Ebook: EU COST C13 Glass and Interactive Building Envelopes

The main objective of the COST Action C13 was to increase the knowledge of properties and possibilities of glazing in order to increase the performance of building envelopes, to reduce the energy consumption and to improve the quality of life with respect to interior space, impact on the environment and human welfare.

This collection of papers presented at meetings and workshops of the COST C13 working groups 1 (Architectural Aspects and Design Integration), 2 (Quality of Interior Space) and 3 (Structural Aspects of Glass) are the result of 5 years of exchange of ideas, experiences and know-how between members, delegates and experts. It represents the body of knowledge from a restricted but representative group of professionals in Europe on the subject of glass building envelopes.

The Steel Structures Laboratory at Ecole Polytechnique Fédérale de Lausanne (CH) and the research group 'Façades' of the faculty of Architecture at Delft University of Technology (NL) have taken the initiative to publish these COST C13 papers in order to disseminate the knowledge to the world of glass façade professionals and to contribute to the development of a new generation of high-performance glass building envelopes.

One of the main conclusions of COST Action C13 is that the three worlds of Architecture, Building Physics and Structural Engineering are today by far not sufficiently integrated. Additional efforts are clearly required to align these three worlds into a holistic entity that is able to design building envelopes that are optimised with regard to all three points of view.

The reader is encouraged to read the separate articles, to reflect on the content and to contribute to the future development of innovative building envelopes by writing novel contributions in the field. Preferably these contributions are interdisciplinary and try to integrate all facets of the subject, from fundamental research topics via technology development issues through to the application in architecture. All of these domains are supposed to stimulate each other rather than living apart together.

Delft, August 2006

Prof. Dr. Mick Eekhout, Prof. Dr. Ulrich Knaack, Ronald Visser; Delft University of Technology, the Netherlands

Lausanne, August 2006

Michel Crisinel, Matthias Haldimann; Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

In 1992 I introduced ‘Zappi’ (Eekhout, 1992) as the yet unknown, fully transparent, structural reliable material for safe structural design in architecture. I have worked in my chair of Product Development in the faculty of Architecture with my staff and students on this topic, which was initiated by the graduation project of Civil Engineering student Rik Grashoff, nowadays alderman in Delft. Since roughly that time I designed numerous experimental glass structures in my design & build company Octatube Space Structures BV. This paper displays the different ‘Zappi’ techniques that have been developed in the different design, development and research processes and particularly for the architectural ‘Blob’ designs of late. The listing contains a mixture of realistic short-term design & development projects I supervised in Octatube and the medium/long term research & developments undertaken in the department of Building Technology.

Modern Architecture and Ventilated Double Glass Façades (VDGF) are often related with each other: the use of enormous quantities of glass, steel and concrete are essential ingredients. VDGF could be briefly defined as façades consisting of two skins (completely transparent or not) with a very particular ventilation strategy. The acoustical European standard EN 12354 (2000) offers the possibility to predict façade insulation and insulation between rooms inter connected with a cavity. It has been concluded from this study that VDGF have an acoustical façade insulation that is far better (up to 10 dB) than that of traditional façades (office buildings). The special ways of ventilating the building and the ‘doubling’ of the skin of the façade play an important role. On the other hand due to transmission of sound through the cavity of a double façade, the acoustical insulation between rooms (on different floors) situated at the façade side, is lower compared to buildings with the same internal construction but with no cavity (values up to 8 dB), when no special measures are taken to prevent the (airborne) indirect transmission. To achieve good calculation matches, some modifications were introduced in the standard, i.e. the calculation per frequency band, taking into account either the “Mass-Spring-Mass” or the “Three Rooms model” depending on the frequency band and adaptations concerning free field conditions.

We refer to: www.bbri.be/activefacades.

This paper demonstrates how advanced façade systems can comply with energy regulations and advocates for a more holistic approach to the selection of the most suitable façade system based on project specific requirements. The impact of different advanced façade systems on the yearly office energy consumption can be determined by using whole building simulation models. A whole building simulation, applied on an office situated in London, demonstrated that double skin façades are able to deliver easier compliance with the new UK energy regulations compared to the more traditional wall systems. The UK energy regulations have incorporated a carbon emission calculation method, which is a flexible method for demonstrating the compliance of offices build with new building materials or innovative envelope systems. This paper highlights that a lot of national regulations do not have such an appropriate assessment methodology, which therefore forms an obstacle of the development and use of innovative building materials and envelope systems. This paper also warns about issues for performing proper building energy comparison such as the choice of the reference façade, the risk of over-estimating the functionality of specific façade components and the progressive impact of the selection of a façade configuration on other façade performance criteria and costs. Finally, the authors urge researchers to use more holistic approaches for the assessment of the ideal façade system. Façade configurations are rarely optimised for one sole parameter but many – and sometimes conflicting – requirements have to be taken into account, e.g. energy performance; acoustic-visual-thermal comfort; initial costs and long-term maintenance and repair costs; architectural appearance; structural performance; durability of façade components; fire resistance and many more. For every new project with its own specific critical requirements, a decision-making matrix and a prediction tool can be used in order to facilitate the optimisation of the most suitable façade system early in the design process.

Starting from a practical design problem related to natural and hybrid ventilation systems, this paper looks at different airflow modeling methods that might be employed to assist in the decision making process of a building design team. The question at hand is whether or not to make use of a double-skin façade system in a new office development. The airflow modeling methods considered are the mass balance network method and computational fluid dynamics (CFD).

The paper gives an overview of the methodology of the design study. The underlying modeling and simulation work is elaborated. The paper finishes with some conclusions, both in terms of the actual performance of the double-skin facade and in terms of the modeling and simulation work.

The main conclusions are that for the foreseeable future the network method is more suited for this type of “everyday” design support work. However there are important areas where the network method in general might benefit from CFD, or vice versa.

The knowledge of luminous flux and illuminance phenomena is necessary for proper and qualitative dimensioning and arrangements of openings and thus for comfortable and efficient living environment. In this paper the concept of luminous efficacy of the solar energy as the basis for the control algorithm is taken into account. Luminous efficacy K is defined as the ratio of luminous flux to radiant flux, K=Φ_V/Φ_e. The idea was to design a flexible dynamic building envelope, which will be able to adjust itself to the changeable environmental conditions regarding the inner illuminance requirements. Changes of the opening's geometry are implemented by moving the roller blind. The roller blind is used to control the inside illumination of human-built environment in real time conditions. We realized the concept of the control with fuzzy elements in the control algorithm for automatically adaptable shadow device on the window. For this purpose a model of a real physical system, a test chamber with an opening equipped with movable roller blind on the south side was build. The fuzzy control system manages the roller blind movement, which enables the optimal use of the available solar energy in order to assure the desired internal illumination.

This paper highlights the integration of the dynamic daylight simulation software DAYSIM [1] into the transient simulation software TRNSYS [2] in order to predict the energy reduction due to the availability of daylighting and lighting control strategies. The simulation of two different envelope configurations (traditional wall with 40% of glazed area versus a fully glazed curtain wall) fitted in an office room is performed on the basis of real weather data (London). The energy consumption and carbon dioxide emissions of both envelope systems are calculated for various lighting control strategies.

This paper analyses the use of an intelligent building envelope as an instrument to manage the variable and sometimes conflictive requirements posed by daylighting in non-domestic buildings. While proper architectural design is a vital precondition for facing these challenges, in a real-time environment it needs to be supported and enhanced by the performance of the building envelope as an environmentally selective filter. The manner in which the building envelope is able to handle the collection, admission and distribution of daylight indoors determines its successfulness in creating an appealing indoor luminous environment with an efficient use of daylight resources. First a short introduction is given on what the authors believe to be the central characteristic of intelligent building envelopes: adaptiveness to and interaction with the environment, with particular focus on the building occupant. This adaptiveness is then evaluated for its ability to manage the complex set of requirements that arise from the use of daylight in non-domestic buildings with a desirable outcome. The analysis combines and compares a selection of secondary literature sources and built examples in their concrete attempts to create solutions to daylighting challenges. The paper is based on the research results of a Ph.D. to be completed in 2005 at NTNU, Trondheim, Norway.

This work deals with the use of glass as a real structural material. An in situ non destructive process is proposed for the control of tempered glass structures, especially in joint zones. A design method is also proposed. It is based on one hand, on a limit state ensuring the structure to be perennial on long duration, and the another hand, on the finite element prediction, on every point of the structure, of both residual stresses due to tempering and stresses due to the mechanical loading. The control consists, during the life of the structure, in comparing images obtained from photoelastic analyses of the loaded structure, to images obtained from finite element analyses of the structure at the ultimate limit state used for the design.

The paper presents experimental observations of glass in contact with inserts of steel, aluminium, polyamide and epoxide resin. Four sets of tests with the different contact materials were carried out in the laboratory of Czech Technical University. Influence of the edge finishing, size and thickness of the glass panel and the corner distance was taken into account. The test results and related FE simulation will allow preparing an analytical prediction model of the contact resistance as well as the bearing resistance of bolted connections.

Engineers are currently using various concepts for the analysis and design of structural elements made of glass. There is no general agreement on a certain design concept yet. All available design concepts suffer from more or less severe drawbacks and their applicability is limited to special cases.

The present paper discusses the present knowledge in the field of structural glass design. The aim is not so much to explain every single detail related to the application of the concepts, but at working out the bases and focusing on difficulties and limitations. The paper provides an overview for anyone interested in the topic and aims at serving as a basis for discussions on desirable improvements to current design concepts and for research work on the topic.

This research paper reports the experimental and theoretical results of the project “Remaining Structural Capacity of Laminated Safety Glass” at ETH Zurich. The aim of the project is to develop new mechanical models and to control the post breakage behaviour of laminated safety glass (LSG). At first the different stages of failure of LSG and the corresponding definitions are given to explain the different capacities of the structure. In broken LSG the poly-vinyl-butyral (PVB) foil works as tension reinforcement and the upper broken glass layer carries the compression forces. Therefore the mechanical properties of the foil were determined using tensile tests. Bending tests combined with impact tests demonstrate that the different glass types, bearings, the type of initial failure and dimensions of the specimen determine the post breakage behaviour. As observed in these tests the fracture behaviour affects the remaining structural capacity (RSC). Different types of yield lines can evolve from initial cracks. The yield line patterns influence the ultimate load decisively.

A method for estimating the strength parameters of the Weibull distribution and the power in Brown's integral from full-scale experiments with rectangular glass panes loaded uniformly at different load rates is proposed.

Knowing the parameters, it is possible to estimate the load capacity of a glass pane subjected to any load history. Provided accurate load histories (load as a function of time during the service life) for e.g. wind and snow loads are available this enables fairly simple and very accurate estimates for the load capacity of a glass pane for different load types.

The method is applied to permanent load and Danish snow load in order to demonstrate the consequences.

A requirement of overhead glazing constructions is that they have a minimum residual bearing capacity. Wide spanned broken laminated glass with a continuous or a discrete support has a risk of falling down. The solution is to fix the laminated glass to the substructure. Wire-cloth or other synthetic fabrics are embedded in the PVB-interlayer or the synthetic resin, near the edges were the glass is supported. The fabric can be fixed to the glazing beads or to the glass fittings with special connections.

The inelastic material behaviour of thermally toughened and heat strengthened Soda-Lime-Silica glass at room temperature was investigated. A four-point bending test configuration was used. The delayed elastic deformations were measured and reveal a significant time-dependent behaviour. The possible origin such as sub-critical crack growth, ion diffusion or absorption of water molecules is discussed briefly. The aim of the study is a better understanding of the nature of glass itself, in order to use this brittle material as a structural member in the field of civil engineering according to appropriate safety requirements.

Thermal stresses of tempered glass in the area of typical boreholes were estimated using Narayanaswamy's structural relaxation model implemented in a Finite-Element-Code (ANSYS 5.5.2). Surface stresses were measured for samples with holes of different float glasses (soda-lime-silica-glass, borosilicate glass) from different commercial tempering processes. The bending strength in the borehole area of the samples and of annealed (float) glass samples was determined using a modified coaxial double-ring bending test. Results are compared and evaluated on a statistical basis. It is shown that the characteristic glass strength of tempered glass in the borehole area is not lower than in the “infinite area” given in European standards. From the results it is also assumed that crack healing plays an important role for the bending strength of tempered float glass.

At different research institutes in and outside Europe, research is in progress to find pieces of the structural glass puzzle. At Ghent University, the focus is on glass beams.

In the present contribution, the authors want to highlight some experiences of the past and current activities concerning buckling problems of glass beams. It is the authors' opinion that at the current state of technological development, glass beams with a rectangular cross-section are by far the most realistic starting-point. Such geometry implies a slender cross-section, which is sensitive to lateral-torsional buckling.

Some concepts are compared in order to deal with lateral torsional buckling of laminated glass beams.

By means of experimental tests on monolithic glass beams and numerical simulations on corresponding models, it has become clear that stability (buckling) instead of strength can be the limiting factor of the load-carrying capacity of glass beams, especially if strengthened or tempered glass is used.

It is also shown that prevention of buckling can increase the load-bearing capacity considerably. Several configurations for buckling prevention are proposed and compared, varying from fixed local supports to continuous supports comparable to elastic foundations. In many cases, the elastic sealants used to connect the beam to the supported glass plates suffice to realize the desired buckling prevention. This support is not brought into account in today's practice.

The following paper summarizes the actions and the results of a research project concerning the behaviour of laminated glass beams.

Due to loading on the strong axis of glass panes, different fracture mechanical behaviour has to be admitted than due to the loading on the weak neutral axis. Existing models to describe the structural safety of glass can not easily be adopted.

The load-bearing of reinforced glass panes varies with the degree of prestressing due to increasing residual stresses of heat strengthened achieved by using reinforced heat strengthened glass.

This article offers a model to calculate the stresses of I-Beams with and without the appearance of cracks (Mode I and II) and the load-bearing behaviour of the adhesive joint of reinforced glass, such as distances between cracks, number of cracks and load introducing length.

Safety-considerations taking into account the load carrying-behaviour of reinforced glasses with different residual stresses due to heat strengthening are shown.

Glass is a material, that is able to resist very high compression stresses and which has special architectural appeal because of its transparency. For this reason, there is a growing trend to extend the use of glass sheets to load carrying elements such as beams, columns and shear panels. Due to their high slenderness and high compression strength, such load carrying elements tend to fail because of instability. The main objective of the research work is the experimental and theoretical study of the fundamental stability problems (column buckling, lateral buckling, plate buckling) for single layer and laminated glass.

Based on stability tests, the load carrying behaviour of simple and laminated glass in the foreseeable dimensions of application was examined and analytic and numeric models were developed. To simulate the buckling behaviour of laminated glass elements, the time and temperature-dependent behaviour of the PVB interlayer was modelled with viscoelastic finite elements.

The main objective of the work is to discuss possible design methods for single layered and laminated glass elements by means of the test results, the developed models and the parametric study.

Undercut anchors were so far seldom used in glass due to brittleness and the risk of subcritical crack growth. The use of tempered glass in combination with a special drilling-process and -geometry of the undercut holes now made it possible to develop a system that allows this method of fastening glass panes. In a parameter study of 3-D numerical tempering simulations with Narayanaswamy's structural relaxation model, the amount of residual stress in the area of the undercut hole was estimated. Temper stresses were compared to surface stresses in the “infinite” area that were measured by photoelastic methods. Pull-out tests and shear-off tests of anchors from annealed and tempered glass were performed and results compared to the numerical results. Both prove that a significant compression stress from tempering exists in the undercut area. Finally load bearing tests showed the performance of the system.

Point bearings are a popular element of modern glass constructions. On the one hand point bearings are elements that can be used to design transparent buildings, on the other hand point fixings allow for a good failure resistance after breakage of a laminated glass panel.

Due to the brittle behaviour of glass the knowledge about the rupture process in the glass hole and the influence of the point bearing geometry is important. The article gives an overview on the investigated research projects of the RWTH Aachen, the results as well as the proposed methods to solve the design problem “point bearing”.

In the planning process it is helpful that rectangular glass panels under uniform loading can be designed very fast. Furthermore it is important to check the results of a FE calculation.

A calculation of many different discretely supported glass panels under an uniform load with the value q=1,0 kN/m² with the help of the FE method results in values of stresses and deformations. The insertion of these values in a 3D-diagramm, results in a surface of results of stresses or deformations. For a pre-design this paraboloidal shell is exact enough. A linear interpolation in this surface is very easy.

With a short number of input data, such as the distance in direction length, the distance in direction across or the thickness of the glass panel, it is possible to interpolate the results of stress in this shape. The interpolation results must be calibrated with other influences for example with the diameter of the the hole and the distance of the hole to the edges and with the shore hardness of the rubber at the support.