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- Volume 55, Issue 1, 2013
Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese - Volume 55, Issue 1, 2013
Volumes & issues
Volume 55, Issue 1, 2013
Estimating car ownership and transport energy consumption : a disaggregate study in Nelson Mandela Bay : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 2 –10 (2013)More Less
This paper investigates energy consumption patterns by households and individuals during travel on a typical day. A methodology is developed to estimate trip-by-trip energy consumption using standard 24-hour travel survey data, and applied to the Nelson Mandela Metropolitan Area using their 2004 household travel survey. Baseline energy consumption patterns by different modes, times of day, and user groups are established. Across the population, energy use is very skewed: 20% of people consume about 80% of transport energy, mainly due to the disproportional contribution of car use to energy expenditure. We then estimate a disaggregate vehicle ownership model and link it to a model of household transport energy consumption to explore the underlying socio-economic and land use variables driving energy consumption. Land use factors (especially job accessibility) significantly affect energy use, but do so differently for low and for high-income households, suggesting that accessibility enhancing land use and transport measures could have unintended consequences for overall energy and environmental management.
Experimental and numerical investigation of the natural frequencies of the composite profiled steel sheet dry board (PSSDB) system : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 11 –21 (2013)More Less
This paper investigates the natural frequencies of the profiled steel sheet dry board (PSSDB) system. Frequency response functions (FRFs), estimated experimentally, were used to determine the natural frequencies of three different PSSDB panels with different screw spacing. Finite element models (FEMs) were developed to predict the natural frequencies of the tested panels. The FEMs were verified by comparing their results with results of the experimental test, and these confirmed the natural frequencies of the system. The effect of screw spacing on the natural frequencies of the system was studied experimentally and numerically. The numerical results uncovered the effect of various parameters, such as the PSS and DB thicknesses and boundary conditions, on the fundamental natural frequency (FNF) of the system. Fifteen finite element models were developed to determine the FNF of the PSSDB system with practical dimensions. When applied as a flooring system these panels are categorised as low-frequency floor (LFF) or high-frequency floor (HFF), to determine occurrence of resonance, design criteria, and whether or not they would be comfortable for humans.
Improving water quality in stormwater & river systems : an approach for determining resources : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 22 –35 (2013)More Less
This paper is a showcase of the approach used to determine the additional resources required to improve inland water quality in the City of Cape Town to an acceptable level. As the improvement of water quality falls in the more complex realm of modern municipal engineering - where many of the issues are so-called "soft" in nature and the problems and solutions are not straightforward â?? the methods discussed in this paper were instrumental in creating an holistic overview of the state of the rivers and wetlands in the City of Cape Town, highlighting the complexity of the problem and assisting to plot a way forward to provide proactive, sustainable measures for the management of water pollution. The paper discusses: the evaluation of water quality data, catchment analysis and determination of pollution sources, a risk assessment, and a prioritisation exercise, and concludes with the novel points and obstacles encountered. In all, the methods discussed provide a significant contribution towards the quest to improve water quality in the City of Cape Town.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 36 –44 (2013)More Less
While agricultural producers on commercially operated irrigation schemes will aim to achieve the recommended high crop yields, those on a smallholder irrigation scheme usually produce moderate to low crop yields. The water demand by these two irrigator types also differs and is reflected in the variations in crop yields.
Because smallholder irrigators produce lower crop yields and use less water, they should use a system suited to this lower water demand. Many irrigation schemes have the opportunity for participants to assess their farming objectives and models. The irrigators can then use the assessment results to determine their water demands, reduce their infrastructure capacity and reduce their capital, operation and maintenance costs.
On many smallholder schemes, the system has been designed for commercial crop yields and water use. If smallholders never achieve commercial levels of production, they have over capitalised and subjected themselves to additional operational strain.
In this study, six irrigation schemes based in the Eastern Cape were evaluated according to three levels of irrigation supply: a commercial irrigator, a smallholder irrigator and the commercial under-utilised irrigator. The irrigation infrastructure for each of the six schemes was designed, and the associated costs determined, for each level of supply. The study investigates the impact of different designs on the amount of water and land used, and resultant costs of the infrastructure. The results show that a smallholder irrigator using a scheme sized for commercial operation can have significantly higher (between 5% and 29%) annual operation and maintenance costs. The study clearly shows that the farmer type should be considered when designing each irrigation scheme.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 45 –59 (2013)More Less
This paper presents a model for the drying shrinkage of South African concretes, developed from laboratory data generated over the last 30 years. The model, referred to as the WITS model, is aimed at identifying the material variables that are the most important predictors of both the magnitude and rate of concrete shrinkage. In comparison with several shrinkage models already in use in the South African concrete industry, namely the SANS 10100-1, ACI 209R-92, RILEM B3, CEB MC90-99 and GL2000 models, the WITS model exhibited the best performance across a range of goodness-of-fit criteria. The ACI 209R-92 model and the RILEM B3 model showed reasonably good prediction. However, since the B3 model could be used to predict just over two-thirds of the data set, it was thus arguably the best alternative to the WITS model for the South African data set. The SANS 10100-1 model performed poorly in its predictive ability at early drying times. This may indicate that its 30-year predictions are more suited to the South African data set than its six-month predictions.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 60 –71 (2013)More Less
The paper presents resistance statistics required for reliability assessment and calibration of limit state design procedures for pile design reflecting southern African practice. The first step of such a development is to determine the levels of reliability implicitly provided for in present design procedures based on working stress design. Such an assessment is presented in an accompanying paper (please turn to page 72). The statistics are presented in terms of a model factor M representing the ratio of pile resistance interpreted from pile load tests to its prediction based on the static pile formula. A dataset of 174 cases serves as sample set for the statistical analysis. The statistical characterisation comprises outliers detection and correction of erroneous values, using the corrected data to compute the sample moments (mean, standard deviation, skewness and kurtosis) needed in reliability analysis. The analyses demonstrate that driven piles depict higher variability compared to bored piles, irrespective of materials type. In addition to the above statistics, reliability analysis requires the theoretical probability distribution for the random variable under consideration. Accordingly it is demonstrated that the log normal distribution is a valid theoretical model for the model factor. Another key asis for reliability theory is the notion of randomness of the basic variables. To verify that the variation in the model factor is not explainable by deterministic variations in the database, an investigation of correlation of the model factor with underlying pile design parameters is carried out. It is shown that such correlation is generally weak.
Pile design practice in southern Africa : part 2 : implicit reliability of existing practice : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 72 –79 (2013)More Less
Limit state design has become the basis of geotechnical design codes worldwide. With the semi-probabilistic limit state design approach, load and resistance factors of (deterministic) design functions are calibrated on the basis of reliability theory. The calibration is done to obtain procedures that will ensure that a target level of reliability is exceeded under the design conditions within the scope of the design function. This is conventionally expressed in terms of the reliability index (β), which is related to the probability of failure (Pf). Acceptable existing design practice is an important source of information on appropriate levels of reliability.
This paper uses the results from a pile load test database to evaluate the reliability levels implied in the current South African pile design approach. The results of the analysis indicate that the reliability index values for ultimate limit state failure of single piles implicit to present design practice vary with the pile class. However, the influence of the probability model applied is more significant. Based on conventional and standardised procedures for reliability analysis, a representative implicit reliability index value βl,Rep 3.5 is obtained, corresponding to a probability of failure Pf = 2.10-4.The values for various sets of pile conditions range from βl = 3.1 (Pf = 1.10-3) to βl = 4.3 (Pf = 1.10-5). This compares well with target levels of reliability for structural and geotechnical performance of βT = 3.0 as set in SANS 10160-1:2011 Part 1 Basis of structural design.
These indicative results provide a useful reference base to establish the reliability of existing and therefore acceptable South African pile design practice. It could also be interpreted as indicative of geotechnical design practice in general. The standard SANS 10160-5:2011 Part 5 Basis for geotechnical design and actions provides the framework for future calibration investigations.
Optimising dosage of Lytag used as coarse aggregate in lightweight aggregate concretes : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 80 –84 (2013)More Less
Lytag, manufactured first by pelletisation of a mixture of fly ash, bentonite and water, and then by sintering the spherical pellets at about 1 300℃, is used as coarse aggregate for producing lightweight plain and structural concrete mixtures. The weight of lightweight concretes is reduced significantly without compromising the structural integrity. The reduced dead load results in significant savings in the cost of foundations and reinforcement, as well as reduction in the sizes of columns, beams and slabs, which in turn reduce the overall volume of concrete and the costs of form work and scaffolding. This paper reports on the results of an experimental study which consisted of designing, preparing and testing different mixtures of lightweight aggregate concrete considering four dosages of Lytag, used as coarse aggregate. It was found that the density and workability of concrete mixtures significantly decreased with increase in the dosage of Lytag. Concrete mixtures containing Lytag were found to be stronger than normal weight concrete. However, the strength of the lightweight aggregate concrete is found to be maximum at an optimum dosage of the Lytag.
Author S.W. JacobszSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 85 –93 (2013)More Less
This paper describes a physical model of a soil nail retained excavation face which was tested in the new geotechnical centrifuge at the University of Pretoria. As centrifuge modelling is new in South Africa, a short introduction to this technique is presented.
The mobilisation of soil nail forces and their maximum values in response to excavation in the model were compared to measurements recently made in an instrumented 10m high soil nail retaining structure for the Gautrain system in Pretoria. Results were also compared to predictions made using a simple failure wedge analysis and a database of eleven full-scale instrumented soil nail walls from the literature. The centrifuge model data compared well with both full-scale situations and theoretical analyses. The results suggest that soil nail forces measured in the centrifuge are conservative due to the mobilisation of a portion of the shear strength of the model soil during the acceleration of the centrifuge, leaving less un-mobilised shear strength available to resist loads resulting from the excavation.
2D Linear Galerkin finite volume analysis of thermal stresses during sequential layer settings of mass concrete considering contact interface and variations of material properties : part 1: thermal analysis : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 94 –101 (2013)More Less
In this research, a new explicit 2D numerical solution is presented to compute the temperature field which is caused due to hydration and thermal conductivity by the Galerkin finite volume method on unstructured meshes of triangular elements. The concrete thermal properties vary, based on the temperature variation and the age of the concrete in the developed model. A novel method for imposing natural boundary conditions is introduced that is suitable for the Galerkin finite volume method solution on unstructured meshes of triangular elements. In addition, the thermal contact is considered at the concrete-rock foundation interface to achieve more realistic simulations in this section. In this work we present the comparison of the thermal analysis numerical results of a plane wall, which had different thermal boundary conditions applied to its edges, with its analytical solution to assess the accuracy and efficiency of the developed model. The applicability of the developed numerical algorithm for thermal analysis is presented by the solution of thermal fields during gradual construction of a typical mass concrete structure.
2D Linear Galerkin finite volume analysis of thermals tresses during sequential layer settings of mass concrete considering contact interface and variations of material properties : part 2 : stress Analysis : technical paperSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 104 –113 (2013)More Less
In this research, a 2D matrix-free Galerkin finite volume method on the unstructured meshes of triangular elements is utilised to compute thermal stress fields resulting from the predefined transient temperature distribution in a mass concrete structure (dam wall). In the developed numerical model, the convergence of the force equilibrium equations are achieved via some iterative solutions for each given computed temperature field. Since the mechanical properties of concrete may vary over time due to concrete ageing, the presented numerical model considers the variation of mechanical properties corresponding to the degree of concrete hydration and concrete temperature. In addition, the geometry of the dam wall and foundation is not considered integrated any longer, so the mechanical contact is considered at concrete rock foundation interface to achieve more realistic simulations of the strain-stress fields in this part. In this work we present the comparison of thermal stress analysis numerical results (of a clamped plane which is exposed to constant temperature) with the results of finite element based LGOR software to assess the accuracy and efficiency of the developed model, and prove that the results correlate well. As an application of the developed model for a real-world problem, thermal stress analysis of a mass concrete structure which is gradually constructed on a natural foundation is performed with regard to variable mechanical properties.
Source: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55, pp 114 –116 (2013)More Less
The effects of placement conditions on the quality of concrete in large-diameter bored piles : discussionSource: Journal of the South African Institution of Civil Engineering = Joernaal van die Suid-Afrikaanse Instituut van Siviele Ingenieurswese 55 (2013)More Less