Load-bearing behavior of multi-storey timber construction – Load-bearing behavior and adaptive stiffness of timber frame walls for earthquake-resistant building bracing in multi-storey timber construction
WHFF Project 2019.07
Project Management: Martin Geiser
The most important facts in brief
- The feasibility of the “walls with openings and adaptive stiffness” approach was investigated using FEM simulations (finite element method) and 1/1 tests on walls with openings to enable statements to be made on stiffness, structural behavior and ductility
- It was found that walls with openings can be realized and it is possible to represent the stiffness of the wall slab well in an FE model
- However, adaptive stiffness has not been found to be feasible for practical applications. The scatter of crack prediction in OSB panel is very large, from which follows that walls with reinforcement are the right way to consider areas with openings
- Damping of wood-frame buildings is another important starting point for cost-effective, earthquake-resistant building bracing in wood construction
Earthquake-resistant construction is mandatory from a technical, normative and legal point of view. A study by the BFH on the relevance of the earthquake design situation in Swiss timber construction has shown “that for residential buildings in timber construction, despite weak to medium seismicity in Switzerland, the earthquake design situation is very often decisive and must be taken into account for the design, as also prescribed by the SIA structural standards”.
The response spectra newly introduced for earthquake design in the SIA 261:2020 standard, as well as the new Z1b earthquake zone, result in even greater earthquake forces for wooden buildings in many cases. With spacious rooms on the first floor and large openings in the facades, contemporary architecture presents a challenge for timber construction in terms of the bracing concept.
The aim of the project was to acquire the necessary basics so that a large-scale project, which is widely supported in the timber construction industry, can be launched. The present project serves to convincingly demonstrate the feasibility of the approach “walls with openings and adaptive stiffness” thanks to appropriate documentation, i.e. FEM simulations and especially 1/1 tests on walls with openings. The overall aim of the project is to promote the use of wood as an earthquake-resistant building material and to close the knowledge gap regarding stiffness, structural behavior and ductility of walls with openings.
The investigations have shown that walls with openings can be realized. Furthermore, it is possible to represent the stiffness of the wall slab well in an FE model (finite elements).
Walls with C-shaped planking panels have not proven practical for several reasons. For example, the performance of walls with C-plates without adaptive stiffness is not sufficiently high at the design level, especially for larger openings. Furthermore, adaptive stiffness has not been shown to be feasible for practical applications. The scatter of crack prediction in the OSB panel is very large. As a result, the residual load-bearing capacity, after it has been mathematically ensured that the panel crack has occurred, is no longer sufficiently high. Furthermore, it must be taken into account that the scatter of the panel crack would become even larger if several panel manufacturers were considered for the determination of the crack value. It follows that walls with reinforcements are the correct way to consider areas with openings. Reinforcements can be either wind-rail strips or laminated veneer lumber strips. Fire design considerations should be added to choose between these two options.
Anchoring walls with openings only in the edge posts is feasible, subject to certain specifications. Due to the sill deformation, a shear anchorage must be found which is very soft to vertical deformation. For further research work, it should be noted that any anchors must be installed during tests. For walls with openings, it is not practical to anchor the wall with the sill for shear transfer. For any calculation models that are adjusted by testing, the tie stiffness must be determined experimentally. The same applies to the shear anchorages to be used. Furthermore, slip in the tension connection must be prevented by greater prestressing or other suitable measures.
The damping of wood-frame buildings is another important starting point for cost-effective, earthquake-resistant building bracing in wood construction. The research conducted as part of this study could not contribute to defining the damping of wood-frame buildings in more detail. Nevertheless, this topic should be further investigated. Two recommendations could be made for further swing-out tests. First, a new release mechanism must be developed. Shear and tension bolts are of particular interest for this purpose. A brittle steel must be used, so that the load drop occurs without much delay. Furthermore, long and low height test setups do not appear to be suitable for this type of investigation due to the short fundamental vibration time.
In follow-up investigations, increased attention must be paid to the failure prediction of the stud frame. No statement could be made on the validity of the FE models with respect to the failure force prediction of the stud frame.
The project was supported by the Swiss Forest and Wood Research Promotion WHFF-CH of the Federal Office for the Environment BAFU.
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For further information please visit the research database ARAMIS.