PhD / Doctoral

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    Open Access
    Lateral-torsional buckling behaviour of monosymmetric steel sections formed with flange upstands
    (2023) Mudenda, Kenny; Zingoni, Alphose
    The study of the lateral-torsional buckling behaviour of steel beams has been largely focused on doubly symmetric beams and less so on monosymmetric sections. Monosymmetric sections, however, do offer some advantages over doubly symmetric beams if the monosymmetry is applied in a way that it can be exploited. This is because the effects of monosymmetry can work to increase or decrease the moment capacity. Identifying these cases is therefore essential. In the current study the behaviour of a monosymmetric section produced by introducing flange upstands as stiffeners is explored. The upstands convert a standard doubly symmetric section into a monosymmetric section. The change in member properties is first explored with the movement of shear center relative to the geometric centroid being of particular interest. It is observed that the shear center initially moves away from the centroid towards the stiffened flange, reaches a stationery point and then starts to move away from this flange. This leads to the shear center then intersecting the centroid so that for a monosymmetric section, at a certain upstand height, the shear center and centroid are coincident. This results in a monosymmetric section taking on a property typically associated with a doubly symmetric section. A study of the change in the monosymmetry constant of the section also reveals that at a given upstand height the monosymmetric section also takes on another property associated with doubly symmetric sections, that of the monosymmetry constant having a value of zero. Further study of the critical elastic moment change with change in upstand height reveals that the section initially has a significant increase in the value of the critical elastic moment with upstand height followed by a trend of diminishing increase rate. In some cases, this trend then reverses so that with an increase in upstand height the critical elastic moment starts to decrease. The region where the monosymmetry acts beneficially to increase section capacity represents a target domain before the monosymmetry then starts working to diminish the member capacity. This second region represents an undesirable domain for the use of these upstands. This observed behaviour, therefore, leads to identification of a domain of upstand heights in which the positive effect of monosymmetry can be exploited. The junction between the two regions, given by a critical upstand height, can be reasonably approximated based on the upstand height at which the shear center and centroid coincide. Engineers who may want to make use of flange stiffeners need to be aware of these trends to better manage the strengthening of doubly symmetric members so that material is used economically and not wasted in regions were the returns are minimal or diminishing. The application of this domain has been demonstrated for linear elastic buckling cases as well as for inelastic buckling cases so that real member behaviour is also simulated. Although applied for strengthening cases in the current study the principle can be used for section optimization at member selection stage should members with upstands be an option. Equations obtained using the Rayleigh-Ritz energy method were employed for the elastic critical buckling study for two transverse load cases. Finite element models were then employed for the nonlinear analysis for the inelastic buckling cases. Residual stresses have not been included in the finite element models. Geometric imperfections as well as material constitutive behaviour were incorporated in the models. This gives a realistic picture of ‘real' member behaviour despite not including residual stresses. The results from the nonlinear study support those from the linear elastic study in demonstrating how the beneficial domain can be exploited.
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    Open Access
    Contribution of anthropogenic climate change to the magnitude of extreme rainfall events and associated synoptic conditions during recent flooding in Kenya
    (2023) Kimutai, Joyce; New, Mark; Wolski Piotr
    The changing probabilities of extreme climate and weather events, in terms of frequency, intensity, spatial extent, duration and timing, are one of the most noticeable and damaging manifestations of human-induced climate change. The Greater Horn of Africa has experienced a number of extreme weather and seasonal climate events over the past two decades. While droughts have predominated, several heavy precipitation events with devastating impacts have also been recorded. During the MarchApril-May (MAM) rainfall seasons of 2012, 2016 and 2018, Kenya experienced high rainfall that caused both widespread and localised flooding, resulting in human and livestock deaths, destruction of infrastructure and property, bursting of riverbanks, submerging of farmlands and emergence of isolated cases of water-borne diseases. This research aimed to assess whether human influence on climate played a role in modifying the rainfall intensity and associated synoptic conditions of extreme rainfall in these years. The work had three specific objectives: (i) Characterise the rainfall magnitude and associated synoptic conditions at the time of the flooding events; (ii) Evaluate the role of human influence on the magnitude of heavy rainfall; (iii) Evaluate the role of human influence on the synoptic conditions associated with heavy rainfall. By using three different attribution approaches, and utilising two observational datasets, one reanalysis data and two independent climate model experiment setups, the study was able to quantify how the local thermodynamic and regional dynamic conditions driving the flood-inducing rainfall in these seasons may have been altered by human-induced climate change. The rainfall magnitudes and associated atmospheric states were first characterised and then differences in the rainfall magnitudes and frequency of the atmospheric states in MAM 2012, 2016 and 2018 were compared to those in preindustrial climate. Three different seasonal heavy rainfall indices were analysed; seasonal maximum consecutive 5-day, 10-day, and 20-day rainfall. The atmospheric states were based on Self Organizing Maps analysis of specific humidity, air temperature, and zonal and meridional wind at 850hPa on surface level pressure and zonal moisture flux.
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    Open Access
    The aerobic, water-assisted selective oxidation of methane over platinum-based catalysts
    (2023) Mahlaba, Sinqobile Vuyisile Lusanda; Van Steen, Eric
    The selective oxidation of methane to C1-oxygenates such as methanol, formaldehyde and dimethylether remains one of the grand challenges of catalysis chemistry, due to the difficulty in attaining high selectivities at high methane conversions. This has been attributed to the higher reactivity of methane oxidation products compared to methane itself, which therefore leads to their facile conversion to CO and CO2 [1]. Strategies that have been adopted to selectively convert methane include product protection (i.e. converting methane to a methylester, which is stable against further oxidation compared to methanol) [2, 3], cyclical methane oxidation using zeolites (i.e. stepwise activation of oxygen, methane and product extraction) [4, 5], the use of mild reaction conditions (i.e. conversion of methane at low temperature using hydrogen peroxide)[6, 7] and the use of copper-exchanged zeolites which enable the catalytic conversion of methane with high methanol selectivities [8-10]. Copper-exchanged zeolites require feeding an excess of methane and still suffer from low methane conversions. The study presented in this thesis is focused on investigating the aerobic, selective oxidation of methane over supported platinum-based catalysts in the presence of water, with the investigation focusing on a) the role of water, b) the effects of the support and c) the effects of alloying platinum on the catalytic activity and oxygenate selectivity. The effects of water were investigated initially in a gas-phase, fixed bed reactor that is equipped with a steam generator. Initial results from the selective oxidation of methane yielded CO2. Accidentally flooding the reactor resulted in the formation of selective oxidation products such as methanol, methoxymethanol and 1,3,5, trioxane, which were observed when the reactor was flooded on purpose. The experiments were then carried out in a specially-constructed trickle-bed reactor that enabled the co-feeding of liquid water. An increase in the H2O/CH4 ratio resulted in an increase in the conversion of methane and in formaldehyde selectivity, which was the favoured product. DFT calculations indicated that formaldehyde was formed from a di s-hydroxy methoxy intermediate, which possibly forms from the hydroxylation of a surface methoxy precursor. Initial catalytic studies involved titania supported catalysts (TiO2, rutile and P25), which deactivated with time, due to deposition of formaldehyde polymeric species. In total, the supports investigated were alumina, carbon, the rutile phase of titania and P25 (a mixture of rutile and anatase). The catalytic activity was in the order of Pt/TiO2-P25 (XCH₄ = 0.6%, Sselective oxidation products = 10%) < Pt/Al2O3 (XCH₄ = 0.5%, SCH₂O = 65%) < Pt/TiO2-Rutile (XCH₄ = 1.0%, SCH₂O = 90%) < Pt/C (XCH₄ = 4.2%, SCH₂O = 99%). Although all catalysts deactivated, alumina underwent phase-transformation to boehmite, which further exacerbated catalyst deactivation. Kinetic experiments indicated that the selective oxidation of methane was limited by the rate of product desorption, which was related to the binding strength of oxygen on the platinum surface. The binding strength of oxygenates on the platinum surface was then tuned by alloying platinum with copper, silver and gold in order to modify the platinum d-band center. The alloys were synthesized targeting a 3:1 Pt:M (M= Cu, Ag or Au) ratio and were supported on the rutile phase of titania. The nanoalloy catalysts were tested in the fixed bed reactor. The selectivity towards formaldehyde was in the order of Pt3Au (64%) = Pt (66%) = Pt3Cu (66%) < Pt3Ag (72%). However, at the highest water partial pressure, the formaldehyde selectivity decreased to 63% for Pt3Ag. The rate of methane conversion was in the order of Pt3Cu (292 µmol/gcat/hr) > Pt3Au (104 µmol/gcat/hr) ³ Pt (95 µmol/gcat/hr) while Pt3Ag was the least active, with a maximum activity of 68 µmol/gcat/hr. Testing Pt3Cu/TiO2 in the trickle bed reactor resulted in a slight increase in the rate of methane conversion (320 µmol/gcat/hr) and an increase in the formaldehyde selectivity (Sformaldehyde = 78%). The performance of the Pt3Ag/TiO2 catalyst was significantly enhanced in the trickle-bed reactor (-rCH₄ = 271 µmol/gcat/hr, Sformaldehyde = 74%). The results presented herein show the possibility of selectively oxidising methane with high selectivity towards formaldehyde. Furthermore, a high conversion of methane (ca. 4-5%) was achieved at fairly high oxygenate selectivities (65-99%), using oxygen as an oxidant.
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    Open Access
    Plaatberg on the Caledon Bastaards: hunters, raiders and traders or pious converts of the Wesleyan Missionary Society?
    (2023) Klatzow, Shelona; Hall, Simon; Chirikure Shadreck
    Plaatberg mission station was established in 1833 by the Wesleyan Missionary Society specifically to minister to a group of people known as Bastaards, under the leadership of Carolus Baatje. As new arrivals in Transorangia who had crossed the boundary from the Cape Colony, the Plaatberg Bastaards came equipped with wagons, horses, guns and ammunition. They showed great skill in adapting to the volatile frontier world in the way that they negotiated the move from colonial farm workers, servants, slaves or disposed Bastaards in the colony to successful traders, raiders and farmers in the Caledon River Valley. Using both written and archaeological evidence, this thesis examines the way that the creolized Plaatberg Bastaards, as inhabitants of the Plaatberg mission station, responded to the Christianising efforts of the Wesleyan Missionaries. The Wesleyans main objective was to transform the Plaatberg Bastaards from “heathen” inhabitants into “civilized” Christian converts, by the imposition of a variety of rules and regulations to achieve this aim. I consider how the very nature of being a creolized mobile group may have influenced the Plaatberg Bastaards responses to the Wesleyan missionaries. These responses may be reflected in the material culture and their use of private and public spaces within the mission as well as in the wider landscape beyond the mission station boundaries. As “heathen” inhabitants of the Wesleyan mission station, the Plaatberg Bastaards had to negotiate their way between and through the aspirations of the missionaries for Christian converts, and the continuity of their own frontier way of life and belief systems. I examine the missionary aspirations for order and control as physically expressed in the public gridlike layout of the mission village itself, being the centre of colonial and religious power. At a finer scale, I address the archaeology of a single domestic precinct and assess the material evidence of dwelling forms, layout and the artefactual mix. At this scale, the order and control desired of the Plaatberg Bastaards by the missionaries was clearly inflected by the layout and utilization of domestic space and hinted at in the use of traded British goods. Additionally, evidence from a rock shelter outside the immediate boundary of the mission, and of hunting, raiding and trading further afield, indicates the private continuity of frontier practice and belief. Selective resistance by the Plaatberg Bastaards to missionary control was strategic and reflected the economic benefits of prior practice, but also the advantages of new practical skills for life within the rapidly changing political landscape of the Northern Cape frontier
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    Open Access
    Epidemiology and genetic risk factors of suicidal behaviour in South Africa
    (2023) Kootbodien, Tahira; Ramesar, Rajkumar; Martin Lorna
    Background: Suicide is an urgent public health problem. Fatal suicidal behaviour (individuals who died by suicide) and non-fatal suicidal behaviour (attempted suicide, self-harm, and suicidal ideation) comprise a complex interplay of individual, social, environmental, and biological factors, that are not fully understood. Given the considerable societal cost associated with suicide and health inequality in South Africa, there is a critical need to determine the burden of suicide and risk factors associated with suicide, to understand who is most at risk to inform effective prevention efforts. Despite clear evidence of multiple risk factors, suicidal behaviour remains difficult to predict and prevent. Prevention efforts in South Africa may be limited by the lack of a national suicide prevention plan and the low base rate of individuals who died by suicide may impede research comparing fatal and non-fatal suicidal behaviour in settings such as ours. In addition, existing suicide data are derived primarily from high-income countries rather than lowand middle-income countries (LMICs) where suicide and poverty levels are high, and the mental health treatment gap is large. The purpose of this study was to broaden our understanding of suicidal behaviour in South Africa by combining various data sources, each representing a unique perspective of the problem, to build on existing knowledge that may inform suicide prevention strategies in South Africa. This thesis is organised into four studies and aimed to investigate risk factors associated with suicidal behaviour and to identify opportunities for targeted suicide prevention. The specific objectives of each study component were as follows: • To describe trends and demographic risk factors in deaths from suicide as well as other conditions that may include suicide to identify populations at risk in South Africa (Study 1). • To investigate the association between environmental and occupational organophosphate pesticide (OP) exposure and attempted suicide in adults admitted to hospital in Cape Town, South Africa (Study 2). • To explore the genetic architecture underlying suicidal behaviour and psychiatric disorders to understand the genetic factors that increase the risk of suicidal behaviour (Study 3). • To describe healthcare utilisation 12 months before suicidal behaviour among individuals who attempted suicide and who died by suicide, to identify opportunities for prevention in Cape Town, South Africa (Study 4). Methods: This thesis included an ecological time-series study of national suicide mortality data from Statistics South Africa using joinpoint regression analysis (Study 1, N=10.3 million recorded deaths from 1997 to 2016; 8,573 deaths from suicide); a conditional logistic regression analysis of an attempted suicide hospital-based case-control study (Study 2, N=400; 200 cases and controls); a genome-wide genetic correlation study of suicidal ideation, self-harm, attempted and fatal suicide (samples [n] ranged from 62,648 to 125,844), and selected psychiatric disorders (n ranged from 9,954 to 386,533) using a genomic structural equation modelling approach (Study 3); and a retrospective cohort of linked electronic health records of individuals who attempted suicide and were admitted to hospital and a case series of fatal suicides on whom forensic autopsies were performed at a mortuary in Cape Town (Study 4, N=484). Results: The key findings in this study show that (i) suicide mortality rates were consistently higher in men than women between 1997 and 2016 (Study 1). Suicide rates increased by 7.7% among young people aged 15 to 29 years. Hanging, poisoning and firearms were the most frequent methods of suicide used. Subgroup analysis showed suicide by hanging and poisoning mortality rates increased by 2.9% and 3.7% across 20 years. Suicide deaths were underreported and may be included among deaths by accidental injuries and undetermined intent. However, these patterns varied by method of death (hanging, poisoning and firearm injury) over the study period. The largest proportion of suicide deaths may be potentially misclassified as accidental hanging and hanging by undetermined intent, and to a lesser extent, accidental poisoning and poisoning by undetermined intent. In contrast, firearm-related deaths were more likely to indicate a homicide than a suicide death. Missing data on select sociodemographic variables limited the accuracy and generalisability of our findings. (ii) Pesticide use in homes and gardens was common (85%); however, there was no association between attempted suicide and environmental (household, garden, and occupational) OP exposure (Study 2). Hazardous drinking and unemployment with no household income were significantly associated with an increased risk of attempted suicide while sharing the house with more than three persons was protective. (iii) We observed strong significant genetic correlations (rg) between suicidal ideation, attempted suicide, and self-harm (rg range, 0.71 to 1.09) and moderate-to-strong genetic correlations between suicidal behaviour traits and a range of psychiatric disorders (Study 3). The strongest genetic correlation was noted for major depressive disorder and suicidal ideation (Ever contemplated self-harm, rg=0.86±0.07, p=1.62x10-36). Multivariate genomic analysis revealed a single (common) factor structure for suicidal behaviour traits, major depressive disorder, attention-deficit hyperactivity disorder (ADHD), and alcohol use disorder. Approximately 2,951 genes and 98 sub-network hub genes were associated with the common factor, and shared biological pathways include involvement in developmental biology, signal transduction and RNA degradation. (iv) Approximately two-thirds of cases had at least one prior visit to a health care facility in the 12 months leading to suicidal behaviour (Study 4). The prevalence of psychiatric disorders was lower for individuals who died by suicide than attempted suicides but both groups interacted equally (approximately 65%) with the healthcare system during the 12 months leading to suicidal behaviour. Patients who used primary care services in the year before dying by suicide attended for the management of their chronic conditions (such as cardiovascular disease and diabetes) and emergency medical care for assault-related injuries. For attempted suicides, common reasons for a healthcare visit were for management of their chronic condition, HIV care, and a psychiatric diagnosis of depression, bipolar, or substance use disorders. Conclusions: This study expands on previous research and shows that while common risk factors are shared by individuals with fatal and non-fatal suicidal behaviour, the degree of risk varies across suicide groups, age, and sex. Findings of increased genetic risk of suicidal behaviour among individuals with psychiatric disorders suggest that identification and early treatment of co-morbid psychiatric disorders (major depression, alcohol use disorder, ADHD, and schizophrenia) should be included in suicide prevention strategies. Evidence of potential misclassification of suicide death within accidental injuries and undetermined intent categories may explain the underestimation of suicide mortality reported in this study. Combined with the high proportion of missing data in the national vital statistics and poor data quality of external causes of death, these findings suggest a critical need for ongoing training on the cause of death certification and further interventions to improve suicide data quality. Further, continued monitoring of suicide mortality data and linking electronic health records may provide opportunities for suicide surveillance that can help identify where prevention strategies should be allocated for maximum benefit, such as primary healthcare outpatient facilities, emergency treatment centres, and antiretroviral clinics.