Carlos Estuardo Ventura

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Evaluating the performance-based seismic design of RC bridges according to the 2014 Canadian Highway Bridge Design Code (2019)

The 2014 edition of the Canadian Highway Bridge Design Code, CSA S6-14, has adopted a performance-based design approach for the seismic design of lifeline and major-route bridges in highly seismic zones. This addition offers many opportunities as well as some challenges with regards to implementing the CSA S6-14 performance-based design provisions in practice. This thesis aims to identify these challenges through a critical review of the CSA S6-14 performance-based design provisions and to address a number of them within the scope of the thesis. The motivation behind conducting the present study is to prepare a reference document for engineers to better comprehend and implement the new provisions in practice. The focus of the thesis is on the performance-based design of new reinforced concrete bridges with ductile substructures.The addressed challenges are related to CSA S6-14 performance verification framework, calibration of performance criteria, and appropriate numerical models to evaluate the established performance criteria. A deterministic and a probabilistic framework are recommended to be used with the CSA S6-14 performance-based design approach. The applications of each of the frameworks are demonstrated through two detailed case studies and the advantages and disadvantages of each framework are discussed. The performance criteria of the code are compared against the recommended criteria in the literature and other design guidelines. Moreover, the strain limits of the code are examined to predict the damage to a number of tested reinforced concrete bridge columns. A thorough comparison of the CSA S6-14 and the updated strain limits of the BC MoTI Supplement to CSA S6-14 is presented. Finally, common modelling techniques for reinforced concrete structures including distributed and concentrated plasticity models are employed to predict the response of a number of tested bridge columns. Mesh-sensitivity issues due to the localization of plastic strains at critical sections or elements of distributed plasticity models are discussed and the methods to rectify the issue are presented and compared. A simple solution is proposed to eliminate the post-processing effort that is required to verify the strain limits of the code in distributed plasticity models, for which material model regularization is used to deal with the mesh-sensitivity issue.

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Damage, collapse potential and long duration effects of subduction ground motions on structural systems (2018)

Current code designs are based on design spectrum which does not account adequately for long duration shaking and therefore they do not reflect the duration effects of subduction earthquakes on seismic demands. The 2015 edition of the National Building Code of Canada (NBCC) probabilistically includes the effects of subduction earthquakes in developing the Uniform Hazard Spectrum (UHS), thus leading to increased interest in the impact of subduction ground motions on seismic demands for design.The research aim of this thesis is to investigate in detail the impact of subduction motions on design, particularly the effect of duration and the evaluation of the damage potential of the subduction motions.The evidence of structural damage observed during reconnaissance after the 2011 Tohoku and 2010 Maule subduction earthquakes was used to investigate the characteristics of ground motions that could be used as consistent indicators of damage potential. Characteristics considered were basic shaking parameters, as well as spectral parameters. Among the parameters considered, the constant strength spectrum appeared to be the best indicator.The effects of the long duration shaking due to subduction motions on the dynamic performance of structures was investigated using Incremental Nonlinear Dynamic Analysis (IDA). The response of several nonlinear Single-Degree-of-Freedom (SDOF) systems designed using a force-reduction factor of 5.0, covering a wide range of fundamental periods and ductility capacities was studied. The effect of duration was isolated by compiling two suites of spectrally compatible motions representing crustal and subduction earthquakes. Based on the drift ratios from IDAs, fragility curves were developed giving the probability of exceeding a prescribed drift ratio. The study clearly showed that the conditional probability of exceeding a prescribed drift for subduction earthquakes is higher than the probability associated with crustal motions. However, the magnitude of this effect depends on the fundamental period, the ductility of the building and the shaking intensity.In order to investigate how the above findings apply to a Multi-Degree-of-Freedom (MDOF) building, the same pair of input motions were used to run IDA on a 6-storey reinforced concrete (RC) moment frame. The resulting fragility curves confirmed the findings obtained with the SDOF analyses.

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Effects of ground motion duration on the seismic performance and collapse capacity of timber structures (2018)

This thesis describes the performance of timber structures under long duration earthquakes. The primary objective of this study is to quantify the effects of long duration ground shaking on wood structures compared to short duration shaking. The investigation is confined to conventional low-rise and modern mid-rise woodframe buildings. Full scale models of the benchmark structures used for this investigation were also constructed and tested on shake tables. Three-dimensional numerical models of the structures were developed using the Timber3D program and validated with the shake table test data. To isolate the effects of duration, two sets of short and long duration records that had approximately the same response spectra were used for nonlinear dynamic analyses of the model structures. Their collapse capacity was evaluated using fragility curves developed by incremental dynamic analysis. The results showed the collapse rate increased under long duration shaking due to a large number of inelastic cycles and higher cumulative energy demands that timber structures experienced compare with the short duration motions. The reduction in median collapse capacity for the low-rise wood structure with engineered oriented strand board (OSB) sheathing and stucco was 26%, for the same structure but without stucco was 29%, and for structure with horizontal board sheathings was 61%, respectively. The reduction in median collapse capacity for the mid-rise woodframe structure was 18%. These results suggest that current design practice based on the response spectra analysis may not adequately characterize the potential collapse of timber structures. This study highlights the need to include ground motion duration effects into current seismic design and assessment provisions.

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Seismic strengthening of unreinforced masonry walls using sprayable Ecofriendly Ductile Cementitious Composite (EDCC) (2018)

Due to their good sound and thermal insulation properties, unreinforced masonry (URM) walls are widely used in partitioning many of the commercial and residential buildings worldwide, either as in-fill or stand-alone. URM is considered one of the most common types of partition wall systems in many of the mid-age, low-rise to mid-rise, school and hospital buildings in North America. URM is still a very common and major building material, in many of the developing countries, being used in many residential and commercial buildings.URM partition walls are known to have very low drift capacity in seismic events and the failure mechanisms are known to be mostly brittle and of catastrophic nature, during earthquake ground motion. Compared to other partitioning systems, URM walls tend to perform poorly during earthquake events, leaving many injuries, casualties, and fatalities behind.This dissertation elaborates on development of a novel, effective, and practical methodology for a robust out-of-plane seismic strengthening technique toward seismically upgrading URM partition walls, using a thin plaster layer of sprayable Ecofriendly Ductile Cementitious Composite (EDCC). The EDCC layer is devoted to secure such walls, which exist in most parts of the world; specially, in developing countries, where world’s most population density is concentrated. In many of these countries, retrofit is the only option, since building replacement is not practical nor an economically feasible solution. The EDCC material can be applied in three different methods: hand troweled, hopper sprayed, or pump sprayed. The thickness of the layer can vary between 10mm to 20mm, depending on the design variables.Full-scale URM walls are built, strengthened, and tested on a shake table, using the strongest real historical earthquake records. The EDCC layer is providing nearly full out-of-plane detention for the wall’s building blocks, as well as minor but uniform shear capacity enhancements for the in-plane action; therefore, holding the masonry units together from falling apart and being thrown during an earthquake generated ground motion. The newly developed high performance material is sprayable, ductile, and resilient, while being affordable, and easy to apply, with much less carbon footprint compared to other similar repair materials.

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Robust statistical subspace-based damage assessment (2017)

The rapid spread and increasing affordability of sensors, are encouraging the government and stake-holders to instrument important infra-structures and structures. These sensors generate vast amount of data which can be used in real-time health monitoring of the instrumented structures by using damage identification methods.A significant component of structural health monitoring is damage identification methods which process the data with the purpose of detecting damages in the structures. One of these methods with a theoretical background is the statistical subspace damage identification method (SSDI). The overarching goal in this thesis is to close the gap between theory and practice, in order to have a method with a strong theoretical background and a credible applicability at the same time.In order to achieve this goal several contributions are motivated in this thesis, which are presented as follows: Firstly, the effect of two challenges faced in the damage detection of structures under real test conditions, namely the measurement noise and duration (length), are theoretically evaluated. It is demonstrated that the measurement noise and length have considerable influence on the statistical subspace damage detection method and they need to be considered based on these proposed theories.Secondly, the statistical subspace damage localization (SSDL) method, is assessed for the first time, in localizing the damage of a real experimental structure, i.e. the Yellow frame, established on the course of this research at UBC. Several methods and theories are developed in order to enable this method in identifying the damage under real test conditions. It was demonstrated that by employing the proposed theories, the SSDL method can robustly locate the damage in a real structure such as the Yellow frame.Finally, two indexes are proposed in predicting the detectability of damage in each element of a structure. These indexes provide valuable information on the sensitivity of SSDL method to the damage in each element. All the proposed theories and methods are demonstrated theoretically; subsequently, they are verified by simple and sophisticated analytical models, and finally, they are validated by real-test data.

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Structural and economic evaluation of self-anchored discontinuous hybrid cable bridges (2017)

The self-anchored discontinuous hybrid cable bridge (SDHCB) is a novel type of bridge system which has the potential to overcome many of the deficiencies of conventional cable bridge structures while preserving their advantages. To date, research on the system is extremely limited. Accordingly, this thesis examines the structural and economic attributes of the system, as well its constructability. These areas of research are vital in evaluating the utility of the system and in advancing its development. The structural attributes of SDHCBs were studied in this thesis using a systematic approach. First, the behaviour of each of the two basic cable types found in hybrid cable bridges was studied under a wide range of parameters. Then, starting with a bare model of a SDHCB, a series of analyses was performed while progressively expanding the model so that the influence of various parameters and bridge components could be isolated, and accurately assessed. The model parameters were further refined through a cost analysis. Thereafter, upon reaching a complete and detailed model, the influence of various structural parameters was re-assessed, the structural benefits of employing various supplemental design components were appraised, and the constructability of the system was addressed. This work is significant in that it has provided a highly generalized and robust model of a SDHCB. Using this model, it is possible to ascertain how various design parameters such as geometric factors, material properties, and loading conditions affect the structural behaviour, cost, and constructability of the system. Many insights were also obtained from this research which led to the formation of a recommended, universal, design space for the system. In addition, the practicality and adaptability of the system were demonstrated through the development of several innovative construction schemes which aim to reduce construction duration and costs through the creation of multiple work fronts and the elimination of large temporary works.

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Directionality effects of pulse-like near field ground motions on seismic response of tall buildings (2015)

Earthquake ground shaking at near-field sites has produced severe damage to buildings and infrastructure, caused large economic and human losses. These earthquakes continue to pose a great threat to many populous urban centers around the world. Over the last 4 decades, major research efforts have been devoted to characterize near-field ground motions and their effects on the response of buildings. A challenging engineering problem that remains unsolved in the seismic design of buildings is the directionality effect of near-field pulse-like ground motions on tall buildings. This dissertation presents a computationally efficient method to calculate the critical displacement response of tall buildings. In this method, the duration and amplitude of the ground velocity pulses contained in the input motion and the building’s first mode translational period are compared to determine the orientation of the Conditional Maximum Velocity (CMV), a new ground motion metric. Nonlinear response history analyses (NRHA) using the CMV ground motion provide a close estimate of the critical displacement response of a tall building along the structural axis. The CMV Method is developed on the basis of a series of parametric studies. First synthetic pulse-like excitations and simple structures were used to systematically investigate the effects of pulse duration and amplitude on the dynamic elastic and inelastic response of structures. It is found that the critical displacement response is influenced significantly by the CMV ground motion. This finding is further validated through series of NRHA of simple structures and several case study tall buildings to near-field ground motion records. It is shown that the NRHA using CMV ground motions result in approximate, but significantly accurate estimates of the critical displacement with small errors and moderate dispersion. Ground motion pairs rotated to fault-normal (FN) orientation do not always result in the critical displacement response. Neither does the maximum direction (MD) ground motion at the fundamental period of the building. Analyses made using FN or MD may result in significant underestimates of critical displacement response compared to the proposed CMV. The use of the CMV Method may lead to a better quantification of potential seismic demands on tall buildings.

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Sliding displacements in reinforced masonry walls subjected to in-plane lateral loads (2015)

Seismic design provisions in the Canadian Masonry Code often lead to indications that the governing yield mechanism for a reinforced masonry wall with a height/length (H/L) ratio below 1.0 and under low axial loads will not achieve the design objective of a flexural yield mechanism and instead, will develop a sliding shear mechanism. In addition to this, results of previous experimental research studies indicate that even for squat walls that yield in flexure, the displacements at the top are the result of both flexure and sliding shear mechanisms. Currently, there is a limited understanding on how sliding shear displacements develop and how they affect the response of a building. The following work sets out to study the sliding shear mechanism and to develop tools for determining the corresponding displacements for seismic design. This study proposes to modify the current definition for a sliding shear mechanism, re-classifying yield mechanisms of Reinforced Masonry (RM) walls with sliding displacements into three separate mechanisms: sliding shear (SS) mechanism, dowel-constrained failure (DCF) mechanism and combined flexural-sliding shear (CFSS) mechanism. In addition, a 2D analytical model is developed and calibrated in this study using the experimental test results of wall specimens with recorded sliding shear displacements. This calibrated model simulates sliding in RM walls based on the effects of frictional resistance, dowel action and flexural hinging, which will be the basis for a procedure that can estimate sliding displacements in an RM wall design.

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Displacement demands for performance based design of skewed bridges with seat type abutments (2014)

Skewed bridges are irregular structures due to the geometry of the deck and bents. Past earthquakes indicate that skewed bridges with seat type abutments exhibit greater damage than their non-skewed pairs. The damage has been attributed to in-plane rotations caused by pounding between the skewed deck and its abutments during strong ground shaking.This thesis combines experimental and analytical approaches to understanding the displacement demands on skewed bridges. As part of the experimental studies, results from ambient vibrations tests help to better understand the importance of directionality in the lateral response of skewed bridges. The predominant direction of the transverse response occurs in the direction of the skew bents; whereas the predominant direction of the longitudinal response is perpendicular to the skew. In addition, the analysis of records from an instrumented skewed bridge confirmed accelerations that could produce in-plane rotations of the deck. A comprehensive parametric study based on nonlinear dynamic analyses was performed to evaluate the effects of different skew angles, abutments types, and soil-foundation-structure interaction. The results demonstrated that elastic methods recommended by current seismic design provisions, and commonly used in standard practice, do not properly capture the in-plane rotations of the deck due to pounding. To overcome this shortcoming, a simple and effective method is proposed here to evaluate the displacement demands of skewed piers accounting for in-plane deck rotations.The proposed method uses validated simplified nonlinear models to generate torsional sensitivity charts for specific bridge prototypes. The charts provide peak in-plane deck rotation estimates as a function of bridge skew angle and the in-plane rotational period. An advantage of this approach is that it requires the designer to only conduct a linear dynamic analysis of the bridge. Nonlinear analysis required to assess the in-plane deck rotation is replaced here by torsional sensitivity charts. The proposed approach is able to predict the displacement response for a comprehensive range of skewed bridge prototypes by capturing the effects of the main parameters controlling the response. The information presented in this thesis will help improve the existing recommendations for performance based design of skewed bridges.

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Effect of pulse-like near-fault ground motions on inelastic response of structures including foundation flexibility (2014)

For the sites in close proximity to a causative fault where formation of near fault long period velocity pulses is conceivable, consideration of flexibility of the foundation system is very important. This is due to the fact that when the flexibility of the base is taken into account the period of the system is longer than the period of the same system assuming a fixed base. Depending on the depth and the stiffness of the underlying soil the period of the system approaches the period of the near fault long period pulses, hence the response of the structure could be much larger.The purpose of this research is to study the nonlinear response of structures to pulse-like near fault ground motions with and without allowing for the foundation system flexibility. To highlight the impact of the near fault ground motions the nonlinear responses of single degree of freedom systems (resembling fixed base structures) to the near fault ground motions are compared to the responses of the same systems to the equivalent far field ground motions.The effects of (translational and rocking) flexibility of the foundation system is also considered using equivalent linear springs and lumped masses added to the base of the single degree of freedom systems. A major parametric study is performed to determine which parameter has the most significant impact on the response of the structure for near fault ground motions when effect of flexibility of the foundation system is explicitly accounted for.An efficient procedure has been developed for predicting the response of a structure with a flexible base to near fault ground motions deduced from the response of an equivalent single degree of freedom system to the equivalent far field ground motions. Validity of the proposed procedure for assessing the effects of near fault ground motions, and the influence of flexibility of the foundation system on the structures’ responses is verified using different analytical models, including a full 3D analysis of a bridge structure; the results proved to be quite satisfactory.

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Investigation of variation of motions between free field and foundation in seismic soil-structure interaction of structures with rigid shallow foundation (2013)

Soil-foundation-structure interaction (SFSI) in buildings during earthquakes is characterized by several aspects including the variation between the free-field and foundation motions. Past procedures for analysing the effect of the foundation on the free-field input motions are all based on the assumptions that the foundation slabs always reduce the motion. Recent guidelines, standard and codes including FEMA-440 and ASCE/SEI41-06 also recognize that foundations with interconnected grade beams or concrete slab will always reduce the free-field motions. It implies that SFSI is beneficial and can be conservatively neglected in regular building design practice. A large number of instrumented buildings that have experienced a numbers of earthquakes in the past provide an opportunity to investigate the SFSI effects and evaluate the methods of estimating the foundation motion based on field data.In this research study, investigation is carried out on the records of past earthquakes from sites of instrumented buildings over a wide range foundation configurations and site conditions in California. Analysis of records among 26 buildings that have shallow rigid foundation shows that foundation motions are reduced in two-third cases and amplified in one-third cases. The estimations of variation of motion by the ASCE procedure are not in good agreement, even for the cases of reduction of foundation-base motion. It was obvious that the amplification of the motion cannot be captured by the procedure.Time history simulations of soil-building system have been carried out for varied parameters in 2D continuum finite element models using computer program ABAQUS. The results from simulations confirm that motion may amplify at the foundation depending on period of the building, soil deposit and the predominant period of input motion. This thesis develops a simple mass–spring–dashpot-based system of soil–foundation–structure interaction, called the 3DOF SFS model, for calculating the foundation-base motion that accounts for dynamic interaction between soil, foundation and building. The 3DOF SFS model was verified for a building-structure system with varied parameters and for different input motions using results from ABAQUS. Both amplification and reduction cases of foundation motion compared with free field were predicted by the model.

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The flexural seismic resistant design of reinforced concrete bridge columns (2012)

Experimental studies about the cyclic response of reinforced concrete bridge columns designedto avoid shear failure and subjected to cyclic, reversible, and increasing displacements have beenperformed in several laboratories around the world. As a consequence there are several force-displacement relationships, called resultant models, that allow to predict the response of thosecolumns. However, the use of the resultant models for earthquake response requires extensivecalibration of several parameters.In this investigation a Finite Fiber Element Model, FFEM, is obtained after calibrating first, theresponse of 30 circular reinforced concrete bridge columns tested under cyclic, reversible, andincreasing displacements. Then a re-calibration is carried out in order to simulate the response oftwo additional columns shake table tested under two earthquake ground motions. After obtainingsatisfactory results the FFEM was used to simulate the seismic response of three bridge columnsdesigned according to the prescriptions of the new seismic design bridge code.The FFEM is able to predict directly four flexural failure mechanisms: cracking and crushing ofthe unconfined and confined concrete, fracture of the longitudinal steel bars due to tension, P-Δeffects, and fatigue of the longitudinal steel bars. Indirectly, the FFEM is able to predict thepossible buckling of the longitudinal bars by capturing the confined concrete strain time-history.In order to capture the low-cyclic fatigue, the FFEM through inelastic dynamic analysis is able tocalculate the number of cycles and the amplitude of the cyclic plastic strains so these quantitiesare introduced into the fatigue equation. The fracture of the bars due to low-cyclic fatigue is afailure mechanism that depends on the accumulation of damage along the severe ground motion.The way to estimate the loss of fatigue life in a steel bar is considering the effect of the durationin the calculations since the materials stress-strain relationships are independent of the durationof the ground motion.In order to determine the accumulation of damage in the bridge column a Cyclic Damage Indexis proposed here. The Index is based on the energy dissipated by the column at the end of theground motion.

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The response of underground power transmission calbes subjected to earthquake-induced permanent ground deformations (2012)

No abstract available.

Methodology for the seismic risk assessment of low-rise school buildings in British Columbia (2011)

This thesis presents a methodology for the seismic risk assessment and risk reduction of schools in British Columbia. The methodology permits school buildings to be ranked by risk levels, and includes information that allows designers to establish the seismic capacity of school buildings and to select appropriate retrofit options. This research includes the treatment of seismic hazard in the province by reference to different types of earthquakes that affect the region, and the development of an extensive database of structural performance of typical school buildings for different types of earthquakes and levels of shaking. The seismic hazard in the province is due to crustal, subcrustal and subduction earthquakes. The ground motion characteristics and the rates of occurrences of these different types of earthquakes are sufficiently different that it justifies assessing their effects separately in the risk calculations. The results of probabilistic seismic hazard analyzes have been combined with incremental nonlinear dynamic analyzes of a variety of structural systems subjected to the three earthquake types. A suite of thirty ground motions representative of these earthquakes has been used for the calculation of seismic risk. This process resulted in a large database of response of structural systems on different types of soils. The database was developed first for systems on firm soils (Site Class C). To account for soft soils (Site Class D) a simplified procedure was developed to convert structural performance on Class C sites to that on Class D sites.This thesis presents information that contributes to the state of knowledge in seismic risk in two forms: research and engineering practice. It provides a better understanding of how the risk in a region can be deaggregated according to the earthquake types, how representative ground motions for each earthquake type can be selected, and how the site conditions can be incorporated in probabilistic risk assessment. The contribution to engineering practice is the development of a ready-to-use methodology for risk assessment and for determining whether or not a retrofit is required for a giving type of structure on a certain type of soil and in a given seismic region.

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Seismic embankment-abutment-structure interaction of integral abutment bridges (2011)

This research thesis is product of a joint study between the Ministry of Transportation and Infrastructure (BCMoT) and the University of British Columbia (UBC) to evaluate the effect of Embankment-Abutment-Structure Interaction (EASI) in the estimation of seismic demands of Integral Abutment Bridges (IABs).IABs consist of a continuous concrete deck integrated with abutments supported on flexible foundations. These structures have become very popular due to the elimination of costly and maintenance prone expansion joints and bearings. Analytical studies and strong-motion earthquake data have shown that the seismic response of the approach embankments in the far field affects the response of IABs. However, current seismic analysis procedures neglect the far-field embankment response because of the complexity in modeling this type of dynamic interaction. Therefore, a simple and accurate model that allows bridge designers to include EASI in the calculation of the seismic demands of IABs is needed.This thesis develops a simple dynamic model, called 3M-EASI, for calculating the seismic response of IABs taking into account EASI. The proposed model consists of two far-field embankment components connected to the bridge structure component by spring-dashpot elements that represent the near-field components. The main contribution of this thesis is the development of the far-field embankment component using equivalent-linear analysis. The 3M-EASI model was verified with time-history analyses of 2D continuum soil finite element models of full-height IABs using the computer program ABAQUS. The analyses indicated that the far-field embankment response affects the response of IABs if the following conditions act simultaneously: (a) the near-field stiffness is greater than 0.4 times the bridge stiffness, and (b) the period of the far-field embankment components is longer than 0.7 times the period of the bridge-near-field system. The 3M-EASI model is shown to be rational, accurate, computationally efficient, and easy to implement in bridge design.

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Multi-hazard risk assessment : an interdependency approach (2010)

This research began with the Joint Infrastructure Interdependencies Research Program (JIIRP). JIIRP was part of an effort by the Government of Canada to fund research to develop innovative ways to mitigate large disaster situations. An interdependency simulator (I2Sim) was developed in the University of British Columbia through this project. This tool was developed to take into account the dynamic changes of the functional conditions of any given system. This thesis makes two major contributions to the capability of the simulator’s methodology, to handle seismic events and events that affect dense concentrations of people. The distinguishing characteristic of an earthquake event can affect the city and all the surrounding regions, causing damage to all lifeline systems. In its original form, I2Sim could model the damage and impact of each system on its own, but was unable to account for the effects of all other systems. The interdependency between systems is a crucial element for determining the impact of an earthquake and the time for recovery. The methodology proposed here can be used to measure Interdependencies and Resiliency in a region.Two cases were studied and implemented to test the methodology and the simulator. The first one was an earthquake hazard in a relatively small region (UBC Campus) in which the interdependencies and resiliency would be revealed to the emergency managers of UBC Campus; the second one, was a localized event in a massive sporting event (Winter Olympics in Vancouver), a black out in a Football Stadium that caused an uncontrolled egress, and related casualties due to a collapsing stage and the evacuation process were modelled. With the methodology and the simulator (I2Sim) it is possible to build up Region models, Disaster Scenarios, Objective Functions and Emergency Planning; and these, along with Interdependency and Resiliency calculations, will help in the preparedness, planning, response and recovery phases of any disaster.

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Master's Student Supervision (2010 - 2018)
Ambient modal identification, finite element model updating, and seismic analysis of bridges on Trans-Canada Highway (2018)

This thesis features finite element model updating of two short-span concrete bridges, namely Gaglardi Way Underpass and Kensington Avenue Underpass. The main objective was to study the effect and determine the importance of finite element model updating by comparing the structural responses for the updated model to the preliminary model. The study was carried out by developing a finite element (FE) model and an operational modal analysis (OMA) model for each bridge. The FE model represented the analytical prototype of the actual structure, while the OMA model was used to extract the modal information for existing structure using the vibration data recorded under normal operating conditions from permanent sensors installed on corners and at mid-span of these bridges. The natural frequencies from OMA were set as a target for the FE model to match. The process of calibrating the analytical FE model to the match the modal information acquired from the experimental model is known as ‘Model Updating’. Having the frequency responses defined, a sensitivity analysis was conducted to determine the parameters that are most sensitive to change, based on which the FE model was automatically updated in an iterative manner. The modal assurance criterion (MAC) and mode shape responses were not used during calibration step since the vibration testing was not dense enough, however, they were solely used as a means of comparing the calibrated FE model to the experimental results. Once the objective of model updating was accomplished, a linear modal time history analysis was carried out using three ground motions having a low, medium range, and a very high peak ground acceleration (PGA), in addition to a fourth very low ambient level ground motion. Comparing the resulting absolute maximum base reactions and the mid-span structural displacements from updated model to the original model, it was concluded that the percentage changes were significantly high, therefore, the chance of original model being uncertain is very high for which model updating is an important and a highly effective technique, where possible, to generate a high confidence FE model that in best possible manner represents the behaviour of an actual structure.

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Performance-based seismic design and evaluation of steel eccentrically braced frames with tubular links as bridge bents (2018)

The latest release of the Canadian Highway Bridge Design Code (CHBDC), S6-14, incorporates performance-based design (PBD) provisions for bridges in Canada for the first time. CHBDC S6-14 establishes two different design approaches, with the PBD being the standard method of design and force-based design (FBD) being permitted for special cases. The focus of this study is on ductile eccentrically braced frames (EBFs) as bridge substructure. For member proportioning, the CHBDC S6-14 refers to the Canadian steel design standard for buildings, CSA S16-14, stating a force reduction factor, R=4. This is a FBD, and there is need to evaluate the design in terms of the performance descriptions and damage states by carrying out the analyses recommended by CHBDC S6-14. For this case study, an existing bridge is considered as a Major Route bridge, and an EBF with built-up tubular shear link has been chosen as an earthquake-resisting system (ERS). Four different cases have been designed including two using FBD and two for PBD approach for comparison purposes. Due to the lack of strain/rotation criteria in CHBDC S6-14 at multiple service states for EBFs as bridge bents, different acceptance criteria for rotations and corresponding damage states have been proposed by using fragility curves from the literature. The link total rotation has been considered as a demand parameter and different methods of repairs consistent with each damage state have also been provided. The response spectrum analysis coupled with inelastic static pushover analysis is used for global displacement demands and for demonstrating local component performance compliance of shear links. Nonlinear time-history analysis is also used to check and provide a comparison of the first approach. The code requires no-yielding for the 475-year return period event. This criterion governs the design and makes the sizes large and inefficient, while the link plastic rotations corresponding to higher return period events are very low compared to the allowable limits provided in the literature for links mainly used in buildings. Through different cases, it is demonstrated that if the links are made replaceable and allowed to have limited yielding at 475-year earthquake, it makes the design more practical.

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Effects of long duration ground motions on the probability of drift exceedance for reinforced concrete frames near the Cascadia Subduction Zone (2017)

There are discrepancies in responses when a structure is subjected to short and long duration ground motions. The probability of drift exceedance has been repeated reported to increase under the influence of long duration records. Although the 2015 National Building Code of Canada has probabilistically taken the Cascadia Subduction Zone seismic hazards into considerations, none is known as what might happen to structures designed using this new Code if subjected to a large magnitude subduction earthquake.The answer is found via computer simulation. Following the general approach adopted in the Seismic Retrofit Guidelines 2nd Edition, incremental dynamic analysis is conducted to investigate discrepancies in the probability of drift exceedance for certain building types under both crustal and subduction ground motion records. These ground motions are selected and scaled to match the 2015 uniform hazard spectrum of Victoria, B.C. A simple shear wall model is first examined to generalize the effects of long duration ground motions. Then a similar study on a reinforced concrete frame is conducted to confirm these generalizations.Long duration ground motions seem to cause a higher probability of drift exceedance in moderately ductile buildings. However, no effect is observed in non-ductile and highly ductile buildings. The system internal capability to dissipate seismic energy by means of hysteretic loops is also contributing to the overall probability of drift exceedance. Its effect is more evident in the long duration as there are more load reversal cycles. Results discussion is provided, and potential ways to account for long duration in structural design are recommended.

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Numerical modeling of seismic performance of light-frame wood buildings (2017)

Light-frame wood structures are the most prevalent construction type in North America, representing over 90% of the residential building stock. Many of these buildings were built prior to the adoption of seismic engineering design practices and thus may be vulnerable in a seismic event. The primary objective of the research is to examine the use of numerical models to predict the seismic behaviour of light-frame wood structures. Models for (i) a full-scale two-storey house, (ii) a full-scale classroom, and (iii) a two-storey school block were created in light-frame wood non-linear analysis packages. The first two models were validated with full-scale shake table tests. The effect of sheathing type, nailing schedule, openings and ground motion characteristics on the seismic behavior of light-frame wood buildings were investigated. A three-dimensional model of a two-storey light-frame timber house with different sheathing configurations was calibrated using non-linear dynamic analysis to the full-scale experimental shake table results. The model of the test structures was able too predict the time-history response of the drift with reasonable accuracy. The contributions of the strength and stiffness from the openings and non-structural sheathing were included in the model. A detailed numerical model (each nail, framing member, hold-down and panel are modeled), as well as a global numerical model was used to predict the seismic behaviour of an additional dynamic shake table testing was also conducted on a full-scale classroom. The effect of openings, sheathing and ground motion duration was further investigated. Finally, the seismic performance of existing structures and the performance of several retrofit options was investigated with the validate modeling techniques using non-linear dynamic analysis of a typical school block built between 1950 – 1960 in Vancouver. The retrofit options met the target performance objectives.

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Operational modal analysis, model updating and response prediction bridge under the 2014 Napa Earthquake (2016)

Bridges constitute a critical and important part of the infrastructure of many cities’ transportation network. They are expensive to build and maintain, and the consequences of a sudden failure are very severe. Therefore, bridges are expected to have a high degree of reliability, which means that they have to perform above a life safety criterion under earthquake excitations. In a continuous effort to improve design guidelines, it is imperative to understand the behavior of existing bridges that are subjected to severe shaking. For this reason, continuous monitoring of bridges has become essential: not only to help determine if a bridge has been damaged but also to understand their response to strong earthquake motions. The work reported here includes an in-depth analysis of the behavior of the Vallejo- Hwy 37 Napa River Bridge during the 2014 California, Napa earthquake (M 6.0). The bridge located in Vallejo California connects Sears Point Road and Mare Island to Vallejo. It was built in 1967. The bridge was instrumented with 12 accelerometers on the superstructure and 3 accelerometers at a free-field site. An analysis of the recorded data of the accelerometers on the superstructure was carried out to determine the maximum displacement at mid-span, and to get the fundamental frequencies of the bridge during the excitation. A finite element (FE) model was developed based on the as-built drawings and model updaitng was perform. Finally, the updated model was used with the recorded ground motion of the 2014 Napa Earthquake to perform a time history analysis. The results were compared to the recorded data of the sensors located on the bridge. The peak displacement at mid-span in the longitudinal and transverse directions of the FE had a good match to the recorded peak displacement. It can be concluded that the updated FE model can capture the peak displacement at the bridge mid-span. It also shows that having a strong motion network can help engineers to better understand the behavior of structures under earthquake loading, by looking at the recorded data and identifying peak values of acceleration, velocity and displacement.

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Operational modal analysis, model updating, and seismic analysis of a cable-stayed bridge (2016)

The Port Mann Bridge is currently one of the longest cable-stayed bridges in North America and the second widest bridge in the world. It is a cable-stayed bridge consisting of 288 cables, two approach spans made of concrete box girders and precast deck panels, and a main span consisting of steel girders and cross beams with precast deck panels. This work sets out to accomplish three main goals: study the dynamic behaviour of the Port Mann Bridge, calibrate the finite element model, and study the effects of model updating using a seismic analysis. The dynamic behaviour of the Port Mann Bridge’s main span is studied using experimental data from field ambient vibration tests and from a structural health monitoring network. A finite element model is created by importing a version of the structural designer’s model and editing it based on design drawings. In order to assess what parameters would be feasible to calibrate, a sensitivity analysis is carried out using various material properties and boundary conditions. The model is then updated to match the experimental analysis results by varying multiple parameters. Finally, the calibrated model is compared to the original model by completing a linear time history analysis. A suite of ground motions were selected and scaled to match specific points on the response spectrum corresponding to the first few periods of the structure. Multiple critical locations are monitored in the time history analysis, and data from these locations are compared before and after calibration to examine the effect of model updating. The study concludes that model updating has a large effect on the predicted seismic behaviour of the bridge, which proves the importance of calibrating finite element models and maintaining physically meaningful parameters. It also shows that having a structural health monitoring program is very important for current and future research endeavours.

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Seismic evaluation of Portage Creek Bridge based on ambient vibration testing (2016)

Constructed in 1983, the Portage Creek Bridge is a three span highway bridge located in Victoria, British Columbia (BC), Canada. This bridge is a part of a smart seismic monitoring program, British Columbia Smart Infrastructure Monitoring System (BCSIMS), which funded by the British Columbia Ministry of Transportation and Infrastructure (MoTI), Canada. The BCSIMS aims to continuously monitor the seismic conditions of the selected bridges on lifeline highways in British Columbia, and as part of this goal, an ambient vibration test was carried out on the bridge in September 2014 in order to update/calibrate the finite element model of the bridge in SAP2000. The updated model was then used to assess the seismic performance of the bridge in accordance with the Canadian Highway Bridge Design Code, 2015. Nonlinear time-history analysis was performed using a finite element model with concentrated plasticity, and results were compared with the performance criteria specified in the code. This thesis presents the overall procedure of the seismic evaluation, as well as the relevant theoretical background and discussion of analysis results.

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Seismic performance evaluation of French Creek Bridge based on Canadian Highway Bridge Design Code 2015 (2016)

Built in 1993, the French Creek Bridge is located on highway 19 on Vancouver Island, BC, Canada. The bridge is part of the British Columbia Smart Infrastructure Monitoring System (BCSIMS), funded by the Ministry of Transportation and Infrastructure (MoTI) BC, Canada. The BCSIMS is a real-time seismic monitoring program that continuously assesses the seismic conditions of the selected bridges in BC. As part of this seismic monitoring program, the seismic performance and nonlinear dynamic behavior of the FCB was evaluated by developing the 3D Finite Element (FE) model of the bridge in SAP2000. The model was updated based on the modal properties extracted from an Ambient Vibration (AV) test. The nonlinear behavior of the bridge was modeled by adding plastic hinges on the ductile components. Then the FE model was used to perform the seismic performance evaluation in accordance with the latest Canadian Highway Bridge Design Code 2015. The evaluation result shows that during major earthquake, no primary members of the bridge were damaged, the bridge will maintains repairable and operational, and should be capable of supporting the dead load and live load after earthquake.

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A simplified methodology for seismic design and assessment of nonstructural elements (2014)

Seismic safety of the nonstructural elements has drawn considerable attention of design and research community over past three decades. It is widely recognized that damage to these elements account for a significant portion of total economic loss after an earthquake. Different building codes put guidelines for assessment of these elements, however, need of a simplified and rational method for their seismic design is still felt by practicing engineers. The existing analysis procedures of floor response spectrum method and modal synthesis method are very detailed and cumbersome to be efficiently used in a design office. Expressions recommended by building codes predict a response which, in many cases, is significantly different from observed behaviour of the components. This thesis proposes a simplified and time-efficient methodology for seismic assessment of these elements which will enable engineers to take rational and consistent design decisions. This methodology is based on the concept of floor response spectrum where the acceleration demand corresponding to a component can be read from obtained floor spectrum. The procedure makes use of a simplified continuous model proposed by an earlier researcher to denote structures. An important feature of this methodology is the way of inputting seismic excitation to structures. The seismic excitation is input in form of ‘Design Ground Response Spectrum’ provided in building codes rather than commonly expected way of using groundmotion time-histories. Some results for floor acceleration demands for two sites in Canada are also presented. The methodology is extended to include base-isolated structures also. An independent procedure is proposed for assessment of components placed in the irregular structures. It is based on scaling of the ‘Reference Floor Response Spectrum’. The methodology presented in this thesis can be developed into an ‘Analyzer’ package to be used by practicing engineers for components’ design and assessment for all places in Canada. A useful guiding line for nonstructural element’s assessment to engineer is to decide whether to retrofit, relocate or replace it. This methodology based on the floor spectrum concept should enable designer in taking this decision rationally.

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Nonstructural component field testing method for the evaluation of seismic demand incorporating floor response spectrum (2014)

Nonstructural component (NSC) failures due to earthquake events can cause significant economic losses and even life-threatening hazards to occupants. In order to mitigate nonstructural seismic damages, it is critical to assess the seismic force demand which can be utilized to optimize the design of the NSC, and/or to assess methods of rehabilitation on anchorages to enhance seismic strength. The existing design codes and standards provide guidelines to calculate the minimum lateral earthquake force for designing a new NSC. However, they do not reflect the in-service condition of an existing NSC, which can vary significantly from when it was first installed. This study is intended to develop an easy-to-implement methodology to assess the seismic force demand of an existing NSC under normal operation. The procedure of the proposed methodology includes two principle phases: 1) field modal identification testing and 2) floor response spectrum analyses using a 3D finite element model (FEM). The practicality of this methodology was assessed through a case study on the U.B.C Hospital Koerner Pavilion building. In this study, the focus is on the machinery and equipment that are critical for the operation of a hospital. During the experimental stage, the fundamental frequencies and damping ratios of eight NSCs were identified. In the second phase, the horizontal floor response spectra (FRS) were constructed from the linear time history analysis results performed on a FEM. Finally, the FRS is used to obtain the lateral seismic force of each NSC corresponding to its dynamic properties. These forces were then compared with those obtained using the NBCC 2010 code equation to demonstrate the effectiveness of this method. Results from the case study provided evidence that the proposed method is overall a simple and effective tool for diagnosing the in-service modal properties of a NSC. The testing results can be easily applied in FRS analysis to obtain a more realistic nonstructural seismic force than that from the NBCC 2010 approach. The potential applications and limitations of the proposed methodology are also discussed in this dissertation to facilitate engineers to determine the suitability of this method to their specific projects.

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Evaluation of soil-structure interaction effects in the dynamic response of instrumented bridges based on seismic records (2013)

The main objective of this study is to gain a better understanding of soil-structure interaction (SSI) and how it affected the response of the bridges under earthquake excitation. Detailed studies of signals recorded at key locations of a bridge –i.e. the bridge deck, pier base, and free field– were conducted. A total of 6 instrumented bridges subjected to 12 earthquakes were selected for the analysis, focusing on the behavior in the transverse direction for these particular cases. The first step of the evaluation process was the investigation of the modal properties by means of a system identification process. Then response spectra were calculated for all the records, and the effects of SSI were determined by comparing the acceleration spectrum of the free field motions with the spectrum of the bridge motions recorded at the foundation slabs or on the pile caps. The frequency contents of the signals were compared based on the Fourier amplitude amplifications of the records. The analysis of the comparisons of column base vs. free-field and column base vs. deck allowed to highlight behaviors related to SSI effects. The possible variability of the fundamental transverse period as a function of the amplitude of shaking with time was investigated using wavelet analysis techniques. The results from both response spectra and Fourier spectra analyses showed clearly that the free field motions are not always de-amplified at the foundation due to the soil-structure interaction effect, as it has been generally accepted. It was also demonstrated that for the same site and bridge, amplification or de-amplification varies from one earthquake to another. For almost all cases in this study the time-frequency analysis results showed that the peak response at the deck corresponds to the natural transverse period. This observation does not hold true only for one case, where the dynamic behavior was highly affected by the approaching embankments.

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Nonlinear response of high-rise buildings: effect of directionality of ground motions (2011)

The response of high-rise buildings to strong ground shaking depends on ground motion parameters namely: intensity, frequency content, duration and horizontal ground motion directionality. The latter has been a concern to engineers for several decades in seismic design. The prediction of the direction where ground motion will hit the building is rendered difficult because in many regions faults are not mapped to a great extent, and for regions were fault locations are known accurate prediction of ground motion directionality is impeded because ground motions have unique wave propagation characteristics along its path. The purpose of this study was to evaluate the influence of ground motion directionality on the nonlinear dynamic response of a high-rise building.The influence of ground motion directionality was evaluated for a building case study. The building was 44 storey, and resembled general features of structural configuration commonly provided to reinforced concrete high-rise in Vancouver city. The nonlinear time history analysis (NLTHA) method was used to estimate seismic response of the building model to bi-directional ground shaking. This method was systematically applied for 40 ground motion component angles of incidence, which accounted for different ground motion directionalities ranging from 0 to 360 degrees. A suite of 3 pairs of horizontal ground motion representative of seismic hazard 2% in 50 years in Vancouver was considered for analysis. The ground motion directionality had significant effect over the calculated building seismic response. In some scenarios at critical angle of incidence the calculated floor displacements and interstorey drifts were 4 times as large as the displacements and drifts calculated for ground motion at 0 degrees angle of incidence. The largest building response envelope was obtained for several critical angles of incidence of the ground motion components. Critical angles of incidence were distributed over the entire building’s height.The relevance of ground motion directionality for seismic design of high-rise buildings was clearly demonstrated. The NLTHA used in conventional design practice still ignores ground motion directionality. It is concluded there is a need to develop the tools engineers can readily use to consider ground motion directionality in seismic design of modern high-rise buildings.

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