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Tall Timber Research at the University of Canterbury

The development of strong and stiff lateral load resisting systems (LLRS) is essential for mid-rise and high-rise timber buildings in seismic regions. Recently the University of Canterbury (UC) timber research group completed two research projects to develop robust LLRS for tall timber buildings.

Conventionally braced timber frames typically can provide high strength and stiffness, but the energy dissipation is limited and concentrated to the connections at the ends of the timber braces. These connections can be damaged severely during a large earthquake and are difficult to repair. In addition, their hysteresis curves show pronounced pinching, thus limiting their energy dissipation capacity. By replacing timber braces with buckling restrained braces (BRBs), more stable and reliable energy dissipation can be achieved. In this project, two full-scale glulam frames with BRBs were designed and tested under quasi-static in-plane loading. The BRBs were designed as the ductile elements and the other elements were over-designed to remain elastic following a capacity design approach. Two types of connections were used for the BRB-timber connections. One was dowelled connections with dowels and inserted steel plates, and the other was screwed connections with inclined self-tapping screws and steel side plates. The test results showed that BRBs improved the ductility by at least two times and energy dissipation significantly when compared with timber braced frames. The connections were strong and stiff enough to engage the BRBs and had negligible damage.

Figure 1 BRB-braced glulam frame tests

In mass timber shear wall structures, energy dissipation is generally concentrated to hold-downs and/or the vertical connections between wall panels. Since mass timber products, such as cross-laminated timber (CLT), have high in-plane stiffness, mass timber shear walls deform mainly in a rocking mechanism and the connections are critical and govern the wall behavior. Core-wall tubular structural forms have a great potential to provide necessary increased strength and stiffness for taller timber structures by developing partial composite action of the orthogonal walls through stiff connections. By replacing conventional hold-downs at the base of a mass timber shear wall with (unbonded) post-tensioning, a strong and stiff wall base connection with re-centering capability and small residual

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displacements can be achieved. Post-tensioned timber technology was originally developed and researched at UC in New Zealand in 2005. There are currently 13 Pres-Lam buildings in New Zealand and around the world. In this research, a post-tensioned C-shaped CLT core-wall mainly using screwed connections was designed and tested under uni-directional and bi-directional cyclic loading. Three four-storey high 2/3 scale factor post-tensioned CLT walls were experimentally tested. These walls included a single wall (Phase I), a coupled double wall (Phase II), and a C-shaped core-wall (Phase III). Different screwed connection methodologies were used to connect the CLT wall panels together at the in-plane and orthogonal joint to develop composite action. It was found that screws installed with mixed angles (screws installed at 90° and at 60° to the timber grain) were the most effective solution. Approximately 2/3 partial composite action was reached and the core-wall system stiffness at 0.33% drift increased more than four times when compared to a decoupled test with only friction between the four CLT panels. The experimental test results confirmed that significant system strength/stiffness and ductility/drift capacity can be achieved in a post-tensioned C-shaped CLT core-wall system with small residual displacements and minimal damage through careful connection detailing.

Figure 2 Post-tensioned C-shaped CLT core-wall tests