Ashraf A. El Damatty

Modeling and Prediction of Failure of Transmission Lines Due to High Intensity Winds

The high intensity winds (HIW) associated with thunderstorm outflows (downbursts, tornados) have profile shapes that differ greatly from the straight line winds in conventional atmospheric boundary layers that form the basis of the design wind loads for electricity transmission line systems. Evidence from line failures, such as Manitoba Hydros (MH) on 5 th September 1996, that caused failure of 19 towers and a 5-day line outage, suggests that many are the result of HIW, due to a combination of extreme wind speed and an off-design wind load distribution. Whilst the complete upgrading of any given line system to withstand the severest storms is not feasible, some critical locations (e.g. major river / road / rail crossings) could benefit from remediation based on a thorough understanding of HIW loads. The authors have completed the first phase of a comprehensive research program aimed at quantifying these HIW wind fields, their loading on transmission line tower and line components and the resultant response of the transmission line system. The main outcomes from that phase were 1 Formulation of a computational fluid dynamics (CFD) model of downburst and tornado wind fields validated by small-scale laboratory experiments and a theoretical / analytical model 2 Derivation of a loading model for downburst winds and application of this model to fully non-linear finite element (FE) models of a MH Type A guyed tower / line system. Identification of some of the critical members and failure mechanisms. A second phase of work is currently in progress that is concerned with the validation and implementation of the previously developed concepts, encompassing 1 Design and construction of a large scale downburst simulator (based on a 2-D wall jet concept) 2 Testing of tower/line physical models in the large-scale downburst simulator, with comparison to loads derived from atmospheric boundary layer wind profile model testing. 3 Improving the downburst and the tornado numerical models by introducing atmospheric parameters and comparison with Doppler radar atmospheric data.

Database for the optimum design of semi-fan composite cable-stayed bridges based on genetic algorithms

The optimum design solution of cable-stayed bridges is a complicated task. The large number of design parameters, strict design constraints imposed by design codes, highly geometrically nonlinear behaviour and robust effect of post-tensioning cable forces make traditional design methods not capable of achieving this task. In this study, an optimisation technique that integrates a finite element model, a genetic algorithm and polynomial functions for evaluating post-tensioning cable forces is adopted to develop a database for the optimum design of cable-stayed bridges. The database describes the variations of the optimum design parameters, including the main span length, height of the pylon, number of stay cables and cross-sectional dimensions of all elements with the total length of the bridge. The study concerns the optimisation of three-span composite cable-stayed bridges with semi-fan cable arrangements. The study covers bridge lengths ranging from 250 to 700 m. The database is presented in the form of simple design charts and tables. Such database is useful in the preliminary design and cost estimation for this type of bridges.

Effect of Geometric Nonlinear Behaviour of a Guyed Transmission Tower under Downburst Loading

Downburst winds, which are a source of extreme wind loading and are referred to as high intensity wind (HIW) loads, have caused numerous transmission tower failures around the world. A previous investigation was conducted to study the performance of a transmission tower under downburst wind loading, where the behaviour of the tower was limited to a linear response. In the current study, a nonlinear frame element is used to assess the performance of the tower under downburst wind loading. The behaviour is studied using downburst wind field data obtained from a computational fluid dynamics (CFD) model. In order to assess the geometric nonlinear behaviour of the tower, the results are compared to a previous linear analysis for a number of critical configurations of a downburst. The nonlinear analysis predicted that peak axial loads in certain members can be up to 34% larger than those predicted by the linear analysis.

Optimum design of composite conical tanks under hydrostatic pressure

Elevated tanks are used all over the world to store water for times of shortage. These tanks can be made of steel, reinforced concrete, or composite, that is, concrete and steel. Composite tanks consist of an external steel shell attached to an internal reinforced concrete wall through steel studs. Composite conical tanks combine the advantages of reinforced concrete and steel tanks as they resist efficiently both tensile and compressive stresses. A comparison showed that the material cost of composite conical tanks is significantly less than that of steel or reinforced concrete tanks having the same layout dimensions. A numerical tool is developed to obtain the optimum design of composite conical tanks under hydrostatic pressure incorporating both finite element and genetic algorithm techniques. This tool is used to obtain the optimum design of a case study composite conical tank that was recently constructed. The developed optimization tool provides the thicknesses of the concrete and steel walls as well as the stud configuration corresponding to the minimum material cost. A comparison between the optimized and unoptimized case study composite tank revealed that a reduction of 32% in the material cost can be achieved. A sensitivity analysis is conducted by changing the price of concrete, steel plate, and studs by (±) 50% of the datum prices and obtaining the corresponding optimum design variables. This analysis showed that the optimum thicknesses of the concrete wall and steel shell as well as studs’ configuration are significantly sensitive to the change in the material prices.

Critical Parameters and Configurations Affecting the Analysis and Design of Guyed Transmission Towers under Downburst Loading

AbstractIn past years, many electrical transmission lines have failed during downbursts. This study is part of a research program aimed at understanding the behavior of transmission lines under localized winds. Most downbursts need to be modeled using computational fluid dynamic (CFD) models; this is not an easy task for engineers in general. This paper focuses on replacing the complicated velocities obtained from CFD models with simplified loads that represent, in a more practical fashion, the critical downburst configurations causing the guyed transmission towers to fail. Different options are considered in terms of the location and nature of the design velocity associated with different configurations. The profile of the horizontal velocity is developed. A practical simplified load corresponding to each of the critical configurations is developed. Detailed steps of the conductors’ reaction calculations are provided. A simplified procedure is developed for designing downburst-loaded guyed transmission t...

Reduced Equivalent Static Wind Loads for Tall Buildings with Tuned Liquid Dampers

The tuned liquid damper (TLD) is a proven and an increasingly popular auxiliary device for mitigating the dynamic effects induced by wind loading on tall buildings. As buildings become taller, lighter, and more flexible, there is a greater contribution from the dynamic component. The most reliable tool for assessing the dynamic component is wind tunnel testing. A boundary layer wind tunnel is capable of accurately calculating an equivalent static wind load (ESWL) acting on a building. The current study investigates the reduction in the ESWL of a lateral-torsional coupled building with a TLD system installed. The building is sensitive to torsion in the first two vibration modes. The current investigation uses three unique multi-modal TLD systems designed specifically for a lateral-torsional coupled building. The building ESWL is evaluated with the TLD systems using measurements from tests conducted at the Boundary Layer Wind Tunnel Laboratory at Western University.