2.1 Self Centering Systems
Modelling of Rigid Rocking of Structures during Earthquake using Linear Functions
P. Yu, S. Gutschmidt & G.A. MacRae
ABSTRACT: New generation seismic structures are likely to suffer no damage in a major earthquake. One of the means of protecting a structure from damage is to allow it to rock. This paper describes the rocking behaviour of a simple cantilever column on various foundation types. It is shown that what is often referred to as “rocking” is in fact a combination of two modes; (i) vibration and (ii) rocking. In vibration, the supports of the structure move vertically depending on the stiffness of the supporting material and its own stiffness, but there is no lift-off. In rocking, there is no deformation in the foundation material or structure, and all lateral deformation is due to lift-off. Equations are developed to describe both of these deformation modes and while the displacement-time response for both modes can have some similarities, there are also major differences. The displacement-time response over one cycle looks similar in both modes; however the velocities and forces are significantly different. Also, if there is energy dissipation, the amplitudes of both modes decrease, but for rocking structures, the “period” of “vibration” also changes. An experimental study is also performed on different foundation types. It is shown that during free vibration from large displacements, rocking almost totally dominates, and then there is a time in which there is a relatively sudden change to the vibration mode.
Recentering Requirements for the Seismic Design of Self-Centering Systems
R.S. Henry, S. Sritharan & J.M. Ingham
ABSTRACT: To achieve the full benefits of a self-centering seismic resilient system, the designer must ensure that the entire structure does indeed self-center following an earthquake. An idealised flag-shaped cyclic hysteresis response is typically used to define the residual drift behaviour of a self-centering member. However, such an idealised cyclic hysteresis response seldom exists and the residual drift of a building subjected to an earthquake is dependent on the actual shape of the cyclic hysteresis response as well as the dynamic loading. To accurately capture the cyclic hysteresis response, the design must consider the inelastic strain in the compression toe of the member and the resulting stiffness degradation of the hysteresis loops.
This paper summarises the current methods that are used to ensure that a self-centering response is achieved during the design of seismic resilient structures. A simple lumped plasticity model was used to demonstrate the inaccuracies of these current procedures and highlight the need to accurately capture the structures dynamic hysteresis response. Additionally, the results were presented for time-history analysis that was performed to investigate the expected residual drift of an example self-centering concrete wall system during an earthquake. Time-history analyses indicated that due to dynamic shake-down the final residual drifts were less than 35% of the maximum possible residual drifts that were observed from the cyclic hysteresis response.
Development of the Self-Centering Sliding Hinge Joint
H.H. Khoo, G.C. Clifton, J.W. Butterworth, C.D. Mathieson & G.A. MacRae
ABSTRACT: The Sliding Hinge Joint is a beam-column connection used in low damage moment-resisting steel frames. It allows inelastic deformation with small but significant losses of elastic strength and stiffness during a major earthquake, and does not always return the joint and overall building to the pre-earthquake position. It thus does not fulfil the optimum requirement of no maintenance. This paper presents the ongoing development of a damage free self-centering Sliding Hinge Joint utilising friction Ring Springs. This work has commenced with tests to determine (1) the effects of steel shims of different hardness on the sliding behaviour, (2) the adequacy of the bolt model previously developed for the SHJ, and (3) the residual joint strength of the post-earthquake joint. The abrasion resistant Grade 400 plate, the hardest steel considered, generated the largest sliding shear capacity, and the most stable sliding characteristics. It is therefore recommended for use in future SHJ construction. The bolt capacities and residual joint strengths are affected by bolt size, length of bolt lever arm and presence of Belleville Springs. These effects are not yet fully understood at the time of writing. More tests of various specimen sizes will be used to develop a more accurate model.
NMIT Arts & Media Building - Damage Mitigation using Post-Tensioned Timber Walls
C.P. Devereux, T.J. Holden, A.H. Buchanan & S. Pampanin
ABSTRACT: The NMIT Arts & Media Building is the first in a new generation of multi-storey timber structures. It employs an advanced damage avoidance earthquake design that is a world first for a timber building. Aurecon structural engineers are the first to use this revolutionary Pres-Lam technology developed at the University of Canterbury.
Conventional seismic design of multi-storey structures typically depends on member ductility and the acceptance of a certain amount of damage to beams, columns and walls. The NMIT seismic system relies on pairs of coupled LVL shear walls that incorporate high strength steel tendons post-tensioned through a central duct. The walls are centrally fixed allowing them to rock during a seismic event. A series of U-shaped steel plates placed between the walls form a coupling mechanism, and act as dissipators to absorb seismic energy. The design allows the primary structure to remain essentially undamaged while readily replaceable connections act as plastic fuses.
This technology marks a fundamental change in design philosophy. Whilst being compliant for earthquake response currently means “a standard that ensures people can walk out alive” this damage avoidance technology ensures that the building “still functions” after an event.
In this era where sustainability is becoming a key focus, the extensive use of timber and engineered-wood products such as LVL make use of a natural resource all grown and manufactured within a 100 km radius of Nelson.
This project demonstrates that there are now cost effective, sustainable and innovative solutions for multi-storey timber buildings with potential applications for building owners in seismic areas around the world.
Design of UFP-coupled Post-Tensioned Timber Shear Walls
M.P. Newcombe, D. Marriott, W.Y. Kam, S. Pampanin & A.H. Buchanan
ABSTRACT: Recent advances in timber design at the University of Canterbury have led to new structural systems that are appropriate for a wide range of building types, including multi-storey commercial office structures. These buildings are competitive with more traditional construction materials in terms of cost, sustainability and structural performance. This paper provides seismic design recommendations and analytical modelling approaches, appropriate for the seismic design of post-tensioned coupled timber wall systems. The models are based on existing seismic design theory for precast post-tensioned concrete, modified to more accurately account for elastic deformation of the timber wall systems and the influence of the floor system. Experimental test data from a two storey post-tensioned timber building, designed, constructed and tested at the University of Canterbury is used to validate the analytical models.
The Demountability, Relocation and Re-Use of a High Performance Timber Building
T. Smith, R. Wong, M.P. Newcombe, D. Carradine, S. Pampanin, A.H. Buchanan, R. Seville & E. McGregor
ABSTRACT: This paper outlines the deconstruction, redesign and reconstruction of a 2 storey timber building at the University of Canterbury, in Christchurch, New Zealand The building consists of post tensioned timber frames and walls for lateral and gravity resistance and timber concrete composite flooring. Originally a test specimen was subject to extreme lateral displacements, in the University structural testing laboratory, and subsequently has been dismantled and reconstructed as offices for the Structural Timber Innovation Company (STIC). In doing this over 90% of the materials have been recycled which further enhances the sustainability of this construction system. The paper outlines the necessary steps to convert the structure from a test specimen into a functioning office building with minimal wastage and sufficient seismic resistance. The feasibility of recycling the structural system is examined using the key indicators of cost and time.