Seismic Upgrading Research Articles

Italian National Seismic Prevention Plan: cost analyses for risk reduction policies

The Italian National Seismic Prevention Plan (NSPP), born following the 2009 Abruzzi earthquake, gave light to an ambitious process aimed at the seismic upgrading of strategic and critical structures overall the Country. The coordination of the Plan is in charge of the Italian Civil Protection Department (ICPD), while Regions are appointed for its execution at a local level. To date, 1,207 structural interventions on buildings have been funded by NSPP, 510 of which so far concluded.The paper describes the state of the art of the Plan, relevant to public buildings, presenting some statistical analyses, compared with those developed in 2021, in terms of general features of the samples, safety indicators and costs sustained, to support the general strategy of seismic risk prevention and define future perspectives. Thanks to the enlargement of the statistical sample (passed from 1.034 to 1.207 items), more robust regression analyses and reliable equations are herein obtained and discussed. The updated sample relevant to buildings is formerly introduced in terms of typological characteristics, intervention types, relevant costs, and associated risk indices (before and after structural interventions). Then, a focus is set on sub-samples with concluded works, related to reinforced concrete buildings, masonry, and mixed structures. Analyses in terms of increment of the seismic safety index and marginal costs defined by unitary costs per percentage point increase of safety index, for the three different structural types are compared with those obtained by the authors in 2021. Finally, some conclusions are drawn from the formulations found and marginal costs obtained. These relationships, calibrated on the upgraded dataset, can be used to support the decision-making in deciding among different risk mitigation strategies, depending on the budget available and on the desired level of safety to be attained, according to a methodology previously developed by the authors.

Seismic upgrading of RC structures through an optimization procedure based on Genetic Algorithm

Nowadays governments are encouraging the upgrading and the reuse (rather than the demolition) of older structures in order to reduce land use and environmental impact due to the construction of new buildings. The choice of the most suitable intervention for the seismic upgrading of existing structures could also be addressed combining member-level (e.g., FRP-confinement of single columns) and structural-level (e.g., insertion of steel bracing systems) techniques, although it may result in a complex technical challenge for engineers, since a huge number of combinations of technically feasible upgrading interventions are theoretically possible to accomplish the desired structural performance. In order to support the intervention choice, an “objective” approach could be implemented making use of recently-invented Artificial Intelligence (AI) procedures. Specifically, the application of Genetic Algorithms (GAs) is usually thought as a suitable optimization procedure in several civil engineering problems. By means of a Genetic Algorithm (GA), the design of upgrading interventions results to be based on one objective criterion related to the cost-effectiveness of the intervention, rather than on the highly subjective “engineering-judgement”. The present paper aims to highlight the capability of a Soft-Computing (SC) procedure in selecting the most cost-effective combination of the aforementioned member-level and structural-level interventions, among the technically consistent ones. To this aim a parametric study on a RC structure based on a similar GA procedure is reported, varying some “engineering parameters” related to the target seismic risk class of the upgraded structure. Relevant differences in the results are observed due to the variation of target of the analyses.

A sub-assembly based technique for calibration of numerical models of infilled r.c. frames according to experimental tests

Most existing buildings in the Mediterranean area suffer from structural deficiencies and need to be retrofitted. However, a proper selection of the seismic upgrading technique stems from a reliable assessment of seismic response based on accurate numerical models. Furthermore, it is widely accepted that the effect of infill panels should be considered to properly reproduce the seismic response of buildings with r.c. framed structure. Indeed, several studies have demonstrated that infill panels significantly increase the lateral stiffness of the frame and influence its dissipative capacity and collapse mechanism. Unfortunately, despite all buildings are endowed with infill panels, the contribution of these non-structural elements to the structural response has been often “conservatively” neglected, in favour of computational simplicity. In this framework, this paper presents a technique for calibration of the finite element numerical model of r.c. frames with infill panels based on the results of a quasi-static cyclic experimental test of a prototype infilled frame. The proposed technique is based on a sub-assembly approach and relies of the fact that (1) the response of the system is mainly governed by the infill panel for small amplitude displacements, while (2) it becomes coincident with that of the bare r.c. frame for large amplitude displacements. Hence, the mechanical properties of the bare frame and those of the infill panel could be separately extracted from the same cyclic response of the infilled frame. This technique has been here applied for the calibration of the numerical model of a one-bay one-storey r.c. frame with infill panel. A full-scale cyclic test of the infilled frame is also executed and the obtained structural response is assumed as target to be fitted.

FEM Analysis of Steel Eccentric Braces for Seismic Retrofitting

The paper discusses issues about the finite element modelling of steel eccentric braces with vertical shear links used as hysteretic dampers to improve the seismic performance of existing reinforced concrete (RC) buildings. To this end, eccentric bracing systems have been designed for seismic upgrading of a case-study existing RC school building in Italy. Multiple finite element modelling approaches are described, using both frame and shell finite elements, various steel stress strain relationships, as well as including both geometrical imperfections and residual stresses. Numerical models were built using two commercial structural analysis programs, SAP2000 and Ansys, and one open-source software, Code_Aster. The numerical study allowed to (i) quantify values of the plastic overstrength factor of the considered shear links; (ii) evaluate the overall system stability and general inelastic response; (iii) compare various modelling approaches.

Post-Earthquake Condition Assessment and Seismic Upgrading Strategies for a Heritage-Protected School in Petrinja, Croatia

Following the Zagreb earthquake in March of 2020, a destructive 6.2 magnitude earthquake struck Croatia again in December of 2020. The Sisak-Moslavina county suffered the most severe consequences; many historical and cultural buildings were badly damaged. In the education sector, 109 buildings were damaged. One such building is the case study of this research. The heritage-protected building of the First Primary School in Petrinja is an unreinforced masonry structure, constructed using traditional materials and building techniques. The historical background of the building and the results of the post-earthquake assessment are presented. A numerical calculation of three strengthening methods was performed in 3Muri software: FRCM, FRP, and shotcrete. Non-linear pushover analysis was performed for each model. Finally, the strengthening methods are compared based on the achieved earthquake capacity, cost, and environmental impact.

Seismic performance upgrading of bridges using superelastic pendulum isolators with variable stiffness considering temperature effects

To enhance the seismic resilience of bridges, this study proposes a novel superelastic pendulum isolator with variable stiffness (SPIVS) equipped with copper-based shape memory alloy (CuSMA) against near-fault earthquakes. The sensitivity of the novel SPIVS to outside temperature effects is considered. A three-span bridge equipped with the proposed SPIVSs incorporating the CuSMA devices is modeled in OpenSees software. A fractional factor based design method is proposed to optimize the parameters of the SPIVS system. The seismic performance of the novel SPIVSs for bridges is assessed considering outside temperature effects by conducting case studies. Results show that the optimum parameters of the novel SPIVS system for various cases are obtained by the proposed method. The novel SPIVS using CuSMA devices can dually control the girder peak displacement and isolator residual deformation for various ambient temperatures. The base force increment of piers is also controlled within 10% to limit the seismic demand. The feasibility and effectiveness of the novel SPIVS system using CuSMA devices, in terms of re-centering and damping capacities for bridge response control at different outside temperatures, are discussed and demonstrated by various cases.

Seismic Energy Upgrading of an Existing Brick Masonry Building by a Cold-Formed Steel Envelope System

In recent years, new solutions have been studied in order to intervene on existing buildings made of load-bearing masonry or reinforced concrete, both from the seismic point of view and the energy one. In recent years, novel modern techniques using lightweight steel profiles made of steel or aluminium alloys, suitably coupled with heat-insulating panels, have been proposed for requalification of existing building stock. In this paper, the Resisto 5.9 system designed by the Progetto Sisma S.r.l. company was studied and applied as a retrofit system of an existing masonry building. First, this new system for seismic energy requalification of the build-up was presented, describing the basic components and the mounting phases. Subsequently, it has applied to a case study building made of full-brick masonry located in the municipality of Casalecchio di Reno, on the outskirts of Bologna in Northern Italy. Finally, the used coating system was designed for the examined building together with other seismic interventions applied to horizontal structures and foundations. The seismic benefits deriving from using the proposed envelope system and other seismic interventions were proved from the seismic viewpoint by linear dynamic and non-linear static analyses using the TreMuri analysis software. From modal analysis it was seen that the dynamic behaviour of the building with Resisto 5.9 is improved, with an increase of the participating mass (68% instead of 49% of the bare building) of the second mode in the longitudinal direction. Moreover, from pushover analysis it was noticed that the seismic safety factor has passed from 0.51 to 1.05 in the X-direction and from 0.78 to 1.27 in the Y-direction. In conclusion, it was demonstrated that Resisto 5.9, together with all the other planned interventions, is able to accomplish the seismic retrofitting of the studied building.

Assessment of a novel hysteretic friction damper for the seismic retrofit of reinforced concrete frame structures

In this paper, a novel hysteretic friction damper, called Prestressed LEad Damper with Straight Shaft (or PS-LED), is presented as a viable solution for the seismic rehabilitation of existing RC framed buildings. The PS-LED is characterized by high stiffness and damping capability combined with a compact design, and low manufacturing cost, which make it suitable for social housing. The constitutive behavior of the PS-LED, assessed experimentally in cyclic tests at different speeds, is formulated in the OpenSees framework through a simple rheological model comprising a parallel of a non-linear spring and a viscous model.To explore the viability of the PS-LED for seismic upgrade of existing buildings, a numerical investigation is performed by considering a RC frame structure, paradigmatic of a category of Italian buildings designed according to outdated codes. The case-study structure retrofit is designed considering two possible damage targets: (i) in the first case, the upgrade is performed in order to achieve elastic response of the structure under the basic design earthquake; (ii) in the second case, a partially dissipative behavior of the RC frame is conceived, with activation of plastic hinges to a repairable level. Non-linear static and Non-linear dynamic analyses are performed, showing a satisfactory agreement between the seismic performance and the design targets.As final stage, a comparison between two retrofit solutions with either the PS-LED or with a conventional steel hysteretic damper (SHD) is performed, demonstrating that the PS-LED, thanks to its high damping capacity, is able to control the increase in internal forces that usually affects frames upgraded with SHDs.

Seismic Performance Upgrade of Elevated Water Tanks Utilizing Friction Dampers

Water storage tanks are essential structures in water distribution supply systems. During an earthquake, the damages incurred to water tanks may lead to disastrous loss of life and property. This is due to the low ductility, redundancy, and low energy dissipation characteristics of these structures in comparison to conventional building structures. In this study, the analytical effectiveness of pall friction dampers is investigated to enhance the seismic resistance capacity of existing and newly constructed elevated water tanks. This study evaluates the performance of three water tanks based on their capacities, namely small (90 m3), medium (600 m3), and large (1,700 m3). The selected elevated reinforced concrete (RC) water tank models are examined for staging heights equal to 16, 20, and 24 m. The main objective of the study is to develop a finite element model of elevated water tanks installed with pall friction dampers considering container weight and water mass using finite element software. Nonlinear time-history analyses of elevated water tanks subjected to an array of different ground motion records are performed. The study evaluates the optimal slip load of the friction dampers required to obtain optimal seismic responses of elevated water tanks. The results exhibit the efficiency of pall friction dampers in mitigating the structural responses of elevated water tanks exposed to seismic loads. The study ultimately concludes that the effectiveness of the pall friction damper is independent of tank capacities, and the efficiency of the damper reduces as the staging height increases.

Condition Assessment and Seismic Upgrading Strategy of RC Structures—A Case Study of a Public Institution in Croatia

In 2020, Croatia was struck by two catastrophic earthquakes, resulting in more than 50,000 damaged structures. The majority of these are masonry buildings, but there are a number of reinforced concrete structures that suffered moderate to extensive damage. In this paper, the seismic condition assessment and upgrading of existing RC structures are presented with a case study building in Zagreb. The assessment procedure includes initial visual inspection, rapid preliminary evaluation, detailed in situ measurements, and non- and semi-destructive methods. New technologies were applied and followed by numerical modeling and verifications. Strengthening proposals are made that respect owner needs and the needs for the energy retrofitting of the existing RC building. As the integrated approach should be respected in the renovation of existing buildings, this case study can represent an example of good practice in the process of seismic and energy retrofitting.

Non-Linear Analysis and Retrofitting by Steelwork of a Precast RC Warehouse

Seismic upgrading and retrofitting of existing constructions is a pressing need for designers and researchers. The necessity of efficient seismic upgrading/retrofitting techniques is, therefore, required in seismic-prone countries, such as Italy. In this framework, steelwork has clearly shown many advantageous applications in the last century. Nonetheless, if compared to other different technologies, steelwork is still limited for consolidation purposes. Moreover, the wide damage provoked by earthquakes to industrial buildings have induced scientific research to investigate the seismic vulnerability of such constructions much more. In the current study, the attention has been, therefore, focused on the use of steelwork systems as anti-seismic intervention techniques from a precast RC industrial warehouse hit by the 2012 Northern Italy earthquakes. Besides the usefulness of steelwork in implementing reliable techniques against earthquakes, the paper has the aim of discussing the different seismic behaviour of the building deriving from dissimilar beam-to-column joint types obtained using steelwork interventions. Other than the widely diffused static scheme with hinges, other types of joints (semi-rigid and rigid), along with the presence of a rigid roof, have been investigated, and the different seismic risk indicators derived from these static schemes have been achieved, highlighting the case of the best seismic behaviour of the warehouse. Finally, the effectiveness of local steel interventions in improving the efficient global response of the building has also been highlighted.

Seismic Assessment and Strengthening of an Existing Non‐Conforming Multi‐Storey Steel Building

AbstractExisting steel structures show a wide variability in terms of structural conception and typology of adopted resisting systems. Moreover, these structures do not often comply with existing codes, showing severe structural inadequacies in many cases. An example of this type of buildings is the existing non‐conforming six‐storey steel building described in this article. The selected building features both Moment Resisting Frames (MRFs) along its longitudinal direction and Concentrically Braced Frames (CBFs) in the transverse direction. The seismic performance of the investigated structure was assessed in accordance with Italian NTC2018 and EN1998‐3, showing unsatisfactory performance and rather poor capacity. Therefore, seismic upgrading solution was designed and its efficiency was assessed and compared to the performance of the original configuration.

Seismic performance upgrade of substandard RC buildings with different structural systems using advanced retrofit techniques

There has been ongoing deliberation to arrive at promising retrofit strategies for pre-seismic code reinforced concrete (RC) structures. Against this backdrop, the research reported in this study puts forth solutions by focusing on advanced retrofit approaches for improving the seismic performance of substandard RC buildings with structural systems common to medium seismicity regions. High-performance reinforced concrete (HPRC) jacket application and self-centering energy dissipative (SCED) braces are the contemporary retrofit alternatives prioritized in this study to upgrade RC structural systems deficient in stiffness, strength and/or ductility. While these retrofit techniques were previously investigated at the member level, this study focuses on implementing thin-HPRC jacket applications, innovative outrigger-belt truss SCED bracing systems, and hybrid approaches on three-dimensional (3D) fiber-based numerical models representing substandard RC structures with different heights. Inelastic pushover analyses and multi-record dynamic response simulations are conducted to assess the retrofit alternatives by monitoring several local damage indices (LDIs) and the related global damage measure (GDM). The study confirms the effectiveness of retrofit measures for a moment-resisting frame building (MRF) deficient in lateral capacity and a high-rise shear wall (SW) building with insufficient global ductility. The presented systematic methodology accounts for the redistribution of seismic demands after retrofitting critical elements and enables selecting effective seismic retrofit solutions, namely SCED bracing system for MRF structure and hybrid retrofit for the high-rise SW building, using probabilistic seismic performance assessment along with cost and practical considerations.

Building retrofit with reduced web (RWS) and beam (RBS) section limited-ductility connections

Softening of beams near the moment connection can be regarded as one of the most convenient methods for retrofitting non-seismically designed buildings. This can be done by drilling the beam web near the connection (aka RWS–Reduced Web Section) or cutting the beam flanges (aka RBS–Reduced Beam Section). Thus, becoming a cost-effective option for retrofitting and erection of new buildings, while optimising use of steel; making the most of this high environmental impactful material. This study presents a comprehensive investigation of the effects of using extended end-plate bolted RWS and RBS connections in building behaviour when subjected to strong ground motion. For that purpose, incremental dynamic analyses (IDA) were performed in a low height building fitted with RWS and RBS connections capable of limited ductility. This was done with the aim of characterising the seismic behaviour of buildings with deficient earthquake design after being subjected to seismic upgrade. It is observed that empirical fragility curves show that probabilities of exceeding 1%, 2% and 4% inter-story drifts are significantly lower (more than 25%) for the building that is designed with RWS connections instead of RBS connections; while the probability of collapse is reduced by at least one third. This indicates that deployment or RWS connections can reduce the risk of both; severe structural damage and, large economical loss due to damage of non-structural elements when seismic demands are moderate.

The Use of SMAV Model for Computing Fragility Curves

Among the buildings with strategic role for civil protection purposes part of them were built before the introduction of modern seismic codes. Aiming to guarantee their operability in case of seismic events, they have to be assessed and, hopefully, seismically upgraded. This aspect arises the necessity of reliable low computational demanding numerical models and probabilistic strategies that can be easily adopted in case of the assessment of conspicuous building stock. Part of the authors proposed and validated, in previous works, a low-demanding seismic model calibrated on ambient vibrations (SMAV) that is engaged in this paper for calculating fragility curves. On the base of dynamic and geometric properties, the SMAV model simulates the dynamic response of buildings. The herein proposed procedure adopts the SMAV model and takes into account the direction of seismic input in a probabilistic manner. The use of the ASI scale as an Intensity Measure parameter is discussed and the results are compared to those obtained with others IM scales. Although SMAV implements an equivalent linear procedure the results are compared with those obtained by more sophisticated FEM models and nonlinear dynamic analyses with a satisfactory agreement. Finally, two real case studies are presented. The fragility curves for both the case studies are calculated and discussed. The results encourage the authors to the use of fragility curves based on SMAV model. Additionally, the fragility curves can be easily implemented in seismic risk assessment procedures at urban or territorial scale and, consequently, the proposed procedure may represent a key aspect in the reduction of seismic risk and rationalization of investments for the seismic upgrading of strategic buildings.

Seismic upgrading of cultural heritage – A case study using an educational building in Croatia from the historicism style

More than two years have passed since the devastating earthquakes struck Croatia in 2020, in which many educational buildings were damaged. In addition, several damaged educational buildings are under heritage protection, representing a special challenge for civil engineers during structural renovation. Strengthening methods that achieve adequate level of seismic capacity are limited in order to preserve the identity of these buildings. In the manuscript, a short overview of the damage to the educational sector is given with a special focus on a renovation of one case study. The case study is an educational building with a masonry structure located in the Lower Town of Zagreb and from the 19th century. An assessment of the structural condition was performed and a model was developed in the 3Muri software. Three variants of strengthening methods were applied to the model – FRCM, shotcrete and a combination of FRCM and shotcrete. Finally, each variant model was analyzed using the pushover method. The results are compared in terms of achieved earthquake capacity, expected cost and environmental impact.

Seismic performance upgrading of containment structures using a negative-stiffness amplification system

As an efficient and crucial energy-generation facility, a nuclear power plant requires a high level of seismic safety as its failure can lead to catastrophic events. In this study, a novel negative-stiffness amplification system-strengthened isolation system (NSAS-IS) is constructed for seismic performance upgrading of containment structures. Moreover, a multiperformance-oriented design principle is developed for the NSAS-IS to enable enhanced energy dissipation and robust control during multi-intensity seismic excitations. The NSAS-IS comprises an NSAS and parallelly arranged isolators; in particular, the NSAS comprises a tuning spring in series with a sub-configuration including a negative-stiffness device and dashpot in parallel. Herein, the mechanical models and physical realization of the NSAS and isolators are established, based on which the equivalent negative-stiffness effect and enhanced energy dissipation capacity are elucidated. A mechanical model of an NSAS-IS-equipped containment structure, as the last safety defense of the inner structure of a nuclear power plant, and its governing equation and finite element model are established. A design principle, aimed at multiperformance upgrading, is developed for the NSAS-IS. Considering the typical containment structure as an example, the advantages of the proposed NSAS-IS and design principle are investigated for multiple seismic performances, including the deformation and shear force responses of the isolation layer as well as the deformation and acceleration responses of the containment shell. The results obtained indicate that installing the NSAS-IS at the bottom of the containment structure provides improved isolation owing to the equivalent negative-stiffness effect; consequently, the multiple responses of the superstructure and isolation layer are reduced more significantly than with conventional isolators. Benefiting from the multiperformance-oriented design and deployed NSAS, the new isolation system exhibits an enhanced energy dissipation efficiency and capacity, simultaneously maintaining the isolation phenomenon. Thus, the total energy dissipation burden on the primary structures can be relieved by the NSAS, which robustly dissipates intense vibrational energies during multi-intensity excitations.

Experimental study on seismic upgrading of RC frames using innovative brace system with two-stage characteristic

Experimental study on seismic upgrading of RC frames using innovative brace system with two-stage characteristic

Experimental Response of Rubble Stone Masonry Walls Retrofitted with Basalt Textile Reinforced Mortar under Compressive-and-Shear Load

Seismic events recently occurred in Europe highlighted the vulnerability of unreinforced masonry structures, representing the majority of the built heritage, to earthquake-induced loads. In particular, the in-plane capacity of shear walls under seismic loadings appeared sensibly modest, both in terms of strength and ultimate displacement, and their failure mode sudden and brittle. Textile Reinforced Mortar (TRM) composites proved to be a proper strengthening solution for the seismic upgrade of such walls. With the aim of gaining a deeper knowledge on the behavior of historical walls retrofitted with TRM and subjected to compression-and-shear, this paper summarizes the results of an experimental campaign characterized by quasi-static cyclic shear-compression tests. The specimens under study reproduced the rubble stone masonry typical of vernacular structures of the UNESCO historical district of Lyon (France). Specifically, the work concerns a full scale masonry wall tested under loading until the development of diagonal cracks and than repaired by a composite constituted by a basalt textile embedded in a lime mortar matrix applied onto the external surfaces as a plaster. The use of a lime mortar plaster to reinforce the structural element permits the development of a solution compatible with the historical substrate. Experimental outcomes revealed how Basalt TRMs succeeded in improving the response of damaged vernacular wall, both in terms of strength and displacement capacity.

Multicriteria Decision-Making Methods in Selecting Seismic Upgrading Strategy of High-Rise RC Wall Buildings

Reinforced concrete (RC) structural walls are widely used in high-rise structures in earthquake-prone areas. Damaged by the earthquakes in the past decades, these buildings need retrofitting in order to increase the resilience of buildings with concrete shear walls. This study aimed to investigate the retrofitting of high-rise RC wall buildings using energy dissipation devices. To this end, a total of four buildings with 15, 20, 25, and 30 stories equipped with concrete shear walls as their lateral load-resisting system were retrofitted using passive seismic control systems. The buildings were subjected to the set of the far-field and near-field records presented in the FEMA standard (P-695), and an index was defined to relate the structural responses of the building, such as drift, acceleration, velocity, displacement, and base shear, to the earthquake records. For this purpose, numerical models, which have been validated with the experimental results, have been performed. The resulting index values were considered as the criteria, and the passive systems were ranked by the efficient Multicriteria Decision Making (MCDM) method. Based on the results from the MCDM method and using the considered criteria, friction damper was ranked first among the available energy dissipation devices for high-rise RC wall buildings.