School of Civil & Environmental Engineering (ETDs)

Permanent URI for this communityhttps://hdl.handle.net/10539/37962

Browse

Start date: 2023End date: 2023

Search Results

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    Developing of a parametrically resonw1t vibrating screen, modelling, simulation and dynamic testing
    (University of the Witwatersrand, Johannesburg, 2023-07) Mohanlal, Mishal; Li, Kuinian
    A novel coupled spring pendulum vibrating screen is proposed with the goal of developing efficient screening using parametric resonance. A simple spring pendulum is initially studied to provide the basis of the dissertation. The theoretical model of the proposed vibrating screen is developed using Lagrangian mechanics which includes damping and generalized forces. Two derivations of the vibrating screen are proposed, the first being a 4DOF (degree of freedom) system and the second being a 3DOF system. The 3DOF system is found to present better numerical stability and is thus utilized for the study. It is shown that the 3DOF system is comparable to the simple spring pendulum for the case where initial conditions are applied to similar coordinates. The proposed vibrating screen presents motion which is not indicative of traditional vibrating screens. It is found that a system where attributes are sized for parametric resonance requires far smaller excitation forces to achieve higher accelerations and displacements compared to traditional vibrating screens. The proposed vibrating screen is an unfeasible design due to the large displacements; high foundation loads and limitations on mechanical components. Discrete element method (DEM) simulations of the proposed vibrating screen are performed to study the efficiency with varying inclinations of the mesh deck. The results are compared to a linear motion vibrating screen. The proposed screen requires far less energy compared to traditional vibrating screens and achieves higher efficiencies with larger deck inclinations. The derived differential equations are verified by experimental testing using free vibrations. The numerical simulations and experimental tests present a good correlation. Signal processing is implemented to compare the natural frequencies from the experimental testing and numerical simulations, the results present a good correlation.
  • Thumbnail Image
    Item
    Use of Multispectral Satellite Imagery to Monitor the Decant Pond of Tailings Dams
    (University of the Witwatersrand, Johannesburg, 2023-08) O’Donovan, Christopher Galen; Adam, Elhadi; Torres-Cruz, Alberto
    Tailings dam failures, such as the Jagersfontein failure in the Free State province and the Fundão and Feijão failures in Brazil, have brought into question the ability of the mining industry to operate safely, threatening its social license. To improve the safety of tailings dams, leading indicators of dam safety should be monitored. The location and historical behaviour of the tailings decant pond provides insight into several such leading indicators and can be used as a proxy to flag potential construction issues. This work investigates the use of public multispectral data collected by the Sentinel-2 satellite mission to monitor the supernatant tailings dam decant pond. This is achieved by leveraging the cloud-based Google Earth Engine platform and open-source GIS tools. Sentinel-2 acquires visible and near infrared spectrum data with a spatial resolution of 10 m and a revisit time of 5 days. Pond data is obtained by visual assessment and automated thresholding of Sentinel-2 imagery. Thresholds of near-infrared (NIR) reflectance and the normalised difference water index (NDWI) obtained by a least square error analysis are investigated. Implementation of the method at three South African tailings dams, constituting four decant ponds, illustrates the capabilities and limitations of Sentinel-2 imagery. High spatial resolution (<5 m) multispectral satellite imagery and natural colour aerial orthophotos (<0.25 m) serve as reference data. Visually assessed Sentinel-2 pond data presented a bias towards slight over estimation of the pond area compared to reference data. Other leading indicators did not show systematic bias across all sites. In general, the deviation between Sentinel-2 and the reference measurements was high, indicating that Sentinel-2 imagery should be used with caution for measurements critical to dam safety. Site-specific thresholds of NIR and NDWI indicated that automated thresholding of the NDWI is superior to NIR reflectance alone. It is shown that Sentinel-2 timeseries imagery can be used in tailings dam monitoring to supplement existing construction surveillance frameworks and provide historical pond data in the absence of such information.
  • Thumbnail Image
    Item
    A Comparative Study of the Efficiencies of Vertical Bracing Practices
    (University of the Witwatersrand, Johannesburg, 2023-11) Saunders, Shawn Wayne Valintino; Elvin, Alex
    The efficiencies of cross sections and configurations applied to vertical bracing are investigated by evaluating reference configurations (RCs), composed of cross braced circular hollow section (CHS) members, against comparative configurations (CCs), consisting of cross-braced Angle members, and single-CHS members. The metrics used to evaluate efficiencies were mass, raw materials costs, and fabrication and erection costs. CCs were found to be more efficient than RCs for most analysed cases, metric and configuration dependent. The following results were found: i. Mass metric a. Crossed-Angle more efficient in 79% of analysed cases. b. Single CHS more efficient in 87% of analysed cases. ii. Raw materials costs metric a. Crossed-Angle more efficient in 92% of analysed cases b. Single CHS more efficient in 88% of analysed cases iii. Fabrication and erection costs metric a. Crossed-Angle more efficient in 90.4% of analysed cases b. Single CHS more efficient in 88.5% of analysed cases Inversions of the efficiency parameter findings, with RCs more efficient than CCs, were observed when: i. RC CHS member slenderness ratios were less than 80-90. ii. CC design loads were greater than 225 kN, 1200 kN and 1500 kN for mass, raw materials and total cost efficiency metrics, respectively
  • Thumbnail Image
    Item
    Water for Firefighting in Sol Plaatje Municipality, Northern Cape, South Africa
    (University of the Witwatersrand, Johannesburg, 2023) Thage, Tumelo MacAurthur; Ilemobade, Adesola
    In South Africa, SANS 10090 (SABS, 2018a) and the Red Book (DHS & CSIR, 2019) recommend that for firefighting municipal water distribution systems must have and maintain the capacity to provide water for firefighting purposes. van Zyl & Haarhoff (2007) state that the provision of water for firefighting is a dominant design consideration as it influences the sizing and outputs of critical components. Scheepers (2012) argues that fire flows acts ‘as the most limiting demand condition’ as when it required it is extracted in large quantities for a short period. The most commonly used fire flow values in the SANS 10090 have largely remained unchanged for several decades. In other words, these values have not been revised to take account of the evolution of municipal water consumption over more than 30 years. This suggests that the recommended water requirements for firefighting may no longer be fit for purpose. The literature review revealed that the fire flows in the Red book violates the same in the SANS 10090 - this is illegal as the SANS 10090 specifies enforceable absolute limits. Furthermore, the Red book and SANS 10090 provide different fire risk classifications and values for the different parameters and as such, inconsistent and lack uniformity. It is important for design engineers to have accurate input data when planning and designing for fire flow requirements as any deficiency in basic design information could lead to an insufficient capacity to fight fires or an over-design of water supply infrastructure. A question arose from these observations, which provided inspiration for this study: Are fire flows in the SANS 10090 and Red book appropriate for current firefighting efforts? In answering this question, the first step of this study was to identify participating municipalities that were willing to release their datasets on firefighting for this study. Sol Plaatje municipality was one of the willing municipalities. This study thereafter categorised and analysed information contained in the 3236 fire incident reports that occurred within Sol Plaatje Municipality during the period 21 July 2017 to 21 August 2020, and compared actual fire flow volumes and flow rate data against the SANS 10090, Red book and previous South African studies. In order to provide context and aid better understanding of the datasets, structured interviews were conducted with municipal officials responsible for firefighting operations at Sol Plaatje municipality. Key highlights of this study indicated: ∼ 93.3% of fires in Sol Plaatje municipality were extinguished using 7 Kℓ or less of water. This result is similar to previous studies- In a 2014 study more than 90% of fires in 5 Western Cape Towns were extinguished using less than 10 Kℓ of water; In a 2019 study, 75% of fires in the City of Johannesburg were extinguished using less than 6.60 Kℓ of water; and In a 2022 study, 87% of fires in the City of Johannesburg were extinguished using 10 Kℓ or less of water. Overall, 75%-93% of fires were extinguished in the various study areas using 10 Kℓ or less of water. For large fires in Sol Plaatje municipality, the average water volume used to extinguish 85 large Category 2 fires was 8.56 Kℓ or less of water. This finding is similar to that of the Western Cape 2014 study and the 2022 City of Johannesburg study that found that 8.6 Kℓ and 9.63 Kℓ or less of water extinguished 77 and 89 large category 2 fires, respectively. What is evident from the results is that the volumes of water used in Sol Plaatje and previous studies are significantly less that the specified values in SANS 10090 and the Red book. An adverse consequence of higher values is that it leads to an over-design of water supply infrastructure as it increases the volume of municipal storage required for firefighting and consequently, increases the total capacity of municipal storage. Increased storage capacity increases resources that are expended, as well as water retention times, which negatively affects water quality. This study validates the recommendations of previous studies that the fire flows in the SANS 10090 and Red book need to be revised to enable the efficient conservation of scarce water resources and optimal design of water systems.
  • Thumbnail Image
    Item
    Passive Vibration Isolation Systems Integrated with Dynamic Vibration Absorber
    (University of the Witwatersrand, Johannesburg, 2023-02) Thakadu, Katlego; Li, Kuinian
    Isolating vibration using an isolator and/or vibration absorber are the two commonly used measures of vibration control in practice. The presented investigation explores multiple passive vibration isolator configurations, integrated with a dynamic vibration absorber, to improve the performance of vibration isolating. In the first section, A Passive vibration Isolator Integrated with a Skyhook and Groundhook Vibrational Absorber is investigated. The dynamic behaviour of the proposed isolators with the lumped mass, fixed to an inertial reference, is investigated using force, and displacement transmissibility equations. The force and displacement transmissibility equations are derived by modelling the dynamic system as a two-degree-of-freedom system. Derivations for both configurations of the isolator, integrated with a skyhook and groundhook are presented, including numerical simulations and parameter sensitivity analyses. The analysis conducted, indicates that both configurations, is indeed a passive linear isolator, which can support a static load whilst achieving significant vibrational isolation, employing a relatively low damping element. Additionally, the practical applications of the proposed configurations are plausible with the currently available material, which makes the research viable within the field of engineering. In the second section, A Passive vibration Isolator Integrated a Dynamic Vibration Absorber with Negative Stiffness Spring is investigated. The dynamic behaviour of the isolator supporting a lumped mass, is investigated using force and displacement transmissibility equations of the isolator. The force and displacement transmissibility are derived by modelling the dynamic system as a two-degree-of-freedom system. A unique constraint on the negative stiffness ratio, α, for the isolator’s stable operation, is developed. The optimal design of such an isolator was demonstrated with a design example. Extensive numerical simulation and parameter studies on the isolator were performed, which revealed attractive dynamic characteristics of the isolator. It is a passive linear isolator, without any non-linear elements, spring or damper. However, it can bear a large static load and, at the same time, achieve a greatly increased vibration isolation. These two effects are considered mutually exclusive in linear isolator and can be overcome in some extent by properly configured nonlinear isolators only. Moreover, numerical simulation and parameters study shows that the negative stiffness ratio and the mass ratio required can be very small, which makes the implementation of such an isolator in practice convenient. All these make the isolator attractive for engineering application. The third section of this research is on the optimal design of a two-stage isolator, a isolator integrated with another isolator. Performance of a passive two-stage vibration isolator and its design was rigorously evaluated and recorded in this study. It is revealed that the vibration isolating performance of a linear passive two-stage vibration isolator depends on the configurational parameters of the isolator. The transmissibility, a non-dimensional parameter used to quantify the effectiveness of an isolation system, of the passive two-stage vibration isolator was derived. A numerical optimization on the transmissibility of the isolator was developed and performed using the Minimax algorithm. It is demonstrated that an optimally designed linear passive two-stage vibration isolator produces significantly better isolating than a single-stage linear isolator at all frequencies, high and low. The proposed linear passive two-stage isolator is not only effective against high-frequency isolation, which used to be the primary use of a passive two-stage isolator but shows outstanding isolation performance at all frequencies. The proposed linear passive two-stage vibration isolator outperforms many complicated nonlinear isolators, including both single-stage or two-stage isolators. Parameter selection, dynamic performance driven optimal design and potential application of the proposed isolator were also investigated and presented.