Mountain Scholar :: Browsing by Author "Mahmoud, Hussam, advisor"

Browsing by Author "Mahmoud, Hussam, advisor"

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Open Access
A framework for life-cycle cost optimization of buildings under seismic and wind hazards
(Colorado State University. Libraries, 2014) Cheng, Guo, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, committee member; Strong, Kelly, committee member
The consequential life and economic impact resulting from the exposure of building structures to single hazards have been well quantified for seismic and wind loading. While it has been recognized that structures are likely to be subjected to multiple hazards during their service life, designing for such scenario has been achieved by as considering the predominant hazard. Although from a structural reliability perspective, this might be a reasonable approach, it does not necessarily result in the most optimal life-cycle cost for the designed structure. Although such observation has been highlighted in recent studies, research is still needed for developing an approach for multi-hazard life-cycle optimization of structures. This study presents a framework, utilizing structural reliability, for cost optimization of structures under wind and seismic hazards. Two example structures, on which the framework is applied, are investigated and their life-cycle cost analyzed. The structures represent typical medium and high rise residential buildings located in downtown San Francisco area. The framework comprises of using the first order reliability method (FORM), programed in MATLAB and interfaced with ABAQUS finite element software to obtain the corresponding reliability factors for the buildings under various loading intensities characterized by the probability of exceedance. The finite element analyses are carried out based on real seismic and wind pressure records using nonlinear finite element time-history dynamic analysis. The random variables selected include hazard intensity (wind load and seismic intensity) and elastic modulus of steel. Once the failure probabilities are determined for the given limit state functions, the expected failure cost for the building service duration considering earthquake or wind hazard, or both, is calculated considering discount rate. The expected life-cycle cost is evaluated using life-cycle cost function, which includes the initial construction cost and the expected failure cost. The results show that the optimal building design considering the wind hazard alone, the seismic hazard alone or a combination of both is different. The framework can be utilized for an optimal design of both wind and seismic load for a given level of hazard intensity.
Open Access
A new hurricane impact level ranking system using artificial neural networks
(Colorado State University. Libraries, 2015) Pilkington, Stephanie F., author; Mahmoud, Hussam, advisor; van de Lindt, John, committee member; Schumacher, Russ, committee member
Tropical cyclones are intense storm systems that form over warm water but have the potential to bring multiple related hazards ashore. While significant advancements have been made for forecasting of such extreme weather, the estimation for the resulting damage and impact to society is significantly complex and requires substantial improvements. This is primarily due to the intricate interaction of multiple variables contributing to the socio-economic damage on multiple scales. Subsequently, this makes communicating the risk, location vulnerability, and the resulting impact of such an event inherently difficult. To date, the Saffir-Simpson Scale, based off of wind speed, is the main ranking system used in the United States to describe an oncoming tropical cyclone event. There are models currently in use to predict loss by using more parameters than just wind speed. However, they are not actively used as a means to concisely categorize these events. This is likely due to the scrutiny the model would be placed under for possibly outputting an incorrect damage total. These models use parameters such as; wind speed, wind driven rain, and building stock to determine losses. The relationships between meteorological and locational parameters (population, infrastructure, and geography) are well recognized, which is why many models attempt to account for so many variables. With the help of machine learning, in the form of artificial neural networks, these intuitive connections could be recreated. Neural networks form patterns for nonlinear problems much as the human brain would, based off of historical data. By using 66 historical hurricane events, this research will attempt to establish these connections through machine learning. In order to link these variables to a concise output, the proposed Impact Level Ranking System will be introduced. This categorization system will use levels, or thresholds, of economic damage to group historical events in order to provide a comparative level for a new tropical cyclone event within the United States. Discussed herein, are the effects of multiple parameters contributing to the impact of hurricane events, the use and application of artificial neural networks, the development of six possible neural network models for hurricane impact prediction, the importance of each parameter to the neural network process, the determination of the type of neural network problem, and finally the proposed Impact Level Ranking System Model and its potential applications.
Open Access
Assessment of potential impacts of climate change on the integrity and maintenance costs of simply supported steel girder bridges in the United States
(Colorado State University. Libraries, 2019) Palu, Susan Mayumi Kock, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, committee member; Senior, Bolivar, committee member
Bridges in America are aging and deteriorating, causing substantial financial strain on federal resources and taxpayers' money. Amid several deterioration issues affecting bridges one of the most common and costly is malfunction and deterioration of expansion joints, due to accumulation of road debris between joints, traffic, and weather. Clogged joints in particular prevent the superstructure from expanding when subject to a temperature increase, giving rise to thermal stresses that are not accounted for during the design phase. These additional demands, in the form of combined axial loads and moments, are expected to even worsen considering potential future changes in climate. Herein, a new framework is developed to assess structural vulnerability and estimate maintenance costs for approximately 80,000 simply supported steel girder bridges across the U.S. The approach aims to aid in establishing a priority order for bridge maintenance and offer insights on how to better allocate funds for a large inventory of bridges. The structural vulnerability is quantified in terms of the reduced capacity resulting from axial load and moment interaction on the girder-slab composite. The projected daily maximum temperatures for future years of 2040, 2060, 2080 and 2100 were processed from the coupled climate model GFDL CM3 under three climate scenarios: RCP 2.6, RCP 6.0 and RCP 8.5. The results showed that the most critical regions for all climate scenarios are: Northern Rockies & Plains, Northwest, Upper Midwest and West. In contrast, the less susceptible regions are the Southeast followed by the Northeast. In addition to vulnerability, life cycle cost analysis was conducted considering the evolution of structural condition of each asset along the years through the interaction equation. The results showed that savings on the order of $4.5 billion could be attained when vulnerability-informed maintenance practice is followed as opposed to its conventional counterpart. It was observed that the climate scenario RCP 2.6, which represents greater efforts to reduce anthropogenic climate change, resulted in the smallest maintenance cost. Moderate efforts over emissions RCP 6.0 implies a $600 million increase, while no intervention under RCP 8.5 results in an additional $2 billion cost over the long term.
Open Access
Collapse simulations of steel buildings under fire
(Colorado State University. Libraries, 2016) Qin, Chao, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, committee member; Kirkpatrick, Allan, committee member
Collapse analysis of steel structures under extreme hazards has been placed on the forefront of research in recent decades. This was primarily motivated by the September 11, 2001, terrorist attacks, which caused the complete collapse of the World Trade Centers (WTCs) including WTC-7. The collapse, attributed mainly to fires resulting from the attacks, raised concerns regarding the level of robustness in steel frames when subjected to fire loadings. While complete collapse of steel buildings under elevated temperature is considered a rare event, as no cases have been reported prior to 9/11, understanding collapse mechanisms of steel buildings under fire conditions can help in developing methods by which future failures can be avoided. One of the main limitations towards evaluating such collapse events is the experimental cost and complexity associated with conducting collapse tests. Numerical simulations, if properly employed, can yield significant dividends in understanding and quantifying structural response under extreme hazards. With the worldwide move toward performance-based engineering, understanding, and quantifying system behavior through advanced numerical simulations, especially during the heating and cooling phases of realistic fire exposures, is essential for establishing proper performance-based provisions for fire engineering that ensure both safe and economical design. To that end, the primary objectives of this research are two folds - 1) to develop a numerical tool that would allow for the evaluation of steel frames under fire loading, or any extreme hazard for that matter, up to and including collapse and 2) to evaluate the demand on steel frames, employing moment frames, braced frames, and gravity frames, under different fire scenarios. These two overarching objectives were realized through the development of advanced numerical models of two 6-story steel-frame buildings with moment frames, gravity frames, and different center bracing systems (one model utilized a concentrically braced frame while the other utilized eccentrically braced frame). The building structures were subjected to two different time-temperature curves and two different fire scenarios. Specifically, the ASTM E119 standard fire curve and the Eurocode 3 parametric fire curve were selected to simulate the fire loadings and were applied independently to the building models under two different contained fire scenarios. The two scenarios included - 1) first floor corner compartment fire and 2) whole first floor fire. This allowed for the assessment of different global system response where collapse is triggered by twist of the entire structure accompanied by lateral deformation in the case of a corner compartment fire and progressive vertical displacement of the entire system in the case of the whole first floor fire. The simulation results of this study show that structural response of steel buildings including collapse mechanism and behavior of structural members and connections during fire events can be predicted with reasonable accuracy using advanced numerical finite element analysis. The results provide substantial insight on the behavior of steel building systems under elevated temperature including the potential for system collapse.
Open Access
Community risk due to wildland urban interface fires: a top-down perspective
(Colorado State University. Libraries, 2021) Chulahwat, Akshat, author; Mahmoud, Hussam, advisor; Ellingwood, Bruce, committee member; van de Lindt, John, committee member; Stevens-Rumann, Camille, committee member
Recent wildfire events, in the United States and around the world, have resulted in thousands of homes destroyed and many lives lost, leaving communities and policy makers, with the question as to how to manage wildfire risk. Wildland urban interface fires have demonstrated the unrelenting destructive nature of these events and signify the need to address the problem. This is particularly important given the prevalent trend of increased fire frequency and intensity. Current approaches to managing wildfires focus on fire suppression and managing fuel build-up in wildlands. Frequent suppression of small scale fires has led to the absence of a natural reduction mechanism, which in turn, results in low frequency high intensity fires. This phenomena has been termed as the Wildfire paradox and it reinforces the ideology that wildfires are inevitable and are actually beneficial; therefore focus should to be shifted towards minimizing potential losses to communities. However, reliance on these strategies alone has clearly proven inadequate. This requires the development of vulnerability-based frameworks that can be used to provide holistic understanding of risk. Mitigation strategies geared towards complete containment of wildfires within the wildlands are unrealistic. Therefore, the primary goal has to be on making communities resilient, with the purpose of minimizing potential losses. There is a paucity of information regarding the interplay between communities and wildfires. Unlike other hazards, for which there exists significant knowledge base, quantification of WUI fires is still an unanswered question for us. To better understand what factors govern the impact of WUI fires, tools to assess and quantify the risk of wildfires to communities are required. In this study, a probabilistic approach for quantifying community vulnerability to wildfires by applying concepts of graph theory is devised. A directed graph is developed to model wildfire inside a community by incorporating different fire propagation modes. Four modes are considered in this study - Convection, Radiation and Embers, and individual ignition models for each are formulated. Through these modes the graph model accounts for relevant community-specific characteristics including wind conditions, community layout, individual structural features, and the surrounding wildland vegetation. The graph model is then used to evaluate vulnerability of each component of the community using shortest path algorithms. The framework is utilized to study the infamous 1991 Oakland fire in an attempt to unravel the complexity of community fires. Centrality measures from graph theory are used to identify critical behavior patterns and evaluate the effect of fire mitigation strategies. Using the vulnerability framework developed, the risk of communities is further quantified. Risk is generally defined by three components - (1) Hazard intensity (2) Degree of exposure and (3) Exposed elements. In context of wildfires, the risk is formulated by combining the following three components - probability of wildland ignition, probability of fire reaching the community and vulnerability of community. Four different communities across the United States are selected and risk analysis is conducted for the months May-September to understand the correlation between community risk and community characteristics. Unlike current practice, the results are shown to be community-specific with substantial dependency of risk on meteorological conditions, environmental factors, and community characteristics and layout. For the final part of this study, an intervention optimization is formulated and applied to the four communities to observe the effect of different intervention measures on community risk. The findings show the need for exploring unique viable solutions to reduce risk for communities independently, as opposed to embracing a generalized approach, which is currently the case.
Open Access
Effect of fire and fire following an earthquake on steel reduced beam section moment connections
(Colorado State University. Libraries, 2013) Turbert, Collin, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, committee member; Kirkpatrick, Allan, committee member
The main objective of this research is to investigate the behavior of steel frames with reduced beam sections (RBS) during a fire as well as during the combined events of fire following an earthquake (FFE). Historical events and recent disasters have clearly demonstrated that the occurrence of these two events (fire and FFE) within steel framed buildings represents a probable scenario that warrants further investigation. Accurate analytical evaluation of the structural behavior of steel buildings under fire, and to a lesser extent an earthquake, is difficult due to the many complex and uncertain phenomena involved. Detailed numerical modeling of the overall structural system has been shown to provide the most reliable simulation results under current research development. However, detailed analysis is generally computationally expensive and as such not practically applicable. In addition, the nonlinear behavior of entire structures is complex and not fully understood. Therefore, detailed numerical models of the overall structural system often have difficulty capturing local failure modes. This research provides a practical analytical approach to perform accurate numerical evaluation of steel structures under fire and FFE and to closely investigate its characteristic behavior. The approach utilized is to limit the focus on localized compartment fires and investigate the behavior of a single beam-column subassembly within the chosen compartment. By limiting the focus of the study the numerical models can be simplified by utilizing specifically appropriate subassembly models for the analysis. Using the finite element program ABAQUS, two different beam-column subassemblies with RBS were created and analyzed. The subassemblies are representative of actual connections in two steel special moment resisting frames that were designed for the highly seismic Los Angeles region. The frames selected for analysis are an 8-story 4-bay frame and a 16-story 4-bay frame and the selected subassemblies are located at the exterior of the frames at the mid and lower levels, respectively. Both subassemblies were analyzed under fire alone to determine their structural behavior during the event as well as allow for a better understanding of the influence the seismic demand has on the behavior of the connection when exposed to FFE. For the FFE simulations both models were analyzed under a suite of earthquake ground motions followed by a fire simulation. For the fire analysis portion of both simulations (fire alone and FFE) a sequentially coupled thermo-mechanical modeling technique, which includes representative constraint elements to simulate the restraint imposed by the frame is employed. The results of the study highlight the significance of including realistic boundary conditions during fire simulations and points towards the possibility for the occurrence of substantial damage in unprotected steel frames during fire as well as protected steel frames during fire following an earthquake.
Open Access
Effect of mixed-mode loading on fatigue and fracture assessment of a steel twin box-girder bridge
(Colorado State University. Libraries, 2019) Irfaee, Mazin M., author; Mahmoud, Hussam, advisor; Heyliger, Paul, committee member; Atadero, Rebecca, committee member; Stright, Lisa, committee member
Steel twin box-girders are considered an attractive option for the construction of bridges due to their basic design, simple form, and ease of creation. Despite their advantages, they are considered fracture critical and as such there is an additional mandate for these bridges to inspected more in depth. This causes their inspection cost to be approximately two to five times greater than that of bridges with non-fracture critical members. The required additional inspection in the U.S. is mainly driven by rare historical events of bridge collapse for bridges that were not steel twin box girders. In addition, the mandated additional inspection does not reflect the inherent level of redundancy in most bridges. Therefore, it is important to quantify the potential for fracture and the level of redundancy in steel two-girder bridges in general, and twin box girders in particular, to minimize their inspection cost. Recognizing the inherently large scatter in fatigue performance, evaluating crack propagation and potential for fracture should, however, be performed in a probabilistic manner using detailed models that represent accurate behavior of the bridge. In this study, a detailed numerical finite element model of steel twin tub-girder bridge is developed and crack growth analysis, potential for fracture of its main tubs, and its overall redundancy is evaluated. The crack growth analysis is performed using multi-mode linear elastic fracture mechanics while accounting for uncertainties in the random variables associated with crack propagation and fracture. The results of the crack growth analysis are utilized to develop fragility functions that specify inspection intervals versus probability of failure where failure is characterized by dynamic crack growth. The analysis conducted to quantify the potential for fracture show distinct possible failure modes that vary from brittle fracture to ductile fracture. The extreme loading case shows that the bridge overall is not at risk of collapse. It is important to note that this conclusion cannot be generalized for all tub girder bridges since the level of redundancy is expected to vary between bridges depending on many factors such as girders geometries, plate thickness, fabrication, among others. However, the presented approach and the corresponding results provide a systematic way by which fracture critical bridges can be evaluated.
Open Access
Evaluation of wind turbine towers under the simultaneous application of seismic, operation and wind loads
(Colorado State University. Libraries, 2013) Smith, Vanessa, author; Mahmoud, Hussam, advisor; Bienkiewicz, Bogusz, committee member; Stansloski, Mitchell, committee member
Wind turbines are widely recognized as a renewable energy resource and as such, their safety and reliability must be ensured. Many studies have been completed on the blade rotor and nacelle components of wind turbines under wind and operation loads. While several studies have focused on idealized wind turbine models, significant advancements on the global and local performance of these models under seismic loads in combination with other loads has been lacking. A study on the evaluation and performance of realistic wind turbine models under wind, operation and seismic loads is proposed and successfully completed. First, the geometry and loading for three wind turbine models are developed. A series of finite element analyses is conducted for each model under a variety of load combinations and earthquake records. Both global results and localized behavior were obtained for each analysis in order to identify areas of improvement within the wind turbine structure. Global results include drift ratios, normalized base shear and fast Fourier transformations to evaluate the stability of the wind turbine during operation. Localized performance focused on the welded connection at the base of the turbine and included Von Mises stresses as well as low-cycle fatigue analyses to determine the number of cycles to failure (initiation of through-thickness crack). These results show that certain turbine models are more susceptible to these loads than others. Several analyses indicate yielding at the turbine base and resonant conditions. The results from these analyses identify several critical issues within the wind turbine design and operation protocol.
Open Access
Experimental assessment of cracked steel beams under mechanical loading and elevated temperatures
(Colorado State University. Libraries, 2016) Ahmadi, Bashir, author; Mahmoud, Hussam, advisor; van de Lindt, John, committee member; Strong, Kelly, committee member
Bridge fire is a major engineering problem that has been gaining attention by researchers and engineers. As reported in the New York Department of Transportation database, there has been approximately 50 cases of bridge collapse due to fire nationwide with many more cases where fires resulted in repairable damage. The fires are typically due to vehicle crash, arson, and in some cases wildfires. The affected bridges are mostly fabricated from steel, concrete, and temper. The problem of bridge fire is further aggravated by the presence of fatigue cracks in steel bridges. Various experimental and numerical studies have been conducted to evaluate the response of steel beams under elevated temperature. However, to date, there is lack of information on the response of steel beams with pre-existing cracks under elevated temperature. The importance of evaluating cracked steel beams under elevated temperature stems from the fact that many steel bridges that are currently in service suffer from major deteriorations manifested in the presence of fatigue cracks that are the result of cyclic loading from daily traffic. With no available data on failure behavior of cracked steel beams under fire, this thesis introduces a new testing protocol for evaluating the response of cracked steel beams under elevated temperature. Specifically, the results of experimental tests, conducted at the structural engineering laboratory at Colorado State University, of four initially cracked W8x24 steel beams under point loading and non-uniform elevated temperature are presented. The cracks are introduced across the bottom flange and the beams are loaded to failure while being subjected to various non-uniform elevated temperature distributions varying from 200 °C to 600 °C. The competition between two different failure modes: excessive deflection and fracture along the crack plane, is evaluated with respect to temperature distributions in the beams. In cases where fracture prevailed, different types of fractures were observed including brittle fracture, ductile fracture, and brittle/ductile transition failure, which depended on the temperature distribution. The results presented include load versus displacement and time versus temperature curves. In addition, digital image correlation method was utilized to develop strain and displacement fields around the cracked regions. This experimental study provides an alternative method for evaluating cracked beams under elevated temperature and will provide engineers with insight into various behavioral aspects of steel beams under the investigated loading demands. Furthermore, the results of this study can be used to calibrate advanced numerical finite element models, capable of capturing large deformations and fracture, which can in turn be used to conduct a parametric study for various sizes of bridge girders under an ensemble of thermal loading scenarios.
Open Access
Experimental fatigue evaluation of underwater steel panels retrofitted with fiber reinforced polymers
(Colorado State University. Libraries, 2019) Hudak, Lauren, author; Mahmoud, Hussam, advisor; Riveros, Guillermo, committee member; Atadero, Rebecca, committee member; Arneson, Erin, committee member
Many steel structures are susceptible to fatigue loading and damage that can potentially threaten their integrity if not monitored and repaired. Steel hydraulic structures (SHS), in particular, experience fatigue loading during operation and are exposed to harsh environmental conditions that can further reduce fatigue life through mechanisms such as stress corrosion cracking and corrosion fatigue. Dewatering to complete inspections or repairs to SHS is time consuming and leads to economic losses, and current repair methods, such as rewelding, often cause new cracks to form after relatively few cycles, requiring repeated inspection and repair. The use of bonded carbon fiber reinforced polymer (CFRP) to repair fatigue cracks in metallic structures has been successfully demonstrated in other industries, and recent work has suggested that the method can also offers a more reliable repair method for SHS. The very few studies regarding CFRP retrofits of SHS indicate that early bond failure often controls the degree of fatigue life extension provided by the repair. This study aims to extend previous experimental studies and further increase the fatigue life of repaired steel components by employing methods to improve CFRP bonding. Additionally, the use of basalt reinforced polymer (BFRP) as an alternative to CFRP is proposed. Limited examples of BFRP used in structural applications are available, but BFRP is attractive for SHS because it does not react galvanically with steel as CFRP does. In this study, four large-scale center-cracked panels were tested under constant amplitude fatigue loading. Of the four specimens, one was retrofitted with CFRP, and one was retrofitted with BFRP. To achieve an environment similar to that experienced by SHS, the two retrofitted specimens and one unretrofitted specimen were submerged in fresh water during testing. Remaining fatigue life was used as the primary metric for assessing the efficacy of the retrofit method. Results indicated that the use of both CFRP and BFRP are effective at extending fatigue life. The extent of fatigue life extension was still controlled by the quality of the FRP bond to steel; however, bond behavior was improved in comparison to previous underwater applications.
Open Access
Fatigue crack propagation in underwater carbon fiber reinforced polymer (CFRP)-retrofitted steel panels
(Colorado State University. Libraries, 2015) Valsangkar, Anuj, author; Mahmoud, Hussam, advisor; Riveros, Guillermo A., committee member; Smith, Frederick W., committee member; Holland, Troy, committee member
Steel structures, such as hydraulic structures and ships operate in harsh wet and corrosive environments and can suffer significant deterioration. The deterioration typically manifest itself in the form of corrosion, fatigue cracking, or a combination of both. While these corrosions or cracks are typically viewed as nuisance, if left unrepaid, they can threaten the integrity of the structure. Repairing these fatigue using the conventional repair methods can be proven to not only be time consuming but also ineffective. Recent advances on the use of CFRP to retrofit structures has shown to be a viable solution for increasing fatigue life of structures made of metals such as different types of steels, aluminum, etc. Although large number of studies have been conducted to evaluate the use of CFRP for retrofitting metal alloys and the promising potential of such has been well-demonstrated, the application has been primarily focused on the aerospace and bridge industries. As a result, very few studies have been concerned with retrofitting metallic structures under wet and corrosive environments. With the above mentioned motivations, there is a clear need to conduct studies to evaluate the viability of using CFRP to repair underwater metal structures. To this end, a new experimental setup is devised to allow for underwater testing of large-scale steel panels. The purpose of this experimental study is to provide a first-of-its-kind benchmark data by which the potential for using CFRP for underwater fatigue repairing metallic structures can be assessed. In this study, four large scale steel panels were tested, three of which repaired with CFRP patches, under different environmental conditions (three remaining to be tested for a total of seven specimens). The main focus is evaluate the effect of CFRP on crack growth rate. Since the application in this study is pertain to water navigation structures used in rivers, the effect of fluvial sediments as well as salt are considered in the study. The use of salt allowed for accelerated corrosion in the specimens to represent actual condition of deteriorated panels. The in-air and underwater results showed an increase in fatigue life with use of CFRP in comparison to bare specimens.
Open Access
Fatigue reliability and post-fracture residual capacity of a two-girder steel bridge
(Colorado State University. Libraries, 2016) Hartung, Lena F., author; Mahmoud, Hussam, advisor; Atadero, Rebecca, committee member; Strong, Kelly, committee member
Due to the immense and always increasing traffic volume, bridges are permanently subjected to repetitive loadings. These high numbers of cyclic loads can cause the initiation of fatigue cracks. If these flaws remain undetected they may become through-thickness cracks and further propagate, if left unrepaired, until they eventually lead to fracture of the entire member. The criticality of a full member fracture is not well defined nor agreed upon. Previous failure cases have demonstrated the ability of two-girder steel bridges to withstand full girder fracture of one of the two girders without structural collapse. Other cases, however, have shown the criticality of a complete girder failure on complete system collapse. Due to uncertainties in bridge redundancy and the ability to develop alternative load path, the American Association of State Highway and Transportation Officials (AASHTO) attempts to prevent fracture or collapse by classifying bridges with respect to their redundancy into fracture critical bridges (FCB) and decreasing their inspection periods. However, this leads to higher construction and maintenance costs for the owners of FCBs. Clearly, the level of uncertainty in bridge performance when one of its two girders suffer complete fracture should be represented in a probabilistic manner to evaluate the probability of fatigue crack growth and system collapse. To that end, thesis uses probabilistic analysis to assess the crack propagation behavior in a girder of a two-girder steel bridge by conducting finite element Monte Carlo simulations. The simulations account for the scatter in the load and the resistance by treating those uncertainties as random variables with predefined statistical distributions. Additionally, the post fracture redundancy is evaluated by comparing the resulting equivalent plastic strain to the failure strain of steel. The results show that the bridge provides sufficient redundancy to redistribute the load after full depth fracture a FC member. Furthermore, the results of the probabilistic analyses provide a basis for choosing the inspection intervals for FCBs.
Open Access
A framework for seismic resilience and recovery of hospital clusters
(Colorado State University. Libraries, 2017) Hassan, Emad, author; Mahmoud, Hussam, advisor
Understanding the behavior of hospitals is essential specially after major earthquakes. A comprehensive framework is presented to estimate losses of hospital clusters, quantity and quality functionality and recovery. The framework includes the recovery of different lifelines and is applied to Shelby County as a testbed to investigate the effect of interdependence on functionality and recovery assessment as well as the mutual effect of the hospitals. A patient-driven model is introduced to estimate the demand on each hospital, which affects the quality of the hospitalization service. This framework can be utilized by emergency planners for pre- and post-disaster recovery management.
Open Access
Integration of graphical, physics-based, and machine learning methods for assessment of impact and recovery of the built environment from wind hazards
(Colorado State University. Libraries, 2019) Pilkington, Stephanie F., author; Mahmoud, Hussam, advisor; Ellingwood, Bruce, committee member; van de Lindt, John, committee member; Zahran, Sammy, committee member; McAllister, Therese, committee member; Hamideh, Sara, committee member
The interaction between a natural hazard and a community has the potential to result in a natural disaster with substantial socio-economic losses. In order to minimize disaster impacts, researchers have been improving building codes and exploring further concepts of community resilience. Community resilience refers to a community's ability to absorb a hazard (minimize impacts) and "bounce back" afterwards (quick recovery time). Therefore, the two main components in modeling resilience are: the initial impact and subsequent recovery time. With respect to a community's building stock, this entails the building damage state sustained and how long it takes to repair and reoccupy that building. In modeling these concepts, probabilistic and physics-based methods have been the traditional approach. With advancements in artificial intelligence and machine learning, as well as data availability, it may be possible to model impact and recovery differently. Most current methods are highly constrained by their topic area, for example a damage state focuses on structural loading and resistance, while social vulnerability independently focus on certain social demographics. These models currently perform independently and are then aggregated together, but with the complex connectivity available through machine learning, structural and social characteristics may be combined simultaneously in one network model. The popularity of machine learning predictive modeling across multiple different applications has risen due to the benefit of modeling complex networks and perhaps identifying critical variables that were previously unknown, or the mechanism behind how these variables interacted within the predictive problem being modeled. The research presented herein outlines a method of using artificial neural networks to model building damage and recovery times. The incorporation of graph theory to analyze the resulting models also provides insight into the "black box" of artificial intelligence and the interaction of socio-technical parameters within the concept of community resilience. The subsequent neural network models are then verified through hindcasting the 2011 Joplin tornado for individual building damage and the time it took to repair and reoccupy each building. The results of this research show viability for using these methods to model damage, but more research work may be needed to model recovery at the same level of accuracy as damage. It is therefore recommended that artificial neural networks be primarily used for problems where the variables are well known but their interactions are not as easily understood or modeled. The graphical analysis also reveals an importance of social parameters across all points in the resilience process, while the structural components remain mostly important in determining the initial impact. Final importance factors are determined for each of the variables evaluated herein. It is suggested moving forward, that modeling approaches consider integrating how a community interacts with its infrastructure, since the human components are what make a natural hazard a disaster, and tracing artificial neural network connections may provide a starting point for such integration into current traditional modeling approaches.
Open Access
Joint elimination retrofits and thermal loading analysis in plate girder bridge using health monitoring and finite element simulations
(Colorado State University. Libraries, 2016) Rager, Karly, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, advisor; Strong, Kelly, committee member
Degradation of United States' public infrastructure has attracted attention from the public and governing agencies alike. A challenge facing transportation departments is management of leaking and clogged expansion joints in bridge structures, which result in significant deterioration to bridge substructures and superstructures. Some agencies have started eliminating these joints. However, technical understanding of which retrofit methodology to employ based on thermal loading and specific characteristics of the structure is lacking. In this study, this problem is investigated with both numerical modeling and analysis of field measurements. Various sensors were installed on the bridge including thermocouples, strain gauges, and linear differential displacement transducers. Following sensor installation, controlled load testing was conducted and the collected data evaluated against numerical and analytical predictions. The installed sensors will allow for long-term monitoring of the bridge to evaluate the effect of seasonal temperature profiles that are characteristic of Colorado on bridge behavior. In addition to gaining technical understanding of site-specific bridge characteristics that influence joint movement using field-testing, numerical finite element analysis was conducted. Specifically, a 3D finite element model was developed and used in a parametric study to assess the effect of various parameters on the stresses occurring in the bridge. The stresses occur due to 1) variation in thermal loading and thermal gradient, 2) clogging of the joint with different materials including gravel and sand, and 3) employment of various repair techniques in eliminating the expansion joints. The results of the numerical models show that clogged joints induce some localized stress but do not significantly affect the global performance of the superstructure. The results also show that a reduction in moment demand on the superstructure is not apparent until a Full-Moment Splice connection is utilized. This study will help engineers to choose the most appropriate method of designing a retrofit for expansion joint removal.
Open Access
Life cycle cost analysis for joint elimination retrofits and thermal loading on Colorado bridges
(Colorado State University. Libraries, 2017) Harper-Smith Kelly, Aura Lee, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, advisor; Strong, Kelly, committee member
Bridge expansion joints are a particularly troublesome component of bridges and many Departments of Transportation (DOTs) are looking for a solution to deteriorating expansion joints on highway bridges. Bridge expansion joints create a break in the structural continuity of a bridge allowing clogging gravels and corroding chlorides to enter. They are designed to absorb thermal movements of the bridge between two bridge elements. There are three main issues regarding expansion joint: maintenance, knowledge about thermal movements, and costs. In order to prevent deterioration due to expansion joints the joints must be cleaned regularly and replaced promptly after failure. However, most DOTs do not have the personnel, time or resources to maintain expansion joints in their districts which leads to bridge deterioration. Other similar maintenance and component issues have been addressed using a Life Cycle Cost Analysis. For this to be used on expansion joints the three main issues of thermal knowledge, maintenance, and costs must first be addressed. The main goal of this project is to help transportation agencies make better decisions about bridge expansion joints. The specific objectives of this study are to 1) expand understanding of thermal loading effects on bridge expansion joints and 2) conduct a LCCA for joint elimination and retrofits for bridges in Colorado. These objectives were accomplished utilizing data from in field instrumentation and finite element models. The study has been developed jointly between the Colorado Department of Transportation (CDOT) and researchers at Colorado State University Three main tasks were conducted to achieve the objectives: 1) collect and analyze long-term thermal loading data from existing bridges to assess thermal loading impacts on joints; 2) perform a parametric study using a calibrated finite element model to further understanding of joint behavior and retrofit options under thermal loads; 3) perform a LCCA for bridge expansion joint retrofitting including impacts on bridge superstructure. The significance of this work includes the results of the data collection and analysis, the parametric study, and the LCCA findings. The preliminary data on the concrete bridge C-17-AT presented in this thesis only accounts for mid-winter temperatures. However, these limited observations do imply that if CDOT is interested in removing an expansion joint, the bridge superstructure and retrofit option would need to support the movement of the bridge. The parametric study and data analysis of thermal gradients indicate a stark need for further research into thermal gradients experienced by bridges. Finally, the LCCA concluded that a retrofit continuous bridge design would provide the most cost effective design by decreasing joint replacement costs and pier cap corrosion.
Open Access
Multi-axial fatigue strength of structural bolts in slip-critical connections under combined cyclic axial and shear demands
(Colorado State University. Libraries, 2018) Rodriguez Lopez, Santiago, author; Mahmoud, Hussam, advisor; Atadero, Rebecca, committee member; Riveros, Guillermo, committee member; Senior, Bolivar, committee member
High-Strength bolts are used extensively in structures and are regarded as the better option for connections subjected to fatigue as compared to welds and rivets. Studies have shown the superior resistance to fatigue and conclude that it should not be an issue when a bolt is properly pre-tensioned. Nevertheless, a recent application of properly pre-tensioned bolts subjected to shear stress reversals shows extensive fatigue cracking and total severing of up to 50% of the bolts in the connections. Sufficient evidence, based on experimental testing and field observations, exist to suggest the possibility of fully pre-tensioned bolts coming loose due to shear stress reversals. The problem of transverse vibrational loosening of bolts has been extensively researched as well as the issue of bolt fatigue. Only recently have they been considered together although no studies of this interaction have been done on high-strength bolts. Certain mechanisms mark the onset of bolt loosening and fatigue when bolts are subjected to cyclic shear or shear combined with tension. In this study, causes of bolt loosening and fatigue failure of bolted connections are explored. Especially the study pertains to structural bolts that are subjected to cyclic loads in multiple directions with shear reversals, which are typical of mitre gate to pintle socket connections. Certain mechanisms mark the onset of bolt loosening and fatigue when bolts are subjected to cyclic shear or shear combined with tension. The actual mechanisms and limits at which this occurs are explored in the literature and experimentally and recommendations are provided.
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
Numerical simulation of out-of-plane distortion fatigue crack growth in bridge girders
(Colorado State University. Libraries, 2014) Miller, Paula A., author; Mahmoud, Hussam, advisor; Heyliger, Paul, committee member; Strong, Kelly, committee member
Aging of the United States infrastructure systems has resulted in the degradation of many operational bridge structures throughout the country. Structural deficiencies can result from material fatigue caused by cyclical loadings leading to localized structural damage. While fatigue crack growth is viewed as a serviceability problem, unstable crack growth can compromise the integrity of the structure. Multi-girder bridges designed with transverse cross bracing systems can be prone to distortion fatigue at unstiffened web gaps. Cracking is exhibited within this fatigue prone region from the application of cyclical multi-mode loadings. Focus of fatigue analysis has largely been directed at pure Mode I loading through the development of AASHTO fatigue classifications for crack initiation and the Paris Law for crack propagation. Numerical modeling approaches through the ABAQUS Extended Finite Element Method offers a unique avenue in which this detail can be assessed. Finite element simulations were developed to first evaluate the applicability of the Paris Law crack propagation under multi-mode loading against experimental data. Following the validation, fatigue crack growth in plate girders with various web gap sizes was assessed due to mixed-mode loadings. Modeling results showed enlargement of horizontal initial crack lengths within stiffer web gap regions arrested crack development. Crack directionality was also seen to change as initial crack lengths were increased. From this research it is hypothesized that deterioration of the transverse stiffener connection can be minimized by increasing the horizontal length of initial fatigue cracks. Enlargement of the crack plane away from regions of localized stress concentrations within the web gap may result in arrestment of the out-of-plane distortion induced cracking.
Open Access
Predicting fatigue life extension of steel reinforcement in RC beams repaired with externally bonded CFRP
(Colorado State University. Libraries, 2014) Sobieck, Tyler, author; Atadero, Rebecca, advisor; Mahmoud, Hussam, advisor; Radford, Donald, committee member
A majority of the United States' transportation infrastructure is over 50 years old with one in nine bridges being considered structurally deficient. Fatigue damage accumulation in bridge structures, generated by cyclic loading of passing traffic, has led to shorter service lives. Over the past few decades studies have shown carbon fiber reinforced polymer (CFRP) repairs to be an effective means of reducing fatigue damage accumulation in reinforced concrete (RC) girders. Despite the abundant research, the results, specifically the increase in fatigue life, vary widely making it difficult to apply them directly to repair designs. Therefore, design codes and guidelines presently in use are insufficient in providing engineers with the proper information to determine the extended fatigue life of the RC bridges repaired with CFRP. Current design codes state FRP repairs should limit the stress range in the reinforcing bars below the threshold where fatigue cracks can propagate. The problem with this philosophy is it essentially designs an overly conservative system with an infinite fatigue life. The proposed approach follows a performance based design philosophy for which the engineer designs for a specified extension in service life by limiting the crack growth rate in the reinforcement so the critical crack length, for which fracture in the reinforcement would occur, is never reached in the extended life. In this thesis, the results of experimental fatigue testing of control and CFRP repaired RC beams are highlighted and the fatigue crack propagation rate in the steel reinforcement is assessed for different repair schemes. The focus on steel reinforcement crack propagation rates was made because similar studies have found the reinforcement to be the limiting fatigue component in RC bridge girders. The results of the experimental study showed an extended fatigue life and a slowed crack growth rate in specimens repaired with both CFRP systems. The crack growth rates were then used to determine the material constants for the Pairs Law, which describes growth of a stable fatigue crack. These results were then used to propose recommendations for design of FRP repair systems for RC flexural members for a specific fatigue life.