Robustness of a Timber Core with a Braced System

Abstract

Timber as a construction material for high-rise buildings is gradually entering the construction industry. Timber is considered a sustainable option, because it is a natural material that absorbs CO2 rather than producing it.
However, using timber as a building material also introduces new challenges, one of which is the problem with lateral stability, due to the relatively low stiffness of the elements and their connections. This research investigates the value of a timber core in a building that has bracings in the façade. The study examines this value in both normal conditions and the accidental limit state in case of a fire situation. The key question being addressed is:
Can a timber core sustain lateral load as a secondary load path, in case of failure of the tability bracing?
First, a literature review is conducted to understand the material and relevant safety mechanisms. A parametric program is used to explore the core’s parameters, alongside a linear elastic 3D FEM (Finite Element Method) program that utilises members and surfaces to analyze the building structure. The considered building is a 10-story rectangular structure (28.8 x 21.6 meters) with glulam beams, glulam columns, and CLT floors. The building was designed on an infinite stiff foundation by using a timber core and timber bracings in the façade. The core consists of cross-laminated timber (CLT) panels, connected with slotted steel plates and dowels. The bracings are glue laminated timber and are connected with two slotted in steel plates and dowels. The CLT panel connections and the bracing connections were calculated by hand and implemented in the FEM model. Which was validated
by hand calculations. Additionally, wind load, variable load and permanent load were applied on the model.
This model was used to answer the following question. What percentage of the lateral force can the core take?
The model with a timber core in the Urban Woods shows an 18% reduction in global deflection compared to the model without a timber core in the ultimate limit state. The deflection of both models where to be within the prescribed limit. Additionally to the deflection reduction, the forces in the bracing are reduced by 33% in the model with a timber core. The core parameters that influence these reductions are the connections between the core panels and the cut-outs in the timber core. A core parameter study has been carried out to answer the following question:
How do the core parameters influence the global deflection?
For the connections, increased stiffness enhances the contribution of the core to reducing global deflection, with reductions ranging from 0% to 22%. Using longer panels results in fewer connections, which makes up for 7% of the global deflection reduction at any given stiffness of the CLT connections. Regarding the cut-outs, a larger cut-out size results in a lower contribution to deflection reduction. The reduction in global deflection for
different cut-out sizes ranged from 5% to 27%. As the number of floors increases, the value of a timber core diminishes.
In the accidental limit state of fire, strong wind and failure of a facade bracing element, the timber core will serve as a sufficient alternative load path. However due to the wind force reduction in the ALS the remaining bracing is also capable of withstanding the lateral wind force. When two elements are removed than the core will be necessary. The unity checks for the CLT elements and connections did suffice.