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Seismic resilient multi-storey timber structures with passive damping

Ashkan Hashemi (supervised by Pierre Quenneville), University of Auckland (EQC funded project 15/U710)

Non-Technical Abstract

Traditional low-rise timber structures usually behave well during an earthquake. This is mainly because of wood’s lower density compared to the other construction materials such as steel and concrete. Nevertheless, when considering multi-story residential or commercial timber buildings, a special attention must be paid to the connections or otherwise the building would suffer from considerable damage during a seismic event. In a seismic active country such as New Zealand, it is not only that the building should resist a large earthquake in magnitude but also should be able to resist multiple shakings including the aftershocks without any reduction in its strength. More than that, the building and its main components should return to their original position after each of the shakings. Otherwise, the building may need to be demolished.

In this research, a resilient seismic solution for multi-story timber buildings is developed that uses innovative connections to dissipate the input energy that the earthquakes introduce to the buildings while providing a self-centring behaviour. This means the buildings would behave as predicted during an earthquake without any significant damage and will return to its original position at the end. A special type of connection called Resilient Slip Friction Joint (RSFJ) is used to obtain this behaviour. The provided solution is investigated by computer simulations and real-scale experimental programs which both verified the expected behaviour and the efficiency of the developed concept. In all of the tests, the timber components remained intact and the required behaviour was provided by the connections (RSFJs).

In addition, design procedures are proposed and validated by the experimental data which the structural engineers can use them to design the buildings with the proposed seismic resilient solution. The results of this research showed a great potential to have seismic resilient buildings by using the RSFJ technology.

Technical Abstract

It is a well-known fact that in a seismic resistant timber structure, the timber elements have to behave elastically during an earthquake and all of the energy dissipation has to occur in the connections. In traditional timber structures in which the connections include conventional metal fasteners such as nails, screws, rivets or bolts, the required ductility and energy dissipation are provided by the connections. However, since energy dissipation in timber connections always involve plastic deformation of the fasteners, irrecoverable damage to the connections is highly probable. Thus, the structure requires serious repair and may need to be dismantled. Moreover, the stiffness and strength of the structure may significantly decrease making it highly vulnerable even to a minor aftershock. As a result, a mixing of the timber members and novel energy dissipation devices is required for the structure to be able to tolerate a severe seismic event and the associated aftershocks.

Conventional slip friction connections with flat plates sliding on each other have always been recognized to have one of the most efficient energy dissipation mechanisms. The provided hysteresis which is close to an elastic-perfectly-plastic one combined with the cost effectiveness of these devices has made them very favourable. Nevertheless, the lack of self-centring behaviour is a major disadvantage which results in considerable residual displacements after an earthquake.

The aim of this thesis is to develop damage avoidance concepts for lateral load resisting systems in multi-story timber and hybrid structures that offer compatibility of deformations for all the elastic timber systems in the structure and that can dissipate the energy through friction. The performance of the conventional slip friction connections in seismic resistant structures is investigated and the consequent damage caused by the lack of self-centring behaviour is studied. To provide the self-centring behaviour, alternative novel solutions are proposed. The performance of the structures containing the proposed solutions is investigated by joint component testing, large scale experimental tests and numerical analyses. Analytical design procedures are proposed and validated by the experimental data. Additionally, a novel shear transferring device is proposed and tested to be used as shear key for rocking structures.

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