing over-heating in summer is the main challenge for most types of

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224 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

building. Thanks to solar-control coatings, glazing can play a role in preventing over-heating in buildings and therefore reduce the use of air-conditioning [92], which is a major consumer of energy in Southern Europe. In addition, solar-control glazing most often comes in double glazed units, which provides insulation in winter. Solar-control and Low-Emissivity properties can be combined into a glazing unit thus further contributing to insulation from both cold and heat depending on the season.

Fig. 21 U-value (W/m2K). Comparison among glazing insulation performances.

This most often provides the best energy saving balance since free solar heat gains are only limited during a few winter months and are compensated by reduced loads on air-conditioning units for the most part of the year. Upgrading to energy-efficient glazing in Southern Europe is thus an efficient way of increasing indoor comfort while making substantial energy savings [21].

Glazing also allows daylight to enter into buildings. Daylighting is needed for lighting up interior spaces within the building but an uncontrolled daylight penetration causes discomfort in glare and brightness. UV rays also provoke a deteriorating effect on organic materials such as colour pigments, plastics and sealants. Such effects are combated with the use of shading device either inside or outside the vision panel or the use of glare, reducing or reflective types of glass which provide relief without restricting vision.

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225 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

7.9 E

NERGY

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EFFICIENCY

S

TRATEGIES FOR

G

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AÇADES

A concept of “green design” is to facilitate the sustainable use of resources, such as energy, water and other materials, for the entire life cycle of the building including its construction [33]. From a building physics point of view, façades represent the control system that delimits the thermodynamic system called "building", since the flow of energy and mass exchanged with the environment pass through its surface. As Herzog states [84], the envelope has the task of creating a balance between internal and external environment, in terms of distribution of temperature, air flow, purity and air humidity, as well as the kind of radiation.

Many studies have even shown that health, comfort and productivity are substantially increased due to well-ventilated indoor environments and access to natural light [47]. On one hand, glazed areas ensure direct contact with the outside, allow the entrance of solar radiation while, on the other hand, they must form a thermal and acoustic barrier, provide robustness and safety requirements, ease of maintenance and durability.

Furthermore, glass is a wholly recyclable material and, thanks to new products and technologies, it supplies unrestricted occasions for designers for the innovative applications in buildings [47]. Glass plays a noteworthy role in accomplishing indoor environmental quality and energy efficiency and in so doing fulfils numerous criteria for green buildings (Fig. 22). The connection between internal and external environment is necessary to ensure the correct air changes and to improve the occupants wellbeing. Then, the presence of natural light and the vision of the outside, are, however, important to improve visual comfort and productivity, as well as to promote energy savings for lighting [33].

However, windows and glass façades play a crucial role for the microclimatic comfort and they are generally considered a weak point in the whole building energy performance, since they also represent a major source of unwanted heat loss, discomfort, and condensation problems. In 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion (WBDG, www.wbdg.org/resources/windows). The direct glare or unwanted reflection

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226 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

must be avoided through shading devices that can control the penetration of solar radiation during the day.

The advantages and the weaknesses of glass envelopes from a building physics viewpoint can be summarized as follows:

ISSUE ADVANTAGE WEAKNESS

1 Daylight and natural interior lighting – Energy savings and improvement of and comfort.

Possibility of glare and strong differences in luminance.

2 Visual connection to the outside. Unwanted and excessive heat losses in winter.

Low surface temperature of inner glazing (discomfort).

3 Ventilation of internal spaces -

Replacement of O2 and improvements of IAQ requirements.

Energy dispersions for excessive and uncontrolled ventilation. Ingress of unwanted noise.

4 Penetration of the solar radiation. Unwanted and excessive heat gains in summer.

High surface temperature of inner glazing (discomfort).

In recent years, windows and glazing systems have undergone a technological revolution. High-performance, energy-efficient systems are now available and can dramatically cut energy consumption and pollution sources: they have lower heat loss, less air leakage, and warmer window surfaces that improve comfort and minimize condensation. These high-performance windows feature double or triple glazing, specialized transparent coatings, insulating gas sandwiched between panes, and improved frames. All of these features reduce heat transfer, thereby cutting the energy lost through windows.

A proper energy-saving strategy must first of all do not waste energy: it implies to reduce winter heat losses and to decrease summer thermal loads. The approach is to first maximise energy-efficient design and operation (minimising demand) and subsequently introduce appropriate renewable energy systems. Low cost photovoltaic technology is an area where the building envelope offers true integration potential and consequently

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227 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

acceptable payback periods. The ability for professionals to introduce such systems appropriately is increasingly important.

Fig. 22 Conceptual scheme of "Green Building"

The feasibility of concepts needs to be tested during the early stages of design in order to allow that the project will meet the required standards, be technically feasible, and the budget is likely to be met. Again, integration and multidisciplinary working is a key-factor for the success and the careful definition of interfaces will prove crucial in order to avoid costly problems downstream, during commissioning and operation.

The development of advanced technology aims at producing adaptive façades, that respond to changes in environmental condition and user behaviour. Clearly such systems represent technological progress and innovation in the sector and, when dealt with properly, technology can facilitate high performance and low impact.

There is however also a risk that certain systems are incorporated because they are seen as advanced technology and have come to symbolise “sustainable design”. Green wash is the term used to describe design which is over-sold and under-delivered as sustainable through high tech gimmicks as opposed to pragmatic, evidence-based solutions [55].

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228 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

7.10 G

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NVELOPES AND

E

NERGY

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ATING

S

YSTEMS

The increased attitude towards green building concepts and practices is confirmed and encouraged by several green building rating systems, such as BREEAM (United Kingdom), LEED (United States, and other countries worldwide), DGNB (Germany), ITACA (Italy), and CASBEE (Japan). They can be applied to most types of projects, including residential, industrial, and commercial buildings in both new construction and existing buildings. Credits are awarded for optional building features that support green design in categories such as location and maintenance of building site, conservation of water, energy, and building materials, and occupant comfort and health [33].

All these criteria demonstrate that sustainable buildings are then strong influenced by climate conditions, since local climate does not suit every building concepts, such as the use of natural ventilation, day-light, solar energy etc. etc. Moreover, the planning process and the related concept development must be improved through an holistic design approach, able to integrate building structure, interior design (fitting out), mechanical services and building envelope. Energy-efficient and sustainable building envelopes play a fundamental role in the whole performance of a building and require a deep integration of the main equipment devices (heating, cooling, ventilation, lighting systems) to minimize energy consumption.

The requirements set out by directives, laws, rating systems, often conflict with architectural trends of increasing curtain walling and transparency of the building envelope; moreover, improvements of building envelope, especially of its transparent elements, can be crucial for an effective reuse and renovation of existing buildings, that are often characterized by underperforming envelopes.

Therefore building physics requirements of improving the energy efficiency of glass façades are mainly:

 Reduce the amount of unwanted heat gains/losses.

 Improve occupants’ comfort and well-being (i.e. enhancing acoustic insulation, reducing glare).

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229 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

Energy efficiency measures include design strategies and technical devices that can reduce the demand loads, such as high-performance systems, careful selection of windows and glazing, sun control and shading devices, daylighting, passive solar techniques, natural, mechanical or hybrid ventilation etc. etc. Using advanced energy analysis tools, design teams can assess and optimize efficient designs and solutions. As a result of advances in construction technologies, renewable energy systems and academic research, creating energy-efficient transparent envelopes is becoming more and more feasible, maximizing both energy efficiency and internal comfort.

Many green building certifications include specifications on “daylight factor” and “direct sunlight availability”. Some countries, such as France, even go further in imposing minimum surfaces of glazed areas within new constructions: since 2005, the French Thermal Regulation imposes minimum glazed areas for new constructions in proportion to the overall surface. In 2011, this minimum surface was in fact increased by 30% to reach a minimum of one sixth of the overall built surface. Thanks to these unique properties of modern glazing, architects now have a new-found freedom to incorporate large areas of glazing as they wish into their designs, knowing there will not be any negative impact on the energy performance of a building.

Fig. 23 LEED strategies to maximise daylighting

Some studies [55] suggest to use less glass, limiting the glazing area to approximately 30%, and use good glass and frames. Windows and curtain walls are the most expensive component in a building and provide the worst energy performance. The more you use, the more energy you burn. This statement is supported by a research carried out by Lstiburek,

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230 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

that illustrated how “bad glass ruins good walls” [55]. The impact of thermal bridging through commercial wall assemblies, and heat flow through window systems can be calculated with relatively good accuracy by calculating an area-weighted average of the R-values of the windows and opaque wall sections (Fig. 23).

The equation takes the form:

Typical curtain wall systems have an R-value of only 2 or 3, with “high performance” systems using highly insulated spandrel panels and best-in-class double glazing may achieve R4.

Curve 1 is for standard U=0.50 thermally broken aluminium punched windows with air-filled double-glazed insulated glazing units in a R12 batt-air-filled steel-stud brick veneer wall system (R6). The overall effective R-value of this wall is around 3 to 4 over the normal range of window-to-wall (WWR) ratios of 25% to 50%.

Curve 2 shows that increasing the R-value of the wall to R11 by adding an inch of foam on the exterior, results in an increase of only R0.5 to R1.5 for the overall R-value for the same range of WWR.

Curve 3 shows how significant an impact window performance can make if a good wall is provided. An externally insulated R16 wall, when mated with poor windows produces a vertical enclosure with an R-value of only R3 to R6 for the normal range of window area.

Curve 4 assumes a good quality window frame with top quality glazing (low-e, argon-filled): the result for the overall vertical enclosure is still only R4 to R7.

These first four curves cover the performance of a wide range of commercial enclosures with a wide range of cladding types. The conclusion is that modern commercial vertical enclosures actually have an R-value that is rarely over 7, and more likely in the range of 3 to 5. Curves 5 and 6 provide an idea of the significant improvements that are possible. Using best-in-class thermally broken aluminium frames and high-performance glazing (U=0.30), Curve 5 shows that even with an R40 wall, the overall R-value will be in the 7 to 12 range for WWR of less than 40% (the highest ratio recommended for high-performance buildings).

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231 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

Even though this is a low-level, it is still significantly more than the alternative. The grey curve below Curve 5 shows the slight benefit gained by increasing wall R-value from 20 to 40, particularly at high glazing ratios.

Curve 6 uses low-e, argon-filled triple-glazed units in an insulated fiberglass frame, to deliver a U-value of only 0.14. Even with a wall insulated to “just” R20, such a combination can deliver an overall R-value of 12 to 14, two to three times more than typical commercial vertical enclosures.

In all cases, it can be seen that high glazing ratios generate enclosure walls that are expensive to purchase with high heat loss and heat gain. This high ratio should be avoided in both individual spaces, such as meeting rooms, as for the whole building on average [55].

Fig. 24 R comparison, in Lstiburek, 2008.

7.11 C

ONSIDERATIONS ABOUT

D

ETAILING

The optimum choice of window and glazing systems will depend on many factors including the building use type, the local climate, utility rates, and building orientation; for instance, in cold weathers (i.e. Northern Europe), mostly winter heat losses must be monitored but, at the same time, light transmittance must be maximized; in warmer climates (i.e. Middle East Europe) or in buildings with high heat loads the solar loads must be

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232 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

especially controlled. In moderate climate (i.e. Mediterranean Area), both heat losses and solar loads must be carefully assessed.

A growing research is the development of actively controlled façades, where the occupants or the building management system can control the transfer of energy through the building envelope, depending on needs [45]. With such efficient envelopes and by integrating renewable energy technologies into façades, glazed buildings will be transformed from energy sink to energy source, while maintaining its properties of full or partial transparency. In order to meet the required reduction in CO2 emissions it is essential that

this innovative technology is made simple and sufficiently cost effective to be used in underperforming buildings.

Historic buildings often require special window detailing [63, 64]. The desire to achieve historical accuracy can sometimes conflict with the desire to provide energy efficiency. Fortunately, several companies are now offering high-performance products that can replicate the appearance of historical windows while maintaining energy efficiency [51].

Careful specification of window and glazing systems is essential to the energy efficiency and comfort of all buildings. In residential, skin-load dominated structures (such as housing) optimum window design and glazing specification can reduce energy consumption from 10%-50% below accepted practice in most climates. In internal-load dominated commercial, industrial, and institutional buildings, properly specified fenestration systems have the potential to reduce lighting and HVAC costs 10%-40% (WBDG, www.wbdg.org/resources/windows).

Glazing choices should be considered holistically. Once the design team and owner agree on the design issue, window and glazing options can be evaluated. Issues to consider include four main aspects, that shall be described and developed in the further paragraphs, where the main significant parameters to fully specify a window system will be outlined:

Thermal characteristics: heat gains and losses. 1. Condensation control and weather tightness.

2. Visual requirements: daylighting, shading and sun control, privacy, glare, view. 3. Acoustic control.

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233 CHAPTER 7-GLASS FAÇADE INTEGRATED PROCESS:SOME DESIGN ISSUES

Lateral wind forces dictate structural design of the façade, especially for high-rise building but wind also causes rain to defy gravity making it flow in all directions on the curtain wall. The weather resistance of a façade is achieved in different ways:

 Single front sealed systems rely on weatherproof outer seals to the infill panel rebates to stop water penetration to the interior.

 Drained and ventilated systems have weatherproof outer and inner seals to the infill panel rebates to stop water penetration. The rebates and cavities are also drained and ventilated to the exterior to prevent the accumulation of any water that may bypass the outer seal.

 Pressure equalised systems are a refinement of the drained and ventilated system, but the ventilation openings are larger to allow rapid pressure equalisation in the cavities with the external pressure. It is important that the inner seals to the cavities are airtight and continuous to resist pressure fluctuations. It is also necessary to compartmentalise within cavities to prevent pressure loss across an elevation to zones exposed to lower wind pressures. Pressure equalization requires the prevision of a ventilated outer wall surface, backed by drained air spaces in which pressures are maintained equal to those outside the wall, with the indoor wall face being sealed against the passage of air (Fig. 25).

This is achieved by ensuring that the outer portions are sealed as tight as possible against rainwater ingress, whilst the inner portions are sealed as tight as possible against airflow. Cavities are made into compartments and each compartment is connected to the exterior by protected openings.

Testing [33] has shown that window and curtain walling systems can achieve better results using pressure equalising principles. However, pressure equalisation cannot counteract other forces such as gravity.

The gaskets also prevent the glass coming into contact with the frame. There are various shapes that display different characteristics when the forces act upon them. Joints in the

Fig. 25 Drainage path of a curtain wall (Courtesy of Pilkington).

Figura

Fig. 1 Ufa Cinema Center - Coop Himmelb(l)au, 1998. Fig. 2 Kunsthaus, Graz - Peter Cook & Colin Fournier, 2003.
Fig. 3 a, b. Norman Fosters Architects, 30 St. Mary Axe Tower, London.
Fig. 4 a, b. Herzog & De Meuron, Prada building, Tokyo.
Fig. 5 a, b. Strasbourg Train Station, NCF DAAB RFR. This surfaces is not a freeform surface in the strict sense since it is  modeled from basic geometric primitives like tori and spheres
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