Browsing by Author "Mudenda, Kenny"

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    An investigation into the effects of the shifting centroid and shear centre in notched steel beams undergoing elastic lateral torsional buckling
    (2022) Aldera, Gianluca; Mudenda, Kenny
    Notched steel sections have been investigated since the late 1970s and early 1980s. Notched sections are interesting since, for a doubly symmetric member, part of the section is singly or monosymmetric – specifically the notched region – while the un-notched portion may be a standard section. Since the notched region is monosymmetric the centroid and shear centre in this region are at different vertical heights. The design for Lateral Torsional Buckling (LTB) in standard prismatic sections accounts for the monosymmetry through the monosymmetry factor which accounts for the additional twisting moment. However, this is not used in the analysis of notched regions for beams that are laterally unrestrained. These regions are stiffened against local buckling failure using horizontal and vertical stiffeners. The horizontal stiffeners make the notched part appear as an H or I shaped section. Using downstands on the horizontal stiffener the centroid and shear centre in the notched region can be shifted relative to each other – and in some cases - to be at the same vertical height (which is the case for the un-notched sections). This study looks into how the addition of stiffeners in the notched region of a doubly symmetric beam affects the centroid and shear centre location for top notched steel sections undergoing elastic lateral torsional buckling, and how the shifting shear centre relates to the critical elastic moment. A parametric study using a Finite Element Model (FEM) using the Finite Element Analysis (FEA) software Abaqus was used to conduct the study. This research tested 5 steel sections (203x133x25, 305x102x25, 533x210x82, 203x203x46 and 305x305x97) with four different spans (1m,3m and 5m,6m) and two loading cases (unit point load 600mm from notched end and a unit point load at mid span) to investigate the influence of the shifting centroid and shear centre. Initially, 13 stiffening techniques were tested on the 203x133x25 section to identify the five best performing techniques. These were improved, then tested on all five sections. Downstands were applied to the horizontal stiffener, their purpose being to shift the shear centre upwards and past the centroid. With each increase in downstand height the shear centre would approach the centroid and eventually move past it. The critical elastic moment increased with the addition of downstands in almost all cases. The increase was independent of changes to the spans or loading configurations. Changes in the trend were evident as changes in gradient or changes in curve type around the centroid shear centre coincidence point. This shows a link between the centroid, shear centre and the critical elastic moment. Further research should be undertaken on the FHEN and FHHVNTS techniques by applying them to many steel sections to see if the results are consistent for a general case. Care should be taken to allow for downstands far beyond the centroid shear centre coincidence point to see if all sections tend to the same result. Future studies should focus on isolating the changes due to the shear centre centroid movement versus the change in overall geometric properties.
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    Assessing possible economic or operational benefits of class 3 webs without stiffeners vs class 4 webs with stiffeners in steel plate girders
    (2021) Tafadzwa, Chikosha Franklin; Mudenda, Kenny
    The aim of this research was to identify a possible range where the use of deep I-plate girders primarily under bending stresses with thick web plates and no web stiffeners has economical or operational benefits than adoption of plate girder cross-sections with slender web plates reinforced with intermediate transverse stiffeners. The research focuses mainly on optimisation for bending moment resistance. Shear capacities of the cross-sections were also computed and compared because shear forces share a close relationship with bending moments being a derivative of the moment. The idea was that upon exhausting the capacity of the largest available rolled I-section, designers should not always tend to lean towards sections with slender web as is common practice encouraged by design standards to get the highest moment carrying capacity. Design codes or standards adopted by different countries and sometimes regions provide structural designers with guidelines on acceptable design practice. The latest South African design standards encourage the use of thin web plates with stiffeners to guard against premature buckling of the web whilst realising high bending moment resistance. This is due to the increased lever arm and having more material in the flange plates as explained in SASCH (2016). Though setting an upper limit on web slenderness βw, the South African steel design standard, SANS10162-1 allows designers to ignore the limit if calculations prove that buckling of the compression flange into the web will not occur under factored load levels. SASCH (2016) alludes to the possibility of there being benefits that can be realised from using thicker web plates than the theoretically recommended ones but it does not give a comprehensive insight as to the nature and extent of those benefits. As shown in the following chapters, not much research has been done to assess the benefits of adopting compact webs in plate girders. For a given cross-section area, different geometrical cross-section configurations were compared in an iterative process. The parameters under investigation were the moment resistance and shear carrying performance to mass ratios. Other design parameters of the elements were not to be compromised following recommended guidelines in the South African steel design standard, SANS10162-1. Additional knowledge gathered from research work detailed in the literature study was incorporated in the optimisation process but guidelines from SANS10162-1 took precedence. The exercise showed that for cross-section area requirements below 35.6x103mm2 to carry moments, wholly class 3 sections provide a competitive alternative to sections with class 4 webs and flanges below class 3 classification. They resist a lesser moment, but the differences are of small magnitudes. The class 3 sections showed significant improvements in terms of shear capacity. Over larger spans, sections with class 4 webs with intermediate transverse stiffeners and of similar mass per unit length become heavier and more expensive because of the increase in the number of stiffeners. The labour requirements and consumables for fabrication also increase because of the stiffener requirements for sections with class 4 webs as shown in the cost estimates.
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    Continuum Damage Mechanics to Model Glulam Beams Reinforced with Glued-In Steel Cables
    (2023) Rhodes, Aaron; Mudenda, Kenny
    This study aims to investigate the use of Continuum Damage Mechanics (CDM) for the computational modelling of timber, a material known for its high variability and anisotropy. Timber has not had a standard way of performing computational mechanics like that of steel and concrete, which has made engineers hesitant to design with the material. Many other concerns, including durability and fire resistance have also hindered its use in modern construction. However, the benefits of timber construction on the environment are well known, and therefore, there is a need for a reliable and accurate computational model for timber. This study utilizes CDM to model the fibrous portion of timber, similar to Fibre Reinforced Polymers (FRPs), in the hopes of encouraging engineers to build sustainable structures with timber as the main structural material. The material model is implemented in the software ABAQUS using a User-Defined Material UMAT written in Fortran coding. This UMAT allows for the simulation of the nonlinear behavior of timber under different loads and boundary conditions. To validate the model, experimental tests on glulam beams internally reinforced with steel cables were conducted. The beams had varying diameters of steel cables (3mm, 5mm and 8mm) to investigate the effect of the diameter of the cables on the strength of the beams. The results were compared to the results obtained from a similar setup in the ABAQUS/CAE software. The results show that the UMAT closely represents the failure, strength, and displacement readings of the control experimental tests. However, it was unable to accurately represent interactions between the reinforcing steels and timber. Despite the lack of accuracy in the interactions between the reinforcing steels and timber, this research concludes that the UMAT is accurate and suitable for solid timber and glulam beams. The UMAT can be used to model the fibrous portion of timber in a similar way to FRPs, and therefore, it will allow engineering predictions to be made with regards to the strength of SA pine timber. This will lead to more sustainable structures and more confidence in timber as a structural material. The model can be used to design and analyze members with more accurate predictions of the structural behavior of timber. Overall, this research aims to encourage a sustainable future by providing engineers with a reliable and accurate computational model for timber, which will lead to more widespread use of timber in construction. The use of CDM for timber modeling opens up new possibilities for sustainable and environmentally friendly construction. Further research in this field can lead to even more accurate and reliable models for timber, making it a viable alternative to traditional building materials.
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    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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    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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    Performance of cold formed welded tubular steel joints under moment loading
    (2008) Mudenda, Kenny; Zingoni, Alphose
    Although exhibiting many superior properties to open sections structural hollow sections (SHS) have seen limited application in structures where joints resist moments. Their application has remained largely in truss type and lattice type structures where pinned joints are assumed. However, SHS have wide potential for use in structures with moment resisting joints. Much still needs to be done, however, to develop an understanding of joint behaviour under moment loads. It is also desirable to come up with means of developing design tables that can simply the design process. The purpose of the research was to investigate the behavior of SHS joints under moment loading through experimental tests as well as using finite element modeling.
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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.