Reliability of statically indeterminate timber structures: impact of connection non-linearity and overstrength

In statically indeterminate timber structures, the stiffness of the members and the semi-rigid behavior of the joints influence the force distribution in the structure, affecting its performance and reliability. In this paper the impact of the non-linear load-deformation behavior of connection on a simple statically indeterminate timber structure is evaluated. The basis of the definition of the non-linear load-deformation behavior of connections in terms of stiffness, load-carrying capacity, and ductility is reviewed. The reliability of a beam supported by rotational spring is determined in a Monte-Carlo analysis. The impact of different typical variation of the connection design as well as overstrength and excessive stiffness are evaluated. It is shown that by considering sufficient ductility of the joints a gain in reliability of structural indeterminate structures compared to the reference condition of a simply supported beam can be achieved. More detailed specifications on how to consider the connection non-linearity in design should be introduced in design codes

Evaluation of parameters influencing the load-deformation behaviour of connections with laterally loaded dowel-type fasteners

Connections made with laterally loaded dowel-type fasteners are important details in timber structures. According to Eurocode 5, their load-carrying capacity can be calculated with the so-called European Yield Model (EYM) and simplified rules for the determination of slip-moduli are given. The slip modulus is given as a mean value for the serviceability limit state and a simple reduction of slip modulus in the ultimate limit state is given in addition. Despite these simple regulations, it is well known that connections with dowel-type fasteners show a considerable non-linear load-deformation behaviour with different degrees of ductility. This ductility can enable the load redistribution in complex and statically undetermined structures and allow to achieve higher capacities compared to linear elastic design. In the paper, the deformation behaviour of connections with laterally loaded dowels dowel-type fasteners is studied based on more than 750 test results of bolted connections. The parameters influencing the slip-modulus, ductility ratio, and ultimate deformation are evaluated. It is focused on the effects of these parameters and the resulting variability in deformation behaviour. Recommendations are given on how different levels of ductility and deformation capacity can be achieved in dependency of the spacing of fasteners and other geometrical parameters

Long-term deformation behaviour of timber columns: Monitoring of a tall timber building in Switzerland

Knowledge on the short and long term deformation behavior of highly loaded components in tall timber buildings is important in view of improving future design possibilities with respect to serviceability, both in the construction and in the operational state. In this paper, we present the results of a monitoring case-study on a tall timber-hybrid building in Switzerland, a 15 storey and 60 m high office building completed in 2019. A fibre-optic measuring system showed an increase of the deformation with increasing load during the construction phase of highly stressed spruce-GLT and beech-LVL columns. However, the highest strain values were not reported in the columns themselves but at the ceiling transitions and in the area near their supports. The measurements on the columns were compared with model calculations for long-term deformation of timber elements in order to differentiate single components of the total deformation caused by load, time, and changes in climate during the construction. Over a monitoring period of a year, good agreement of the modelled deformations could be confirmed, which indicates that such models could be well suited for future usage in serviceability design of tall timber buildings

Design for adaption - Making timber buildings ready for circular use and extended service life

The construction sector has a significant share of Green House Gas emission and raw material consumption globally. Among common construction materials, timber has a long tradition of numerous applications as a renewable material. The implementation of the concept of circularity in the process of the construction of timber buildings has a high potential sustainability impact. The concept of design for adaption is to extend the service life of timber building to the maximum in several life cycles. In this paper the demand regarding circularity is analysed from interviews with different stakeholders and their economic, social and environmental incentive. The possibility to assess the sustainability impact of design for adaption are evaluated. Different examples of existing practices and potential solutions for design for deconstruction and adaption are summarized

A holistic framework for designing for structural robustness in tall timber buildings

With the ever-increasing popularity of engineered wood products, larger and more complex structures made of timber have been built, such as new tall timber buildings of unprecedented height. Designing for structural robustness in tall timber buildings is still not well understood due the complex properties of timber and the difficulty in testing large assemblies, making the prediction of tall timber building behaviour under damage very difficult. This paper discusses briefly the existing state-of-the-art and suggests the next step in considering robustness holistically. Qualitatively, this is done by introducing the concept of scale, that is to consider robustness at multiple levels within a structure: in the whole structure, compartments, components, connections, connectors, and material. Additionally, considering both local and global exposures is key in coming up with a sound conceptual design. Quantitatively, the method to calculate the robustness index in a building is presented. A novel framework to quantify robustness and find the optimal structural solution is presented, based on the calculation of the scenario probability-weighted average robustness indices of various design options of a building. A case study example is also presented in the end

Experimental study on partial compression parallel to grain of solid timber

This paper describes results and analysis of experimental testing of entire and partial compression strength and stiffness parallel to the grain of solid timber (Japanese cedar). To investigate the spreading effects, the size effect of strength and stiffness, and the mechanism of the damage zone located close to the loading plates, tests on 90 specimens were performed. As a result, it was observed that damage zones existed near the loading plates. The observed spreading effects in the compression parallel to the grain were very small which justifies neglecting them regarding strength and stiffness. Although a presence of a size effect of the compression strength parallel to the grain of glulam specimens with knots was reported, the compression strength parallel to the grain of solid wood specimens without knots does not have a size effect. The height and width of the cross-section of the specimens have an influence on the damage zones and, hence, on the effective modulus of elasticity of the full height of the specimens. The compression strength has a correlation with the density of the timber and the width of the annual rings

Experimental study of compressive properties parallel to grain of glulam

This paper describes results and analysis of experiment of compression behavior parallel to grain of glulam (Norway Spruce, Japanese cedar, and Japanese cypress), especially in damage zones near loading plates and joints. To investigate the influence factors of physical properties and the mechanism of the damage zone near the loading plates or the joints, compression tests on 90 specimens and surface-measurement tests on 48 specimens were performed. As a result, it was observed that damage zones existed near the loading plates and the joints. The lengths of the damage zone of wood–wood joints are larger than that of wood–steel joints. The length of the damage zone was independent of the load and the height of the specimens. However, the length of the damage zone and its scatter increase as the width of the cross-section increases, as does the roughness on the contact surface. It was considered that the cause of the damage zone is the roughness on the contact surface and the length of the damage zone depends on the roughness on the contact surface. Therefore, the length of the damage zone depends on the processing accuracy on the contact surface and has an increasing trend as the contact-surface area increases. There was the size effect of the height of the specimens on the compressive strength because of knots

Designing timber connections for ductility – A review and discussion

This paper discusses the design principles of timber connections for ductility with focus on laterally-loaded dowel-type fasteners. Timber connections are critical components of timber structures: not only do they join members, but they also affect load capacity, stiffness, and ductility of the overall system. Moreover, due to the brittle failure behaviour of timber when loaded in tension or shear, they are often the only source of ductility and energy dissipation in the structure in case of overloading, much like a fuse in an electrical circuit. This paper addresses current challenges in connection design for ductility, reviews selected best-practice design approaches to ensure ductility in timber connections, suggests simple performance-based design criteria to design connections for ductility, and aims to stimulate a discussion around potential solutions to implement safe design principles for ductile connections in future design codes and connection testing regimes

Robustness in fire

With the increasing number of complex and tall timber buildings with a significant area of unprotected timber surfaces, questions arise about the robustness of these buildings in extreme fire scenarios. In recent building projects, measures for robustness have been implemented on an ad hoc basis in agreement between the designers and the authorities. This chapter discusses general approaches to achieve structural robustness with regard to fire design and evaluates them to give guidance for robust fire design of timber structures