Assessment, Reconstruction and Decision Procedures for the Preservation of Existing Structures after Earthquakes

A topical collection in Buildings (ISSN 2075-5309). This collection belongs to the section "Building Structures".

Viewed by 35862
Printed Edition Available!
A printed edition of this Special Issue is available here.

Editors


E-Mail Website
Collection Editor
Faculty of Civil Engineering, University of Zagreb, 10000 Zagreb, Croatia
Interests: structural assessment; historic structures; timber structures; masonry structures; scan to fem; seismic risk; drones
Special Issues, Collections and Topics in MDPI journals

E-Mail Website
Collection Editor
ISISE, Department of Civil Engineering, University of Minho, 4800-058 Guimarães, Portugal
Interests: seismic assessment of historic masonry structures; dynamic of structures; scan to fem; composite structures; fracture mechanics
Special Issues, Collections and Topics in MDPI journals

E-Mail Website
Collection Editor
Chair of Concrete and Masonry Structures, Department of Structural Engineering, Faculty of Civil Engineering, University of Zagreb, HR-10000 Zagreb, Croatia
Interests: civil engineering; composites; construction; structural analysis; earthquake engineering; construction materials; construction engineering; finite element analysis; mechanical properties
Special Issues, Collections and Topics in MDPI journals

E-Mail Website
Collection Editor
Department of Earthquake Engineering, Bogazici University, 34342 Bebek, Turkey
Interests: structural dynamics; nonlinear modeling and analysis; earthquake risk and loss modeling

Topical Collection Information

Dear Colleagues,

In 2020, there were destructive earthquakes all around the world, causing loss of lives, building collapses, and severe structural and non-structural damage and economic losses (e.g., M7.4 Oaxaca (Mexico), M7.0 Aegean Sea (Turkey–Greece), M 6.7 Elazig (East Turkey), M5.3 and M6.4 Croatia, M5.8 Khoy (Iran–Turkey) earthquakes). All these events have made it evident once again that society’s preparedness is crucial for better responding to natural disasters. Identification of vulnerability characteristics and earthquake performance assessment of existing structures are essential steps in reducing earthquake losses and eventually making the societies and cities resilient.

In the past, most of the initial work on structural assessment was based only on the visual inspection of buildings, NDT methods, and engineering judgement. The last decade, however, has seen a growth in the technological development of various tools which can greatly support the prediction of structural safety and behavior of existing structures. The mentioned topics and issues are greatly expressed in pre- and post- earthquake assessments, reconstructions, and renovations of buildings and cities. Case studies of real examples of assessment and reconstruction of existing buildings represent a bridge between practice and research. They provide a method for research in which the complexity of the process and outcome of results contribute to a valuable database for all techniques in preserving existing building stock. The preservation is of particular interest for buildings of cultural value and/or under heritage protection.

This Topical Collection aims at collecting original and high-quality papers and good practices discussing case studies on the assessment, monitoring, reconstruction, maintenance, and preservation of existing structures. Descriptive papers addressing the existing masonry, concrete and timber structures are welcome. Case studies can refer to any processes preserving the built environment, from architectural digitalization to modern techniques of reconstruction of individual buildings and society as a whole.

The main idea of this Topical Collection is to gather a database of case studies regarding the:

  • Assessment of existing masonry, timber, and concrete structures;
  • Heritage preservation;
  • The role of material, component, and assembly aspects in the behavior of existing structures;
  • Seismic risk and vulnerability;
  • Immediate response after an earthquake;
  • Maintenance of structures;
  • Use of emerging technologies in the prediction of structural behavior;
  • Advances in numerical modeling of existing structures;
  • Decision-making process and reconstruction of existing structures;
  • Retrofit and mitigation strategies for existing structures;
  • Energy renovation of existing structures;
  • Teaching initiatives preservation of the built environment.

Dr. Mislav Stepinac
Dr. Chiara Bedon
Dr. Marco Francesco Funari
Prof. Dr. Tomislav Kišiček
Dr. Ufuk Hancilar
Collection Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the collection website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Buildings is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • case studies
  • existing structures
  • seismic design
  • assessment
  • NDT
  • structural updating
  • seismic risk
  • immediate response
  • numerical modeling
  • reconstruction

Related Special Issue

Published Papers (12 papers)

2024

Jump to: 2023, 2022, 2021

16 pages, 3793 KiB  
Article
Development of Seismic Fragility Function for Underground Railway Station Structures in Korea
by Sun Yong Kwon, Jongkwan Kim, Dongyoup Kwak, Seunghoon Yang and Mintaek Yoo
Buildings 2024, 14(5), 1200; https://doi.org/10.3390/buildings14051200 - 24 Apr 2024
Cited by 1 | Viewed by 847
Abstract
This study describes the methodology employed to construct a seismic fragility function based on a pre-existing numerical model tailored for underground stations. Employing a dynamic numerical model, a comprehensive analysis encompassing 110 distinct cases was conducted, each varying in soil depth and classification. [...] Read more.
This study describes the methodology employed to construct a seismic fragility function based on a pre-existing numerical model tailored for underground stations. Employing a dynamic numerical model, a comprehensive analysis encompassing 110 distinct cases was conducted, each varying in soil depth and classification. Seismic waves, conforming to the standard design spectrum, were utilized within these numerical analyses. The formulation of the fragility function within the constructed model follows a structured approach, segmented by damage indices and severity levels. This systematic breakdown serves to outline the fundamental framework for establishing the fragility function, providing insights into its development process. Subsequently, the derived fragility function underwent a rigorous comparative analysis against established seismic fragility functions from prior studies. This comparative assessment serves as a critical evaluation tool, allowing for an appraisal of the suitability and robustness of the newly developed fragility function in relation to existing benchmarks. Full article
Show Figures

Figure 1

2023

Jump to: 2024, 2022, 2021

17 pages, 7830 KiB  
Article
Post-Earthquake Housing Reconstruction Management and Implementation in Rural Areas: Review and Lessons from Dujiangyan, Wenchuan Earthquake
by Qiushan Li, Kabilijiang Wumaier and Koide Osamu
Buildings 2023, 13(9), 2251; https://doi.org/10.3390/buildings13092251 - 5 Sep 2023
Cited by 1 | Viewed by 1776
Abstract
Housing reconstruction plays a crucial role in renovating disaster-hit areas. Rural areas are considerably different from urban areas in terms of geographic environment, building size, residential culture, and social organization. Therefore, post-disaster recovery and reconstruction models for urban areas cannot be applied directly [...] Read more.
Housing reconstruction plays a crucial role in renovating disaster-hit areas. Rural areas are considerably different from urban areas in terms of geographic environment, building size, residential culture, and social organization. Therefore, post-disaster recovery and reconstruction models for urban areas cannot be applied directly to disaster-hit rural areas. This study, based on the experience of rural housing reconstruction after the Wenchuan earthquake, identified key strategic issues in housing reconstruction that must be addressed to achieve the goal of “building back better” in the future. By taking the experience of Dujiangyan as our reference, the study found that the following strategies are important for successful housing reconstruction in rural areas: (1) actively involve disaster victims through a participatory institutional design; (2) coordinate the interests of governments, markets, and disaster victims and the functions of living, production, and ecology through a classified housing reconstruction system; and (3) activate the quota for rural collective construction land and create a new source of funding for housing reconstruction through the market circulation of urban-rural land. Additionally, in the context of urban-rural integration, changes in land use can lead to rural spatial reconstruction and sustainable regional development, providing a reference for formulating optimal post-disaster reconstruction strategies. Full article
Show Figures

Figure 1

20 pages, 8950 KiB  
Article
The Assessment and Retrofitting of Cultural Heritage—A Case Study of a Residential Building in Glina
by Karlo Ožić, Ivan Markić, Antonela Moretić and Luka Lulić
Buildings 2023, 13(7), 1798; https://doi.org/10.3390/buildings13071798 - 14 Jul 2023
Cited by 5 | Viewed by 2717
Abstract
The focus of the study is on the renovation of a specific case study, which is a 19th century building under cultural heritage protection. It highlights the particular challenges faced by civil engineers in the structural renovation of buildings that are under heritage [...] Read more.
The focus of the study is on the renovation of a specific case study, which is a 19th century building under cultural heritage protection. It highlights the particular challenges faced by civil engineers in the structural renovation of buildings that are under heritage protection. Preserving the identity of these buildings limits the available methods for strengthening their seismic capacity. At the beginning, information about the seismic activity and the different post-earthquake evaluation procedures are presented to identify the damage and take appropriate further steps. Then, basic information about the building is given and supported by graphic attachments. In the following, the methods and materials are explained, focusing on in situ testing with the semi-destructive flat-jack method and the analysis of the structure with the nonlinear method implemented in the software. Subsequently, the obtained results are presented and discussed, accompanied by graphics. An approach for strengthening the structure is presented, which includes a combination of traditional methods and innovative solutions suitable for the preservation of cultural heritage. The discussion and conclusions emphasize the importance of assessing and retrofitting existing masonry structures due to their vulnerability, especially in earthquake-prone areas. Finally, this article also provides insights into the local context, cultural significance, and historical background of the building, along with the specific retrofitting solutions employed to address its unique requirements. Full article
Show Figures

Figure 1

36 pages, 9648 KiB  
Article
Seismic Retrofitting of Mid-Rise Unreinforced Masonry Residential Buildings after the 2010 Kraljevo, Serbia Earthquake: A Case Study
by Predrag Blagojević, Svetlana Brzev and Radovan Cvetković
Buildings 2023, 13(3), 597; https://doi.org/10.3390/buildings13030597 - 24 Feb 2023
Cited by 5 | Viewed by 2302
Abstract
There is a significant building stock of post-WWII low- and mid-rise unreinforced masonry (URM) buildings in Serbia and the region (former Yugoslavia). Numerous buildings of this typology collapsed due to the devastating 1963 Skopje, Yugoslavia earthquake, causing fatalities, injuries, and property losses, as [...] Read more.
There is a significant building stock of post-WWII low- and mid-rise unreinforced masonry (URM) buildings in Serbia and the region (former Yugoslavia). Numerous buildings of this typology collapsed due to the devastating 1963 Skopje, Yugoslavia earthquake, causing fatalities, injuries, and property losses, as well as experienced damage in a few recent earthquakes in the region, including the 2010 Kraljevo, Serbia earthquake (MW 5.5) and the 2020 Petrinja, Croatia earthquake (M 6.4). These buildings are three- to five-stories high, have clay brick masonry walls, and rigid floor slabs, usually with an RC ring beam at each floor level. This paper presents a case study of a URM building which was damaged due to the 2010 Kraljevo earthquake and subsequently retrofitted. A comparison of seismic analysis results, including the capacity/demand ratio and displacement/drift values, for the original and retrofitted building according to the seismic design and retrofit codes which were followed in Serbia as well as some of the neighboring countries for several decades and Eurocode 8 has been presented. The results of this study show that the selected retrofit solution that satisfied the Yugoslav seismic code requirements is not adequate according to the Eurocode 8, primarily due to significantly higher seismic demand. Full article
Show Figures

Figure 1

30 pages, 8958 KiB  
Article
Adriseismic Methodology for Expeditious Seismic Assessment of Unreinforced Masonry Buildings
by Giorgia Predari, Lorenzo Stefanini, Marko Marinković, Mislav Stepinac and Svetlana Brzev
Buildings 2023, 13(2), 344; https://doi.org/10.3390/buildings13020344 - 26 Jan 2023
Cited by 7 | Viewed by 1854
Abstract
The paper describes a novel Adriseismic method for expeditious assessment of seismic risk associated with unreinforced masonry buildings. The methodology was developed for the Adriseismic project of the Interreg ADRION programme, with the aim to develop and share tools for increasing cooperation and [...] Read more.
The paper describes a novel Adriseismic method for expeditious assessment of seismic risk associated with unreinforced masonry buildings. The methodology was developed for the Adriseismic project of the Interreg ADRION programme, with the aim to develop and share tools for increasing cooperation and reducing seismic risk for six participating countries within the region surrounding the Adriatic and the Ionian Seas. The method is applicable to unreinforced masonry buildings characterised by three main seismic failure mechanisms, namely masonry disintegration, out-of-plane failure, and in-plane damage/failure. Depending on the input parameters for a specific structure, the assessment yields a qualitative output that consists of the masonry quality index, the index of structural response, the level of seismic risk, and the most probable collapse mechanism. Both input and output of the method are applied in the spreadsheet form. The method has so far been applied in urban areas of participating countries in the project, including Mirandola, Italy; Kaštela, Croatia; Belgrade, Serbia. In parallel, the methodology has been validated by performing a detailed seismic assessment of more than 25 buildings, and the results have been compared with the results of the proposed expeditious method. The results show a good correlation between the two methods, for example, the structural response index obtained from the expeditious method and the capacity/demand ratio obtained from the conventional assessment method. Full article
Show Figures

Figure 1

34 pages, 19819 KiB  
Article
Seismic Retrofitting of Dual Structural Systems—A Case Study of an Educational Building in Croatia
by Mario Uroš, Marija Demšić, Maja Baniček and Ante Pilipović
Buildings 2023, 13(2), 292; https://doi.org/10.3390/buildings13020292 - 18 Jan 2023
Cited by 2 | Viewed by 3067
Abstract
On 29 December 2020, a devastating Mw6.4 earthquake struck near the town of Petrinja, Croatia. The main earthquake was preceded by a Mw4.9 foreshock the day before. The earthquakes caused extensive damage to buildings, especially historic buildings made of unreinforced masonry but also [...] Read more.
On 29 December 2020, a devastating Mw6.4 earthquake struck near the town of Petrinja, Croatia. The main earthquake was preceded by a Mw4.9 foreshock the day before. The earthquakes caused extensive damage to buildings, especially historic buildings made of unreinforced masonry but also to buildings of other typologies and to critical infrastructure. Today, recovery efforts in Croatia focus primarily on reconstruction and seismic retrofitting. Family homes and public, cultural, educational, and other facilities are top priorities. In this paper, a comprehensive study of existing building in the educational sector is presented as a case study. The seismic performance of the building is evaluated using numerical methods, first for the as-built condition and then for the retrofitted building. For each condition, the collapse mechanisms of the building were determined and critical structural elements were identified. The presented retrofit strategy of the dual structural system consisting of RC frame system and masonry walls aims to reduce the displacements of the RC frame system to a level sufficient to prevent the early brittle failure of the concrete. Additionally, the discrepancies when using different modelling approaches are discussed. Full article
Show Figures

Figure 1

2022

Jump to: 2024, 2023, 2021

28 pages, 17576 KiB  
Article
Post-Earthquake Condition Assessment and Seismic Upgrading Strategies for a Heritage-Protected School in Petrinja, Croatia
by Aida Salaman, Mislav Stepinac, Ivan Matorić and Mija Klasić
Buildings 2022, 12(12), 2263; https://doi.org/10.3390/buildings12122263 - 19 Dec 2022
Cited by 6 | Viewed by 2580
Abstract
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 [...] Read more.
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. Full article
Show Figures

Figure 1

22 pages, 11538 KiB  
Article
A Design Methodology for the Seismic Retrofitting of Existing Frame Structures Post-Earthquake Incident Using Nonlinear Control Systems
by Assaf Shmerling and Matthias Gerdts
Buildings 2022, 12(11), 1886; https://doi.org/10.3390/buildings12111886 - 4 Nov 2022
Viewed by 2070
Abstract
A structural design methodology for retrofitting weakened frame systems following earthquakes is developed and presented. The design procedure refers to frame systems in their degraded strength and stiffness states and restores their dynamic performance using nonlinear control systems. The control law associated with [...] Read more.
A structural design methodology for retrofitting weakened frame systems following earthquakes is developed and presented. The design procedure refers to frame systems in their degraded strength and stiffness states and restores their dynamic performance using nonlinear control systems. The control law associated with the employed systems regards the gains between the negative state feedback and the control force, which consists of linear, nonlinear, and hysteretic portions. Structural optimization is introduced in designing the nonlinear control systems, and the controller gains are optimized using the fixed-point iteration to improve the frame system’s dynamic performance. The fixed-point iteration method relates to first-order PDE equations; hence, a new state-space formulation for weakened inelastic frame systems is developed and presented using the frame system’s lateral force equilibrium equation. The design scheme and optimization strategy differ from designing passive control systems, given that the nonlinear control system’s force consists of linear, nonlinear, and hysteretic portions. The utilization of the fixed-point iteration in the structural design area is by itself a novel application due to its robustness in addressing the gains of any type of nonlinear control system. This paper’s nonlinear control system chosen to exhibit the application is Buckling Restrained Braces (BRBs) since force consists of linear and hysteretic portions. The implementation of hysteretic control force is rare in structural control applications. In the case of BRBs, the fixed-point iteration optimizes the cross-sectional areas. Two system optimization examples of 3-story and 15-story inelastic frames are provided and described. The examples demonstrate the fixed-point iteration’s applicability and robustness in optimizing control gains of nonlinear systems and regulating the dynamic response of weakened frame structures. Full article
Show Figures

Graphical abstract

17 pages, 1940 KiB  
Article
The Role of Disaster Risk Governance for Effective Post-Disaster Risk Management—Case of Croatia
by Zvonko Sigmund, Mladen Radujković and Josip Atalić
Buildings 2022, 12(4), 420; https://doi.org/10.3390/buildings12040420 - 31 Mar 2022
Cited by 8 | Viewed by 3061
Abstract
Risk governance is mostly viewed through the lens of disaster or emergency management departments, agencies, or organizations. Visible in times of crises, risk governance is rarely seen as part of everyday public or private functions such as planning, social welfare, investments, or fiscal [...] Read more.
Risk governance is mostly viewed through the lens of disaster or emergency management departments, agencies, or organizations. Visible in times of crises, risk governance is rarely seen as part of everyday public or private functions such as planning, social welfare, investments, or fiscal responsibilities. This paper emphasizes the importance of disaster risk governance in disaster risk management activities on the example of the post-disaster recovery of Croatia after a series of strong seismic events in mainland Croatia. The analysis is made based on a thorough review of national documents of Croatia and other selected countries overlapped with the national journals reporting on the situation from the affected areas. In accordance with the authors’ opinion, the necessary elements of disaster risk governance are clearly stated through the four Sendai framework priorities, and this statement is supported by the facts from the case study. Without either the political will or the enabling surrounding the disaster, risk management is next to impossible. The Croatian case study emphasizes the importance of disaster risk governance, showcasing the adaptation process for the post-disaster recovery process to start. Full article
Show Figures

Figure 1

24 pages, 15393 KiB  
Article
Post-Earthquake Rapid Damage Assessment of Road Bridges in Glina County
by Anđelko Vlašić, Mladen Srbić, Dominik Skokandić and Ana Mandić Ivanković
Buildings 2022, 12(1), 42; https://doi.org/10.3390/buildings12010042 - 4 Jan 2022
Cited by 13 | Viewed by 4604
Abstract
In December 2020, a strong earthquake occurred in Northwestern Croatia with a magnitude of ML = 6.3. The epicenter of this earthquake was located in the town of Petrinja, about 50 km from Zagreb, and caused severe structural damage throughout Sisak-Moslavina county. [...] Read more.
In December 2020, a strong earthquake occurred in Northwestern Croatia with a magnitude of ML = 6.3. The epicenter of this earthquake was located in the town of Petrinja, about 50 km from Zagreb, and caused severe structural damage throughout Sisak-Moslavina county. One of the biggest problems after this earthquake was the structural condition of the bridges, especially since most of them had to be used immediately for demolition, rescue, and the transport of mobile housing units in the affected areas. Teams of civil engineers were quickly formed to assess the damage and structural viability of these bridges and take necessary actions to make them operational again. This paper presents the results of the rapid post-earthquake assessment for a total of eight bridges, all located in or around the city of Glina. For the assessment, a visual inspection was performed according to a previously established methodology. Although most of the inspected bridges were found to be deteriorated due to old age and lack of maintenance, very few of them showed serious damage from the earthquake, with only one bridge requiring immediate strengthening measures and use restrictions. These measurements are also presented in this paper. Full article
Show Figures

Figure 1

2021

Jump to: 2024, 2023, 2022

25 pages, 28000 KiB  
Article
Seismic Vulnerability Assessment for Masonry Churches: An Overview on Existing Methodologies
by Mattia Zizi, Jafar Rouhi, Corrado Chisari, Daniela Cacace and Gianfranco De Matteis
Buildings 2021, 11(12), 588; https://doi.org/10.3390/buildings11120588 - 26 Nov 2021
Cited by 13 | Viewed by 3453
Abstract
The present manuscript deals with the seismic vulnerability assessment of existing masonry churches, which is a fundamental process for risk and consequent prioritization analyses, as well as application of effective retrofitting strategies. In the past, different approaches with various levels of accuracy and [...] Read more.
The present manuscript deals with the seismic vulnerability assessment of existing masonry churches, which is a fundamental process for risk and consequent prioritization analyses, as well as application of effective retrofitting strategies. In the past, different approaches with various levels of accuracy and application ranges have been developed to assess the vulnerability to damage of such structures in case of seismic events. Based on the classification provided in the Italian Guidelines for the Cultural Heritage, in this paper a review of seismic vulnerability assessment methodologies for existing masonry churches is presented. The main goal of the current study is to provide a critical comparative overview about these procedures, highlighting the main issues related to the application of each detail level. Moreover, particular attention is focused on the applications present in literature, allowing for the definition of a potential systematic procedure for smart management policy aimed at preserving cultural, architectural and historical heritage. Full article
Show Figures

Figure 1

23 pages, 16025 KiB  
Article
Assessment and Rehabilitation of Culturally Protected Prince Rudolf Infantry Barracks in Zagreb after Major Earthquake
by Mija Milić, Mislav Stepinac, Luka Lulić, Nataša Ivanišević, Ivan Matorić, Boja Čačić Šipoš and Yohei Endo
Buildings 2021, 11(11), 508; https://doi.org/10.3390/buildings11110508 - 27 Oct 2021
Cited by 15 | Viewed by 4867
Abstract
Recently, Zagreb was struck by a strong earthquake. Damage throughout the city was tremendous due to numerous aged and vulnerable masonry buildings. Many damaged buildings are under a certain level of cultural heritage protection. Hence, reliable assessment and effective rehabilitation are important to [...] Read more.
Recently, Zagreb was struck by a strong earthquake. Damage throughout the city was tremendous due to numerous aged and vulnerable masonry buildings. Many damaged buildings are under a certain level of cultural heritage protection. Hence, reliable assessment and effective rehabilitation are important to preserve cultural significance and mitigate risk for human life. With that in mind, the procedure of a detailed condition assessment of the building under heritage protection is presented. A detailed historical background of the case study building is shown, and observed damage and conducted in situ tests are discussed. The nonlinear static seismic analysis performed in the 3Muri software is extensively elaborated. Four different levels of reconstruction according to new Croatian law are briefly presented. Additionally, several strengthening scenarios are proposed with various strengthening techniques. Full article
Show Figures

Figure 1

Back to TopTop