Optimised structural system for multi-storey timber buildings

In this project, researchers developed a structural system for multi-storey timber structures that is optimised for horizontal action (wind, earthquakes). They focused on areas with low to medium seismicity and moderate exposure to wind. By proposing new and standardised layouts for joints and further developing existing design and test methods, they make a significant contribution to making timber structures more competitive compared to rivaling construction methods. This could make multi-storey timber structures more reliable and economical and increase their planning certainty and reliability. Based on the data collected in the experimental tests on joints, wall elements and buildings, the designers can now estimate with greater reliablilty the stiffness, load-bearing and displacement capacity.

In future, an increasing number of residential, office and school buildings up to medium storey height (3 to 8 storeys) will be built completely out of timber. For this reason, established multi-storey timber structural systems need to be further developed, particularly the lateral load resisting elements and their design in view of withstanding earthquakes and strong winds. Serviceability, fire safety, noise protection and the use of resources are additional boundary conditions in this study.

In this project, researchers set out to develop a structural system for multi-storey timber structures that is specifically designed for horizontal actions as they occur during earthquakes and strong winds. The design method used is deformation based and builds on the principles of capacity design: zones that behave elastically or inelastically during an extreme event are defined in the structure. The project was divided into three modules: module 1 studied the behaviour of joints under cyclical loading, module 2 the behaviour of wall elements and relevant adjacent structural elements, and module 3 the behaviour of the entire structure and its design method.

A timber system for multi-storey wood structures that are optimised for earthquakes and wind loads will make wood more competitive compared to other building materials. The standardisation of joints and further development of design and test methods envisaged in this project could make multi-storey timber structures more reliable and economical and increase their planning certainty.

The results of the tests on joints showed that design rules according to Eurocode 5 for nailed and stapled OSB panel-to-timber connections need to be re-examined regarding the influence of forces at an angle to the grain, prescribed spacings and edge distances of connectors and equations to estimate the load-bearing capacity.
On-site shaking tests of a multi-storey light-frame timber building with timber-concrete composite slabs revealed considerable differences in structural stiffness between experiment and design model. This is the case because the stiffness of the structural light-frame timber walls and the non-structural walls was underestimated. In addition, friction was not included in the model. Neglecting friction, which is the common practice in design, is not suitable in research when comparing test and numeric model. The researchers therefore performed tests to study friction between components of light-frame timber walls; the numerical model could then be improved by accounting for the experimentally determined static and kinetic friction coefficients.
The researchers developed adapted loading protocols for cyclical tests of joints and wall elements. Compared to protocols for areas with high seismicity, these protocols include fewer load cycles and smaller cumulative damage demands. This opens the door for more competitive solutions in areas with low to medium seismic activity. With the help of a parametric study on single-degree of freedom systems, the researchers could establish new relationships between behaviour factor and ductility. This relationship makes it possible to apply the method used in the design of reinforced concrete structures (N2 method according to Eurocode 8) to light-frame timber structures.

Earthquake resistant wood structures for multi-storey buildings