Innovative Timber Constructions

Practical and material orientated academic research has become increasingly important for architectural practice. This is due to several reasons. First of all, it contributes to contemporary concepts in architecture and improves their implementation. Today’s architects are looking for a deeper understanding of technical and technological questions related to architecture: technology, construction methods as well as structural considerations are no longer seen as bothersome necessities, as it was often the case in the past. Second, research is in charge for finding answers to one of the biggest architectural challenges of our time, namely sustainable building. Society’s burgeoning awareness of the urgent need to use renewable materials for building construction is undeniable and has become an important parameter for architectural production. As a result, timber constructions experience a new popularity and the importance of research on timber increased. The potential of this research becomes evident with some of the latest developments and innovations in this domain. The here presented research lines ‘Innovative Timber Constructions’ pursues an interdisciplinary approach and links the three domains of architecture, structural engineering and timber construction. The research not only opens the way to a new era of innovative timber constructions and timber construction techniques. It initializes the unprecedented exploration and study of timber related structures. Indeed timber has the dual capacities to be formed and to retain a given form. Aesthetic and structural qualities of the shown projects have raised a wide range of questions and many still remain to be addressed. There appears to be something remarkable in the interaction of the material and the formal qualities that produces a distinguished quality of design.

Reconsidering traditional timber joinery for sustainable structures

Jan Friedrich Georg Brütting, Corentin Jean Dominique Fivet

Timber construction plays an important role in architectural history, in modern construction methods and in sustainable design, for which wood-only joinery techniques have been used for centuries. Traditional joinery typologies are the product of successive, empirical improvements over generations of artisans [1]. However, over the industrialization era, many traditional wood-only construction techniques were abandoned in favor of more profitable connections with mechanical fasteners [2]. This shift of practice lead to the development of connections with quantifiable capacity but neglecting the intrinsic anisotropic properties of natural timber. Specialized knowledge of carpenters and typical building traditions were lost. Yet, examples of historic timber buildings with a life span of multiple centuries have demonstrated many advantages of interlocking timber joints, e.g. in severe loading cases as in the case of earthquakes [2]. New technologies and recent developments in digital fabrication allow the custom and automated manufacturing of traditional wood-only connections for new buildings [3]. A big potential of such contact-only connections is their fast assembly and non-destructive disassembly. Remarkable examples include the Japanese Ise-Shrines, which are rebuilt in a 20-year frequency for the past 1300 years. Traditional joinery techniques allow for sustainable solutions through the replacement, long-term maintenance and repair of structural elements [4]. They also ease recycling and energy-recovery. In times of global environmental problems where the building sector is highly responsible for resource depletion, greenhouse-gas emissions and waste pollution, these advantages will become even more important for regenerative architecture or transformable building design. Buildings need to become more adaptable to frequent changes in ownership or use to avoid superfluous demolition. For example in Japan in the past, entire cities made from timber were constructed for disassembly and transport in order to accommodate for recurrently changing administrative centers [3]. Some joint typologies were designed to allow the later vertical or horizontal extension of buildings [2]. In Japan also, contemporary buildings are not understood as permanent edifices but as structures made to be replaced or removed. This could only be achieved through highly developed timber joinery techniques. A combination of traditional joinery with contemporary post and beam construction and CNC prefabrication is already well established in Japan in the construction of residential buildings [3]. However, a straightforward application of traditional joinery techniques in today’s construction is not always possible. In the past, the structural integrity of traditional joinery techniques was proven mainly through in-situ, de-facto testing. Today, building codes include little-to-no information about the application of wood-only carpentry connections in comparison to those with mechanical fasteners [2]. Research on interlocking joints concentrates on a very precise analysis of few different joint typologies. There are limitations in literature in terms of analyzing a variety, or a library of ancient interlocking wood joints to develop an extensive relative comparison between them. As a first step, the present research initiates a FEM campaign to catalog a library of Asian joints. To align traditional techniques with modern construction and typically used North American timber, solid Douglas fir was considered as the material. The goal of the analysis is to extract the relative capacity and stiffness of different joints to understand their mechanical behavior. This comparative record, which should be extended to additional Asian and European joint types, will allow academics and designers to push the capabilities of these ancient techniques while benefitting from modern materials, construction techniques and fabrication tools. Other research will also address the optimization of joint geometries to customize demanded capacity or stiffening. In addition, Life Cycle Assessment of constructions with conventional and advanced traditional joinery technology will be carried out to quantify potential environmental savings.

2018

Tragverhalten und Bemessung von Holz-Glas-Verbundträgern unter Berücksichtigung der Eigenspannungen im Glas

Klaus Kreher

The construction principles of "Timber-Glass-Composite-Girders" demand practical and theoretical research to ensure reliable designs. Toward this objective the mathematical description of load-bearing and the non-rigid bond is the subject of this work, i.e. to define the contributing parameters for the design of these girders. The non-rigid-bond of the adhesive joint is of great importance because of its influence on the load-bearing of the system as a whole. The loading of girders is dominated by permanent forces thus leading to the obligation of knowing the long-term behaviour of the system and its components. The definition of the characteristic values, taking into account the long-term load-bearing, is an intrinsic part of this investigation. It can be shown that the load-bearing of the girders depends on the fracture of the glass panes. The properties of glass-fractures are a function of the residual internal prestresses due to the heat-strengthening of the panes. The load-bearing, particularly the post-cracked behaviour, changes with respect to the intensity of these internal prestresses. The timber is able to reinforce the cracked glass, leading to a ductile load-bearing behaviour as in the girders, with a dependency upon the size of the remaining fractures. A model based on the differential equations of the non-rigid bond is defined in order to calculate the loading-peaks of the adhesive joint and the material loading itself. The characteristic values of the non-rigid bond, such as the number and distances between cracks and the load introducing length, were evaluated in tests on composite slabs and small scale girders. The existing calculation models which take into account non-rigid bonding were modified to adapt the influence of the width and thickness of the adhesive joint. In order to calculate the stressing of the material the existing calculation models were adapted to calculate the post-cracked situation. The load-bearing behaviour of glass-panes bent with respect to its strong neutral axis needs other safety-considerations than panes, bent off their plane. It is shown that the existing safety considerations subject to the use of glass can not easily be adapted onto the composite girders. The bending with respect to the strong neutral axis and the reinforcement of the glass of the timber demands a different hypothesis to adapt the fractural mechanical analysis and to establish a safe limit conception. This is part of the performed research of this document. The following descriptions present briefly the obtained results: Depending on the quality of the glass (residual stresses due to heat-strengthening, e.g. annealed glass, heat-strengthened glass, fully tempered glass), the ductility and the mode of failure of the girders does change. As a result of its failure mode, annealed glass without internal prestresses offers the highest remaining load-carrying potential after the first crack has appeared. This ductility and thus the structural safety, diminishes with an increasing degree of internal prestressing due to thermal treatment. Heat-strengthened glass (with various degrees of prestressing, various residual stresses) shows a decrease in remaining load-carrying capacity with an increasing degree of prestressing until it fails in a brittle mode as fully toughened glass does. This has to be well considered in respect to safety considerations. Girders with panes made of glass having internal residual stresses due to heat strengthening below 50N/mm2 are considered to collapse ductily, residual stresses bigger than 50N/mm2 cause brittle failure. The effective stiffness of the girders decreases under permanent loads in function of the bondage. Both, timber and adhesive take part in this decrease; the influence of both of the involved materials has been defined. Since the design of conventional glass constructions which uses the concept of principle stresses cannot be adapted, unidirectional shear stresses had to be determined for the shear strength of glass, which is defined as 25N/mm2 for annealed glass. The characteristic values of the adhesive to define the non-rigid bonds have been determined, likewise the influence of the width and thickness of the adhesive joint. The load introducing length, the distances between cracks and the number of cracks are defined with tests on composite slabs. A model to describe the theoretical bond with the defined parameters based on the differential equations was developed. This model allows the calculation of the material stressing at the load introduction and on both sides of a crack. To calculate the stressing of glass and timber existing calculation models which take into account non-rigid bonding were modified to adapt the influence of the width and thickness of the adhesive joint. In order to calculate the stressing of the material the existing calculation models were adapted to calculate the post-cracked situation. The safety factors for the influences of the glass surface, environmental conditions, load duration and the parameters of fracture mechanical analysis as developed as they are known for semi-probabilistic safety concepts for glass constructions cannot easily be adopted. The bending with respect to the strong neutral axis demands a different hypothesis in order to adapt the fractural mechanical analysis and to develop the safety parameters. For the reliable design of timber-glass-composite-girders a contribution to the appropriate use of probabilistic, semi-probabilistic and deterministic safety concepts are given. A realised construction (Hotel "Palafitte" in Monruz (NE)) shows an example and the experience made in designing girders.

EPFL2004

Structural Timber Fabric: Applying Textile Principles on Building Scale

Markus Hudert, Yves Weinand

Textiles are interesting from both a structural and an architectural point of view. Their patterns and textures, created by the interworking of yarn elements, are not only highly appealing on an aesthetical level, they also possess load bearing qualities. The research project “Structural Timber Fabric: Applying Textile Principles on Building Scale” sets out to investigate this potential and proposes to develop a new family of timber constructions based on the logic and principles of textile techniques. In this context, and within the scope of several case studies, one of the core objectives is to create an innovative structural system with concise aesthetic, spatial and structural qualities. Thus, the proposal addresses important challenges at the frontiers of the fields of architecture and civil engineering. The here presented paper outlines selected parts of this research.