Retrofit of Rocking Structures

Lead Research Organisation: University of Cambridge
Department Name: Engineering

Abstract

Numerous structures exhibit rocking behaviour when loaded dynamically, including unreinforced masonry structures, monuments, towers, bridge piers, sculptures, etc. The collapse of these structures due to dynamic loading has caused global destruction, as recently exhibited by earthquakes throughout the world. In the UK, collapse of masonry bridges during intense traffic loading is also a large concern. Thus, there is a national and international need to prevent the devastation caused by the collapse of these structures.Despite a significant amount of research in this area, engineers still misunderstand the fundamental difference between the dynamic response of rocking structures and typical elastic structures, and therefore assess rocking structures from a flawed perspective. The typical solution is to prevent rocking behaviour instead of controlling it. Prevention is usually achieved by tying structures down or reinforcing them. In the case of masonry structures, this is accomplished by drilling through structures and adding steel reinforcing, or by wrapping structures in Fibre-Reinforce Polymers (FRP). While these methods can be effective, they can over-stiffen structures and be destructive. Adding stiffness drastically changes fundamental dynamic behaviour, and can cause high stresses which lead to local damage. Such damage could be prevented with alternate retrofit solutions.The primary goal of this research is to develop new methods of controlling rocking motion using optimized damping solutions (e.g. shock absorbers). Instead of adding stiffness to the structure, damping is proposed because it allows some motion while dissipating unwanted energy. Thus, both devastating collapse of structures which have not been reinforced, and unnecessary local damage due to over-stiffening, could be prevented.In this context, this research will aim to characterize the fundamental behaviour of damped rocking motion through analytical modelling. A single rocking block analytical model will serve as tool to determine the type of damping which best controls rocking motion, and then to optimize the specific characteristics of damping mechanisms. Subsequently, more complex analytical models which describe the rocking behaviour of masonry arches will be created. Arches are typical components of masonry buildings and bridges, so understanding their dynamic behaviour is critical in developing appropriate retrofitting solutions. Analytical arch models will be used to test a variety of retrofit schemes which incorporate optimized damping mechanisms.While analytical models are critical for characterizing behaviour and designing retrofit solutions, experimental testing is essential to evaluate their accuracy. Results of analytical modelling will first be used to inform the design and construction of optimized spring-damper elements. These elements will enable the retrofit of blocks and arches which will be tested under horizontal ground motion using a small scale shake table. Experimental results will be used to evaluate analytical modelling results and to determine the effectiveness of retrofit solutions.Finally, analytical modelling is effective for simple structures, but it is typically not feasible for more complicated ones. Thus, the final aim of this work is to use commercial Discrete Element Modelling (DEM) software to predict experimental results. DEM is an appropriate tool for this purpose because it is tailored to model the interaction of multiple distinct blocks. If DEM is determined to be accurate, it could be an essential tool for designing and testing retrofit solutions for more complicated structures.In summary, new retrofit solutions are needed. This research aims to lay the foundation for the development of a new class of retrofit solutions which exploit clever damping systems. In the process, scientific progress will be made regarding the control of non-smooth dynamic systems in general.

Planned Impact

Widespread implementation of improved retrofit techniques which are based on this research would achieve the societal and economic benefits envisioned. To accomplish this, a significant continued effort beyond the duration of this project is planned. First, an effort will be made to connect with industrial partners to facilitate the transfer of knowledge from the academic to the industrial environment. Expected partners include structural engineering firms who are tasked with retrofitting structures, architectural conservation organisation who are tasked with preserving structures, and manufacturing companies who would be interested in producing custom damping devices. Second, collaboration with other academics will improve theoretical advancements and enable future experimental testing. Initially, this will involve collaboration with project partner Dr. Andrei Metrikine, Professor of Structural Mechanics at the Technical University of Delft. Professor Metrikine will be doing theoretical research to better identify structures which are most vulnerable to rocking collapse, and are therefore most in need of retrofit. Beyond the scope of this proposal, collaborations will be sought with researchers with the facilities and expertise to carry out large scale testing of possible retrofit solutions. Large scale testing will be an important step leading to industrial application. In order to assist in achieving the envisioned impacts, a board of advisors has been assembled. This board will provide access to a much larger network of possible industrial and academic partners who would be interested in the collaborations outlined above. The board of advisors will also serve to provide feedback and new ideas for the research, which will ultimately improve the project and provide quality assurance. The scientific impact of this work will propagate through dissemination of results in typical academic media. Results will be published in international refereed journals, conference proceedings, and Master of Engineering reports. Presentation of results at international conferences will also enable connection with new collaborators who could push the work in different directions. Finally, the positive impact of this work on the post-doctoral researcher and Master of Engineering students will be achieved through bi-weekly meetings with the Principal Investigator and periodic meetings with the board of advisors and project partner.

Publications

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DeJong M (2014) Dynamically equivalent rocking structures DYNAMICALLY EQUIVALENT ROCKING STRUCTURES in Earthquake Engineering & Structural Dynamics

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DeJong M (2012) Seismic response of stone masonry spires: Analytical modeling in Engineering Structures

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Dimitrakopoulos E (2012) Revisiting the rocking block: closed-form solutions and similarity laws in Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences

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Dimitrakopoulos E (2012) Overturning of Retrofitted Rocking Structures under Pulse-Type Excitations in Journal of Engineering Mechanics

 
Description The primary project contributions include:

1) The analytical results necessary to rapidly produce rocking response spectra for near-source earthquakes.

2) A fundamental understanding of the effect of added viscous damping on the rocking response.

3) A fundamental understanding of the effect of flexibility on the rocking response.

4) Further identification of the advantages and limitations of discrete element modeling for the prediction of rocking response of historic masonry structures.

5) Further identification of the potentially devastating effects of near-source earthquake ground motions on rocking structures.

6) Models that can be used to optimize the damping and stiffness of retrofitted, or newly designed, rocking systems.
Exploitation Route The fundamental analytical research on rocking structures could be used to assess and design rocking structures in practice. In particular, it could be used to create rocking spectra for the prediction of maximum rocking response during expected earthquake events. This is an essential step towards the more widespread use of rocking as an effective isolation measure for design or retrofit.

The computational DEM modeling results provide the insight necessary to use these methods in practical design and assessment.

The experimental data generated was used to validate computational and analytical models, but could also be used by other researchers as benchmark results.
Sectors Construction,Other

 
Description The findings have been used by other researchers in the development of new seismic systems and the assessment of exisiting structures. The findings are currently being used, together with colleagues in Italy, to develop a practical analysis tool for seismic assessment of existing masonry buildings that would be available to consulting engineers. The research is also being used to assess heritage structures in collaboration with Historic England. The findings have been applied in consulting to the seismic design new structures and the assessment of existing structures.
First Year Of Impact 2012
Sector Construction,Culture, Heritage, Museums and Collections,Other
Impact Types Societal

 
Description Collaboration - New Zealand 
Organisation University of Auckland
Country New Zealand 
Sector Academic/University 
PI Contribution The research conducted as part of this grant has provided the basis for a research exchange with the University of Canterbury and the University of Auckland in New Zealand. A PhD student continuing related research is now spending a year at the these two institutions to conduct further experimental and analytical modelling. It is envisioned that this collaboration will expand in the future.
Start Year 2012
 
Description Collaboration - New Zealand 
Organisation University of Canterbury
Country United Kingdom 
Sector Academic/University 
PI Contribution The research conducted as part of this grant has provided the basis for a research exchange with the University of Canterbury and the University of Auckland in New Zealand. A PhD student continuing related research is now spending a year at the these two institutions to conduct further experimental and analytical modelling. It is envisioned that this collaboration will expand in the future.
Start Year 2012
 
Description Collaboration - Roma Tre University, Italy 
Organisation Roma Tre University
Country Italy 
Sector Academic/University 
PI Contribution The modelling results in the project led to a research project to investigate collapse mechanisms of masonry structures in Italy, in an attempt to improve building code requirements. The collaboration will involve research meetings to discuss results, and the exchange of PhD and post-doctoral researchers between Italy and the UK. For summer 2013, our research team hosted a post-doctoral researcher who implemented and extended our previous research outputs to more directly apply to seismic assessment of buildings in Italy. This led to a journal paper. From summer 2014, a PhD student here at Cambridge has been extending this work further to create a computational tool that will implement the analytical research.
Collaborator Contribution For summer 2013, a post-doctoral researcher visited Cambridge and implemented and extended our previous research outputs to more directly apply to seismic assessment of buildings in Italy (as noted above). This led to a journal paper. From summer 2014, a PhD student at Roma Tre University has been doing complementary research to the student at Cambridge. We meet regularly to discuss the research.
Impact This collaboration led to a publication: Mauro A, de Felice G, DeJong MJ, (2015). The relative dynamic resilience of masonry collapse mechanisms, Engineering Structures, 85, 182-194. This collaboration also led to 4-day short course for graduate students entitled "Seismic assessment of masonry structures", held in Rome Italy from April 7-10, 2015. It was attended by approximately 60 graduate students from all over Europe.
Start Year 2012
 
Title Shake table 
Description As proposed in the grant, a shake table was constructed in the Structures Laboratory at the University of Cambridge to test the dynamic response of scale model structures. The table has the capability to provide impulse and harmonic motions, as well as full earthquake time histories. The shake table has been used to test the rocking response of rocking blocks, masonry arches, and masonry spires. 
Type Of Technology Physical Model/Kit 
Year Produced 2010 
Impact The creation of the shake table enable experimental testing that was an essential part of validated analytical and computational models. 
 
Description Invited Lecture: Rocking of Structures During Earthquakes (Bologna) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Presented research results from this project and previous work on rocking to graduate students, professors, and consultants involved in the retrofit of masonry structures after earthquakes in Northern Italy.

Educated others on how to assess buildings on rocking collapse. Changed views of listeners on how to assess certain buildings. Inspired others with new research ideas. Generated interest for future collaboration.
Year(s) Of Engagement Activity 2012
 
Description Invited Lecture: UK Society for Earthquake and Civil Engineering Dynamics (SECED) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited lecture for the UK Society for Earthquake and Civil Engineering Dynamics (SECED). The lecture summarised results from this research project as well as other research on rocking structures.

The talk generated interest in the subject, particularly from the Nuclear industry. It also generated good questions, which resulted in a publication in the SECED society Newsletter. It also resulted in consultancy questions related to the nuclear industry, which stemmed a new research direction.
Year(s) Of Engagement Activity 2013
 
Description Rocking of Structures During Earthquakes: From Collapse of Masonry to Modern Design (Rome) 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Invited speaker to present results from this research grant and previous research on rocking structures.

Talk generated a lot of interest, and started a collaboration between myself and a Professor in Rome, Italy. This collaboration resulted in a post-doctoral visitor from Italy for two months, another research publication, and a grant application.
Year(s) Of Engagement Activity 2012
 
Description Short Course: Seismic Assessment of Masonry 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact The activity was a Short Course entitled "SEISMIC ASSESSMENT OF MASONRY STRUCTURES", and was held at University Roma Tre, Department of Engineering, on April 7-10, 2015.

The event was attended by 60-80 people who were both post-doctoral researchers and people from professional practice. The course was approved for continuing education ( or the equivalent in Italy) credit for practioners. The course disseminated the results of the research grant, as well as other research by Dr Matthew DeJong (University of Cambridge) and Prof Gianmarco De Felice (University Roma Tre).
Year(s) Of Engagement Activity 2015