Browsing by Author "Silva, Vitor"
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- Earthquake loss estimation for the Kathmandu ValleyPublication . Chaulagain, Hemchandra; Rodrigues, Hugo; Silva, Vitor; Spacone, Enrico; Varum, HumbertoKathmandu Valley is geologically located on lacustrine sediment basin, characterized by a long history of destructive earthquakes. The past events resulted in large structural damage, loss of human life’s and property, and interrupted the social development. In recent years, the earthquake risk in this area has significantly increased due to uncontrolled development, poor construction practices with no earthquake safety provisions, and lack of awareness amongst the general public and government authorities. In this context, this study explores the realistic situation of earthquake losses due to future earthquakes in Kathmandu Valley. To this end, three municipalities: (a) Kathmandu Metropolitan City, (b) Lalitpur Sub-Metropolitan City and (c) Bhaktapur Municipality are selected for a case study. The earthquake loss estimation in the selected municipalities is performed through the combination of seismic hazard, structural vulnerability, and exposure data. Regarding the seismic input, various earthquakes scenario considering four seismic sources in Nepal are adopted. For what concerns the exposure, existing literature describing the construction typologies and data from the recent national census survey of 2011 are employed to estimate ward level distribution of buildings. The economic losses due to the earthquake scenarios are determined using fragility functions. Finally, the ward level distribution of building damage and the corresponding economic losses for each earthquake scenario is obtained using the OpenQuake-engine. The distribution of building damage within the Kathmandu Valley is currently being employed in the development of a shelter model for the region, involving various local authorities and decision makers.
- Evaluation of Analytical Methodologies to Derive Vulnerability FunctionsPublication . Silva, Vitor; Varum, Humberto; Crowley, Helen; Sousa, Romain; Pinho, RuiThe recognition of fragility functions as a fundamental tool in seismic risk assessment has led to the development of more and more complex and elaborate procedures for their computation. Although vulnerability functions have been traditionally produced using observed damage and loss data, more recent studies propose the employment of analytical methodologies as a way to overcome the frequent lack of post-earthquake data. The variation of the structural modelling approaches on the estimation of building capacity has been the target of many studies in the past, however, its influence in the resulting vulnerability model, impact in loss estimations or propagation of the uncertainty to the seismic risk calculations has so far been the object of restricted scrutiny. Hence, in this paper, an extensive study of static and dynamic procedures for estimating the nonlinear response of buildings has been carried out in order to evaluate the impact of the chosen methodology on the resulting vulnerability and risk outputs. Moreover, the computational effort and numerical stability provided by each approach were evaluated and conclusions were obtained regarding which one offers the optimal balance between accuracy and complexity.
- Generation of spectrum-compatible acceleration time history for NepalPublication . Chaulagain, Hemchandra; Rodrigues, Hugo; Varum, Humberto; Silva, Vitor; Gautam, DipendraThe 25 April 2015 Gorkha earthquake in Nepal caused severe structural damage in central Nepal, including the capital city Kathmandu. Both seismic vulnerability analysis of affected buildings and nonlinear time history analysis need increasing time history. However, no adequate records are available in Nepal, thus structural engineering practices follow Indian guidelines. The Nepal building code allows time history analysis along with the seismic coefficient and response spectrum methods, so nonlinear time history analysis can be used to assure adequate seismic safety. To fulfill the gap of recorded accelerograms, we developed artificial acceleration time histories considering the seismic hazard model of Chaulagain et al. (2015). We compared the synthetic accelerograms developed in this study with the records of Gorkha earthquake; the recorded maximum peak ground acceleration during the Gorkha earthquake showed comparable acquaintance only to the peak amplitudes of synthetic accelerograms generated for 300- and 475-year return periods.
- Multiple Description of Coded Video for Path Diversity Streaming AdaptationPublication . Correia, Pedro; Assunção, Pedro A.; Silva, VitorThis paper extends the current concept ofmultiple description coding (MDC) to the compressed domain, by proposing efficient splitting of standard single description coded (SDC) video into a multi-stream representation. A novel multiple description video splitting (MDVS) scheme is proposed to operate at network edges, for increased robustness in path diversity video streaming across heterogeneous communications chains. It is shown that poor performance of existing methods is mainly due to distortion accumulation, i.e., drift, when decoding is carried out with missing descriptions. The proposed scheme is able to effectively control drift distortion in both intra and inter predictive coding, evenwhen only one description reaches the decoder. This is achieved by generating a controlled amount of relevant side information to compensate for drift accumulation, whenever any description is lost in its path. The simulation results show that any individual description can be decoded on its own without producing drift, achieving significant quality improvement at reduced redundancy cost. The overall performance evaluation, carried out by simulating video streaming over lossy networks with path diversity, also demonstrates that MDVS enables higher quality video in such heterogeneous networking environments, for a wide range of packet loss rates.
- Multiple description video transcoding with temporal drift controlPublication . Correia, Pedro; Assunção, Pedro; Silva, VitorThis paper proposes a multiple description (MD) transcoding scheme capable of preventing drift by distortion accumulation in temporally predicted motion compensated slices. Drift compensation is achieved by generating a controlled amount of side information to be used for decoding whenever a description fails to reach the end user terminal. The side information is generated by re-encoding the transcoding residue with an independent quantisation parameter which also controls redundancy. A simplified architecture is devised to reduce transcoding complexity in regard to the number of processing functions and buffer requirements. The experimental results show that temporally predicted frames do not suffer from drift and their quality is significantly improved at reduced redundancy cost in comparison with a classic MD transcoding scheme.
- Optimal priority MDC video streaming for networks with path diversityPublication . Correia, Pedro; Ferreira, Lino; Assunção, Pedro; Cruz, Luis; Silva, VitorThis paper proposes a robust video streaming scheme for priority networks with path diversity, based on a combined approach of multiple description coding (MDC) with optimal picture classification into two priorities. A binary classification algorithm is proposed to define high (HP) and low (LP) priority network abstraction layer units (NALU), which in turn define the packet priorities. An optimisation algorithm is used to find HP pictures, based on dynamic programming and relying on minimisation of the packet loss concealment distortion. The paper shows that the proposed algorithm is able to effectively improve the decoded video without increasing the MDC stream redundancy. The overall performance evaluation, carried out by simulating MDC video streaming over lossy networks with path diversity, demonstrates that the proposed algorithm yields higher video quality for a wide range of packet loss rates (PLR). Comparing with no-priority MDC video streaming schemes, the simulation results show that the proposed algorithm can improve the average PSNR results in 0.7-3.2dB for packet loss rates between 3% and 15%.
- Seismic risk assessment and hazard mapping in NepalPublication . Chaulagain, Hemchandra; Rodrigues, Hugo; Silva, Vitor; Spacone, Enrico; Varum, HumbertoSeismic risk in the form of impending disaster has been seen from past records that moderate-to-large earthquakes have caused the loss of life and property in all parts of Nepal. Despite the availability of new data, and methodological improvements, the available seismic hazard map of Nepal is about two decades old. So an updated seismic hazard model at the country level is imperative and logical. The seismic hazard and risk model constitute important tools for framing public policies toward land-use planning, building regulations, insurance, and emergency preparedness. In fact, the reliable estimation of seismic hazard and risk eventually minimizes social and economic disruption caused by earthquakes. In this frame of reference, the seismic risk assessment at a country level is elementary in reducing potential losses stemming from future earthquakes. Thus, this study investigates structural vulnerability, seismic risk, and the resulting possible economic losses owing to future earthquakes in Nepal. To this end, seismic risk assessment in Nepal is done using an existing probabilistic seismic hazard, a newly developed structural vulnerability, and recently released exposure data. The OpenQuake-engine, the open-source platform for seismic hazard and risk assessment from the Global Earthquake Model initiative, was used to calculate the seismic hazard and risk in Nepal. The seismic hazard and mean economic loss map were formulated for the 1, 2, 5, and 10 % probability of exceedance in 50 years. Finally, the distribution of building damage and corresponding economic losses due to the recurrence of the historical 1934 earthquake was presented in this study. © Springer Science+Business Media Dordrecht 2015.
- Video transcoding from H.264/AVC to MPEG-2 with reduced computational complexityPublication . Moiron, Sandro; Faria, Sergio; Navarro, António; Silva, Vitor; Assunção, PedroThis paper addresses video transcoding from H.264/AVC into MPEG-2 with reduced complexity and high rate-distortion efficiency. While the overall concept is based on a cascaded decoder-encoder, the novel adaptation methods developed in this work have the advantage of providing very good performance in H.264/AVC to MPEG-2 transcoding. The proposed approach exploits the similarities between the coding tools used in both standards, with the objective of obtaining a computationally efficient transcoder without penalising the signal quality. Fast and efficient methods are devised for conversion of macroblock coding modes and translation of motion information in order to compute the MPEG-2 coding format with a reduced number of operations, by reusing the corresponding data embedded in the incoming H.264/AVC coded stream. In comparison with a cascaded decoder-encoder, the fast transcoder achieves computational complexity savings up to 60% with slightly better peak signal-to-noise ratio (PSNR) at the same bitrate.