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- Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese
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- Volume 56, Issue 3, 2014
Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese - Volume 56, Issue 3, 2014
Volumes & issues
Volume 56, Issue 3, 2014
Conduit-hydropower potential in the City of Tshwane water distribution system : a discussion of potential applications, financial and other benefits : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 2 –13 (2014)More Less
In water distribution networks, water is often fed under gravity from a higher reservoir to another reservoir at a lower level. The residual pressure head at the receiving reservoir is then dissipated through control valves (mechanically or hydraulically actuated), sometimes augmented by orifice plates where there is a propensity for cavitation. There are possibilities to add turbines in parallel and generate hydroelectricity at these locations using the flow and head available.
The benefit of this hydropower generating application is that minimal civil works need to be done, as the control valves are normally inside a control room/valve chamber. No negative environmental or social effects require mitigation, and the anticipated lead times should be short.
From a topographical perspective the City of Tshwane has a lower elevation than the bulk service reservoirs of Rand Water, which is the main water supply. Water is distributed through a large water system that includes 160 reservoirs, 42 water towers, 10 677 km of pipes and more than 260 pressure reducing stations (PRS) that operate at pressures of up to 250 m.
The top ten hydropower potential sites in the City of Tshwane water distribution network have a total energy generating capacity of approximately 10 000 MWh/a. A number of potential conduit-hydropower sites have shown promise of short payback periods. The identifying and development of these sites in Tshwane to convert water pressure to electricity is ongoing and exploited further.
Various challenges currently exist with reservoir communication in isolated areas due to vandalism and theft of necessary infrastructure, including electricity cables and solar panels.
Because conduit-hydropower systems can be housed completely inside chambers, vandalism and theft can be mitigated. Therefore, one of the major benefits of hydropower turbines at these sites is that the hydroelectric potential could be exploited to power telemetry, pressure management, flow control and monitoring/security systems.
Alternatively or additionally, other local demand and/or (depending upon the quantum of energy available) off-site energy demand clusters, or even a municipal or national grid, could also be serviced by these power stations. The capacity of hydroelectric installations can vary to suit the application for the amount of power needed or to be generated.
Short payback periods, especially when using pumps as turbines, also make conduit-hydropower systems attractive.
Evaluating the prioritisation of South African dams for rehabilitation with special focus on risk to human lives : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 14 –24 (2014)More Less
Existing decision criteria for the prioritisation of dam rehabilitation in South Africa are evaluated. In this context risk to human lives and economic considerations are the most important decision drivers, although other considerations are also taken into account by the Department of Water Affairs. The Department's assessment of risk to human lives is compared to international best practice and prediction models to show that the assessment is currently somewhat inconsistent, resulting in over-conservative decisions for low-consequence situations and possibly too low safety levels for severe-consequence situations.
Reduction of the life-safety risks comes at significant costs, which may be better spent elsewhere. Society's Willingness to Pay is used to determine the lower boundary for acceptable investments in life safety. Investments for improved safety could also be made for economic reasons. Economic optimisation would often imply higher safety levels than required by Society's Willingness to Pay. These concepts are applied to case studies of actual South African dam rehabilitation projects, allowing evaluation of the quality of decisions taken.
Based on the above, life-safety criteria that incorporate a measure of the economic efficiency of the proposed rehabilitation are developed, and suggestions are made to improve the current decision criteria used by the Department of Water Affairs.
Author M. OpanSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 25 –29 (2014)More Less
The aim of this study was to determine the effects of safety factors on the deflections of a concrete gravity dam. Overturning and sliding safety factors for a selected concrete gravity dam with base width b and height H were specified, using pseudo analysis for the b/H ratios and earthquake acceleration values between 0.1 g and 0.4 g. Deflection values for specified parameters were obtained from the structural analysis program SAP2000. Deflection safety factor curves, determined from the b/H ratios, were obtained, as well as the earthquake acceleration values. The results of this analysis showed that safety factors reduced while strain values increased.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 30 –33 (2014)More Less
The behaviour of collapsible soil is often quantified by conducting a Collapse Potential Test or double oedometer test. However, these tests interpret the soil behaviour in terms of total stress, while it is well known that the behaviour of soil is governed by its effective stress. This article shows the results of a test conducted on undisturbed residual granite to study the collapse of the soil in terms of effective stress. The test was conducted in a modified oedometer which allowed the incremental addition of water to the sample during the test. The matric suction was quantified as a function of the moisture content of the soil during the test. The results indicate that the reduction in effective stress plays an important role during soil collapse. In addition it was found that much of the collapse settlement may be due to creep which occurs after the reduction in effective stress.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 34 –43 (2014)More Less
A composite Y-shaped bridge was analysed as a three-dimensional structure, using commercially available software. A finite element model to simulate the structural behaviour of the bridge was used to study its static and dynamic behaviour in order to gain a better understanding of the interaction between the main bridge and the ramp. An extensive parametric study was conducted, in which a typical composite Y-shaped bridge model was analysed and tested. The key parameters considered in this study were: loading conditions, ramp radius of curvature, ramp longitudinal slopes, bifurcate diaphragm plate stiffness, and loading cases. The structural behaviour of composite bridges is discussed in detail. The results from this study could enable bridge engineers to design complex composite Y-shaped bridges more reliably and economically.
Effect of activator dosage, water-to-binder-solids ratio, temperature and duration of elevated temperature curing on the compressive strength of alkali-activated fly ash cement pastes : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 44 –52 (2014)More Less
In this paper the effect of sodium oxide concentration, the water-to-binder-solids ratio, temperature, and the duration of elevated temperature curing on the compressive strength of alkali-activated fly ash cement pastes was investigated. Alkali concentration varied between 3% and 15% Na2O of fly ash mass. An increase in Na2O from 3% to 9% greatly improved the compressive strength of the pastes from 26.1 MPa to 50.8 MPa at 28 days. A further increase in Na2O up to 15% did not provide an increase in the strength, but a decrease was observed, as well as higher strength variation. The paste activated with 9% Na2O had the highest strength at 28 days and a low standard deviation, and 9% Na2O was thus considered as the best value in the present study. The temperature and the duration of elevated temperature curing were found to be critical factors affecting the compressive strength at early age, but their effect decreased significantly in the long term. The water-to-binder-solids ratio affected the compressive strength considerably. An increase in the water-to-binder-solids ratio of the pastes from 0.18 to 0.29 resulted in a decrease in the compressive strength from 49.3 MPa to 21.3 MPa.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 53 –64 (2014)More Less
Directional wave spectra provide detailed information about wave climates some of which can be important for coastal vulnerability assessments and design applications. Spectral data is also important for calibrating and validating spectral wave models that are widely used in coastal engineering. This paper discusses directional spectra derived from measurements on the east coast of South Africa (16-year data set). A variety of spreading functions are applied and compared. The Cosine-2s and Gaussian distributions produce similar results and seem to give an appropriate representation of directional spreading at the case study location. The spectra show some seasonal variations, with the broadest directional spreading in summer and the narrowest (from the southeast) in winter. The winter season also has the highest wave energy of the seasons. The spectral data has no clear evidence of cyclone activity contributing significant northeasterly wave energy as has often been conjectured for this location. The occurrence of wave energies above a threshold that causes significant coastal erosion varies seasonally, and mainly comprises long period swell waves linked to distinct weather systems. The analysis and results reported here provide insights for modelling coastal vulnerability and designing coastal infrastructure.
Author S.J. Van VuurenSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 65 –76 (2014)More Less
Eccentric reducers are traditionally recommended for the pump suction reducer fitting to allow for transportation of air through the fitting to the pump. The ability of a concentric reducer to provide an improved approach flow to the pump while still allowing air to be transported through the fitting is investigated. Computational fluid dynamics (CFD) were utilised to analyse six concentric and six eccentric reducer geometries at four different inlet velocities to determine the flow velocity distribution at the inlet to the pump. It was found that eccentric reducers with angles greater or equal to 15° and concentric reducers with an angle greater or equal to 20° did not pass the assessment criteria related to the inlet conditions. Air could be hydraulically transported through all of the concentric reducers modelled except for the 20° concentric reducer. A correctly designed concentric reducer will not only provide a more uniform velocity distribution in comparison to an eccentric reducer, but will allow for the hydraulic transportation of air through the reducer.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 56, pp 77 –87 (2014)More Less
Underspanned suspension bridges are structures with important economical and aesthetic advantages, due to their high structural efficiency. However, road bridges of this typology are still uncommon because of limited knowledge about this structural system. In particular, there remains some uncertainty over the dynamic behaviour of these bridges, due to their extreme lightness. The vibrations produced by vehicles crossing the viaduct are one of the main concerns.
In this work, traffic-induced dynamic effects on this kind of viaduct are addressed by means of vehicle-bridge dynamic interaction models. A finite element method is used for the structure, and multibody dynamic models for the vehicles, while interaction is represented by means of the penalty method. Road roughness is included in this model in such a way that the fact that profiles under left and right tyres are different, but not independent, is taken into account. In addition, free software (PRPgenerator) to generate these profiles is presented in this paper.
The structural dynamic sensitivity of underspanned suspension bridges was found to be considerable, as well as the dynamic amplification factors and deck accelerations. It was also found that vehicle speed has a relevant influence on the results. In addition, the impact of bridge deformation on vehicle vibration was addressed, and the effect on the comfort of vehicle users was shown to be negligible.