Mountain Scholar :: Browsing by Author "Jia, Gaofeng, committee member"

Browsing by Author "Jia, Gaofeng, committee member"

Now showing 1 - 9 of 9

Open Access
A modeling toolkit for comparing AC vs. DC electrical distribution efficiency in buildings
(Colorado State University. Libraries, 2021) Othee, Avpreet, author; Cale, James, advisor; Young, Peter, committee member; Herber, Daniel, committee member; Jia, Gaofeng, committee member
An increasing proportion of electrical devices in residential and commercial buildings operate from direct current (DC) power sources. In addition, distributed power generation systems such as solar photovoltaic (PV) and energy storage natively produce DC power. However, traditional power distribution is based on an alternating current (AC) model. Performing the necessary conversions between AC and DC power to make DC devices compatible with AC distribution results in energy losses. For these reasons, DC distribution may offer energy efficiency advantages in comparison to AC distribution. However, reasonably fast computation and comparison of electrical efficiencies of AC-only, DC-only, and hybrid AC/DC distributions systems is challenging because DC devices are typically (nonlinear) power-electronic converters that produce harmonic content. While detailed time-domain modeling can be used to simulate these harmonics, it is not computationally efficient or practical for many building designers. To address this need, this research describes a toolkit for computation of harmonic spectra and energy efficiency in mixed AC and DC electrical distribution systems, using a Harmonic Power Flow (HPF) methodology. The toolkit includes a library of two-port linear and nonlinear device models which can be used to construct and simulate an electrical distribution system. This dissertation includes a description of the mathematical theory and framework underlying the toolkit, development and fitting of linear and nonlinear device models, software implementation in Modelica, verification of the toolkit with laboratory measurements, and discussion of ongoing and future work to employ the toolkit to a variety of building designs.
Open Access
A study on bridge inspections: identifying barriers to new practices and providing strategies for change
(Colorado State University. Libraries, 2021) Abdallah, Abdelrahman M., author; Atadero, Rebecca A., advisor; Ozbek, Mehmet E., advisor; Jia, Gaofeng, committee member; Chai, Dae Seok, committee member
Bridge inspections are one of the key elements required for a successful bridge management process to ensure adequate bridge performance. Inspections significantly inform maintenance decisions and can help in managing maintenance activities to achieve a reliable bridge network. In the United States (U.S.) routine visual inspections are required for most bridges at a maximum interval of 24-months regardless of the bridge condition. However, limitations of current bridge inspection practices impact the quality of information provided about bridge condition and the subsequent decisions made based on that information. Accordingly, the overarching goal of this research project is to support bridge inspection practices by providing a systematic and rational framework for bridge inspection planning and identifying the factors that can facilitate innovation and research transfer in the bridge inspection field. To do so, this dissertation includes three separate yet related studies; each focusing on essential aspects of bridge inspection planning. Much research in bridge inspection has been conducted to improve the inspection planning process. The first study provides an overview of current bridge inspection practices in the U.S. and conducts a systematic literature review on innovations in the field of bridge inspection planning to identify research gaps and future needs. This study provides a background on the history of bridge inspection in the U.S., including current bridge inspection practices and their limitations, and analyzes the connections between nondestructive evaluation techniques, deterioration models and bridge inspection management. The primary emphasis of the first study is a thorough analysis of research proposing and investigating different methodologies for inspection planning. Studies were analyzed and categorized into three main types of inspection planning approaches; methods that are based on: reliability, risk analysis, and optimization approaches. This study found that one of the main barriers that may be preventing the implementation of new inspection planning frameworks in practice is that the approaches presented focus on a single bridge element or deterioration mechanism in the decision-making process. Additionally, it was concluded that approaches in the literature are either complex to apply or depend solely on expert judgement. Limitations of the uniform calendar-based approach used to schedule routine inspections have been reported in the literature. Accordingly, the objective of the second study is to provide a new systematic approach for inspection planning that integrates information from bridge condition prediction models, inspection data, and expert opinion using Bayesian analysis to enhance inspection efficiency and maintenance activities. The proposed uncertainty-based inspection framework can help bridge owners avoid unnecessary or delayed inspections and repair actions, determine the inspection method, and consider more than one deterioration process or bridge component during the inspection planning process. The inspection time and method are determined based on the uncertainty and risks associated with the bridge condition. As uncertainty in the bridge condition reaches a defined threshold, an inspection is scheduled utilizing nondestructive techniques to reduce the uncertainty level. The framework was demonstrated on a new and on an existing reinforced concrete bridge deck impacted by corrosion deterioration. The results showed that the framework can reduce the number of inspections compared to conventional scheduling methods, while also reducing the uncertainty regarding the transition in the bridge deck condition and repair time. As identified through the first study, over the last two decades many researchers have focused on providing new ideas to improve conventional bridge inspection practices, however, little guidance is provided for implementing these new research products in practice. This, along with resistance to change and complexity of the proposed ideas, resulted in a lack of consistency and success in applying new technologies in bridge inspection programs across state departments of transportation (DOTs). Accordingly, the third paper presents a qualitative study set out to identify the factors that can help improve research products and accelerate change and research transfer in bridge inspection departments. This study used semi-structured interviews, written interviews, and questionnaires for data collection and engaged with twenty-six bridge staff members from different DOTs. The findings of this study are expected to be both specific to changes in bridge inspection practice and have some generalizability to other significant changes to engineering practice at DOTs. To improve research products, this study suggested that researchers need to collaborate more with DOT staff members and provide relevant research products that are not specific to certain bridge cases and can be applied on different bridges. Also, to facilitate change in transportation organizations, change leaders should focus on showing the need for change, gaining support from the FHWA, allocating the required resources, and enhancing the capacity of DOT staff members through training and effective communication. The investigation also presented participants' opinions on some of the aspects related to conventional inspection practices such as their support of a uniform inspection interval over a variable interval, and the main barriers limiting the use of NDE methods. This study contributes to the body of knowledge in the bridge inspection field by providing a new inspection planning approach that depends on the uncertainty and the risks associated with the bridge condition and uses both computational methods and expert judgment allowing bridge owners select inspection time and method while considering more than one deterioration process or bridge element. In addition, this study presents some of the factors that can help reduce the gap between research and practice and facilitate innovation and change in transportation organizations.
Open Access
Chloride binding and desorption mechanism in blended cement containing supplementary cementitious materials exposed to de-icing brine solutions
(Colorado State University. Libraries, 2024) Teymouri Moogooee, Mohammad, author; Atadero, Rebecca, advisor; Fantz, Todd, advisor; Jia, Gaofeng, committee member; Bailey, Travis, committee member
Concrete, the most widely used construction material globally, faces significant challenges due to its porous nature, particularly from chloride-induced corrosion. This corrosion, primarily caused by chloride ions penetrating concrete, affects over 7.5% of U.S. concrete bridges, incurring annual costs ranging from $5.9 to $9.7 billion. Chlorides enter concrete from various sources, including de-icing salts. Maritime infrastructures also suffer from severe chloride-induced corrosion because seawater contains a high concentration of chloride ions. Irrespective of how chlorides enter the concrete, chlorides can exist in concrete in two forms: free and bound chlorides. While bound chlorides are beneficial, they can be released due to environmental factors like carbonation and chemical attacks, exacerbating corrosion rates. These attacks cause pH reduction in concrete and subsequently can result in the release of bound chlorides (chloride desorption).This dissertation aims to address three main objectives: (1) investigate factors influencing chloride binding measurements due to lack of a standardized method for chloride binding measurements, (2) study chloride desorption mechanisms in different cementitious systems exposed to de-icing brines, and (3) analyze pH and compositional changes in blended pastes under chloride contamination and carbonation. First, factors impacting chloride binding measurements were identified, such as sample form and saturation level, solution composition, and solution volume. Vacuum-saturated samples exhibited higher chloride binding than partially saturated or dried samples, with powdered samples showing the highest binding. Secondly, chloride desorption mechanisms were investigated in both Ordinary Portland Cement (OPC) pastes and pastes containing supplementary cementitious materials (SCMs) like fly ash, slag, and silica fume. Results indicated that the type of cation in the brine solution influenced bound chloride levels, with SCMs improving chloride binding capacity. Slag inclusion was effective in promoting chloride binding, while silica fume showed the least effect. The degree of chloride desorption under acid attack depended on the acid-to-paste mass ratio. The results reveal that inclusion of fly ash and slag is favorable in terms of chloride desorption, and silica fume is not recommended for use when chloride-induced corrosion is a concern. MgCl2 and CaCl2 de-icers demonstrated a lower chloride desorption compared to NaCl. Finally, the synergistic effects of chloride contamination and carbonation were examined in OPC and fly ash-containing pastes. Carbonation led to over 95% chloride desorption after two weeks, with fly ash-containing pastes exhibiting lower pH levels due to reduced portlandite content. Incorporation of fly ash is not recommended when carbonation is a concern. Therefore, caution should be exercised when considering fly ash inclusion in mixtures where both chloride contamination and carbonation are simultaneous concerns. This dissertation contributes to understanding chloride desorption in cementitious systems, essential for enhancing the durability and service life of concrete structures. This dissertation shed lights on primary factors influencing chloride binding measurements, enhancing the accuracy and comparability of chloride binding results. The results reveal that type of cation present in the solution and type of SCMs have significant influences on the pH, chloride binding capacity, and chloride desorption rates.
Open Access
Investigation of mechanistic deterioration modeling for bridge design and management
(Colorado State University. Libraries, 2017) Nickless, Kyle, author; Atadero, Rebecca, advisor; Jia, Gaofeng, committee member; Shuler, Scott, committee member
The ongoing deterioration of highway bridges in Colorado dictates that an effective method for allocating limited management resources be developed. In order to predict bridge deterioration in advance, mechanistic models which analyze the physical processes causing deterioration are capable of supplementing purely statistical models and addressing limitations associated with bridge inspection data and statistical methods. A review of existing analytical models in the literature was conducted. Due to its prevalence throughout the state of Colorado and frequent need for repair, corrosion-induced cracking of reinforced concrete (RC) decks was selected as the mode of deterioration for further study. A mechanistic model was developed to predict corrosion and concrete cracking as a function of material and environmental inputs. The model was modified to include the effects of epoxy-coated rebar, waterproofing membranes, asphalt overlays, joint deterioration, and deck maintenance. Probabilistic inputs were applied to simulate inherent randomness associated with deterioration. Model results showed that mechanistic models may be able to address limitations of statistical models and provide a more accurate and precise prediction of bridge degradation in advance. Preventative maintenance may provide longer bridge deck service life with fewer total maintenance actions than current methods. However, experimental study of specific deterioration processes and additional data collection are needed to validate model predictions. Maintenance histories of existing bridges are necessary to predicting bridge deterioration and improving bridge design and management in the future.
Open Access
Multi-scale & multi-resolution experimental and analytical methods for mitigating blast risk with barrier walls
(Colorado State University. Libraries, 2024) Sullivan, Kellan M., author; Mahmoud, Hussam, advisor; Puttlitz, Christian, advisor; Gadomski, Benjamin, committee member; Jia, Gaofeng, committee member; Stephens, Catherine, committee member; Pezzola, Genevieve, committee member
Over the last decade, interest in blast resistance and protection has increased as a result of the perpetual threat of terrorist groups around the world. In evaluating the Department of State (DOS) reports on terrorism since 2007, an estimated 330,000 fatalities and 430,000 injuries have been caused by terrorist attacks worldwide (2022). In the United States, various large scale explosive attacks have occurred over the years including the World Trade Center bombings in 1993, the Alfred Murrah Federal Building bombing in 1995, and the coordinated September 11th attacks in 2001. More recently, there has been a shift in the tactics of terrorist groups to use improvised explosive devices (IEDs) to target civilians due to regulations put in place after the September 11th, 2001, attacks that made it difficult for them to obtain a large amount of explosive material among other factors contributing the rise of terrorist activity. Attacks such as the Boston Marathon bombing in 2013 and the Madrid train bombing in 2004 demonstrate this shift in tactics. The upward trend of the use of IEDs around the globe since the September 11th, 2001, attacks presents a catalyst for a shift in research methods for blast mitigation techniques to provide protection to people rather than just structures. Therefore, developing methods to provide protection for people from blast effects is necessary to minimize the impact these terrorist groups have on our communities. Of the existing blast mitigation strategies, perimeter walls or barriers are specifically advantageous in that they increase standoff distances and provide an obstacle to the propagation path of the blast wave as well as primary fragmentation. The use of perimeter walls or barriers to protect structures has been well established in literature, however the use of barriers to protect people has not. The ability to predict airblast effects accurately and efficiently over a large variation in scaled ranges, within a complex environment, is important to characterize the potential severity of damage to structures and casualties among personnel in both military and civilian settings. Many different techniques have been used over the years to perform blast prediction of various airblast parameters such as pressure and impulse and blast resistant design research. While experimentation remains a valuable and powerful tool, in recent years, computational and numerical models have grown in popularity for their accurate evaluation capabilities. Advanced numerical software such as hydrocodes and computational fluid dynamic programs are often used to model airblast propagation and its impact on structures. However, in more complex environments, where blast loading in large areas of interest may occur, using high-fidelity computational modeling software could be inefficient due to the computing power required. The goal of this dissertation was to develop a performance-based design framework for predicting the probability of survivability of a double-barrier system under blast loading, and the probability of different bodily injuries for personnel from the blast wave itself. In this dissertation, the gaps in research for protecting civilians from IED attacks in large open areas, understanding the impact of multiple barriers on the blast shockwave and pressures around the barriers, and investigating an absorption focused barrier were addressed. A combination of analytical, numerical, and experimental methods at multiple scales was used to develop and validate the various elements needed to conduct the performance-based design. This dissertation developed rapid computational models to predict the pressure field around a double-barrier system, analyzed a new barrier design that focuses on reducing the energy of the shockwave in order to protect people, and accounted for the uncertainty and variability in multiple parameters to establish potential risk for various scenarios for both the barrier and for people. The analyses combined numerical, analytical, and experimental methods at multiple scales, to create models to predict and assess the pressures associated with person-borne-improvised-explosive-devices (PBIEDS). The developed models used to predict and quantify the pressures around a rigid double-barrier system and the response of the wood barrier to blast loading were coupled with small- and full-scale experimental testing to validate and assess the accuracy and efficiency of the models. From the results of dissertation, it can be observed how the implementation of a double-barrier system can significantly reduce the pressures experienced around the barriers, which can lead to less potential for serious injury or damage from blast events. Additionally, it showed that the distance between the barriers plays a critical role in the pressures and therefore the potential for injury between the barriers. In addition, adopting an innovative approach to blast barrier design to consider the use of more lightweight, commonly available, non-rigid materials to increase the energy absorption to attenuate the blast shockwave rather than just reflect was proven to be beneficial.
Open Access
Optimizing resilience decision-support for natural gas networks under uncertainty
(Colorado State University. Libraries, 2019) Ameri, Mohammad Reza, author; van de Lindt, John W., advisor; Chen, Suren, committee member; Jia, Gaofeng, committee member; Shields, Martin, committee member
Community resilience in the aftermath of a hazard requires the functionality of complex, interdependent infrastructure systems become operational in a timely manner to support social and economic institutions. In the context of risk management and community resilience, critical decisions should be made not only in the aftermath of a disaster in order to immediately respond to the destructive event and properly repair the damage, but preventive decisions should to be made in order to mitigate the adverse impacts of hazards prior to their occurrence. This involves significant uncertainty about the basic notion of the hazard itself, and usually involves mitigation strategies such as strengthening components or preparing required resources for post-event repairs. In essence, instances of risk management problems that encourage a framework for coupled decisions before and after events include modeling how to allocate resources before the disruptive event so as to maximize the efficiency for their distribution to repair in the aftermath of the event, and how to determine which network components require preventive investments in order to enhance their performance in case of an event. In this dissertation, a methodology is presented for optimal decision making for resilience assessment, seismic risk mitigation, and recovery of natural gas networks, taking into account their interdependency with some of the other systems within the community. In this regard, the natural gas and electric power networks of a virtual community were modeled with enough detail such that it enables assessment of natural gas network supply at the community level. The effect of the industrial makeup of a community on its natural gas recovery following an earthquake, as well as the effect of replacing conventional steel pipes with ductile HDPE pipelines as an effective mitigation strategy against seismic hazard are investigated. In addition, a multi objective optimization framework that integrates probabilistic seismic risk assessment of coupled infrastructure systems and evolutionary algorithms is proposed in order to determine cost-optimal decisions before and after a seismic event, with the objective of making the natural gas network recover more rapidly, and thus the community more resilient. Including bi-directional interdependencies between the natural gas and electric power network, strategic decisions are pursued regarding which distribution pipelines in the gas network should be retrofitted under budget constraints, with the objectives to minimizing the number of people without natural gas in the residential sector and business losses due to the lack of natural gas in non-residential sectors. Monte Carlo Simulation (MCS) is used in order to propagate uncertainties and Probabilistic Seismic Hazard Assessment (PSHA) is adopted in order to capture uncertainties in the seismic hazard with an approach to preserve spatial correlation. A non-dominated sorting genetic algorithm (NSGA-II) approach is utilized to solve the multi-objective optimization problem under study. The results prove the potential of the developed methodology to provide risk-informed decision support, while being able to deal with large-scale, interdependent complex infrastructure considering probabilistic seismic hazard scenarios.
Open Access
Resilience of transportation network during post-earthquake emergency response and recovery stages
(Colorado State University. Libraries, 2023) Wu, Yangyang, author; Chen, Suren, advisor; Bradley, Thomas, committee member; Mahmoud, Hussam, committee member; Jia, Gaofeng, committee member
Earthquakes can cause casualty, injuries, and extensive infrastructure damages and significantly disrupt transportation networks. Disrupted transportation networks resulting from damaged bridges or debris may cause delays on emergency response activities and impact traffic efficiency and safety during the post-earthquake recovery stage. A functioning post-hazard transportation network is the backbone to support the effective emergency response and maintain efficient post-hazard recovery plans of the whole community. The main purpose of this dissertation is to model and improve the resilience performance of transportation networks during both post-earthquake emergency response and recovery stages. It is expected that the proposed methodologies in this dissertation will help making risk-informed decisions in terms of pre-hazard mitigation planning, emergency medical service management, and post-earthquake restoration planning to enhance the resilience of transportation networks. A suite of novel methodologies is proposed to evaluate and enhance the resilience performance of transportation networks subjected to major earthquakes in this dissertation. Firstly, a resilience modeling framework of traffic networks is developed to simulate the transportation performance during post-earthquake emergency medical response considering interactions between infrastructures, people, and hazard. Secondly, a new approach is proposed to quantify the comprehensive redundancy of transportation networks during post-earthquake emergency medical response considering search-and-rescue efforts and life vitality decay. Thirdly, a methodology is proposed to evaluate the resilience performance of traffic networks in private-vehicle-based post-earthquake emergency medical response considering bridge failure, building debris, and emergency traffic flow. Fourthly, a novel methodology is proposed to assess the post-earthquake resilience of transportation networks considering link functionality, travel time and traffic safety. Finally, a model to simulate time-dependent resilience of degraded transportation networks during post-hazard recovery period is developed to incorporate the time-evolving travel demand of the community.
Open Access
Shake table testing of hybrid wood shear wall system
(Colorado State University. Libraries, 2019) Anandan, Yeshwant Kumar, author; van de Lindt, John, advisor; Jia, Gaofeng, committee member; Chong, Edwin K. P., committee member
Cross-Laminated Timber (CLT) is an engineered, prefabricated mass timber product that has shown excellent structural and mechanical properties. With the growing application of CLT in industry, there have been a number of research projects carried out to introduce CLT in tall buildings located in high seismic regions. The concept of post-tensioning mass timber has been adopted from concrete systems and this led to development of seismically resilient structural systems that can undergo multiple earthquake and continue to re-center. This thesis presents the results of a shake table test program that focused on testing of a one-story full-scale hybrid wood shear wall system comprised of a post-tensioned CLT wall panel with Light-frame wood shear (LiFS) wall panels on each side. The testing was conducted at CSU's Engineering Research Center shake table. The objective of this study was to combine the advantages of the post-tensioned CLT systems with those of LiFS walls. The hybrid shear wall system in the testing had two LiFS walls on either side of a post-tensioned rocking CLT wall panel. Mild steel rods were used as post-tensioning rods in this experiment and the test structure also included gravity frames constructed with wood studs (but no sheathing) and a CLT floor diaphragm to support a seismic weight of 12,000 lbs. The structure was subjected to the 1989 Loma Prieta ground motion record scaled to different intensities. The final test used the original 1994 Northridge ground motion record from the Rinaldi record station, with a slight reduction to be able to be accommodated by the 20 inch shake table actuator stroke. This test was conducted to understand the collapse mechanism of the structure and demonstrated the ability of the post-tensioned CLT to re-center the structure after 5% inter-story drifts and also the ability of the LiFS walls to act as energy dissipation and lateral force resisting systems.
Open Access
The effects of design decisions on service life and life cycle cost for a concrete slab in a parking garage
(Colorado State University. Libraries, 2017) Badr, Ali, author; Atadero, Rebecca, advisor; Jia, Gaofeng, committee member; Valdes-Vasquez, Rodolfo, committee member
Parking garages are unique structures that are useful and common as part of the transportation infrastructure system in the US. Large percentage of these structures is open which expose them to ambient environment and in some cases deleterious chloride exposures. Corrosion of embedded steel is the main cause for concrete deterioration and chloride exposure is one of the major causes for corrosion. Therefore, designing these structures for durability is essential to extend their service life and reduce their degradation status and future repair costs. Improving the durability of these structures can be a costly process at the construction phase that might leave owners of parking garages reluctant about increasing the upfront costs. Therefore, Life-365 software has been used in this study to investigate the service life and life cycle cost impacts of different design decisions throughout the lifetime on a reinforced concrete slab element in a parking garage. Life Cycle Cost Analysis "LCCA" is a process that weights the trade-offs of different phases cost including initial construction and subsequent maintenance and repair throughout the design life period and can help understand the long-term value of additional upfront costs. In Life-365, service life is the sum of two periods: the initiation period and the propagation period while LCC is the sum of three cost phases :the construction phase, the barrier phase and the repair phase which starts at the end of the service life period and extends the remainder of design life. The design decisions or design variables that are investigated in this study include varying the concrete cover distance to the embedded steel, varying the w/cm ratio, using different supplementary cementitious materials, using different barriers and reinforcement types. The geographic location of the parking garage was chosen as Denver, Colorado. Corrosion is likely to occur in this city where harsh environmental conditions are present, including snow falling into parking garages' decks or using deicing salts to melt accumulated snow on roads which can be carried by tires or underneath automobiles. Results of this study showed that using supplementary cementitious materials are the best design variables to consider in terms of saving money for the concrete slab during its design life, besides increasing the concrete cover distance. In addition, a combination of SCMs with a low w/cm ratio has proven to be very effective in terms of reducing costs especially when it is used with higher concrete cover. This study can help designers and owners of these structures in managing and allocating the resources they have more effectively.

Browsing by Author "Jia, Gaofeng, committee member"

Results Per Page

Sort Options