Browsing by Subject "Structural Engineering and Materials"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Open Access
    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.
  • Thumbnail Image
    Item
    Open Access
    Examination of flexural crack width prediction in concrete: comparison of analytical and numerical models
    (2020) Fredericks, Brandon; Beushausen, Hans-Dieter
    The reliability of crack prediction methods in concrete design plays a role in the degree of confidence with which durability can be ensured. Bond failure between concrete and the embedded reinforcing steel exposes the steel surface within a crack path. This relative slip results from differential tensile strain between concrete and steel that allows harmful ions to reach and then react along the rebar length. A reliable prediction method should therefore account for the loss of bond in crack propagation. Researchers question the significance of the role played by surface cracks in structural deterioration, therefore casting doubt on the need for exhaustive crack analysis. The applicable fundamental theory of cracking, namely non-slip or slip determines the steel exposure and therefore the likelihood of reduced structural service life attributable to crack behaviour. However, while cracks originate at the surface of reinforcement through bond failure, simultaneously a cover distance away no cracks could appear on the concrete tension surface or they could be twice to ten times the crack width at the rebar level. Due to the heterogeneous composition of concrete, some commentaries state it impossible to accurately predict crack widths. Design standards therefore provide estimates of maximum crack widths to a degree of probability. This study examines the methods available for predicting cracks on the tension surface and the degree to which this is indicative of weakened bond between concrete and reinforcement. In this examination, it will be seen that concrete has ductility due to tension softening behaviour. The addition of steel to the tension area transforms the fracture process zone problem to the definition of a bond-slip relationship. Bond stresses generated at the rebar perimeter define the analytical relationships in design codes. These stresses control crack width at the tension surface. Results from the analytical code-based models are compared for increasing section depths, bar sizes and maximum spacings in the tension zone. A significant variation in the predicted maximum crack width is observed for deeper members. For very large concrete sections, the analytical models appear to provide unreasonable crack width values. The analytical equations in design codes focus on the bond relationship and ignore the size effect of concrete inherent in its microstructure. The concentration of flaws increases in larger members; hence size effect would play a greater role. Numerical modelling for crack prediction is therefore recommended for crack analysis in larger concrete members.
  • Thumbnail Image
    Item
    Open Access
    Parametric study on the buckling behaviour of singly and doubly curved concrete arch dams
    (2011) Mokhuthu, Batho; Zingoni, Alphose
    This study explores the buckling behaviour of thin walled and curved concrete arch dams investigated as a function of various parameters of interest, the focus being on the influence of dam thickness on the elastic buckling behaviour.