Mechanical Characterization of Timber Structural Elements Using Integral Mechanical Attachments

Automation is increasingly present in the construction industry, whether at the design, manufacturing or assembly stage. Thanks to new technologies, such as robotics, new ways of designing structural elements can be imagined and implemented. Complex methods and sequences of assembly can be set up quickly as well. Numerous studies have been carried out in this direction at the laboratory for timber constructions IBOIS (EPFL) especially on wood-wood connections called Integral Mechanical Attachments (IMA). This paper is focused on the mechanical characterization of a prefabricated structural element entirely made of Oriented Strand Board (OSB) panels produced by a fully robotized line. In order to avoid bonding process due to cost, ecological and time reasons, IMA using OSB have been chosen to connect each prefabricated element. Experimental tests and numerical models have been developed to determine the mechanical response of such structural elements.

Development of Novel Standardized Structural Timber Elements Using Wood-Wood Connections

Julien Gamerro

Traditional wood-wood connections, widely used in the past, have been progressively replaced by steel fasteners and bonding processes in modern timber constructions. However, the emergence of digital fabrication and innovative engineered timber products have offered new design possibilities for wood-wood connections. The design-to-production workflow has evolved considerably over the last few decades, such that a large number of connections with various geometries can now be easily produced. These connections have become a cost-competitive alternative for the edgewise connection of thin timber panels. Several challenges remain in order to broaden the use of this specific joining technique into common timber construction practice: (1) prove the applicability at the building scale, (2) propose a standardized construction system, (3) develop a convenient calculation model for practice, and (4) investigate the mechanical behavior of wood-wood connections. The first building implementation of digitally produced through-tenon connections for a folded-plate structure is presented in this work. Specific computational tools for the design and manufacture of more than 300 different plates were efficiently applied in a multi-stakeholder project environment. Cross-laminated timber panels were investigated for the first time, and the potential of such connections was demonstrated for different engineered timber products. Moreover, this work demonstrated the feasibility of this construction system at the building scale. For a more resilient and locally distributed construction process, a standardized system using through-tenon connections and commonly available small panels was developed to reconstitute basic housing components. Based on a case-study with industry partners, the fabrication and assembly processes were validated with prototypes made of oriented strand board. Their structural performance was investigated by means of a numerical model and a comparison with glued and nailed assemblies. The results showed that through-tenon connections are a viable alternative to commonly used mechanical fasteners. So far, the structural analysis of such construction systems has been mainly achieved with complex finite element models, not in line with the simplicity of basic housing elements. A convenient calculation model for practice, which can capture the semi-rigid behavior of the connections and predict the effective bending stiffness, was thus introduced and subjected to large-scale bending tests. The proposed model was in good agreement with the experimental results, highlighting the importance of the connection behavior. The in-plane behavior of through-tenon connections for several timber panel materials was characterized through an experimental campaign to determine the load-carrying capacity and slip modulus required for calculation models. Based on the test results, existing guidelines were evaluated to safely apply these connections in structural elements while a finite element model was developed to approximate their performance. This work constitutes a firm basis for the optimization of design guidelines and the creation of an extensive database on digitally produced wood-wood connections. Finally, this thesis provides a convenient design framework for the newly developed standardized timber construction system and a solid foundation for research into digitally produced wood-wood connections.

EPFL2020

A Calculation Method for Interconnected Timber Elements Using Wood-Wood Connections

Jean-François Bocquet, Julien Gamerro, Yves Weinand

Wood-wood connections, widely used in the past, have been progressively replaced by steel fasteners in timber constructions. Currently, they can be manufactured and implemented more efficiently thanks to digital fabrication techniques. In addition, with the emergence of new timber plate engineered products, digitally produced wood-wood connections have been developed with a strong focus on complex free-form geometries. The gained knowledge through research and building implementations have pushed the development of more standardized structural elements. As a result, this work presents a new concept of building components using through tenon connections based on the idea of transportable flat-packs directly delivered and assembled on site. The main objective of this research is to develop a convenient calculation model for practice that can capture the semi-rigid behavior of the connections and predict the effective bending stiffness of such structural elements. A case study is used as a reference with three large-scale slabs of a 8.1 m span. Bending and vibration tests are performed to study the mechanical behavior and assess the proposed calculation method. The results show the high influence of the semi-rigid behavior of connections on the bending properties and, therefore, on the serviceability limit state. The model is in good agreement with the test results, and further improvements can be made regarding the local behavior of the connection. This study demonstrates the feasibility of the proposed construction system and the applicability of the developed calculation model to design practice.

2020

Fire endurance of multicellular panels in an FRP building system

Craig Tracy

Fiber-reinforced synthetic polymers (FRP) are the building materials that may permit both the improvement of long-term building performance and the simplification of the construction process. Thanks to their high specific strength, low thermal conductivity, good environmental resistance, and their ability to be formed into complex shapes, FRP materials are well-suited to fulfilling many building functions. By integrating traditionally separate building systems and layers into single function-integrated components and industrially fabricating those components, the amount of on-site labor can be greatly reduced and overall quality can be improved. In order to profit from the advantageous qualities of FRP, however, it is essential to address the unique weaknesses and disadvantages of the material. Most notably, the problems of poor fire safety and high material costs must be overcome. In response to these challenges, a new multiple-story building system employing FRP materials is proposed. Within this system, fire safety is ensured through the use of an internal liquid cooling system, which circulates a cooling medium through the load-bearing FRP elements to maintain their temperature within a safe operating range. This system is made cost-effective through the integration of the building's heating and cooling system. By controlling the temperature of the circulating liquid, the building's structural elements can serve as heating or cooling emitters (radiators). Further, the addition of the liquid within the cells of the FRP elements helps maintain a more constant interior climate through the "thermal flywheel" effect, which improves energy efficiency and comfort. Experimental investigations were performed to explore the fire safety aspects of the proposed system. An existing FRP cellular bridge deck material was adapted to incorporate an internal liquid cooling system. After several preliminary investigations, large-scale experiments involving structural and fire loading were conducted on both liquid-cooled and non-liquid cooled specimens. The experiments demonstrated the efficacy of the system in protecting load-bearing FRP elements from the weakening effects of high temperatures, especially those that are stressed in compression. Structural fire endurance times were improved from less than one hour to more than two hours (EC1 Part 1.2) through the implementation of the liquid cooling system. Alongside the experimental program, a series of mathematical models were developed. Numerical thermochemical and thermomechanical models simulate the response of loaded liquid-cooled FRP panels in fire, while analytical models predict the post-fire mechanical behavior of fire-damaged sections. All models provide predictions that are within 10% of experimentally measured values.