Embedment test specimens for long-term loading experiments. Photo by: Eva Binder /Michael Schweigler

Project: Long-term mechanical behavior of dowel connections in timber structures as the basis for an extended service life and reuse

The project aims at gaining more knowledge on the long-term behavior of dowel connections, as the key factor for an efficient connection design and as basis for an extended service life and reuse of timber structures, to foster a more sustainable built environment.

Project information

Project manager
Michael Schweigler
Other project members
Eva Binder, Romain Lemaitre, Björn Johannesson, Anders Alrutz
Participating organizations
Linnaeus University
FORMAS, Annual open call 2022 (Early-career researchers)
1 January 2023 – 31 December 2026
Building technology (Department of Building Technology, Faculty of Technology)
Research groups
Wood Building Technology, Connections in Timber Structures
Linnaeus Knowledge Environment
Green Sustainable Development

More about the project

One of the largest contributor to CO2-emissions, responsible for the climate change, is the building sector, and thus it is also the sector with one of the highest potential to reduce future CO2-emissions. One promising opportunity is to foster the use of the sustainable building material timber. The trees as the source of timber do not only take-up CO2 during their growth in the forests, but timber also acts as CO2-storage during the service life in the buildings. It is not only of highest relevance to promote the use of timber for our constructions, but also to keep timber elements as long as possible in the system, before the stored CO2 is released to the atmosphere. This can be done by extending the service life of buildings, or by reusing elements at the end of their service life. The purpose of this project is to enhance engineering design rules, to allow for efficient design of timber structures, by gaining more knowledge on the long-term behavior of timber structures, and especially of their connections, which are in many cases the design-decisive components.

The development of engineered wood-based products, like glulam and cross-laminated timber (CLT) allows constructing large-spanning timber structures and high-rise timber buildings. A crucial factor in designing efficient modern timber structures are the connections between various elements. These connections are usually designed using mechanical fasteners like nails, screws, and especially dowels for high-performance structures. In such structures not only the short-term behavior, looking at extreme forces acting on the structure, but also the long-term behavior, considering forces typically acting over the service-life of the structures are of highest relevance. Load variations, like high wind loads in addition to the self-weight of the structure, might cause local crushing of the wood fibers in the connections. This continuously weakens the connections with the number of large load events, and thus, leads to a reduced connection capacity. In addition, wood shows the phenomena of increasing deformations when exposed to a constant load, like the self-weight of the structure, called creep deformations. Thus, also the deformations in the connections increase over the service life, which might reach an unacceptable limit. The magnitude of these deformations is strongly influenced by the moisture content of the timber and its variation, as a consequence of variation of the ambient climate, like it can be seen when changing from winter to spring. The interplay of moisture content and its variations, load level and load variation, and its effect on the capacity and deformation behavior of dowel connections will be investigated in this project.

A novelty is the combination of experimental investigations on the dowel embedment level, to be used as input to computer models to predict the connection long-term behavior. The long-term dowel embedment behavior exposed to constant loading in constant ambient climate will be investigated by experiments in a first step of the project. This allows to understand the basic mechanical behavior, which is crucial for developing creep models. In a second step, the same experiments will be conducted for varying ambient climate, which allows to calibrate the creep models for in service life situations of timber buildings. In parallel, a moisture transport model will be developed to allow for prediction of the moisture distribution and moisture variation inside the wood in the connection area. This enables to couple the information from the creep model with the typical moisture profiles inside the timber structure elements. All this information come together in the developed computer model for prediction of the long-term connection behavior. The big advantage of the proposed computer model is its calculating efficiency, which allows for a large number of simulations over the entire service life of the timber structure (typically 50–100 years), in a short time. This makes it very suitable for development of enhanced design rules for the long-term deformation and capacity of dowel connections.

The overall goal of the project is to contribute to a reduced carbon footprint of our current and future built environment, while increasing the structural efficiency and safety. Within the project, knowledge on mechanical properties of connections and calculation models for enhanced design rules will be developed.

The project is part of the research in the research groups Wood Building TechnologyConnections in Timber Structures and in the Linnaeus Knowledge Environment Green Sustainable Development.

Structure of the project: “Long-term mechanical behavior of dowel connections in timber structures as the basis for an extended service life and reuse”. Creator: Michael Schweigler
Structure of the project: “Long-term mechanical behavior of dowel connections in timber structures as the basis for an extended service life and reuse”. Image: Michael Schweigler Click for a larger image.