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Post-tensioned rocking structures have received much attention in recent years due to their ability to minimize residual drift and structural damage after an earthquake. Because of these two characteristics, they have been widely recognized as a high-performance structural solution to resist seismic forces compared with current conventional codified systems in which the formation of irreparable plastic hinges in the structural members could result in high economic losses due to demolition, reconstruction costs and downtime. The concept of controlled rocking mechanism was first applied to precast-concrete structures [1] and then successfully extended to steel [2] and timber [3] buildings. When applied to timber, it is known as the prestressed laminated timber (Pres-Lam) system and consists in mass timber structural elements jointed together through the use of unbonded post-tensioned tendons. During an earthquake, the unbonded post-tensioned tendons provide the system with self-centering capacity which brings the structure back to its original position. In addition, external energy dissipation devices may be included in the design to obtain a re-centering/dissipative system with its typical “flag shape” hysteretic loop.
Beam–column joint detail Young Hunter House (Christchurch, New Zealand) and flag-shaped moment-rotation response of a hybrid connection.
Although Pres-Lam structures have shown promising structural performance, the effect of the rocking motion on the seismic demand of nonstructural components should be also taken into account to assess the overall effectiveness of this system. Large seismic demand on nonstructural components can compromise the life safety of building occupants and led to high economic losses as these components represent 75-85% of the original construction cost of a typical commercial building [4]. Furthermore, in some cases a significant portion of economic losses can be associated with post-earthquake downtime or reduced productivity due to nonstructural damage.
Therefore, evaluating the performance of nonstructural components is a crucial aspect to assess the overall effectiveness of Pres-Lam buildings and provide guidance on the seismic design of nonstructural components according to a performance based seismic approach. Past studies suggested that post-tensioned rocking structures can experienced an increase in member forces as results of the structure vibrating in its higher mode during the rocking motion [5]. Therefore, a first concern for nonstructural components in post-tensioning rocking structures is the increased acceleration demand that could result from higher-mode forces. Wiebe and Christopoulos [6] have also demonstrated that large acceleration should be expected in self-centering systems when abrupt stiffness change occurs at high velocity. Consequently, another concern is that potential spikes in floor acceleration could cause damage to nonstructural components during the rocking motion, as the column impact when the frame becomes grounded with the foundation.
However, although a high seismic demand on nonstructural components could have a significant impact on the safety and economic performance of Pres-Lam structures, few studies have assessed the overall effectiveness of this system considering the performance of both structural and nonstructural elements. This research project focuses on Pres-Lam frames with the purpose of assessing the overall seismic performance of this system in a holistic way, considering both structural and non-structural components. In order to give a comprehensive representation of the investigated seismic force-resisting system and provide information on which design parameter influence nonstructural component demand in Pres-Lam frames, a set of prototype buildings is designed by selecting different system attributes and design parameters such as building height and amount of energy dissipation. The seismic response of nonstructural components is evaluated using a cascading analysis method in which the structural response obtained from time history analysis is used as input for the evaluation of the nonstructural component’s response. The FEMA P-58 methodology [7] is then applied to establish if the previous mentioned concerns on the performance of nonstructural components in Pres-Lam frames truly have a significant impact on the expected annual loss in terms of direct economic losses and downtime. To provide design recommendation for nonstructural components in Pres-Lam frames, for each building prototype, the expected annual loss is calculated considering different potential scenario of nonstructural component’s seismic upgrade. The cost-benefit analysis of each of these upgrades is performed using the optimization methodology developed by Steneker [8]. According to this approach, the most efficient seismic upgrade strategies can be determined through the comparison between capital investments associated with each upgrade and the expected increase in resilience obtained from the loss estimation.
The described research methodology is currently being implemented and is expected to: quantify nonstructural seismic demand in Pres-Lam frames compared with conventional structural systems; provide information on which design parameters have a major impact on this nonstructural seismic demand; provide recommendations on the prioritization of the seismic upgrades of nonstructural building components.
References:
[1] Priestley, M. N., Sritharan, S., Conley, J. R., & Pampanin, S. (1999). Preliminary results and conclusions from the PRESSS five-story precast concrete test building. PCI journal, 44(6), 42-67.
[2] Christopoulos, C., Filiatrault, A., Uang, C. M., & Folz, B. (2002). Posttensioned energy dissipating connections for moment-resisting steel frames. Journal of Structural Engineering, 128(9), 1111-1120.
[3] Palermo, A., Pampanin, S., Buchanan, A., & Newcombe, M. (2005). Seismic design of multi-storey buildings using laminated veneer lumber (LVL).
[4] Taghavi, S., & Miranda, E. (2003). Response assessment of nonstructural building elements. Pacific Earthquake Engineering Research Center.
[5] Wiebe, L., Christopoulos, C., Tremblay, R., & Leclerc, M. (2013). Mechanisms to limit higher mode effects in a controlled rocking steel frame. 1: Concept, modelling, and low‐amplitude shake table testing. Earthquake engineering & structural dynamics, 42(7), 1053-1068.
[6] Wiebe, L., & Christopoulos, C. (2010). Characterizing acceleration spikes due to stiffness changes in nonlinear systems. Earthquake engineering & structural dynamics, 39(14), 1653-1670.
[7] FEMA (2012a). Seismic Performance Assessment of Buildings - methodology. Report P-58 Federal Emergency Management Agency, Vol. 1, 1-278, Washington, D.C.
[8] Steneker, P., Filiatrault, A., Wiebe, L., Konstantinidis, D. (Forthcoming). Integrated Structural- Nonstructural Performance-Based Seismic Design and Retrofit Optimization of Buildings. Journal of Structural Engineering, DOI: 10.1061/(ASCE)ST.1943- 541X.0002680.
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