Off-diagonal 2-4 damping technology using semi-active resetable devices

Semi-active resetable devices are an emerging and effective method of minimising structural degradation due to environmental loads. Of particular importance in implementing supplemental damping, such as resetable devices, is the ability to retrofit existing structures. However, supplemental damping also tends to increase base shear demand, limiting practical gains. The use of a two-chamber resetable device enables a control law to be used that adds damping only into quadrants 2 and 4 of the force-deflection plot, adding damping forces on the opposing diagonals to the structural force. Thus, base shear can be reduced, creating significant potential for retrofit applications. The impact of off-diagonal 2-4 damping on the displacement structural response, structural force and total base shear is investigated through spectral analysis. The 2-4 control law is shown to be the only law that can reduce the structural force as well as the total base shear for a structure; a unique result. Off-diagonal damping equal to 100% additional stiffness reduced both the structural force and total base-shear by 20-35%. Therefore, semi-active enabled off-diagonal damping could be incorporated into large scale retrofit applications where present passive approaches have significant limitation

Innovative seismic solutions for multi-storey LVL timber buildings

Describes an extensive experimental program at the University of Canterbury, for the development of new structural systems and connections for multi-storey laminated veneer lumber (LVL) timber buildings in earthquake-prone areas. The proposed innovative ductile timber connections are conceptually similar to recent seismic solutions successfully developed for precast concrete multi- storey buildings. The paper gives an overview of the research program, and the results of quasi-static cyclic tests on frame subassemblies, including exterior beam-column joints and cantilever columns, as well as pseudo-dynamic tests on cantilever columns. The experimental results showed significant dissipation of hysteretic energy, good self-centering capacity and no appreciable damage of the structural elements, confirming the expected enhanced performance of the proposed structural systems

Code provisions for seismic design of multi-storey post-tensioned timber buildings

Recent developments and successful preliminary experimental validations of innovative types of ductile connections for multi-storey seismic-resisting laminated veneer lumber (LVL) timber buildings have opened major opportunities for extensive use of structural timber in seismic regions. These particular solutions, named jointed ductile connections or hybrid systems are based on post-tensioning techniques to assemble structural LVL members for both frame and shear wall systems which are designed to exhibit controlled rocking deformations during seismic loading. These systems have been proposed and uccessfully tested using concepts developed for high-performance seismic-resisting precast concrete buildings, currently being approved in major seismic codes and design guidelines worldwide. The extremely satisfactory results of quasi-static cyclic and pseudodynamic experimental tests on exterior beam-column joint subassemblies, column-to foundation connections and shear wall systems have provided valuable confirmation of the high seismic performance of these LVL systems, as well as the reliability of the adopted design criteria and methodology. In this paper, after a brief introduction to the concept of post-tensioned seismic-resisting LVL structures and an overview of experimental results, particular focus will be given to seismic design aspects, within a performance-based design approach, as a sound basis for the preparation of seismic design code provisions

Fifteen years of performance-based design in New Zealand

The paper gives an overview of performance-based design in New Zealand with particular reference to fire, seismic, and durability design of timber structures. The performance-based code came into force 15 years ago when it superseded the previous prescriptive codes. Major changes were introduced, particularly in fire engineering, where every specific limitation, such as that on the maximum number of storeys for timber buildings, was removed. The compliance of each design with the performance requirements may be demonstrated using either of an approved verification method (specified calculations), an acceptable solution (prescriptive requirements), or an alternative solution (special oneoff design). This new regime allowed more freedom in design and the use of new construction systems, but with the simultaneous deregulation of the building industry, a reduction in quality followed, with poor details and sloppy construction leading to a number of problems such as “leaky buildings”. For this reason, more stringent regulations were recently introduced, with a much more rigorous system of quality control

Re-Shaping Hysteretic Behaviour - Spectral Analysis and Design Equations for Semi-Active Structures

Semi-active dampers offer significant capability to reduce dynamic wind and seismic structural response. A novel resetable device with independent valve control laws that enables semi-active re-shaping of the overall structural hysteretic behaviour has been recently developed, and a one-fifth scale prototype experimentally validated. This research statistically analyses three methods of re-shaping structural hysteretic dynamics in a performance-based seismic design context. Displacement, structural force, and total base-shear response reduction factor spectra are obtained for suites of ground motions from the SAC project. Results indicate that the reduction factors are suite invariant. Resisting all motion adds damping in all four quadrants and showed 40-60% reductions in the structural force and displacement at the cost of a 20-60% increase in total base-shear. Resisting only motion away from equilibrium adds damping in quadrants 1 and 3, and provides reductions of 20-40%, with a 20-50% increase in total base-shear. However, only resisting motion towards equilibrium adds damping in quadrants 2 and 4 only, for which the structural responses and total base-shear are reduced 20-40%. The spectral analysis results are used to create empirical reduction factor equations suitable for use in performance based design methods, creating an avenue for designing these devices into structural applications. Overall, the reductions in both response and base-shear indicate the potential appeal of this semi-active hysteresis sculpting approach for seismic retrofit applications - largely due to the reduction of the structural force and overturning demands on the foundation system

Quasi-static cyclic tests on seismic-resistant beam-to-column and column-to-foundation subassemblies using Laminated Veneer Lumber (LVL)

This paper describes part of an extensive experimental programme in progress at the University of Canterbury to develop Laminated Veneer Lumber (LVL) structural systems and connections for multistorey timber buildings in earthquake-prone areas. The higher mechanical properties of LVL, when compared to sawn timber, in addition to its low mass, flexibility of design and rapidity of construction, create the potential for increased use of LVL in multi-storey buildings. The development of these innovative ductile connections in LVL, proposed here for frame systems, have been based on the successful implementation of jointed ductile connections for precast concrete systems, started in the early 1990s with the PRESSS Program at the University of California, San Diego, further developed in Italy and currently under further refinement at the University of Canterbury. This paper investigates the seismic behaviour of the so-called “hybrid” connection, characterised by the combination of unbonded post-tensioned tendons and either external or internal energy dissipaters passing through the critical contact surface between the structural elements. Experimental results on hybrid exterior beam-to-column and column-to-foundation subassemblies under cyclic quasi-static unidirectional loading are presented. The proposed innovative solutions exhibit a very satisfactory seismic performance characterised by an appreciable energy dissipation capacity (provided by the dissipaters) combined with self-centring properties (provided by the unbonded tendons) and negligible damage of the LVL structural elements

Real-time integral based structural health monitoring

An algorithm has been developed to provide real-time structural health monitoring during earthquake events. For a given input ground acceleration the algorithm matches the Bouc-Wen hysteresis model to structural response data using piecewise least squares fitting. The methodology identifies pre-yield and post-yield stiff-ness, elastic and plastic components of displacement and final residual displacement. This approach is particularly useful for rapid assessment of structural safety by owners or civil defense authorities. The algorithm is tested with simulated response data using the El Centro and Kobe earthquake records. Using simulated data for a two degree of freedom shear building model, the algorithm captures stiffness to within 2% of the real value and permanent deflection to within 5% when significant non-linear response occurs. This is achieved with acceleration data sampled at 1KHz and displacement data sampled at 10H

Real-Time Structural Health Monitoring of a Non-linear Four Storey Steel Frame Structure

Structural health monitoring (SHM) is a means of identifying damage from structural response to environmental loads. Real-time SHM is of particular use for rapid assessment of structural safety by owners and civil defense authorities. This paper presents an algorithm for real-time SHM during earthquake events using only acceleration measurements and infrequently measured displacement motivated by global positioning system. The algorithm identifies a nonlinear baseline model including hysteretic dynamics and permanent deformation using convex integral-based fitting methods and piecewise linear least squares fitting. The methodology identifies pre and postyield stiffness, elastic and plastic components of displacement, and final residual displacement. It thus identifies key measures of damage affecting the immediate safety or use of the structure and the long-term cost of repair and retrofit. The algorithm is tested with simulated response data using the El-Centro earthquake record and with measured response data. Both data sets are based on a four-story nonlinear steel frame structure using the El-Centro ground motion record. Overall, the algorithm is shown to provide accurate indications of the existence, location, and magnitude of structural damage for nonlinear shear-type buildings. Additionally, the identified permanent displacement is a particularly useful damage measure for the construction of probabilistic fragility functions

Re-shaping hysteretic behaviour using resetable devices to customise structural response and forces

Semi-active dampers have significant capability to reduce wind and seismic structural response. A novel resetable device with independent valve control laws that enable semi-active re-shaping of the overall structural hysteretic behaviour has been recently developed and validated. Three methods of re-shaping structural hysteretic dynamics are statistically analysed in a performance-based seismic design context. Response reduction factors from the uncontrolled case are obtained across a spectrum of structural natural periods for displacement response, structural force, and total base shear. Overall results indicate that reduction factors are suite (near-field versus far-field) invariant. Resisting all motion and resetting at zero velocity adds damping in all four quadrants and showed 40-60% reductions in the structural force and displacement at the cost of a 20-60% increase in total base-shear. Resisting only motion away from equilibrium adds damping in quadrants I and 3, and gave reductions of 20-40%, with a 20-50% increase in total base-shear. However, only resisting motion towards equilibrium added damping in quadrants 2 and 4 only,, for which the structural responses and total base-shear were reduced 20-40%. The reductions in both response and base-shear indicate the appeal of this unique, semi-active hysteresis sculpting approach for seismic retrofit applications largely due to the reduction of the force and overturning demands on the foundation syste

Semi-active rocking wall systems for enhanced seismic energy dissipation

Rocking walls are an effective method of dissipating structural response energy and mitigating damage during seismic events. Similarly, semi-active resetable devices have shown significant potential to dissipate energy, customize hysteresis behavior and reduce damage. Hence, the addition of a resetable device within a rocking wall can further improve the overall energy management during seismic events. A rocking system, designed for a large open structure, is modeled and the response to a suite of Earthquake records examined for the uncontrolled and device added system. Performance metrics of peak reduction factors, equivalent effective/viscous damping and area enclosed in the Cc vs theta curve are presented for a suite of 10 Earthquakes. Results show that semi-active rocking walls have 3-4x more effective equivalent viscous damping and can provide it every cycle, rather than for 1-2 cycles as with pre-stressed, pre-tensioned passive designs. Such semi-active rocking walls could also be used as supplemental, low-footprint response energy management systems in retro-fitting a variety of structure