Wood as a construction material has in recent years increased, in particular concerningpedestrian bridges. By utilizing wood, the ecological footprint can be reduced,and the material can be designed to comply with the increasing aesthetic demandbridge designers are facing. However, as the material weighs little with respect toits bearing capacity, combined with design becoming more slender, human inducedvibrations are becoming a problem.Having this in mind, the objective of the thesis is to conduct a case study on anexisting timber pedestrian bridge and assess its dynamic parameters by means ofexperiential testing and numerical modelling. The case study concerns the Hägernäsbridge, an arch bridge located in Hägernäs, Täby. The thesis also considers seasonaleffects by conducting experiments on two separate occasions. In addition, the thesisevaluates influencing parameters on the dynamic behaviour by conducting a sensitivityanalysis. To aid the above mentioned objective, a literature review coveringsimilar type of analysis is conducted. The literature review also studies the seasonaleffect, mainly from the asphalt layer, as its stiffness contribution is temperaturedependant.The results from the dynamic parameters showed that not all modes fall above therecommended values concerning damping ratio (with values above 1-1.5%). However,all modes fulfill design criteria concerning the magnitude of the natural frequencies.Furthermore, results showed that the natural frequencies are highly temperaturedependant. The measured values during warm weather (+17C) resulted in lowervalues than those from the cold weather experiment (-10C). Moreover, the greatestdifference, by 21% was on the 1st transverse mode and the lower difference was onthe 1st vertical mode, that decreased by merely 5%. Moreover, the damping ratiowas calculated and it was not possible to find any correlation between warm andcold temperature. Instead, decreased temperature caused some modes to increase indamping ratio and others to decrease. The most affected mode was once again the1st transverse mode which increased by 146% going from warm to cold temperature

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This dissertation discusses the potential for using a conventional main lateral-force resisting system, combined with the reserve strength in the gravity framing, and or auxiliary collapse-inhibiting mechanisms deployed throughout the building, or enhanced shear tab connections, to provide adequate serviceability performance and collapse safety for seismic and wind hazards in the central and eastern United States. While the proposed concept is likely applicable to building structures of all materials, the focus of this study is on structural steel-frame buildings using either non-ductile moment frames with fully-restrained flange welded connections not specifically detailed for seismic resistance, or ductile moment frames with reduced beam section connections designed for moderate seismic demands. The research shows that collapse prevention systems were effective at reducing the conditional probability of seismic collapse during Maximum Considered Earthquake (MCE) level ground motions, and at lowering the seismic and wind collapse risk of a building with moment frames not specifically detailed for seismic resistance. Reserve lateral strength in gravity framing, including the shear tab connections was a significant factor. The pattern of collapse prevention component failure depended on the type of loading, archetype building, and type of collapse prevention system, but most story collapse mechanisms formed in the lower stories of the building. Collapse prevention devices usually did not change the story failure mechanism of the building. Collapse prevention systems with energy dissipation devices contributed to a significant reduction in both repair cost and downtime. Resilience contour plots showed that reserve lateral strength in the gravity framing was effective at reducing recovery time, but less effective at reducing the associated economic losses. A conventional lateral force resisting system or a collapse prevention system with a highly ductile moment frame would be required for regions of higher seismicity or exposed to high hurricane wind speeds, but buildings with collapse prevention systems were adequate for many regions in the central and eastern United States. / Ph. D.

This thesis presents the results of basically two separate studies. The first study involved identifying structural and earthquake parameters that influenced seismic structural collapse. The parameter study involved nonlinear dynamic analyses using single-degree-of-freedom (SDOF) bilinear models. Four parameters were associated with the SDOF models — the lateral stiffness, the post-yield stiffness ratio, the yield strength, and the stability ratio (P-Delta effects). Then, three parameters were associated with the ground motions — the records themselves, the lateral ground motion scales, and the vertical ground motion scales. From the parameter study, it was found that the post-yield stiffness ratio augmented by P-Delta effects (rp) in conjunction with the ductility demand was the best predictor of collapse. These two quantities include all four structural parameters and the seismic displacement demands. It was also discovered in the parameter study that vertical accelerations did not significantly influence lateral displacements unless a given combination of model and earthquake parameters was altered such that the model was on the verge of collapsing. The second study involved Incremental Dynamic Analysis (IDA) using bilinear SDOF models representative of low rise buildings in both the Western United States (WUS) and the Central and Eastern United States (CEUS). Models were created that represented three, five, seven, and nine story buildings. Five sites from both the WUS and CEUS were used. Four different damage measures were used to assess the performance of the buildings. The IDA study was primarily interested in the response of the structures between the earthquake intensities that have a 10 percent probability of occurring in 50 years (10/50) and 2 percent probability of occurring in 50 years (2/50). The results showed that all structures could be in danger of severe damage and possible collapse, depending on which damage measure and which earthquake was used. It is important to note, though, that the aforementioned is based on a damage-based collapse rule. The damage-based rule results were highly variable. Using an intensity-based collapse rule, proved to be more consistent. Due to the nature of the bilinear models, only those structures with negative rp values ever collapsed using an intensity-based collapse rule. Most of the WUS models had positive rp values and many of the CEUS models had negative rp values. While many of the CEUS structures had negative rp values, which made them prone to collapse, most of the CEUS structures analyzed did not collapse at the 2/50 intensity. The reason was that the periods of the CEUS models were much longer than the approximate periods that were required to determine the strengths. Consequently, the strength capacity of most of the CEUS models was much greater than the seismic strength demands. While many of the CEUS models did have sudden collapses due to the large negative rp values, the collapses happened at intensities that were generally much higher than the 2/50 event. In the IDA, it was also shown that vertical accelerations can significantly affect the ductility demands of a model with a negative rp post-yield stiffness ratio as the earthquake intensity approaches the collapse intensity. Since IDA is concerned with establishing collapse limit states, it seems that the most accurate collapse assessments would include vertical accelerations. / Master of Science

No / A vehicle that deviates laterally from its intended path of travel when the brakes are applied is considered to demonstrate ‘instability’ in the form of an unexpected and undesirable response to the driver input. Even where the magnitude of lateral displacement of the vehicle is small (i.e. ‘drift’ rather than ‘pull’) such a condition would be considered unacceptable by manufacturers and customers. Steering ‘drift’ during braking can be caused by several factors, some of which relate to vehicle design and others to external influences such as road conditions. The study presented here examines the causes and effects of steering drift during straight-line braking. A comparative analysis is made between two types of vehicle model: one built with rigid suspension components and the other with flexible components. In both the cases, the vehicle behaviour is simulated during braking in a straight line, and responses including lateral acceleration, yaw rate, and lateral displacement of the vehicle are predicted and analysed under fixed steering control. Suspension/steering geometry characteristics, namely toe steer and caster angle, have been studied to understand how the effect of variations in these parameters differs in models with rigid or flexible components drift during straight-line braking. Results from both vehicle models show that differences between rigid and flexible components can affect the predicted steering drift propensity. The differences between the two models have emphasized the importance of using flexible (compliant) components in vehicle handling simulations to achieve better correlation between prediction and experiment.

<p dir="ltr">Dynamic analysis of embedded firmware is a necessary capability for many security tasks, e.g., vulnerability detection. Rehosting is a technique that enables dynamic analysis by facilitating the execution of firmware in a host environment decoupled from the actual hardware. Current rehosting techniques focus on high-fidelity execution of the entire firmware. Consequently, these techniques try to execute firmware in an emulated environment, with precise models of hardware (i.e., peripheral) interactions. However, these techniques are hard to scale and have various drawbacks. </p><p dir="ltr">Therefore, a novel take on rehosting is proposed by focusing on the application components and their interactions with the firmware without the need to model hardware dependencies. This is achieved by rehosting the embedded application as a Linux application. In addition to avoiding precise peripheral modeling, such a rehosting technique enables the use of existing dynamic analysis techniques on these embedded applications. The feasibility of this approach is demonstrated first by manually performing the rehosting on real-world embedded applications. The challenges in each of the phases – retargeting to x86-64, peripheral handling, and fuzzing the rehosted applications are elaborated. Furthermore, automated steps for retargeting to the x86-64 and peripheral handling are developed. The peripheral handling achieves 89% accuracy if reserved regions are also considered. The testing of these rehosted applications found 2 previously unknown defects in driver components.</p>

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[pt] Os fluxos de detritos são perigosos riscos naturais, que afetam países com intensas precipitações e terrenos montanhosos. Tais eventos configuram alto perigo para a vida humana e danificação de infraestrutura, resultando em importantes perdas econômicas. O estudo de fluxos de detritos envolve um mecânismo complexo e suas técnicas de previsão são baseadas na calibração de modelos, que devem ser delimitados por tentativa e erro de eventos anteriores. Tais previsões são ferramentas valiosas para delimitar as potenciais áreas de risco e, dessa forma, projetar medidas de mitigação e convivência. O principal objetivo deste trabalho foi analisar o comportamento de quatro fluxos de detritos deflagrados por precipitações de alta intensidade em regiões tropicais utilizando modelagem numérica em 2D e 3D. Foram analisados os casos de Lajas e Llano de la Piedra na Costa Rica e os casos de Córrego D Antas e Hospital São Lucas no Rio de Janeiro. Os principais parâmetros utilizados, na avaliação do risco deste tipo de movimentos de massa, são: a distância percorrida, a área de impacto, a velocidade e profundidade do fluxo. Os casos foram calibrados utilizando a reologia de Voellmy. A definição dos parametros na calibração é vital, pois oferece a possibilidade de previsões de primeira ordem, feita sobre escorregamentos acontecendo em condições semelhantes. Os resultados da análise dinâmica mostram valores consistentes entre os valores observados e as modelagens numéricas em 2D e 3D para os principais parâmetros avaliados, corroborando o uso destas ferramentas para análises de risco e projeção de medidas de mitigação e convivência. / [en] Debris flows are dangerous natural hazards affecting countries with steep terrains and heavy rainfall. They are associated with serious risks to human lives and infrastructure leading to important economic losses and fatalities. Debris flows involve complex mechanics and prediction techniques that are based on the calibration of models that must be constrained by trial-and-error back-analysis of previous landslides. Such predictions are a valuable tool for outlining potential hazard areas and the development of mitigation strategies and design of protective structures. The main goal of this work was to analyze the behavior of four debris flows triggered by heavy rainfall in tropical regions with numerical modelling. The Voellmy rheology was used to calibrate the cases occurred in Lajas and Llano de la Piedra in Costa Rica, and Córrego D Antas and Hospital São Lucas in Rio de Janeiro. The main parameters used for landslide risk assessment are runout distance, potential impact area, flow velocity and flow depth. The definition of appropriate calibrating parameters is important because it provides the possibility of first order predictions to be made about the motion of future landslides happening under similar conditions. The results of the dynamic analysis showed that consistent values were obtained for the main parameters evaluated in the 2D and 3D runout models, verifying the usefulness of these tools for landslide risk assessment and the project of protection structures.

<p dir="ltr">Shape memory alloys (SMAs) belong to the category of smart materials due to their unique shape memory properties induced by a thermomechanically-triggered phase transformation. This phase changing process is also associated with a pronounced energy dissipation capacity. In recent years, the shape-recovery and energy-dissipating capabilities of SMAs have been object of extensive studies with particular focus on the opportunities they offer for the design of smart composites. The restoring stress of constrained SMAs as well as the modulus change, following thermal loading, can be leveraged to improve the static and dynamic performance, such as the pre/post-bulking behavior, the aerodynamic stability, and the impact resistance of composite materials embedded with SMA wires or fibers. The nonlinear damping resulting from the nonlinear material behavior associated with the ferro-elastic and pseudo-elastic phases was explored in a few studies focusing on vibration suppression in composites. Nonetheless, existing research mainly focused on either SMA wire or fiber reinforced composites, while the understanding of the dynamics of hybrid composites integrating SMA layers still presents several unexplored areas. In part, this technological gap might be explained by the fact that the most common SMA alloy, the so-called Nitinol, is expensive and hence not amenable to be deployed in large scale applications. With the most recent advancements in low-cost SMAs (e.g. Fe-based and Cu-based alloys), new applications that make more extensive use of SMAs are becoming viable. It follows that the understanding of the dynamic response of composites integrating SMA laminae becomes an important topic in order to support the development of innovative hybrid composite structures.</p><p dir="ltr">This dissertation explores the design and the nonlinear dynamic response of hybrid composites integrating SMA laminae, with a particular emphasis on the damping performance under different operating conditions. The dynamic properties of SMA monolithic beams and hybrid composite beams integrated with SMA laminae are investigated via one-dimensional constitutive models. Monolithic SMA beams are investigated to understand the fundamental aspects of the damping capacity of the material as well as possible bifurcation phenomena occurring under different types of harmonic excitations and different levels of pre-strain. The study then focuses on hybrid composite beams, highlighting the effects of design parameters, such the thickness, position, and pre-strain level of SMA layers on the transient and forced dynamic characteristics.</p><p dir="ltr">To further explore the potential of embedding SMA laminae to tailor the damping capacity of the hybrid composite and optimize the distribution of SMA materials, hybrid composite plates (HCPs) assembled by stacking fiber composites and SMA layers (either monolithic or patterned) are explored. The damping capacity of the HCP is assessed under different operating conditions, with emphasis on the effect of pre-strain levels in the SMA layers. The optimization study focuses on understanding the distribution of SMA materials and the synergistic role of patterning and pre-straining individual SMA layers within the HCP. The damping capacity of the HCP is also estimated as a function of the SMA total transformed volume fraction in order to identify the types of patterns and the pre-strain profiles capable of improving the overall damping capacity of the HCP.</p><p dir="ltr">The investigation on the dynamics of SMA hybrid composites continues with the optimal design of sandwich composite beams with elastic face sheets and SMA cellular cores. A deep learning-based surrogate model is proposed to efficiently predict the nonlinear mechanical response of the SMA sandwich beams subject to transverse loading, hence enabling the optimization of the SMA cellular core. The multi-objective optimization of the energy-dissipating capacity and of the overall stiffness is then performed by taking advantage of evolutionary algorithms. Once the optimal geometric parameters of the SMA cellular cores are obtained, finite element simulations are conducted to numerically validate the optimal configurations of the sandwich beams.</p><p dir="ltr">Finally, the numerical models are validated via experimental measurements conducted on monolithic SMA beams. Tests include both tensile and vibration measurements in both the ferro-elastic and pseudo-elastic regimes. The stress-strain relations obtained from tensile tests are used to calibrate the constitutive model of SMAs. Subsequently, experimental vibration tests are performed on clamped-clamped SMA beams to assess the effect of pre-strain levels on the damping capacity of SMA beams via a dedicated experimental setup to apply and maintain the pre-strain levels. The theoretical, numerical, and experimental results provided in this dissertation can serve as important guidelines to design lightweight SMA smart composites with customizable dynamic behavior.</p>