Gregory A. MacRae

POST-EARTHQUAKE RESIDUAL DISPLACEMENTS OF BILINEAR OSCILLATORS

Structures subjected to large inelastic deformations during violent ground shaking do not always return to their initial ‘at-rest’ position but may have residual displacements. Even if collapse does not occur, large residual displacements may render them unusable or irreparable. In order to investigate the likely magnitude of residual displacement many bilinear single-degree-of-freedom oscillators with specified ductilities of 2·0, 4·0 and 6·0, stiffness ratios ranging from −0·25 to 1·0 and fundamental periods from 0 to 3·0s were subjected to 11 earthquake records from various ground types. It is shown that bilinear oscillators with positive stiffness ratios generally have small residual displacements, while those with negative stiffness ratios tend to undergo little inelastic reversal of deformation and have larger residual displacements. Reasons for this behaviour were able to be explained by means of a ‘hysteresis centre curve’. A design example for structures able to be modelled as single-degree-of-freedom oscillators is provided.

P‐Δ Effects on Single‐Degree‐of‐Freedom Structures in Earthquakes

A vertical force, P, moving through a lateral displacement, Δ, causes extra “P-Δ” forces on a structure that are not accounted for in analyses based on the undeformed structural geometry. If the effect of these P-Δ forces are large, then displacements during an earthquake may be greater than those when the P-Δ effect is not considered and the likelihood of overturning increases. This paper discusses previous studies to understand the problem of P-Δ. The likely change in response of bilinear oscillators is found from knowledge of the behaviour of simple single degree of freedom bilinear oscillators and the effect of P-Δ on a hysteresis loop. Using the “hysteresis centre curve” (HCC) concept, recommendations are made for the design of single degree of freedom structures with general shape hysteresis curves considering the P-Δ effect.

Damage Avoidance Design Steel Beam-Column Moment Connection Using High-Force-to-Volume Dissipators

Existing welded steel moment frames are designed to tolerate substantial yielding and plastic rotation under earthquake loads. This sacrificial design approach can lead to permanent, and often irreparable damage when interstory drifts exceed 2%. The experimental seismic performance of a 50% full-scale damage avoidance designed structural steel beam-column connection is presented. The beam-column joint region consists of a top flange-hung beam connected to the column by an angle bracket. High-force-to-volume (HF2V) devices are attached from the column to the beam to provide joint rigidity and energy dissipation as the joint opens and closes. The HF2V devices are connected either below the beam flange or concealed above the beams lower flange. Reversed cyclic lateral load tests are conducted with drift amplitudes up to 4%. No damage is observed in the principal beam and column structural elements. The need for stiff device connections to achieve optimal device performance is demonstrated, and potential design solutions presented. Stable hysteresis and repeatable energy dissipation for a large number of cycles up to the 4% drift level is observed. It is concluded that superior and repeatable energy dissipation without damage can be achieved for every dynamic motion cycle, in contrast to conventional sacrificially designed welded moment frame connections.

LMS-based approach to structural health monitoring of nonlinear hysteretic structures

Structural health monitoring (SHM) algorithms based on adaptive least mean squares (LMS) filtering theory can directly identify time-varying changes in structural stiffness in real-time in a computationally efficient fashion. However, better metrics of seismic structural damage and future utility after an event are related to permanent and total plastic deformations. This study presents a modified LMS-based SHM method and a novel two-step structural identification technique using a baseline nonlinear Bouc—Wen structural model to directly identify changes in stiffness due to damage as well as plastic or permanent deflections. The algorithm is designed to be computationally efficient; therefore it can work in real-time. An in silico single-degree-of-freedom (SDOF) nonlinear shear-type structure is used to prove the concept. The efficiency of the proposed SHM algorithm in identifying stiffness changes and plastic/permanent deflections is assessed under different ground motions using a suite of 20 different ground acceleration records. The results show that in a realistic scenario with fixed filter tuning parameters, the proposed LMS-based SHM algorithm identifies stiffness changes to within 10% of true values within 2s. Permanent deflection is identified to within 14% of the actual as-modeled value using noise-free simulation-derived structural responses. This latter value provides important post-event information on the future serviceability, safety, and repair cost.

Experimental Studies on Cyclic Performance of Column Base Strong Axis–Aligned Asymmetric Friction Connections

AbstractThis paper describes experimental testing of an asymmetric friction connection (AFC) at the base of a steel column such as may be used in a moment-frame. Friction/sliding surfaces were parallel to the column-strong axis. In-plane, out-of-plane and 2D clover leaf–cyclic tests were conducted of columns both with and without applied axial force. Tests were conducted both with and without bolts passing through the plates to provide compression on the sliding/friction interface. It was found that this type of rocking base–friction connection can tolerate high levels of drift without significant strength degradation, although some stiffness degradation occurred, especially after the cycles in the weak-axis direction. A simple procedure proposed to estimate the moment resistance is verified. The friction-base connections, having similar cost to conventional connections and exhibiting low-damage performance, have the potential to become widely used in aseismic construction.

Experimental Study of Full-Scale Self-Centering Sliding Hinge Joint Connections with Friction Ring Springs

This article presents 10 tests on a self-centering Sliding Hinge Joint (SHJ) subassembly. The flexural capacity was generated with a combination of ring springs and Asymmetric Friction Connections, with the proportion varied between tests. The joints produced stable and repeatable hysteretic behavior and minimal damage to the floor slab, with self-centering capability improving with increasing ring spring contribution. There was negligible difference between tests undertaken at static and seismic-dynamic rates of loading. The SHJ had a 20% reduction in strength under a near fault pulse type motion. A simple mathematical model of the SCSHJ rotational behavior is also developed.

Christchurch Women's Hospital: Analysis of Measured Earthquake Data during the 2011–2012 Christchurch Earthquakes

A network of acceleration and displacement sensors installed in the Christchurch Womens Hospital (CWH) in July 2011 captured an extensive range of earthquake signals, allowing for a unique opportunity to analyze the performance of the New Zealand South Islands only base-isolated structure. Key characteristics of a range of earthquake signals, including frequency spectra and response patterns, are identified, with particular focus on the swarm of earthquakes on 23 December 2011, including four earthquake events greater than magnitude 5.0 on the Richter scale. The findings indicate that the response of the isolators and the superstructure was essentially elastic for the events analyzed during this period. Accelerations measured above and below the isolators were similar, indicating that the behavior of the devices resembled that of rigid blocks. No significant rocking or torsional motion of the building was observed.

Christchurch Women’s Hospital: Performance Analysis of the Base-Isolation System during the Series of Canterbury Earthquakes 2011–2012

AbstractLive monitoring data and simple dynamic reduced-order models of the Christchurch Women’s Hospital (CWH) help explain the performance of the base-isolation (BI) system of the hospital during the series of Canterbury earthquakes in 2011–2012. A Park-Wen-Ang hysteresis model is employed to simulate the performance of the BI system and results are compared to measured data recorded above the isolation layer and on the sixth story. Simplified single, two, and three degree-of-freedom models (SDOF, 2DOF, and 3DOF) show that the CWH structure did not behave as an isolated but as a fixed-base structure. Comparisons of accelerations and deflections between simulated and monitored data show a good match for isolation stiffness values of approximately two times of the value documented in the design specification and test protocol. Furthermore, an analysis of purely measured data revealed very little to no relative motion across the isolators for large events of moment magnitude scale (Mw) 5.8 and 6.0 that occ...

Seismic performance of non-structural components and contents in buildings: an overview of NZ research

This paper summarizes the research on non-structural elements and building contents being conducted at University of Canterbury in New Zealand. Since the 2010-2011 series of Canterbury earthquakes, in which damage to non-structural components and contents contributed heavily to downtime and overall financial loss, attention to seismic performance and design of non-structural components and contents in buildings has increased exponentially in NZ. This has resulted in an increased allocation of resources to research leading to development of more resilient non-structural systems in buildings that would incur substantially less damage and cause little downtime during earthquakes. In the last few years, NZ researchers have made important developments in understanding and improving the seismic performance of secondary building elements such as partitions, facades, ceilings and contents.

Linear and Nonlinear Seismic Structural Impact Response Spectral Analyses

This paper describes analyses of single-degree-of-freedom structures with different spacing, coefficients of restitution, structural periods, and both linear and nonlinear cases, to a suite of earthquake records with equivalent probability of occurrence. A methodology to relate the probability of impact, and the probability of different levels of percentage peak, spectral displacement increase over a suite of events with equivalent probability of occurrence. Thus, both analyses provide a design risk assessed for these different design parameters, which provides a framework for risk analysis and design that is developed and illustrated. It is shown that smaller gaps between structures and greater difference between structural periods independently lead to greater probabilities of impact. Also, smaller gaps, greater coefficients of restitution and structural linearity (i.e. less yielding) lead to increases of structural displacement as a result of impact. The overall results provide significant insight into the design parameters and their sensitivity around structural impact, and provide these results within a risk based framework amenable to designers and the profession. The approach developed may be generalized to other cases with more degrees of freedom, different masses or damping values.