BMe Research Grant
My research focuses on the seismic behavior of Hungarian road bridges. My goal is to assess the seismic risk of different damage states in case of an earthquake. The applied methodology is capable of determining critical components, evaluating the effectiveness of retrofit methods, thus allowing optimal economic decisions. A framework has been worked out for the automatic evaluation of seismic risk even covering the whole national bridge inventory.
I carry out my research at the Department of Structural Engineering which is responsible for the design subjects of the structural engineering students. Significant research is done in the accredited Structural Laboratory. The department is also a strong industrial partner as consultant, co-designer or independent inspector. My supervisor, László Gergely Vigh is the leader of the seismic research group and is also the member of the ECCS-TC13 committee which works on the improvement of the European seismic design standard (Eurocode 8 [1,2]).
In high seismicity regions, such as the western coast of the US, Japan, New-Zealand, the Mediterranean region of Europe, people are prepared against significant earthquakes. Seismic design is a common task; the structures are designed for seismic actions. Contrarily, in moderate and low seismic regions (eastern coast of the US, northern parts of Europe) this approach only gained ground in everyday design from the 90’s. In lack of seismic design, even smaller earthquakes can cause severe damage in these areas, which may lead to significant economic consequences. Accordingly, the uniform European standard (Eurocode 8) prescribes the seismic design of all new or retrofitted structures even in moderate (e.g. Hungary) or low seismic regions. As per the formal Hungarian standard (ÚT ) , only bridges with spans over 50 m had to be designed for seismic actions, thus most of them lacks of seismic design. Recently, the shortcomings have been revealed in case of several new bridge or retrofit designs. Previous results indicate that bridges may be vulnerable to earthquake loads calculated in accordance with the Eurocode 8 standard.
Beside earlier results, our design and research experiences [S1,S2,S3,S4] confirmed that in case of specific bridges, seismic load may be critical due to the lack of seismic design. Therefore, revealing their seismic behavior is of primary importance. The goal of the research is to establish a nationwide evaluation framework. Similar regional or national seismic bridge assessment was carried out in the US , in Italy , in Turkey , but we are not aware of any research in European moderate seismic region. The source of the uncertainty in the seismic response of the structure is the seismic load itself; therefore it is key step to determine proper seismic loads. The assessment of the whole bridge inventory requires a database the structure of which has to be determined and all the necessary data have to be defined. With the evaluation of the database, bridges can be classified; input parameters of typical bridges can be obtained with statistical tools. With these input parameters a parametric study should be carried out to recognize critical layouts and bridge components (piers, bearings etc.). These studies outline vulnerable bridges, however, the economic consequence of possible damages can only be estimated with the calculation of seismic risk. For this calculation probabilistic analysis methods are needed, the framework of the methodology should allow for assessing the whole national road system. As the last step, in case of bridges with high risk, retrofit methods should be evaluated and compared; the efficiency of these methods should be evaluated in the aspect of risk mitigation.
Road bridge database, numerical models
In cooperation, the Hungarian Transport Administration (HTA) provided me with the data structure of the Integrated Bridge Database (IBD) which allowed creation of an own database for research purposes. I classified bridges (Fig. 1a) on higher level roads based on their structural type and material, relative number and value. Concrete bridges have the highest representation; and their most commonly used structural type is the precast girder bridges (Fig. 1b).
Fig. 1 a) Classification of bridges on higher level roads. b) Classified bridges on the map of Hungary (black dots mark the precast girder bridges).
Fig. 2 Numerical model of typical precast girder highway bridges.
The goal is not only to work out a methodology, but also to provide an automatic analysis procedure. For the seismic analysis and simulation, finite element numerical model [S5,S6] has to be built for each bridge. For this, an integrated system was built in MySQL, Matlab  and OpenSees [8,S7] which works as follows: queries essential data from the database; automatically generates numerical model; and automatically carries out a predefined seismic analysis. The numerical model of a typical precast girder highway bridge can be seen in Fig. 2. The IBD is created for administrative purposes, complete structural description of the bridges is missing. These missing parameters are determined by analyzing existing bridge plans and design principles.
Determination of seismic load
Seismic analysis can be carried out in the frequency domain with response spectrum analysis, or in the time domain with time-history analysis using artificial or real ground motions. I performed probabilistic seismic hazard analysis to refine the standard spectrum , and also determined hazard curves for different sites. The latter gives the probability of exceedance of different spectral intensities (Fig. 3a). Spectral intensities with the same probability (standard value is 10% in 50 years) compose the site-specific spectrum, which I also determined for the sites. In Fig. 3b it can be seen that this refined spectrum is less conservative than the one proposed by the standard.
Fig. 3 a) Hazard curve for Komárom. b) Standard and site specific spectra and the selected records.
I also worked out an algorithm to create artificial ground motions fit to a predefined spectrum [S4,S8] for time-history analysis. These ground motions follow the expected spectral intensities, but their other intensity parameters, like the energy content can be significantly different from those of real earthquakes. Therefore, a holistic record selection algorithm [10,11] was adopted to match Hungarian circumstances. Of thousands of records, those with intensity characteristics (not only spectral, but e.g. energy content) adhering to the distributions calculated for the site, were chosen. Those selected records can be seen in Fig. 3b.
Critical layouts and risk analysis of seismic damage
In order to determine critical layouts and components, a parametric seismic analysis was carried out using response spectrum analysis [S9,S10]. Response spectrum analysis is a fast and efficient tool, enabling a wide parametric range can be covered; and an upper bound of the internal forces and deformations can also be calculated. The analyses focused on the most commonly used precast girder bridges. The input parameters were determined with statistical evaluation of the bridge database. For the reliable estimation of different damage occurrences, cyclic-plastic material behavior reflecting the real behavior of bridge components should be used. The numerical model was updated accordingly, taking into account the aspects of automatic model generation. The risk of damage occurrences was calculated with seismic risk analysis method (Fig. 4). Proper number of nonlinear time-history analyses was carried out on the structural numerical model with different seismic intensities. The maximal structural responses (forces, deformations etc.) were registered in case of each analysis, then they were compared with the capacity of the predefined damage state; the probability of exceedance was determined as the function of the intensity levels. As the obtained fragility curve provides conditional probability; the total probability could be calculated by integrating the multiplication of the fragility and the hazard curve (Fig. 3a). Latter gives the probability of exceedance of the intensity levels. Fragility curves of a typical highway bridge can be seen in Fig. 5a. Components fragility curves can be compared and the most critical component can be recognized; while system fragility can be used to evaluate the seismic risk associated with the damage of the whole bridge system.
Fig. 4 Illustration of seismic risk analysis.
Expansion of the road bridge database
The current road bridge database lacks of essential data, it is only sufficient to determine main input parameters for the seismic analysis. In my research, the existing database structure was analyzed, the missing parameters were determined, and the structure of the expanded database was designed [S9].
Determination of seismic load
With the determination of site specific spectra, the seismic load can be refined. It was shown through analyses using ground motions fit to site specific and standard spectra (Fig. 5b) that the calculated seismic risk can be reduced with more appropriate seismic load [S8].
Fig. 5 a) Fragility curves of a typical highway bridge in case of total damage. b) Fragility curves created with ground motions fit to standard and site specific spectrum (pier flexural failure).
During the determination of expected seismic intensities, it was also shown that the significant duration of expected earthquakes in Hungary is lower than 10 seconds which is proposed by the standard [S8]. This is beneficial in case of plastic systems due to the possibly mitigated cumulative plastic deformations.
System wide evaluation of highway bridges
In case of precast girder bridges, it was shown that the shear resistance of piers and the shear resistance of superstructure-abutment joint are insufficient. Using interpolation on the parametric results to obtain system wide estimation of critical components, the maximal acceptable peak ground accelerations was determined for each bridge and each component, then using the location of the bridge and the standard peak ground acceleration map (Fig. 6a); the number of possibly critical components was identified (Fig. 6b). The results indicate that the pier shear resistance is not adequate for more than half of the multi-span bridges. The study sketches the number of critical components and the locations of the bridges provide essential information for a retrofit plan. With the results indirect system wide evaluation of the seismic risk can be also achieved [S10].
Fig. 6 a) Critical (red) and adequate (green) bridges for pier shear failure plotted on the seismic hazard map of Hungary. b) Relative number of critical components of multi-span highway bridges.
Seismic risk analysis of existing highway bridges
A total of 30 precast girder highway bridges are used for seismic risk analysis. It was confirmed that the pier shear resistance is insufficient, while in most of the cases the risk of flexural failure is under a prescribed level. The seismic risk of the whole structure was calculated based on the system fragility curves, and it was concluded that the risk can be mitigated only by increasing the shear resistance. Evaluating three different retrofit methods, it was shown that a fibre-reinforced polymer retrofit is the most economical solution [S11].
The refinement of seismic load allows for more economical structures to be designed. It is our goal to create a web-based application for practicing engineers where site-specific spectra as well as the artificial record generator and the record selection algorithm can accessed. The parametric seismic analysis revealed the critical components and layouts; the maximal acceptable intensities can be used by structural engineers to focus on these critical elements. The risk of damage occurrences can be applied for economic analyses, optimal decisions can be made for a retrofit plan. These analyses can be extended to national level, if proper amount of data is provided for the developed seismic risk analysis framework. It should be noted that the aim of the research is to provide practical solutions and results for engineers, and the results can be later integrated into the standard. The developed framework has already been used in practice for the evaluation of various seismic retrofit strategies of the existing Hárosi Danube bridge [S12]. A further goal is the extension of the bridge database based on its current structure we developed. With this extension, the developed integrated framework would be capable of evaluating not only the seismic risk of the whole bridge inventory, but other structural analysis could also be carried out; e.g. route planning for heavy traffic could be easily solved.
[S1] Simon J, Vigh LG. (2013). Hidak megerősítése szeizmikus hatásokkal szemben. Építményeink védelme 2013. Ráckeve, Hungary. pp. 1–15., ISBN:978–963–89016–5–1.
[S2] Simon J, Martinovich K, Dani B, Ájpli B, Sapkás Á, Vigh LG. (2013). Hidak állékonyságának biztosítása szeizmikus terhekre – esettanulmányok. Útügyi Lapok 1:(1). Paper 2.
[S3] Simon J, Vigh LG. (2012). Seismic assessment of Hungarian highway bridges – A case study. Proceedings of the First international conference for PhD students in Civil Engineering. Cluj-Napoca, Romania, 2012., pp. 155–162. ISBN:978–973–757–710–8.
[S4] Simon J, Vigh LG. (2013). Seismic assessment of an existing Hungarian highway bridge, Acta Technica Napocensis – Civil Engineering and Architecture 56:(2), pp. 43–57.
[S5] Simon J. (2012). Numerical model development for seismic assessment of continuous girder bridges. Proc. of Conference of Junior Researchers in Civil Engineering, BME, Budapest, Hungary, pp. 34–41, ISBN 978–963–313–061–2.
[S6] Simon J. (2013). Parameter identification for dynamic analysis of pile foundation using non-linear p-y method. Proceedings of the Second Conference of Junior Researchers in Civil Engineering, Budapest, Hungary, pp. 161–170.
[S7] Simon J, Vigh LG. (2014). Multi modal response spectrum analysis implemented in OpenSEES. OpenSees Days Portugal 2014: Workshop on Multi-Hazard Analysis of Structures using OpenSees, Porto, Portugal, pp. 39–42.
[S8] Simon J, Vigh LG. (2015). Seismic Vulnerability Assessment of an Existing Hungarian Highway Bridge Using Hazard Compatible Ground Motions. 12th Hungarian Conference on Theoretical and Applied Mechanics. Miskolc, Hungary.
[S9] Simon J, Vigh LG. (2015). Magyarországi hídadatbázis alkalmazhatósága meglévő közúti hidak földrengésvizsgálatához. Útügyi Lapok. 5: pp. 1–24.
[S10] Simon J, Vigh LG. (2015). Parametric seismic analysis of prestressed multi-girder bridges in Hungary, Bulletin of Earthquake Engineering, (submitted).
[S11] Simon J, Vigh LG. (2015). Preliminary seismic vulnerability assessment of pre-code multi-girder bridges in Hungary. SECED 2015 Conference: Earthquake Risk and Engineering towards a Resilient World, Cambridge UK.
[S12] Simon J, Vigh LG, Horváth A, Pusztai P. (2015). Application and assessment of equivalent linear analysis method for conceptual seismic retrofit design of Háros M0 highway bridge. Periodica Polytechnica Civil Engineering, 59:(2) pp. 109–122.
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