Navid Ataei

Response Sensitivity for Probabilistic Damage Assessment of Coastal Bridges Under Surge and Wave Loading

The susceptibility of coastal bridges to damage during hurricane-induced storm surge has been illustrated along the U.S. Gulf Coast in several hurricane events. This factor poses a significant threat to the safety of nationwide transportation systems, effectiveness of postevent emergency response and recovery activities, and socioeconomic stability further afforded by functioning transportation infrastructure. Nationwide risk and loss assessment packages currently lack any reliable input models of bridge fragility to assess the risk to the transportation infrastructure posed by hurricane-induced storm surge and wave. However, these tools are essential for comparing the vulnerability of different bridge types, conducting regional risk assessment or loss estimates, and supporting decision making on risk mitigation activities. As a first step in the development of probabilistic models of bridge vulnerability subjected to hurricane scenarios, sensitivity studies are conducted to assess the significance of varying hazard and bridge parameters on the dynamic response of coastal bridges. Three-dimensional nonlinear finite element models are used to assess these demands under varying input modeling parameters, and an analysis of variance is conducted to evaluate the significance of each parameter. The sensitivity study reveals that the potential variation in wave parameters has the most statistically significant impact on the response of the bridge. Additionally, a second-level sensitivity study reveals that the most critical structural parameter is the deck mass followed by connection modeling parameters. The results of this study provide insight into modeling parameters that should receive careful treatment in probabilistic analysis of bridge vulnerability.

Probabilistic Modeling of Bridge Deck Unseating during Hurricane Events

Although coastal bridges have exhibited significant susceptibility to damage during hurricane-induced wave and surge events, probabilistic models are lacking to quantify the vulnerability of bridges under a range of structural or hazard parameters and support hurricane risk assessment and mitigation activities. This paper introduces a computationally efficient methodology to assess the fragility, or conditional failure probability, of bridges, targeting the deck shifting or unseating failure mode, which is a predominant severe mode of failure for vast inventories of simply supported span bridges. The method propagates uncertainties in parameters affecting the structural capacity and demand, such as mass density, connection strength, material properties, workmanship, and wave parameters. Fragility surfaces are derived in which failure probability is presented over a range of relative surge elevation and wave heights. The application of this technique is shown through a regional fragility assessment of 136 bridges in the greater Houston area. The results reveal that a failure zone emerges in the fragility surfaces of the bridges studied, with a dramatic transition in probability of failure. Additionally, bridges with similar trends in fragility surfaces can be readily classified based on estimates of their mean mass per span length. Finally, application of the new vulnerability models in a case-study regional risk assessment for Hurricane Ike reveals consistency with observed damage and also offers an opportunity for future studies to investigate alternative scenarios for risk-mitigation planning or extensions of the methodology to consider other regions or failure modes.

Sensitivity of Dynamic Response of Bridges under Multiple Hazards to Aging Parameters

Continued aging and deterioration of bridges poses a threat to bridge performance not only under regular service loads, but also results in pronounced vulnerability under extreme dynamic loads, such as seismic or hurricane induced surge and wave loading. In fact, aging of bridges in the form of corrosion attacks load paths critical under dynamic loads, including the superstructure-substructure connection elements and the reinforcing steel in column plastic hinge zones. This paper investigates the effect of aging on the dynamic response of multiple span concrete girder bridges when subjected to seismic as well as coupled surge and wave loading induced by hurricanes. The paper highlights the key differences and similarities in the nature of loads under the two natural hazards, the demand placed on key components, and the resulting dynamic response and failure modes of aging bridges. Nonlinear dynamic analysis is conducted using 3- dimensional bridge models with time-varying model parameters due to corrosion of reinforcing bars in decks and columns and degradation of elastomeric bearings with steel dowels. The sensitivity of component response, such as column demands, bearing deformations, or deck displacement, to variation in aging parameters is investigated in the study. Findings indicate that while the nonlinear dynamic behavior and select failure modes of the bridges may differ between the seismic and surge/wave loading cases, there is some consistency in the impact and criticality of aging parameters affecting dynamic response under the extreme loading cases, such as corrosion of bearing dowels and column reinforcement. These results form the foundation for multi-hazard vulnerability assessment of bridges considering the present in field condition.

Influential fluid–structure interaction modelling parameters on the response of bridges vulnerable to coastal storms

Analysis of coastal bridges under hurricane-induced wave and surge loads is essential for safety and performance assessment of water crossing bridge inventories. A reliable numerical model that can be employed to study the behaviour of bridges in hurricane events is beneficial because it reduces the cost and effort required for experimental models. Furthermore, it is important to identify modelling parameters that have a significant effect on the simulated response in order to guide uncertainty treatment for future bridge reliability studies. To address these needs, a high fidelity numerical model for simulation of coastal bridges is utilised that takes into account the fluid–structure interaction and includes contact surfaces that permit deck shifting and unseating during surge and wave passage. After validation of the model with experimental test data, it is implemented to examine the response of a typical water crossing bridge in the Houston area, revealing the values of wave and surge loads and also the potential of deck unseating under extreme wave and surge conditions. A sensitivity study is conducted to determine the uncertain structural modelling parameters that significantly affect the bridge response when subjected to surge and wave. Concrete strength and density, coefficient of friction between super- and substructure and soil shear strength are found to influence the bridge response and should be considered in probabilistic analyses and reliability assessments of coastal bridges.

Coastal Bridge Reliability during Hurricane Events: Comparison of Fragility Curves by Static and Dynamic Simulation

Post-hurricane functionality of the transportation system is vital for emergency response and recovery activities. Coastal bridges are among the most critical components of the transportation system susceptible to hurricane induced wave and surge loads that may hamper this functionality. This study explores alternative approaches to develop probabilistic statements of coastal bridge damage potential conditioned upon hazard intensity, known as fragility. Fragility surfaces for a representative bridge structure of the Houston/Galveston area are developed using two different approaches: static analysis and nonlinear dynamic simulation. The static approach is efficient and can provide sufficient fragility models for simply supported bridges with limited connectivity between the super structure and substructure, since their predominant mode of failure is deck unseating or shifting due to connection failure. Nevertheless, for bridges with strong connection between sub and superstructure, the force transfer can cause other components to exceed their limit state. Hence the full dynamic behavior of structure becomes more critical in the fragility modeling of bridges with high strength connections.

Fragility Assessment of Coastal Bridges under Hurricane Events Using Enhanced Probabilistic Capacity Models

The susceptibility of the transportation network, in particular coastal bridge, along the US Gulf Coast to hurricane damage has been demonstrated in previous hurricane events. Accordingly, it is vital to have an approach for reliability assessment of coastal bridges to evaluate the vulnerability of existing bridges, prospective retrofit measures, or even new designs. Probabilistic assessments are required in order to capture the uncertainties in the performance of bridges stemming from variability in the loading and the natural hazard itself, which include surge and wave parameters, as well as structural design and construction. The conditional probability of failure of a structure under a given hazard condition, known as the fragility, has become a well accepted tool in risk assessment of structures; yet an approach for derivation of the fragility of coastal bridges is required to support risk assessment in these regions. This paper reports on a methodology for construction of fragility surfaces for bridges with high strength connections between super- and substructure subjected to coastal storms. Although these configurations are less common than typical simple supports with minimal connectivity, they do exist among coastal bridge inventories and moreover, such provision for enhanced connectivity has been suggested for retrofit or new design techniques. Construction of these fragility surfaces requires probabilistic estimates of structural capacity and demand. This research provides a consistent methodology for capacity limit state appraisal based on the numerical simulations of bridges under hurricane induced wave and surge loads, and global performance objectives for bridges related to post event loss of stiffness or load carrying capacity. Predictive probabilistic models are developed for the demand parameters identified as primary indicators of capacity loss (e.g. columns axial strain) and the bridge fragility is constructed for each damage state to demonstrate the application of the proposed method.