Stiffness Modelling of Large Timber Connections in Stability Trusses

Abstract

The demand for timber buildings is rising because of the need to make the construction industry more sustainable. Eurocode 5 provides guidance in the design process of timber structures. Numerical modelling is generally used for complex structures like multi-storey timber buildings. Currently, there is limited understanding of the serviceability behaviour of multi-storey timber buildings that are stabilized by glulam trusses. This is partly because the load-slip behaviour of large timber connections is not well understood.

The aim of this thesis is to develop a better understanding of the influence of the connections on the serviceability behaviour of timber buildings with glulam stability trusses. In the first part of this thesis, the state-of-the-art of timber connection design is discussed. The second part is dedicated to the development of a finite element model. A modelling approach is developed that allows for the specification of the load-slip behaviour of the truss connections in the end nodes of the modelled members. A sensitivity study focused on the serviceability stiffness of the truss connections is performed using the FE model. Next, variants on the base design are modelled and studied for comparison, which provides insight into the effect that changing geometry has on serviceability behaviour.

This thesis shows that the load-slip behaviour of the truss connections has a significant influence on the serviceability behaviour of a timber building. It is also found that the current design rules are not suitable for predicting the serviceability stiffness of these connections. For timber connections with slotted-in steel plates and dowels, their load-slip behaviour can be idealized by an initial slip followed by its elastic stiffness. The initial slip is caused by the hole clearance in the steel plates in combination with various non-linear effects like densification of the timber. The hole clearance in the steel plates is also responsible for the sequential activation of the dowels, which is considered to be the primary cause for the reduction in elastic stiffness per dowel per shear plane as the number of dowels increases. This so-called ’group effect’ results in a much lower stiffness of the truss connections than the current design formula suggests.

The results from the numerical modelling show that the load-slip behaviour of the connections between the diagonals and columns has a large influence on the serviceability behaviour of the truss. A distinction is made between diagonal connections and vertical components. The initial slip in the connections causes an increase in the maximum displacement of the structure. The elastic stiffness of the truss connections decreases the global stiffness of the structure. This resulted in an increase in maximum displacement, a decrease in the natural frequency, and an increase in peak accelerations.

The serviceability behaviour of large timber connections should be explored further. Current design codes are not suitable for predicting their serviceability behaviour, which can have consequences for the design of complex structures like multi-storey timber buildings with glulam stability trusses. To ensure the proper design of these buildings, our understanding of their serviceability behaviour is critical. This thesis is anticipated to be a valuable contribution to bridging the knowledge gap. It shows the shortcomings of the current design rules and provides practical information about how to predict the serviceability behaviour of a timber building that is stabilized using glulam trusses.