Low-damage System Research Articles

A self-centering multilevel rocking wall-frame system for mitigating seismic damage in precast concrete buildings

In order to improve seismic resilience and reparability of precast concrete buildings, different forms of rocking wall building systems have been proposed in recent decades. The base-rocking single wall-panel system incurs limited damage under lateral loading, but it experiences high floor acceleration and storey forces for mid- and high-rise buildings due to the effect of higher vibration modes. A multiple-rocking wall system has been proposed to reduce the effect of higher modes in storey forces, but this system leads to larger drifts, thereby increasing the damage to drift-sensitive non-structural elements. To overcome these limitations, the present study proposes a Self-centering Multilevel Rocking Wall-frame (SMRW) system that simultaneously reduces all forms of seismic demands. The SMRW system consists of multiple small rocking wall units, semi-rigid horizontal precast concrete panels and post-tensioned base-rocking boundary columns at each floor level. To enhance the energy dissipation capacity of the proposed system, external tension-compression yield steel dampers are provided across each rocking interface. Superior seismic behaviour of the proposed SMRW system is validated by comparing the seismic performance of 3, 9, 15 and 21-storey building models with the proposed SMRW system together with the base-rocking, multiple-rocking and conventional cantilever structural wall systems. For this purpose, nonlinear static and dynamic (time-history) numerical analyses are conducted on the building models using finite-element and fiber-element modelling, which are validated using experimental results. As the numerical results demonstrate, the proposed SMRW system, while being a low-damage system, can simultaneously mitigate the inter-storey drift, floor acceleration and storey forces/moments compared to other systems. Therefore, the proposed SMRW system can significantly reduce seismic damage to a building’s structural and non-structural elements, and can offer a truly low-damage option for mid and high-rise buildings in seismic regions.

Effect of serviceability limit state and over-strength mechanism on seismic performance of self-centring low damage structures with friction connections

Low damage design is an alternative to conventional seismic design, where earthquake-induced damage in the building is controlled and minimised. The most common approach to implement this concept is to design the structural system in a way that damage occurs in a series of specified components. This makes the designer able to predict the performance of the building under earthquakes by deliberately designing these components weaker than the other parts of the structure. These components can be replaced, repaired, or even designed to remain functional after earthquakes. Therefore, the performance of such components (e.g. weak links) is critical for evaluting the performance of the building as a whole.The New Zealand standard defines two extra limit states in addition to the design level earthquake (Ultimate Limit State (ULS)). First is the Serviceability Limit State (SLS), where the system is expected to remain linear and elastic, and the second is the collapse limit state (also known as MCE limit state), where damage is accepted, but the building should not collapse. For a low damage system, the weak links should be designed to remain linear and elastic under SLS actions. Also, they should have adequate reserve capacity to not completely fail under MCE actions. The latter is also called the over-strength mechanism (or the upper bound design level) for designing the low damage components. This study investigates the effect of the over-strength and serviceability limit state on the overall seismic performance of low damage systems. Two types of generic structural responses are considered for the study, self-centring braced frames and self-centring rocking wall systems. Numerical models for both concepts are developed and subjected to nonlinear dynamic time-history analysis with different intensities up to large, rare events (MCE). The results showed that braced frames' appropriate over-strength capacity is relatively smaller than rocking walls. The findings of this study help designers and engineers to perform more efficient and optimised designs for low damage systems. Also, this research helps them to have a better understating of the performance of such systems under large, rare earthquake events.

Seismic assessment and finite element modeling of traditional vs innovative point fixed glass facade systems (PFGFS)

In the last decades, recent earthquakes have further highlighted the high vulnerability of non-structural components. Post-earthquake damage due to building envelope, equipment and building contents can lead to substantial economic losses in terms of repair costs and daily activity interruption (downtime). Moreover, non-structural damage can represent a life-safety threat for both occupants and pedestrians. These considerations confirm the crucial need for developing low-damage systems for either structural or non-structural elements. This paper aims to assess the seismic performance of glazed facade systems, widely adopted in modern buildings, focusing on point fixed glass facade systems (PFGFSs), also referred to as “spider glazing”. In this work, a numerical investigation is developed to study the seismic performance of such systems at both local-connection level through a 3D FEM in ABAQUS as well as at global system level through a simplified lumped plasticity model in SAP 2000 to assess the overall in-plane capacity of the facade. Based on the local connection and global facade system behavior, a novel low-damage connection system is herein proposed, and a parametric study is carried out on the key parameters influencing the facade capacity. The benefits of implementing low-damage connection details are highlighted by an increase of the in-plane capacity of the facade system when compared to a traditional solution. To further investigate the potential of the proposed low-damage details in preserving the integrity of the facade system itself, non-linear time history analyses have been carried out on a case-study building equipped with the innovative PFGFSs.

Quasi-Static Cyclic Testing of Hybrid Posttensioned Bridge Column Supported on a Monopile Foundation

In this research, a low-damage posttensioned rocking solution referred to as dissipative controlled rocking (DCR) connection is proposed for bridge columns supported on monopile foundations. The DCR connection is a low-damage system that replaces the plastic hinge mechanism in a column and aims to minimize and potentially eliminate the repair time and costs after an earthquake. A DCR bridge system combines unbonded posttensioning and replaceable dissipaters to provide self-centering and energy absorption capabilities, respectively. Current research on DCR column systems has focused on the performance of bridge bents founded on a fixed foundation, neglecting soil damping effects. In this research, a one-third scaled bent supported on a monopile with passive soil resistance was tested under quasi-static cyclic loading to validate the lateral seismic response of a DCR bridge bent with the contribution of the soil–foundation–structure interaction. When developing the details for the DCR connection, particular emphasis was made on utilizing conventional construction materials and forms for a low-damage bent that yields a similar construction cost to a monolithic joint. Nevertheless, the proposed DCR connection can be easily and quickly repaired or replaced after a significant seismic event. The performance of the bent was compared against a benchmark structure with an emulative connection type that resembles a monolithic cast-in-place joint. Results from testing suggest a greater performance of the low-damage DCR column. The dynamic response of the DCR column built on a flexible foundation differed from that of a fixed-base structure, which is primarily due to the energy dissipation capability of the flexibly supported structure. Flexibility in the soil foundation increased the structural period of the structure and reduced the seismic response of the DCR column. Damage to the DCR bent was limited to concrete crushing at the rocking interface, which proved to be easily repairable. In contrast, the bent with an emulative connection developed plastic hinging at the bottom of the column, which was nonrepairable.

Shake table tests of concrete anchors for non-structural components including innovative and alternative anchorage detailing

In recent years, the growing need for reducing non-structural damage after earthquakes has stimulated a dedicated effort to develop innovative types of fasteners for anchoring non-structural components (NSCs) to reinforced concrete (RC) host-structures. To contribute to such need, and building on previous research, this paper presents the results of a series of uni-directional shake-table tests of simulated NSCs anchored to concrete via: (1) expansion, and (2) chemical anchors; post-installed into: (a) uncracked, and (b) cracked concrete. Considering different construction details, the experimental investigation focused on traditional anchorage systems, alternative solutions comprising mortar filling into the gap clearance, and a low-damage system relying on supplemental damping devices, capable of reducing the acceleration of the NSCs as well as the force of the anchorage during seismic shakings. The experimental tests provided significant evidence on the beneficial effects of a dissipative anchorage protecting both the non-structural component and the anchorage itself, even during strong earthquakes. Moreover, when construction details allow to close the fixture clearance with a mortar filling, this stiffer solution provide an additional reduction of NSCs seismic accelerations and forces. Therefore, suggestions for further improvements of the adopted low-damage solution are also proposed.

Large-scale testing of low damage rocking Cross Laminated Timber (CLT) wall panels with friction dampers

The use of controlled ductile rocking shear walls with low damage connections can be an efficient alternative to traditional high damage design in order to mitigate the earthquake induced damage. Nowadays there is an increased demand for seismic low damage systems that can offer a high resistance level against the severe seismic events, allowing the buildings to be swiftly returned to service. This paper introduces a friction-based energy dissipation hold-down connection for rocking Cross Laminated Timber (CLT) structures with potential for re-centring behaviour. The results of component experimental tests on the device and results of large-scale tests on a rocking CLT wall with the proposed hold-down connectors have shown that the proposed system has the potential to be used in earthquake-resistant low- to mid-rise CLT structures. Furthermore, capacity prediction equations are provided for this system that are verified by comparing the analytical data with the experimental results.

Wigram–Magdala Link Bridge: Low-Damage Details for a More Efficient Seismic Design Philosophy

Research on low-damage systems has been significant in recent years. These systems combine post-tensioning, which provides self-centring; and replaceable devices, which provide energy dissipation. Research by the University of Canterbury, New Zealand, on low-damage bridge piers has been used and developed into a real structure: the Wigram–Magdala Link Bridge. This is believed to be the first bridge in New Zealand, and possibly worldwide, to adopt such a system. This paper uses the Wigram–Magdala Link Bridge as a case study to describe the low-damage details adopted. The paper will then discuss the opportunities which exist for future incorporation of low-damage detailing on bridge structures. For example, the unique features of controlled damage, excellent post-damage performance and ease of repair offer the potential for a change in seismic design philosophy. The low-damage connections provide an opportunity to permit controlled and easily repairable damage to occur in a lower return period earthquake than currently allowed. This shift in seismic design philosophy enables a more efficient use of materials, while still preventing collapse of the structure.

Quasi-static cyclic testing and analytical investigation of steel plate shear walls with different post-tensioned beam-to-column rocking connections

Self-Centering Steel Plate Shear Walls (SC-SPSW) have been recently proposed to help achieve possible immediate occupancy following an earthquake for buildings located in regions of high seismicity. For this purpose, like other recently developed low-damage systems, SC-SPSWs seek to: (1) eliminate damage to the surrounding gravity frame of the seismic force resisting system; (2) provide frame recentering, in order to return the building back to its pre-earthquake near vertical alignment, and; (3) concentrate hysteretic energy to replaceable elements, providing rapid and practical reparability.A testing program was conducted investigating the quasi-static cyclic behavior of SC-SPSW systems detailed with three different beam-to-column connections and two SPSW infill configurations. Specimens consisted of one-third scaled single-bay three-story frames. Results are presented for SC-SPSWs detailed with three different post-tensioned (PT) beam-to-column rocking connections that: either (1) rock about the top and bottom beam flanges; (2) rock about the top beam flanges only referred to as the NewZ-BREAKSS connection, or; (3) rock about the beam centerline. The latter two connections were investigated to mitigate floor system damage due to PT boundary frame expansion (i.e., beam-growth) that occurs with the first mentioned rocking connection detail. Experimental and numerical results are presented for both a solid infill web plate and infill web strip configuration. Results presented show that solid infill web plates provide some additional strength and energy dissipation due to their strength in compression and affects frame recentering for quasi-static loading conditions of progressive increasing displacement cycles. In contrast, infill web strips are essentially tension-only and have the benefit that they are not susceptible to tearing from the boundary frame and does not affect frame recentering. Numerical results show that the compression strength of the solid infill web plate in these experiments was approximately 10 percent that of the yield strength of the infill web plates in tension. Furthermore, lessons learned from this testing program are presented that show that for a multiple actuator configuration along the height of a multi-story SPSW specimen, a top level displacement control with force control of the lower actuators is necessary to avoid undesired actuator interaction along the frame heights.

Low Damage Moment Resisting Connection Using Blind Bolts

Concrete Filled Steel Tubes (CFSTs) are being used as columns in moment resisting frames in many parts of the world. Because of their aesthetic appearance, favourable ductility and large load bearing capacity, they are popular with architects and engineers. The use of CFSTs is limited in some countries (like Australia) due to the problems and cost associated with the connection of steel beams to the closed column section, unlike open H-shaped columns where ordinary structural bolts can be used. In this paper, a structural system is proposed which uses moment resisting frames as the lateral load resisting system. This system eliminates the use of welds at the site, which is the most common method, although expensive, which is used to achieve a moment resisting connections. The proposed system uses double T-stub connections to connect universal beams on opposite sides of the CFST column, and headed anchored blind bolts are used to connect those T-stubs to the column. This system provides sufficient stiffness and strength to be used in low to mid-rise buildings in low to moderate seismic regions. The proposed system uses a capacity design method to limit the load and non-recoverable damage in the connection (especially in the anchorage). One of the components in the connection, the web of the bottom T-stub, is designed as a fuse to create a low damage system for very rare earthquakes.

Probabilistic safety assessment of self-centering steel braced frame

The main drawback of conventional braced frames is implicitly accepting structural damage under the design earthquake load, which leads to considerable economic losses. Controlled rocking self-centering system as a modern low-damage system is capable of minimizing the drawbacks of conventional braced frames. This paper quantifies main limit states and investigates the seismic performance of self-centering braced frame using a Probabilistic Safety Assessment procedure. Margin of safety, confidence level, and mean annual frequency of the self-centering archetypes for their main limit states, including PT yield, fuse fracture, and global collapse, are established and are compared with their acceptance criteria. Considering incorporating aleatory and epistemic uncertainties, the efficiency of the system is examined. Results of the investigation indicate that the design of low- and mid-rise self-centering archetypes could provide the adequate margin of safety against exceeding the undesirable limit-states.

Low-Damage Posttensioned Segmental Bridge Columns with Flexible End Joints for Seismic Accelerated Bridge Construction

A computational parametric study leading toward the development of a new type of low-damage precast concrete segmental column for accelerated bridge construction in seismic areas is outlined. In contrast to conventional monolithic concrete construction, accelerated bridge construction significantly reduces on-site construction time, total project delivery time, weather-related construction delays, and environmental impact, and it provides improved work zone safety for the traveling public and contractor personnel and high product quality and durability. Low-damage systems provide significantly reduced postearthquake downtimes, repair costs, and casualties. The proposed column design will incorporate internal unbonded posttensioning and flexible joints at the column ends, where large flexural demands occur. These joints will be (flexurally) deformable enough to accommodate large column lateral deformations (drift ratios of 10% or higher) without damage, and (axially) stiff enough to limit initial axial deformations due to initial posttensioning and gravity loads. To investigate this concept, a three-dimensional finite element model of a cantilever segmental concrete column incorporating a flexible joint segment at the bottom was generated, and a comprehensive parametric study was conducted. The parametric study investigated three types of polyurethane elastomers for the flexible joint and various geometric properties of the flexible joint in reference to the column geometry. The findings of this study, as supported by preliminary material testing, have shown that hyperelastic materials, such as stiff rubbers and polyurethanes, are good candidates for the proposed application.

Accelerated Bridge Construction (ABC) and seismic damage resistant technology

Although New Zealand bridges performed well structurally during recent Canterbury earthquakes, some critical arterial routes lost their functionality. Life Safety is still our primary objective but nowadays we are moving towards new societal needs which also, at minimum, aim to limit business disruption. Building designers are already moving towards low-damage system technology for both structural and non-structural components. Bridge engineers have to inherit those enhanced concepts and technologies. In fact, in order to protect the economy and save lives, it is vital that bridges remain drivable after a natural disaster, such as an earthquake. More importantly asset managers and networks’ owners want rapid response, design flexibility, quick construction and limited maintenance costs. This should be possible to be achieved by contractors and designers with limited budgets. In very populated urban centres or a critical network location and moderate-to-high seismicity an Accelerated Bridge Construction (ABC) technology which combines durable materials and low-damage technology, seems to be the only viable solution to minimize traffic disruption during the bridge life. The American Association of State Highway Transportation Officials (AASHTO) started in 2002 a long-term strategic bridge plan which aims to cover all these issues. Similar research strategy was initiated in Japan, Taiwan and Europe which is slowly going towards adaptation of ABC as a standard bridge practice. The question would be what is New Zealand vision for the next twenty-thirty years? This paper aims to overview the current international trends and challenges and gives innovative concepts which can be contextualized for New Zealand bridges.