S. Pampanin

PERFORMANCE-BASED SEISMIC RESPONSE OF FRAME STRUCTURES INCLUDING RESIDUAL DEFORMATIONS PART II: MULTI-DEGREE OF FREEDOM SYSTEMS

The role of residual deformations when evaluating the performance of multi-storey frame structures subjected to ground motion is investigated in this paper. The limitations of damage indices available in the literature, either based on ductility, energy dissipation or a combination of both, in capturing such a significant aspect of the seismic response of frame structures are discussed. The concept of residual deformations as a critical complementary indicator to cumulative damage, introduced in a companion paper (Part I) for single-degree-of-freedom (SDOF) systems, is herein extended to multi-degree-of-freedom (MDOF) frame systems. The seismic performance of multi-storey frame structures, either representative of new designed or existing structures, is investigated, focusing on the response in terms of residual deformations. Residual deformations are shown to be sensitive to the hysteretic rule adopted, to the system inelastic mechanism as well as to the seismic intensity. The influence of higher modes and P-Δ effects on the final residual deformations is addressed. A combination of maximum drift and residual drift in the format of a performance matrix is used to define the systems global performance levels and is then extended to a framework for an alternative performance-based seismic design and assessment approach.

ANALYTICAL MODELLING OF THE SEISMIC BEHAVIOUR OF PRECAST CONCRETE FRAMES DESIGNED WITH DUCTILE CONNECTIONS

Pure precast beam-column systems incorporate unbonded reinforced at the critical sections, causing strain incompatibility between steel and concrete. As a result, classical section analysis method, well know for characterising monolithic concrete members, cannot be directly applicable to these systems. This paper provides a section analysis method suitable for precast members, incorporating, through an analogy with equivalent cast-in-place solution named “monolithic beam analogy”, an additional condition on the member global displacement. The proposed method was first validated with the experimental data from tests on beam-column Hybrid subassemblages. Using appropriate hysteresis rules and the response envelopes defined by the section analysis method, a prediction of the behaviour of the PRESSS test building was carried out. Satisfactory agreements obtained between the analytical and experimental results confirm the validity of the suggested methodology. Derivation of the method and experimental validation are herein presented.

“Multi-storey prestressed timber buildings in New Zealand.”

This paper describes recent research and development of a new system for multi-storey prestressed timber buildings in New Zealand. The new system gives opportunities for much greater use of timber and engineered wood products in large buildings, using innovative technologies for creating high-quality buildings with large open spaces, excellent living and working environments, and resistance to hazards such as earthquakes, fires and extreme weather events.

Relevance of beam-column joint damage and collapse in RC frame assessment

The role of joint damage and collapse in the seismic response assessment of existing reinforced concrete frame buildings is herein investigated. Based on recent results from experimental investigations on frame system and subassemblies designed for gravity-load-only, considerations on structural performance based on hybrid local and global failure mechanisms related to joint damage are provided, with particular attention to displacement demand, interstorey drift and damage distribution. Effects of bond deterioration and slip of reinforcing bars passing through an interior joint are discussed in terms of local hierarchy of strength and sequence of events. A simple analytical model for joint response is proposed and adopted for preliminary investigations on frame systems with substandard structural details. The occurrence of a “shear hinge” in the joint might protect to some extent soft-storey mechanisms, reducing the interstorey drift demand, with no significant effects on the global displacement demand. On the other hand, typical inadequacies of structural details (i.e. end-hook anchorage in beam bars combined with use of smooth bars) might cause severe strength degradation leading to particularly brittle failure mechanism.

CONCEPT AND DEVELOPMENT OF HYBRID SOLUTIONS FOR SEISMIC RESISTANT BRIDGE SYSTEMS

The development of alternative solutions for precast concrete buildings based on jointed ductile connections has introduced innovative concepts in the design of lateral-load resisting frame and wall systems. Particularly efficient is the hybrid system, where precast elements are connected via post-tensioning techniques and self-centring and energy dissipating properties are adequately combined to achieve the target maximum displacement with negligible residual displacements. In this contribution, the concept of hybrid system is extended to bridges as a viable and efficient solution for an improved seismic performance when compared with monolithic counterparts. Critical discussion on the cyclic behaviour of hybrid systems, highlighting the most significant parameters governing the response, is carried out. The concept of a flexible seismic design (displacement-based) of hybrid bridge piers and systems is proposed and its reliability confirmed by quasi-static cyclic (push-pull) and nonlinear time-history analyses based on lumped plasticity numerical models.

Assessment and Design Procedure for the Seismic Retrofit of Reinforced Concrete Beam-Column Joints using FRP Composite Materials

In this study, an analytical procedure for the evaluation of the expected performance of existing reinforced concrete (RC) beam-column joints before and after being retrofitted using fiber-reinforced polymer (FRP) composite materials is presented. Focus is given on the evaluation of the shear-strength versus deformation properties of the panel zone region either in the as-built or FRP-retrofitted configuration. Based on experimental and numerical evidence as well as on physical models representing the mechanics of the joint region, principal tensile stresses versus joint shear deformation relationships are adopted and preferred to more traditional nominal shear-strength rules, to evaluate, within a step-by-step iterative procedure, the combination of the joint shear contribution provided by the FRP composite material and that provided by the concrete core alone. The use of principal stresses allows one to directly account for the effects of variation of axial load, typically neglected in the assessment and retrofit of beam-column joints. The hierarchy of strength and sequence of events (damage mechanisms) expected within a beam-column subsystem are visualized via M-N interaction performance domains, used as a basis for a performance-based retrofit philosophy. Specific limit states or design objectives are targeted, with attention given to both strength and deformation limits. The proposed analytical procedure is validated on the results of a set of experimental tests available in the literature. With the intention to provide a simple design tool that can be easily implemented by practicing engineers, a worked example for the evaluation of the expected performance of an FRP retrofitted beam-column joint is provided and used as a basis for a parametric study to illustrate the effects of different strengthening schemes on the behavior of strengthened exterior joint panels under various axial load levels.

Section Analysis and Cyclic Behavior of Post-Tensioned Jointed Ductile Connections for Multi-Story Timber Buildings

A new seismic resisting system for multi-story timber buildings has been recently developed and proposed by the authors based on refinement and adaptation of jointed ductile connections, using post-tensioning technology, originally developed for precast concrete construction. Successful implementation of the concept and confirmation of the high structural and seismic performance of such systems has been obtained through extensive experimental testing on system subassemblies carried out at the University of Canterbury in the past few years. In this contribution, a simplified analytical model for the analysis of the section and overall connections behavior is proposed based on the extension of existing procedures proposed and adopted for the design and analysis of jointed ductile precast concrete walls and frames. Particular focus will thus be given to the specific modifications required when dealing with the peculiarity of post-tensioned connections using orthotropic timber engineered wood materials such as Laminated Veneer Lumber or Glu Laminated systems. The modified procedure, suitable for both analysis and design, is herein verified using the aforementioned extensive experimental test data from quasi-static cyclic tests on wall, column, and beam-column subassemblies. Based on detailed analytical-experimental comparison, looking at both local and global connection behavior, suggestions are given in terms of (a) strain penetration contribution when dealing with internally glued mild steel reinforcement, (b) actual stiffness of the connection when accounting for the bearing effects in the parallel or perpendicular-to-the-grain directions. These considerations are then implemented into the design procedure in order to guarantee control over the re-centering/dissipative system mechanism

Development of Probabilistic Framework for Performance-Based Seismic Assessment of Structures Considering Residual Deformations

Recently, the importance of considering residual (permanent) deformations in the performance assessment of structures has been recognized. Advanced structural systems with re-centering properties as those based on unbonded post-tensioning tendons are capable of controlling or completely eliminating residual deformations. However, for more traditional systems, which count for the vast majority of buildings, residual deformations are currently considered an unavoidable result of structural inelastic response under severe seismic shaking. In this article, a probabilistic framework for a performance-based seismic assessment of structures considering residual deformations is proposed. The development of a probabilistic formulation of a combined three-dimensional performance matrix, where maximum and residual deformations are combined to define the performance level corresponding to various damage states for a given seismic intensity levels, is first presented. Combined fragility curves expressing the probability of exceedence of performance levels defined by pairs of maximum-residual deformations are then derived using bivariate probability distributions. The significance of evaluating and accounting for residual deformations within a Performance-based Earthquake Engineering (PBEE) approach is further confirmed via numerical examples on the response of Single Degree of Freedom (SDOF) systems, with different hysteretic behavior, under a selected suite of earthquake records. Joined fragility curves corresponding to various performance levels, defined as a combination of maximum and residual response parameters, are derived while investigating the effects of hysteretic systems and strength ratios. It is observed that stiffness degrading Takeda systems result in lower residual deformations than elasto-plastic systems and show lower probability of exceeding a jointed maximum-residual performance level. For a chosen performance level, Takeda systems with higher strength ratios show better performance, particularly with lower intensity of excitations.

Experimental evaluation of the in-plane stiffness of timber diaphragms

The seismic response of unreinforced masonry (URM) buildings, in both their as-built or retrofitted configuration, is strongly dependent on the characteristics of wooden floors and, in particular, on their in-plane stiffness and on the quality of wall-to-floor connections. As part of the development of alternative performance-based retrofit strategies for URM buildings, experimental research has been carried out by the authors at the University of Canterbury, in order to distinguish the different elements contributing to the whole diaphragms stiffness. The results have been compared to the ones predicted through the use of international guidelines in order to highlight shortcomings and qualities and to propose a simplified formulation for the evaluation of the stiffness properties.

Quasi-static cyclic testing of two-thirds scale unbonded post-tensioned rocking dissipative timber walls

AbstractPosttensioning low-damage technologies were first developed in the late 1990s as the main outcome of the U.S. Precast Seismic Structural System (PRESSS) program coordinated by the University of California, San Diego, and culminated with the pseudo-dynamic test of a large-scale five-story test building. The extension of posttensioned techniques to timber elements led to the development of new structural systems, referred to as Pres-Lam (prestressed laminated timber). Pres-Lam systems consist of timber structural frames or walls made of laminated veneer lumber, glue laminated timber (Glulam), or cross-laminated timber (CLT). Pres-Lam walls consist of a rocking timber element with unbonded posttensioned tendons running through the length and attached to the foundation, which provides a centering force to the wall, while energy dissipation is supplied by either internal or external mild steel dissipaters. Previous tests carried out on posttensioned timber walls focused on small-scale (one-third) speci...