Question

Provide a detailed report on below topics and submit all the files (Excel, word and SAP2000)

Design of Transmission towers using SAP2000

Answer 1

1. Introduction
Traditionally, building high-voltage power lines has had few obstacles during their construction
phase. Currently, this type of infrastructure is facing a number of setbacks: it has a considerable impact
on the environment, on economic activities, and on the expansion of cities, besides its economic cost,
including inspections and maintenance. All of these problems have led the companies that use and
maintain this infrastructure to consider making the best use possible of the existing lines before placing
new lines. Old lines were designed according to the standards of the time in which they were built and
they were designed to bear a certain load. In many cases, these towers were designed over forty years
ago, so the increased loads that will be placed on them will be far greater than the one for which they
were designed. In addition to this fact, the design and execution data of the towers has, in most cases,
disappeared, and in other cases, building regulations did not even exist at the time. Due to this,
addressing the re-use of existing power lines requires a geometric and structural analysis of the towers to
assess their current state.
Formerly, the towers’ dimensional analysis was performed through expeditious and manual methods
(through the use of a gauge and a measuring tape) that required direct contact with the structure and,
therefore, meant high risks and high costs. Afterwards, in search of a remote non-invasive measuring
method, classic topographic measuring allowed thorough, notably intense, field work taking angular
measurements and determining singular points indirectly through angular intersections. More recently, the
existence of reflectorless electromagnetic surveying equipment has allowed direct measurement of
distances and angles from a single point, making field work easier and more efficient, although it solely
focuses on extracting unique and specific measurements determined by the topographer [1]. This has meant
great uncertainty upon the elements of the tower, since the data was only taken at the point where the
measurement is performed. In order to have the full representation of the geometry of the structure, in the
last years laser scanning has presented as an interesting solution [2,3,4,5], due to the fact that they
generate dense real-time point-clouds of the tower’s geometry from a distance [6]. However, one of the
major limitations of these terrestrial geotechnologies is the overall height of the tower, impossible to
cover completely from the ground, which has led to the use of robotic unmanned aerial systems that
take aerial images and, through photogrammetric procedures, obtain dense three-dimensional models
of this type of infrastructure [7].
As for structural assessment, this kind of structure has been analyzed from different points of view
as presented in the literature: the effects of loading in the stability of the tower [8–11]; the effects of
the stiffness of connections [12–14]; and causes of failure [15–17]. No previous work was performed
in order to evaluate the effect of geometric imperfections such as misalignments of structural members
at joints or assembly imperfections.Geometric Modeling
The geometric modeling of the transmission towers was performed following four steps:
1. Cleaning and segmentation of point clouds in order to remove undesired data, such as
reflections, noise or sensor artifacts. This step was performed manually.
2. Alignment of the point clouds from each scan under a common coordinate system. An
automated registration method, iterative closest point (ICP) [25], was applied, supported by the
identification of matching points and minimizing the Euclidean distance between
corresponding point clouds. Initial approximations (three points) were manually identified by
the user, trying to guarantee a good distribution around the area of interest and along the three
main directions (X, Y, Z). A solid-rigid transformation based on the three points identified was
executed. Afterwards, an automatic iterative process to align the different scans was performed
taking the Euclidean distance as a minimization criterion.
3. Generating cross-sections and technical drawings of the electrical towers, focusing on the steel
profiles that make up each section of the tower and the arrangement of the connections used to
define the linkage of these profiles. Different profiles were automatically generated along each
main direction (X, Y, Z) in order to obtain vector information of the main sections of the towers
and, thus, initial approximations to support the technical drawings and CAD model generation.
4. Obtaining a computer aided design (CAD) model. Since the structural analysis based on a FEM
model does not cope with dense laser models, an important step which allows us to pass from
the 3D point clouds to a solid geometric model was performed. This step consists in extruding the
sections obtained in the step before along its normal direction. Manual interaction is required in
this step in order to solve the different intersections between profiles and their connections. In
addition, specific existing libraries based on standard steel profiles (i.e. L-shaped and U channel
profiles) were used for modeling the towers. Geometric modeling was done using Geomagic
Spark, 2013 version.

3.3. Structural Analysis
The three towers are formed by angular steel profiles of different dimensions, and given the age of
the towers and only for the purpose of the methodology developed in this article, we assume the lower
specification for a steel material enabled by [26], type S-235.
This brings the following mechanical properties: Young’s modulus of 2.1 × 108 kN/m2
, specific
weight of 76.9729 kN/m3
, Poisson’s coefficient of 0.3, and yield stress of 235 MPa.
Furthermore, the data of the power line, support type of the tower, and the mechanical
characteristics of the electrical drivers are detailed below:
- High voltage power line with rated voltage of 132 kV and 50 Hz AC
- Two duplex-circuit line with alignment support.
- Span: 300 m between supports.

Remote Sens. 2015, 7 11558
- Electrical driver of aluminum galvanized steel, type LA-280.
The boundary conditions of the three towers are assumed to be articulated supports in each of the
legs that make up the outer frame of the towers, so that they have only limited movements according to
the global axes (X, Y, Z). The constraints upon the members are explained in each of the structural
models discussed above.
The different load conditions are obtained according to [18]. The following descriptions summarize
the loads, always bearing in mind that the towers are located in the province of Guadalajara (Spain)
and that they are power lines with alignment support and with suspension insulator strings [18].
- Permanent loads: The self-weight of the steel profiles that comprise the towers, electrical
conductors, fittings, insulators, and the grounding wire.
- Wind load: It acts upon steel members of the towers, the insulators, and the suspension
insulator strings.
- Imbalance of tensile forces: A longitudinal force equivalent to 25% of all unilateral tractions of
electrical drivers and grounding wire. This tensile force will be applied at the point where the
electrical conductors and the grounding wire are attached to the support, thus taking into
account the torsion that these forces could create.
- Electrical conductor failure: A unilateral tensile force related to a single electrical conductor or
a grounding cable’s failure. The minimum admissible value for the failure is 50% of the broken
cable’s tension in the power lines that have two conductors per phase.
Taking into account aforementioned load patterns, the current standards [18] refer to certain
calculation hypotheses that establish the load cases, shown in Table 1.
Table 1. Load cases considered in structural analysis of towers.
Tower Type Force Direction Hypothesis 1 Hypothesis 2 Hypothesis 3
Alignment support
and suspension
insulator strings
Vertical
Permanent loads, considering the electrical
conductors and the grounding cables to withstand
wind load according to a 120 km/h wind speed
Transversal
Wind load (120 km/h) on
electrical conductors, cables,
grounding cables and
supports of tower
Not applicable Not applicable
Longitudinal Not applicable
Imbalance of tensile
stress
Electrical
conductor and
grounding cable
failure
In order to clarify such load cases considered, in Figure 3 is detailed upon finite element model of
one of the case studies herein analyzed (Tower 1), the arrangement of the loads in each of the three
scenarios previously commented. Similarly, loads are arranged in Towers 2 and 3

Experimental Results and Discussion
4.1. Case Studies
Since this study arises as a consequence of the need to analyze the structural viability of a series of
electricity transmission towers that serve as support to an old power electricity line located between the
cities of Guadalajara and Torija (Spain), three different cases studies were chosen for the development
of the aforementioned methodology.
The electricity transmission towers chosen correspond to a type of tower known as
“support alignment” which are disposed over different sections of electricity line. Additionally,
in order to extend the study over different structural typologies, each tower corresponds to a
different morphology.
The first tower is formed by both a main body support and another principal body (comprising
horizontal bracings and diagonal bracings according to a St Andrew’s disposition) and three horizontal
symmetrical bodies for the support of the cables. The second tower only has a support body (formed
by horizontal bracings and secondary diagonal bracings according to a St Andrew’s disposition) and
three asymmetric horizontal bodies. The third tower is similar to the second one, with exception in the
diagonals forming the support body which are not arranged according to a St Andrew’s disposition.
Figure 4 shows a photograph of the three towers that composes cases studies described above.

Geometric Modeling
Following all steps described in Section 3.2, the point cloud data obtained as a result of a laser
scanning survey was subsequently transformed into a CAD model valid for its implementation in the
finite element software package SAP2000.
This is a key step required in this kind of reverse engineering process, since data obtained from
laser scanning technology do not represent any valid information by itself for the purpose of finite
element analysis without suitable data processing [27].
Therefore, taking this into account, CAD models for each one of towers analyzed together with
drawings about its current disposition and assembly information were obtained.
Figure 5 shows CAD wire models obtained for each one of towers analyzed. Once such geometrical
models were obtained, they were directly imported as a DXF file to SAP2000 software for the finite
element analysis stage.
Main geometric data concerns to dimensions of the base, height of the tower, and length of
horizontal bodies are displayed in Table 2. The length of the horizontal bodies of the towers is
measured from the main body of the tower up to the farthest node. Tower 1 has three horizontal bodies
with different dimensions, while in towers 2 and 3, all the horizontal bodies have similar dimensions.
Table 2. Main geometric data for transmission towers analyzed.
Tower Base Dimensions (m) Height (m) Length of Horizontal Body (m)
1 7.25 × 7.25 37.25
3.95
6.25
5.0
2 1.5 × 1.5 18.50 2.0
3 1.5 ×1.5 18.50 2.15

Work in excel sheet

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