Browsing by Subject "Structural Engineering"

Now showing 1 - 11 of 11
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    Open Access
    Alternative patch repair materials for rebar corrosion damage
    (2017) Jassa, Primesh; Beushausen, Hans-Dieter; Ngassam, Ines Tchetgnia
    Reinforced concrete (RC) is extensively used in the construction industry. It is particularly used to guarantee that infrastructure assets around the world last for multiple years whilst ensuring that the structural integrity and serviceability of the structure is maintained. However, in practice countless RC constructions are failing prematurely due to a large number of factors of which the corrosion of steel embedded within concrete is the most significant (Matthews et al., 2003). Steel corrosion is particularly pernicious to concrete due to the expansive nature of the corrosion by-products formed, which commonly leads to cracking and spalling. One of the most common methods adopted in the rehabilitation of corrosion damaged concrete is the patch repair procedure. However, in practice this method has shown to often be unreliable as a consequence of the widespread occurrence of shrinkage cracking and poor substrate-patch adhesion leading to debonding of the patch repair. From a practical point of view, such failed repair systems essentially restore the repaired concrete back to a deteriorated state. The underlying cause of poor durability in patch repairs is attributed to a range of reasons including, the lack of understanding of the substrate-patch composite system and the limited availability of appropriate design standards. Furthermore, there is a lack of understanding in the repair industry on the critical material properties actually required for durable patch repairs. There is a common belief that repairing concrete with specialised proprietary repair materials would guarantee durability. However the widespread premature failure of patch repairs conducted using such materials has proven the contrary. A proper patch repair process includes treatment of the corroded steel, adequate substrate surface preparation, installing sacrificial anodes (at least for chloride contaminated concrete) and surface coating. In principle, if this process is correctly followed then the material requirements for a durable, non-structural repair would be to fill in the cavity created by removing contaminated concrete, resist shrinkage induced cracking and/or debonding and provide protection against chloride ingress (in chloride environments). The material used for patch repairs could be any appropriate repair material and it does not specifically need to be a specialised cementitious repair mortar. This dissertation presents an understanding of the materials and issues concerning the durability and serviceability of patch repairs, with the aim of identifying alternative non-structural patch repair materials for the durable repair of corrosion-damaged concrete structures. The potential patch repair materials studied in this dissertation were rubberised waterproofing bitumen, polymer (copolymer of vinyl acetate and ethylene) with 5% cement replacement and 60%, 80% and 100% fly ash (FA) mortar. Patch repairs were conducted on substrate moulds to test application and observe cracking/debonding occurrence. Furthermore, compressive strength, durability index, accelerated drying shrinkage, restrained shrinkage, workability and SEM tests were conducted. It was concluded that the 60% FA repair material had the best overall performance with the polymer-cement concrete exhibiting good bonding and crack resistance properties. This research established that innovative alternative repair materials such as a 60% FA or polymer-cement concrete material, can be developed for non-structural patch repairs with improved long-term performance relative to conventional materials. The research has further provided a foundation for the development and design of durable repair mortars by identifying the principal material performance properties required of such materials.
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    Open Access
    An investigation into the effects of early propping removal on the deflection of reinforced concrete beams
    (2018) Rockstroh, Benjamin Andreas; Beushausen, Hans
    In today’s fast paced construction industry, there is an ever present need to increase productivity and to complete projects as quickly as possible. Reinforced concrete is a popular and widely used construction material. However it has the unfortunate drawback in that the concrete requires time to set and gain sufficient strength before loads may be applied and the formwork and props can be removed. It is therefore desirable to keep propping times to a minimum. If the propping is removed too early, there is a risk of the member deflecting excessively and exceeding the maximum allowable limits, or in severe cases it could even lead to a structural failure or collapse. The SANS 2001 code provides recommended propping times for beams and slabs, which can be used as a guideline by building contractors and structural designers. These propping times present a universal approach, which does not consider all the factors that affect deflection. This simplified approach may be considered to be conservative as shorter propping durations could be possible without a loss in performance. The aim of this dissertation is to look into the effects of early propping removal on the longterm deflections of concrete members. This was done by modelling the deflection of a typical reinforced concrete beam at different ages of loading, using three code-based deflection calculation methods. The codes that were used are the South African National Standard (SANS), Eurocode (EC2) and American Concrete Institute code (ACI 318). A detailed literature-based investigation was conducted to determine the factors which affect deflection in reinforced concrete members, as well as the theory behind the code-based deflection calculation procedures. This was followed by the modelling of deflections using the abovementioned methods. Three case studies were performed to determine the effects of early propping removal under different scenarios. The first case study only deals with the effects of early age loading on long-term deflection. As an added point of interest, two different concrete mixes were used, made with two different types of cement. The second case study compares the effect that different levels of relative humidity have on the long term deflection at early ages of loading. Lastly, the effects of concrete strength on long-term deflections at early ages of loading was modelled. The results of the first case study indicated that a reduction in propping time is possible without causing excessive deflections. In the second and third case study is was observed that both the relative humidity and concrete strength respectively have an effect on the long term deflection and therefore also influence the propping time. The study concluded that based on the obtained estimated deflection values using the codebased methods, the propping times provided in the SANS 2001 code may in certain applications be conservative. According to the results obtained from the code-based deflection calculation procedures, it is possible to reduce the propping duration. It was suggested that an alternative method should be developed which would allow structural designers to determine the required propping time more accurately.
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    Open Access
    Fatigue Behaviour of CFRP Strengthened Reinforced Concrete Beams
    (2020) James, Valontino Ruwhellon; Moyo, Pilate
    The performance of reinforced concrete (RC) structures, such as bridges in the heavy haul industry, may be severely impacted by fatigue when subjected to repeated cyclic loading. Fatigue not only reduces load carrying capacity and serviceability limit states (SLS), but it can cause structural failure even when the components are subjected to low stress range cyclic loading. Corrosion damage exacerbates fatigue related problems as chloride induced pitting corrosion facilitates the formation and gradual propagation of cracks under cyclic loading. A common rehabilitation and retrofitting approach that involves patch repairing and fibre reinforced polymer (FRP) strengthening has proven effective to not only restore structural capacity, but also to enhance infrastructure service life. The structural repair process involves the replacement of deteriorated cover concrete with a less permeable patch repair mortar. The patch repair only restores durability of the structure; to restore or enhance structural capacity the repair process further involves bonding of FRP laminates. Particularly in the case of FRP's with a low elastic modulus, the design is often guided by serviceability limit states as opposed to ultimate limit states (ULS), resulting in an over-reinforced structural member. In addition, the reinforcement area of commercially available FRP strengthening may exceed the design requirements, especially at low levels of corrosion damage. In both the abovementioned considerations the design may result in an over-reinforced section. At the time when this researched was proposed, the effect of increasing damage extent on fatigue behaviour of over-reinforced RC beams was not clear and merited further investigation. A scientific experimental approach was developed to investigate the long-term performance of fifteen (15) full-scale 40MPa RC beams with dimensions 155x254x2000mm and ultimate capacity of 62.3kNm. Accelerated corrosion damage was induced in varied extents which included 450mm, 800mm, 1300mm and 1800mm length to a constant degree of 10% on all specimens. Specimens from each damage extent were patch repaired using SikaCrete214 and subsequently strengthened with externally bonded with SikaCarboDurS512 carbon fibre reinforced polymer (CFRP) laminates. Four-point bending monotonic loading tests were conducted on one (1) specimen from each damage extent. The results obtained from the quasi-static tests were used to determine two (2) cyclic loading stress ranges at which the remaining 2 specimens from each damage extent would be tested under. Under the 40% and 60% stress ranges four-point bending cyclic loading tests were carried out at a test frequency of 4Hz. Information was acquired on key performance indicators that included fatigue life, crack development, failure mode and stiffness degradation, where stiffness was assessed in terms of midspan deflection, composite material strains and neutral axis shift. Information on these parameters were collected using strain gauges, linear variable differential transducers (LVDT), DEMEC strain targets and digital image correlation (DIC). Ultimate failure loads under monotonic loading showed that despite having the highest degree of corrosion, the 450mm damage extent specimen had the highest failure load of 325kN. The failure load gradually reduced to 290kN as the damage extent was increased to 1800mm and the 0mm (control) specimen failed at the lowest load of 274kN. In contrast to the static behaviour, the specimen fatigue life enhanced by 106.3% as the damage extent was increased from 450mm to 1800mm. As expected, the 40% stress range tests yielded much longer fatigue lives than their 60% stress range counterparts. Furthermore, the experimentally obtained fatigue lives were compared to three fatigue life prediction models and the Helgason and Hanson model yielded the closest correlation with the experimental results. IV ABSTRACT Crack densities were found to increase with a longer fatigue life. An increase in damage extent was found to positively affect crack development and overall stiffness of the specimen during longterm fatigue testing. This finding was further substantiated by an assessment of midspan deflection, compression concrete strain and carbon fibre strain results, which all suggested a lower neutral axis and a lower stiffness reduction rate under fatigue loading as the damage extent was increased from 450mm to 1800mm. Furthermore, the tension concrete cracks propagated gradually during longer fatigue tests periods, while the tension steel and carbon fibre were comparably less affected by the resultant internal forces. Unfortunately, the neutral axis strain measurements using DEMEC targets were unable to assess the relative effect of an increase in damage extent as well as the compression concrete and carbon fibre strains were able to. During this experimental period, it was established that the laboratory layout was not conducive for carrying out the DIC process of long-term cyclic loading tests. The area in which testing took place did not adequately protect the camera against the environment and therefore required daily storage of the equipment. Regular movement of the camera for storage purposes introduced measurement inaccuracies which accumulated over longer test periods of up 20 days. However, for the short-term tests that did not require movement of the camera, the DIC process yielded favourable results. It was possible to capture the crack patterns early in the test period when the crack growth rate and development of new cracks was high using DIC. It was found that the high strain cracks coincided with the points of maximum vertical deflection (obtained through DIC) and eventual failure location of the specimen. The points of maximum deflection obtained from the DIC process were often not at midspan, which in the absence of the DIC process, would not have been possible to predict accurately. The results have shown that the specimens with the longer damage extents exhibit improved fatigue performance than their shorter counterparts. This revealed a stark contrast to their monotonic loading performance which favoured shorter damage extents. Furthermore, DIC holds potential to predict failure location more accurately than conventional approaches used for structural health monitoring (SHS).
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    Open Access
    Influence of geometry on the dynamic behaviour of steel tubular towers for onshore wind turbines
    (2017) Folster, Kaylee; Mudenda, Kenny; Zingoni, Alphose
    South Africa has recently experienced challenges regarding electricity consumption and availability. As part of the country's Integrated Resource Plan, these challenges are to be addressed. This involves a 20 year plan which aims to increase electricity supply capacity as well as reduce the reliance on coal power as part of the global trend to become more environmentally friendly. Wind power, specifically, is to account for a large portion of the renewable energy that is expected to become available by 2030. This results in the need for the understanding of wind turbine design by South African engineers. The dynamic analysis of wind turbine structures, is of particular interest to Civil Engineers. Wind turbine towers are recently of the monopole or tubular type tower, predominantly constructed of either concrete or steel or a combination of both. Steel tubular towers above a height of 80m are generally not recommended for wind turbines owing to cost concerns as well as difficulties in meeting dynamic behaviour requirements. Concrete towers and steel-concrete hybrid towers are recommended for this height regime. The aim of this study was to assess the prospective use of steel tubular towers of varying geometric shape for wind turbines with tower heights of 80m or greater. The study focussed on the analysis of natural frequency and assessing the applicability of steel tubular towers of geometric shapes that have not been previously explored or reported. The turbine of choice for this study was the Vestas V112 3MW type as this is one of the most commonly used and more efficient turbines for towers of this height regime. The results of this study showed that steel monopole towers of heights of 80m and more are still viable options for wind turbine towers. Various geometric tower cases of heights varying from 80m to 120m, produced acceptable fundamental natural frequencies within the allowable frequency range for a Vestas V112 3MW turbine.
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    A numerical study of the suitability of rigid inclusion ground reinforcement beneath caisson quay walls
    (2017) Holmwood, Andrew Graham; Kalumba, Denis
    The objective of this study was to determine whether rigid inclusions are suitable for reinforcement of the foundation of a caisson quay wall functioning as a container terminal. Apart from their brittle behaviour under lateral loading, rigid inclusions are well suited to the large uniform loads and stringent post-construction deflection tolerances associated with container terminal structures. Their inherent strength and stiffness means they have certain advantages over other stiffening columns commonly used for ground reinforcement in port expansion projects. Their mechanical properties allow construction to unrestricted heights at any construction rate and, in theory, RIs can be applied to all soil types. Additionally the locations of many ports coincide with rivers, deltas and estuaries which are associated with poor soil conditions often requiring ground improvement. Their suitability is of practical significance to port planners and engineers who are faced with the challenge of providing satisfactory foundation performance that is cost effective. The addition of RI ground reinforcement for this structural application would allow for greater flexibility in meeting these challenges. The literature review for this study was broad in its scope with emphasis placed on describing the mechanics of the problem, analysis methods and suitable installation methods for execution in the marine environment. One of the key outcomes of the literature review was identifying the problem of lateral loading due to "free-field" lateral ground movements. In light of this, suitable strategies for limiting and accommodating lateral loading of the RIs were proposed. A numerical study of the proposed ground improvement scheme was undertaken using the 3D finite element method. The key model outputs were caisson deflections and RI forces, moments and stresses, for the various simulated construction phases up to operational conditions. The model results were assessed in terms of the key foundation performance criteria which were related to STS crane rail tolerances and limiting tensile stresses in the RIs. This study found that for a firm clay subsoil condition the proposed RI ground reinforcement scheme met the foundation performance criteria for this structural application provided (i) strategies to limit lateral loading were implemented and (ii) the RIs were reinforced over the length where they were not fully compressed. While this study provided insights into the behaviour of RIs for this structural application, ultimately suitability is a function of range of factors, in addition to the limited technical performance criteria derived for this study.
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    Open Access
    A parametric investigation into the membrane stresses of hydrostatically loaded circular and elliptic toroidal shells
    (2017) Govender, Nishalin; Zingoni, Alphose
    This study explores the membrane stresses of hydrostatically loaded elliptical and circular toroidal tanks. Equations are derived, using the membrane theory of shells, to obtain equations which can accurately describe the meridional and hoop stress behaviour at locations sufficiently far away from any bending disturbance occurring within the shell. The derived expressions are validated using the finite element software ADINA, indicating excellent agreement between the analytical and numerical solutions. A parametric study is undertaken, whereby the membrane profiles for prolate, oblate and circular toroidal shells is investigated. Parameters which are varied are the opening and aspect ratio of toroidal shells. Stress resultant profiles are shown for numerous cases in order to aid designers on suitable ratios to minimise membrane stresses for use when designing hydrostatically loaded toroidal shells. Lastly, numerical examples are investigated, keeping the volume constant and comparing the surface area due to a variation of opening and aspect ratios. It was found that when investigating toroidal shells, considerations are required when choosing the aspect ratio and opening ratios. Based on the results obtained, compromises between prolate and circular cross-sections with relatively small opening ratios are recommended in order to minimise the cost and maximise the structural efficiency, based on the membrane stresses occurring within the shell.
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    Open Access
    Parametric study of stiffened steel containment shell structures
    (2008) Masendeke, Rugare B; Zingoni, Alphose
    A FEM-based parametric study is undertaken to investigate the buckling behavior of meridionally and circumferentially stiffened steel cylindrical and conical shell frustum subjected to different load cases. This situation arises in different steel shell applications such as storage vessels (liquid, solid and gas) and in certain configurations of industrial process facilities. The stiffeners are flat strips of rectangular section welded on to the outer surface of the shell, either over the whole length of the shell meridian or around the circumference of the shell. It is required to establish how the elastic buckling load and mode shapes vary with respect to certain key parameters of the problem. The parameters of interest in the study include the number of stiffeners around the shell circumference and along the meridian, the stiffener-depth to shell-thickness ratio, and the stiffener depth-to-width ratio. This thesis reports the findings of the parametric study and also presents some results of experimental tests on laboratory small-scale models of stiffened cylindrical and conical frusta.
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    A Proper Orthogonal Decomposition-based inverse material parameter optimization method with applications to cardiac mechanics
    (2016) Moodley, Kamlin; Skatulla, Sebastian
    We are currently witnessing the advent of a revolutionary new tool for biomedical research. Complex mathematical models of "living cells" are being arranged into representative tissue assemblies and utilized to produce models of integrated tissue and organ function. This enables more sophisticated simulation tools that allows for greater insight into disease and guide the development of modern therapies. The development of realistic computer models of mechanical behaviour for soft biological tissues, such as cardiac tissue, is dependent on the formulation of appropriate constitutive laws and accurate identification of their material parameters. The main focus of this contribution is to investigate a Proper Orthogonal Decomposition with Interpolation (PODI) based method for inverse material parameter optimization in the field of cardiac mechanics. Material parameters are calibrated for a left ventricular and bi-ventricular human heart model during the diastolic filling phase. The calibration method combines a MATLAB-based Levenberg Marquardt algorithm with the in-house PODIbased software ORION. The calibration results are then compared against the full-order solution which is obtained using an in-house code based on the element-free Galerkin method, which is assumed to be the exact solution. The results obtained from this novel calibration method demonstrate that PODI provides the means to drastically reduce computation time but at the same time maintain a similar level of accuracy as provided by the conventional approach.
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    The Use of The Sectorial Coordinate Approach to Demonstrate Unique Shear Centre Properties of Nonstandard Monosymmetric Steel Sections
    (2020) Muukua, Mervin Mbakekua; Mudenda, Kenny
    The study looks at how the sectorial coordinate approach can be used to solve the problem of unique shear centre property of non-standard monosymmetric sections. In solving the unique shear centres, the behaviour of the shear centre with reference to the centroid is carefully studied as the geometry of the section is changed. The study shows investigations of the sections which are 152x152x30 UC H-section, the same H-section with 2/3 as well as 1/3 of the bottom flange width and a 203x178x30 T-section. Vertical plates of 8mm thickness are added to the ends of the upper flanges of the sections instigating increments of 12.5mm from 0 to 100mm height. From the computations done, the following is observed: • The difference between the sectorial coordinate approach results and those from Prokon is at most 3.1%. Also, as the end plate heights are increased, the difference in the results increases. • Shear centres change in position with reference to the centroids as a result of the change in geometry of the sections. • For the H-section the shear centre is initially with the centroid at zero plate height. It then moves upward (higher than the centroid) to a certain peak point, then decreases steadily, intersecting the centroid again and eventually ending up being lower than the centroid with upstanding plate height increases. • A similar pattern follows the H-sections with reduced bottom flange widths, with the only difference being that the shear centre is initially higher than the centroid. It slightly increases to a peak then gradually decreases, intersecting the centroid at a certain point and ending up lower than the centroid. • As for the T-section, the shear centre is initially at the highest point (furthest from the centroid) and decreases gradually, intersecting the centroid and ending up lower than the centroid. • The H-section with upstanding plates offers itself as a section that has an envelope (gap) between the shear centre and centroid when the shear centre is above the centroid which is much lesser than the other sections. With the usage of excel spreadsheets, the sectorial coordinate approach is an efficient and accurate method to find shear centres and related section properties.
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    Track-Bridge Interaction Effects in Heavy Haul Railway Viaducts
    (2022) Ngwenya, Mixo; Moyo, Pilate
    When continuously welded rails are placed over a bridge, the track and the bridge interact via the ballast in the case of ballasted track or track slab in the case of non-ballasted track. This interaction, commonly referred to as track-bridge interaction results in force transfer between the track and the bridge. With the demand to increase freight haulage on heavy haul railway lines intensifying to meet mineral export demands, there is a need to understand the manifestation of rail bridge interactions in heavy haul railway bridges. Understanding the manifestation of these forces is critical for the management of the infrastructure during operation. Whilst track-bridge interactions effects design limits in high-speed rail have been documented, to the author's knowledge there has been no documented report that addresses track bridge interactions in the design of new heavy haul railway bridges and the management old heavy haul railway bridges. Resultantly, this study explored the observed patterns of rail forces, longitudinal deck displacements, ambient temperature, concrete temperature and rail temperature on the Olifants River Railway Bridge. Thereafter, the observed patterns were used investigate the effect of rail temperature variation on rail forces and the longitudinal displacement of the deck. Examine the effect of variation in concrete temperature on the longitudinal deck displacement, rail forces and variation in rail temperature as well as the effect of longitudinal deck displacement on rail forces. The effects of the presence of a train on the longitudinal displacement of the deck, rail forces and concrete temperature will also be investigated. Finally, this study developed a predictive multiple linear regression model that will assist in the management and maintenance heavy haul railway bridges. This study demonstrated that rail temperature variation is inversely proportion to the rail forces in the rail, longitudinal deck displacement is directly proportional to concrete temperature variation and that longitudinal deck displacement of the bridge deck and rail forces in the rail are inversely proportional. However, the correlation between the longitudinal deck displacement and the rail temperature, rail temperature in the track and concrete temperature in the deck, concrete temperature in the deck and the rail forces in the track could not be established conclusively. The effect of the presence of the train on the longitudinal displacement of the deck, rail forces and concrete temperature could also not be established conclusively
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    Vibration based assessment of Kalbaskraal rail bridge
    (2017) Hendricks, Moghammad Sameeg; Moyo, Pilate
    The focus on the condition and performance of existing structures has increased due to the growing number of structures approaching or exceeding their design life. The challenges associated with the assessment of existing structures include deterioration, changes in loading conditions, changes in the structures function or the structure reaching the latter portion of its designed service life. In order for authorities to better determine how to deal with existing structures, there must be a coherent means of determining, measuring and benchmarking the current condition and performance of the structure. The current study proposes and demonstrates the integration of a visual based condition assessment with vibration based assessment techniques for railway bridges. The methodology suggests a systematic visual assessment combined with the development of a finite element model which is calibrated by using modal parameters ascertained from vibration based testing. The bridge which was used as a case study was the Kalbaskraal Railway Bridge located in Malmesbury. The proposed methodology consists of the following steps: 1) Initial Assessment 2) Development of a Finite Element Model 3) Detailed assessment and Ambient Vibration Field Testing 4) Analysis of Modal Parameters 5) Calibration of FEM using Modal Parameters 6) Setting up Load Configurations 7) Assessment of structural response 8) Assessment of Serviceability limit state of bridge The overall outcome of the study yielded an effective result in that the conclusions drawn from the outcomes of the methodology correlated well with previous studies on the bridge. The structure under its current operational load of 16ton/axle wagons performed within the allowable serviceability limit state. A proposed increase to 22.5ton/axle loads identified that the bridge would be performing on the boundary or above the allowable serviceability limit state and that retrofitting may have to be considered for the bridge to effectively support the additional load. The results derived from this study can be extremely valuable in the bridge management process as the information on the condition of the bridge can aid bridge authorities in their decision making processes.