Influence of a timber outrigger system on cross laminated timber core buildings

Abstract

The construction industry contributes significantly to global emissions. In response to the growing concern about climate change, new methods are being developed to reduce these emissions. Building with timber is one such approach, as it can be CO2 neutral if reforestation is practiced. Timber is a lightweight and flexible material compared to traditional construction materials like steel and concrete, which can result in high accelerations and deflections, negatively affecting user comfort. Engineered wood products such as CLT and GLT make it possible to construct larger panel sizes and spans, which can lead to more efficient building design. Many buildings utilize core systems to provide lateral stability and facilitate vertical transportation of goods and people. Therefore, replacing a conventional concrete core with a CLT core could be a viable solution to reduce the building’s environmental impact. Timber, being a flexible material, often lacks the necessary stability to meet user requirements for deflection and acceleration in high-rise buildings. An outrigger, similar to those used in concrete and steel structures, can provide the solution to enhance user comfort. The outrigger transfers bending moments from the core to the columns, reducing forces in the core and increasing overall stiffness. However it is unclear what the influence of the addition of a timber outrigger on a CLT core building is and how such a system can be improved. Therefore the purpose of this thesis is to evaluate the impact of a timber outrigger on a CLT core building, and provide guidance for structural engineers on how to improve the structural behaviour of such a system. The impact of adding an outrigger is evaluated through the comparison of different numerical case studies with and without timber outriggers. Additionally a sensitivity analysis is performed in which the influence of core joint design and the impact of outrigger design parameters is evaluated. By introducing a timber outrigger to a CLT core building bending moments are transferred from the core to the column, partially restricting rotation of the core. This will have the following effects on the behaviour of the structure. Firstly, the deflections due to lateral wind loading are reduced due to the increased global stiffness. Secondly, the increased stiffness of the structure will result in a reduction of peak lateral acceleration. Thirdly, by transferring loads from the core to the columns the strength requirements on the connections of the core are reduced. Finally, by introducing a timber outrigger system to a CLT core building horizontal forces are generated in the core at outrigger level, which should be accounted for in the design phase of the vertical core joints. Increasing the stiffness of the outrigger will further reduce deflections, peak lateral acceleration and strength requirements on the core joint, however strength requirements on the vertical core joints at outrigger level will further increase. An exponential relationship has been derived between the lateral deflections and the ratio between the effective bending stiffness of the outrigger and the effective bending stiffness of the core, a similar exponential relationship has been derived for the peak lateral accelerations Additionally it was investigated what the critical design parameters are and how the performance of a CLT core building with timber outriggers can be improved. For the design of a CLT core the connection between the core and foundation showed to be critical, having large effects on the lateral deformation. While increasing the stiffness of the vertical connections showed to have little influence on the interaction between core walls due to their respective high interaction factors. In the design of the outrigger it was shown that a two storey outrigger is able to more than double the reduction in deflections compared to a single storey outrigger, further increasing the overall effectiveness of the system. Finally the effectiveness of the system was found to be significantly influenced by the configuration of the outrigger truss, demonstrating a greater impact than increasing member size or connection stiffness.