Rozprawy doktorskie na temat „Telecommunications tower”

El texto completo de este trabajo no está disponible en el Repositorio Académico UPC por restricciones de la casa editorial donde ha sido publicado.<br>This article proposes a work method reform in the production line of self-supporting telecommunication towers of a Peruvian metalworking enterprise. The problem raised considers the cost overruns caused by the delivery and dispatch of incomplete tower kits. The main process evaluated is the assembly of tower kits, which presented multiple deficiencies that we analyzed with multiple engineering tools using the eight steps of practical process improvement. We evaluated by a large scale definition the work table in the assembly and stacking of parts, defining by specific zones each part type and profile, and optimizing the total process times so as not to affect the scheduled dispatches. Thus, it is possible to minimize the number of customer returns.

The reliability analysis of telecommunication networks requires an estimate of the probability of failure of antenna-supporting structures. Lacking such estimates, the network planners tend to assume that the probability of failure of towers is negligible. On the other hand, reliability concepts implicit in the codes are not always compatible with those used in reliability analysis of the network, in particular, the implicit probabilities associated with partial load and resistance factors are obtained for idealized structural members and do not address the reliability of the structure as a system. Advances in structural reliability, combined with more extensive climatologic data, can be used to implement a probabilistc approach for the design of towers.<br>The objective of this study is to propose a procedure for calculating the probability of mechanical failure of self-supported telecommunication towers. The procedure introduces the concept of calculating the conditional probability of failure which can be used with different joint distributions of wind and ice with a minimum of computations. As an example, the methodology is applied to the CEBJ tower in James Bay. The structure is assumed to behave linearly and to be statically determinate. In consequence, the structure can be modelled as a weakest link model.<br>The study demonstrates the possibility of estimating the probability of failure for the whole structure using a rational approach. The critical members of the structure and the relative importance of each of the design parameters with respect to the probability of failure are identified in order to simplify the reliability analysis. The probability of failure is most sensitive with respect to the joint probability distribution function of wind speed and ice thickness. Upper and lower bound estimates of the probability of failure are presented for different assumption in the joint distribution. These results indicate the need for a better model for the environmental loads.

This thesis deals with the assessment of seismic accelerations in buildings and the seismic analysis of components installed on building rooftops, with special focus on operational telecommunition towers during and after earthquake shaking.<br>First, acceleration data recorded during the 1999 Chi Chi earthquake from 11 instrumented buildings located in Taiwan were studied. Fundamental building periods were extracted by system identification and compared to those evaluated according to the equations proposed in the 2005 edition of the National Building Code of Canada (NBCC). Next, rooftop acceleration spectra and time histories were evaluated using 3-D finite element building models; three models of instrumented buildings were calibrated using accelerograms from the Chi Chi earthquake and the fourth model is a building located in downtown Montreal. The building models were subjected to 44 historical strong motion accelerograms and 30 synthetic accelerograms compatible with the target uniform hazard spectra specified in NBCC 2005 for Montreal. Based on both the experimental and numerical results, a maximum rooftop acceleration amplification of 4 is proposed for low/medium rise buildings and 3 for flexible high-rise buildings (T > 1.7 s).<br>In the second stage, a simplified method for the prediction of seismic shear forces and overturning moments at the base of self-supporting steel lattice telecommunication towers mounted on building rooftops is presented. The proposed method involves the estimation of four parameters: the rooftop seismic acceleration, the mass distribution profile of the tower along its height, the maximum acceleration amplification at the tower top, and the fundamental sway mode shape of the tower on a rigid base. The method was validated by means of numerical results of nine generated building-tower combinations composed of three towers assumed to be mounted on three of the building models studied in the first stage of the research. The building-tower combinations were subjected to the same sets of earthquake records used for the prediction of accelerations. It was found the proposed method yields conservative results in all the cases analyzed.<br>In addition, the empirical component force amplification factor for telecommunication towers as proposed in the NBCC 2005 was compared to the factors evaluated for the towers of the 16 building-tower combinations. Improved component force amplification factors based on rational analysis are proposed.

Thesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.<br>This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.<br>Cataloged from student-submitted PDF version of thesis.<br>Includes bibliographical references (pages 71-73).<br>In recent years, metal halide perovskite solar cells have gained traction as a potential competitor to the mature silicon-based solar cells in terms of both cost and performance. Being a young technology, however, means little is known about its true market value. In order to understand this, it is important to both get an accurate estimate of manufacturing cost and explore potential applications. In this thesis, we develop a Monte Carlo cost analysis method and apply it towards a realistic perovskite module manufacturing sequence today. We determine a nominal cost estimate of $101.7/m2, which for a 15% efficient module comes out to $0.68/W. Compared to silicon, which hovers around $0.40/W, this is rather competitive, since most of the installed system cost comes from non-module components such as installation labor and racking. With the lightweight and flexible form factor of perovskite solar modules, new applications become possible. One such application is the installation of perovskite photovoltaics (PV) vertically on telecommunications towers. Since such towers cannot support the weight of conventional silicon PV, this is a potential market for perovskite PV that silicon cannot satisfy. Using HOMER microgrid simulation software, we determine that it is financially feasible to install vertical PV in countries with high diesel prices and low grid reliability, such as India.<br>by Justin Xiao.<br>M. Eng.

Technical Report. Structural Engineering Series No. 2012-15<br>The author and her supervisor, Prof. Ghyslaine McClure, recently worked on a research aimed at validating computational seismic response predictions of a guyed telecommunication mast with ambient vibration measurements. This report summarizes telecommunication towers damages due to recent earthquakes (from 1999 to 2011) as reported in damage reconnaissance accounts openly accessible.

From the first generation, 1G, to the fourth generation, 4G, the development and technological advancements in telecommunications network systems have been remarkable. Faster and better connections have opened up for new markets, ideas and possibilities, to that extent that there now is a demand that surpasses the supply. Despite all these advancements made in the mobile communications field most of the concept of how the technology works and its infrastructure has remained the same. This however, is about to change with the introduction of the fifth generation (5G) mobile communication. With the introduction of 5G much of the technology introduced will be different from that of previous generations. This change extends to include the entire infrastructure of the mobile communications system. With these major changes, many of the tools available today for telecommunications network evaluation do not really suffice to include the 5G network standard. For this reason, there is a need to develop a new kind of tool that will be able to include the changes brought by this new network standard. In this thesis a simulation framework adapted for the next generation telecommunication standard 5G is set to be developed. This framework should include many of the characteristics that set 5G aside from previous generations.

Torres metálicas treliçadas tem sido largamente utilizadas como suporte de antenas de microondas. Devido ao baixo peso e a inexistência de terremotos significativos no Brasil, a força de vento é o fator determinante do projeto destas estruturas. Com o objetivo de determinar estas forças e seus efeitos em torres treliçadas foram realizadas investigações numéricas e experimentais. Uma torre de 100 metros foi dimensionada com base nos padrões existentes no Brasil. Examinou-se a resposta dinâmica da estrutura ao longo do vento. Investigaram-se a resposta ressonante, não ressonante e o fator resposta de rajada. Concluiu-se que a resposta ressonante não é significativa para este tipo de estrutura. Compararam-se os modelos de DAVENPORT (1993), da norma Brasileira NBR6123/88 e o processo do vento sintético de FRANCO (1993). Uma investigação experimental foi realizada para analisar os coeficientes de força em uma seção da torre estudada. Os experimentos foram realizados no “Boundary Layer Wind Tunnel Laboratory” (BLWTL) da “University of Western Ontario” (UWO), Canadá. Foram analisados o ângulo de incidência do vento; o índice de área exposta; o efeito de proteção; o fator de interferência no coeficiente de arrasto de antenas de microondas, devido à proximidade da torre e a influência da turbulência do vento. Os resultados mostraram boa concordância com os valores obtidos em diferentes normas existentes.<br>With today’s expanding communication systems, a large number of lattice towers to support cellular and microwave antennas are being constructed in Brazil. Due to the lightweight of these structures, wind forces are the primary concern in the design. With the objective of determining these forces and their effects upon latticed towers, a numerical and experimental investigation is accomplished. A 100-meter tower is designed based on existing patterns of latticed towers in Brazil. The along wind dynamic response is analysed. The resonant, background response and gust factor are examined. It is concluded that the resonant response is not significant for this type of structure. The DAVENPORT (1993) method, the Brazilian code NBR6123/88 and the process of the synthetic wind of FRANCO (1993) are compared. An experimental investigation is accomplished to analyse the force coefficients on a section of the lattice tower studied. The tests were performed at the Boundary Layer Wind Tunnel Laboratory, University of Western Ontario (UWO), Canada. The wind incidence angle; the tower solidity; the shielding effect; the interference factor upon the drag coefficient of the microwave antennas, due to the tower proximity and the influence of the wind turbulence are analysed. The results are ii good agreement with the values obtained in different existent codes.

Durante a implantação do sistema de telefonia celular no Brasil, milhares de torres foram projetadas por profissionais que utilizaram modelos inadequados para a consideração dos efeitos dinâmicos do vento. Devido a este fato, problemas estruturais têm sido freqüentes. Este trabalho é motivado pela falta de material na literatura para auxiliar o projetista a especificar as características geométricas e mecânicas deste tipo de estrutura. São abordados todos os aspectos envolvidos no projeto de uma torre de telecomunicações: normas, características de cada tipo de torre, de aço ou de concreto, além de uma pesquisa junto a empresas fabricantes com o objetivo de levantar dados sobre o peso dos diferentes tipos de torre. Este tipo de informação é de grande valia para a avaliação da viabilidade de projetos ou para o desenvolvimento de novos tipos de estruturas. Diante das vantagens e desvantagens das soluções existentes, este trabalho apresenta um modelo de torre que une a alta relação momento de inércia - área de seção transversal das torres em treliças ao baixo carregamento devido ao vento dos postes, para alturas de 40 m e 70 m. A viabilidade destes projetos é indicada pela comparação de seu peso com o de estruturas pesquisadas. Após descrever natureza do vento e as formas de obter e aplicar as forças dinâmicas associadas, além da obtenção de resposta de estruturas a estas excitações, é proposto um código numérico para a realização deste tipo de análise. Neste código, o processo de integração numérica é executado tanto pelo método de Newmark, como por Diferenças Finitas. Pode ser considerada não-linearidade geométrica pelo efeito do esforço axial e da variação das coordenadas nodais sobre a matriz de rigidez. São ainda avaliados por este algoritmo incertezas de modelo inseridas pela forma de determinação do vento turbulento e pela consideração da flexibilidade das fundações. A inclusão de amortecedores de atrito à estrutura é, da mesma forma, analisada.<br>During the implantation of cellular telephony in Brazil, thousands of towers were designed by professionals who used models unsuitable for the consideration of wind dynamic effects. Because of this, structural problems have been frequent. This work is motivated by data lack in literature to help the designer to specify geometric and mechanical characteristics of this structure category. It is discussed every aspect involved in telecommunications tower design: standards, characteristics, materials and a search in the manufacturers to obtain data about weight of the different types of towers. These informations are important to evaluate the project feasibility or to develop new models of structures. Front advantages and disadvantages of existing solutions, this work presents a tower project that adjoins the ratio moment of inertia - cross section area from truss towers and the low wind loading from the poles, to 40 and 70 m towers height . The viability of these projects is estimated by comparing its weight with data searched. After describing nature of the wind and how to obtain and apply the dynamic forces involved, how to obtaining response of structures to these excitation, it is proposed a numerical code to carrying out this analysis. In this code, the integration process is performed by the Newmark and finite differences numerical methods. It can be considered the geometrical non-linearity considering on the stiffness matrix the axial stress effect and nodal coordinates variation. Uncertainties inserted by turbulent wind determining and the foundations flexibility consideration are also evaluated by this algorithm. Friction dampers inclusion is, similarly, examined.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-12-12T15:55:02Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação de mestrado - Análise dinâmica de torres para telecomunicações (Revisão Definitiva).pdf: 5572516 bytes, checksum: 3aeb29a09ea2819f49316a79c5388a73 (MD5)<br>Made available in DSpace on 2016-12-12T15:55:02Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Dissertação de mestrado - Análise dinâmica de torres para telecomunicações (Revisão Definitiva).pdf: 5572516 bytes, checksum: 3aeb29a09ea2819f49316a79c5388a73 (MD5) Previous issue date: 2015-12-11<br>Na última década, o crescimento da telefonia celular no Brasil venceu os limites das grandes capitais, levando a rede a cobrir municípios antes desprovidos de tal tecnologia. A transmissão do sinal pode ocorrer de diversas formas, mas, de fato, a mais comumente utilizada é por meio de antenas sustentadas por torres metálicas. Na prática, as antenas são instaladas em qualquer momento após a montagem da torre, mesmo anos após sua construção, sem qualquer verificação estrutural e, por vezes, excedendo a capacidade de carga nominal. Além disso, na maioria dos casos, não são levados em conta certos cuidados com relação ao posicionamento de suportes e das próprias antenas. Portanto, é comum encontrarmos antenas instaladas de forma inadequada, sendo colocadas fora da silhueta da torre. Este posicionamento, aliado com a ação dinâmica ocasionada pelo vento, pode trazer problemas graves à estrutura. Neste trabalho, são apresentados dois casos reais de torres metálicas que sofreram danos estruturais devido ao problema em questão. Este trabalho tem como principal objetivo apresentar uma solução de amortecimento para as vibrações causadas pelo vento em antenas para telecomunicações. A proposta visa utilizar elastômeros, como o Neoprene, entre a cantoneira do montante e as abraçadeiras dos suportes, reduzindo assim o período de vibração e, consequentemente, os efeitos da fadiga.<br>In the last decade, the growth of mobile telephony in Brazil won the boundaries of the great capitals, leading the network to cover cities before devoid of such technology. The signal transmission can occur in various ways, but, in fact, the most commonly used is through antennas supported by steel towers. In practice, the antennas are installed at any time after assembly of the tower, even years after its construction, without any structural verification and sometimes exceeding the rated load capacity. Moreover, in most cases, care is not taken into account in relation to the positioning brackets and their antennas. Therefore, it is common to find antennas installed improperly, being placed outside the tower silhouette. This positioning, combined with dynamic action brought by the wind, can bring serious problems to the structure. In this work, we present two real cases of metal towers that have suffered structural damage due to the problem at hand. The objective of this work is to present a solution for damping the vibrations caused by the wind in telecommunications antennas. The proposal to use elastomers, such as neoprene, between the bracket and the tower column, thus reducing the vibration period and, consequently, the effects of fatigue.

A Engenharia de Ventos Baseada em Desempenho (Performance-based Wind Engineering - PBWE) é uma filosofia de projeto que preconiza identificar e quantificar as incertezas envolvidas no projeto estrutural a fim de assegurar níveis previsíveis de desempenho às edificações, não mais gerenciando o risco através da clássica abordagem determinística. Contudo, devido à recente proposição da metodologia, ainda há poucos estudos relacionados à PBWE, cada qual apresentando diferentes limitações. Assim, o presente trabalho propõe uma adaptação da metodologia da Engenharia de Ventos Baseada em Desempenho à análise probabilística do comportamento de torres metálicas, avaliando diferentes modelos de cálculo para estimativa das forças do vento neste tipo de estrutura. Para tanto, investigou-se as incertezas envolvidas na caracterização do campo de ventos e da resistência estrutural e foram analisados quatro métodos distintos para a estimativa das forças de vento em torres metálicas: dois procedimentos de cálculo correspondentes à norma brasileira de ventos ABNT NBR 6123:1988 (ABNT, 1988), a metodologia de Davenport (1993) e a de Holmes (1994). Um estudo de caso envolvendo a estimativa da confiabilidade de uma torre de telecomunicação também foi conduzido. Constatou-se que ambos os procedimentos de cálculo admitidos conduzem a níveis de segurança de mesma ordem de grandeza e que a elaboração de projetos de torres considerando a direção de incidência do vento como sendo a mais desfavorável à estrutura é demasiadamente conservadora. Como contribuição, verifica-se que o projeto ótimo de torres pode ser alcançado com base no nível de segurança desejado para diferentes velocidades máxima de vento associadas a intervalos de recorrência específicos.<br>Performance-based Wind Engineering (PBWE) is a design philosophy that aims to identify and quantify the uncertainties involved in the structural design in order to ensure predictable performance levels to buildings, no longer managing risk through the classical deterministic approach. However, due to the recent proposal of the methodology, there are few studies related to PBWE, each presenting different limitations. Thus, this paper proposes an adaptation of the Performance-based Wind Engineering methodology to the probabilistic analysis of the behavior of steel towers, evaluating different calculation models for estimating wind forces on this type of structure. To this end, uncertainties involved in the characterization of the wind field and structural strength were investigated and four different methods for the estimation of wind forces on steel towers were analyzed: two procedures relative to the Brazilian winds standard ABNT NBR 6123:1988 (ABNT, 1988), and the methodologies of Davenport (1993) and Holmes (1994). A case study concerning the reliability estimation of a telecommunication tower was also conducted. It was found that both assumed calculation procedures lead to security levels of the same order of magnitude and that the design of towers considering that the wind always blows from the worst direction is too conservative. As a contribution, it is found that the optimum design of towers can be achieved based on the desired security level for different maximum wind speeds associated to specific recurrence intervals.

碩士<br>國立成功大學<br>建築學系碩博士班<br>92<br>In Taiwan, the development of seismic design provisions for non-building structure components has lagged behind that of building structures. If codes provide good design suggestion, not only many seismic hazard can be reduced and the lost of fortune can be avoided, but also restoration and reinforcement can be rapidly proceeded after a seismic event. Targeting roof-top telecommunication towers on buildings, this thesis utilizes a structural analysis software to study their dynamic behavior, and provides a revision for roof-top tower seismic design code. 　　This thesis first performed a literature review for tall telecommunication towers in order to comprehend tower’s requirement at planning stage. Afterwards, this thesis utilizes analysis software “SAP2000” for a telecom office building in Tainan County to perform structural simulation.Comparing with the result of the system identification from real EQ and ambient vibration time history to the computer model, a set of appropriate parameters of buildings and roof towers were established. This set of parameters were applied on another real roof-top towers’ vibration modeling. By alternating models of the towers to various heights, this thesis studies resonance effect of buildings and roof towers. According to the conclusions from SAP2000 analysis, and refering to formula about telecom’s seismic design in other papers, a revised formula is suggested.

碩士<br>國立屏東科技大學<br>土木工程系碩士班<br>92<br>The design and construction of telecommunication towers are growing since the use and the need of cellular phones are increasing. In addition to dead loads like tower weight itself, antenna weight or accessory equipments weight, the loads concerned in a tower structural design are wind and earthquake loads. For the publications of wind load in Taiwan so far, there are Tsai in 1996 and Cheng in 2002 studying the wind force codes of building. For earthquake load, there are 1997 version of seismic design code by Architecture and Building Research Institute, Ministry of the Interior, and several studies for revising this code. For telecom tower structural design, the only publication known so far is the telecom tower design codes by Chung-hwa Telecom Co. in 1997, which mainly focuses on the calculation of wind load, but not as much on earthquake load. The thesis is a further study for this telecom tower design code. For wind load calculation, it also employs the above wind force codes of building. The earthquake load calculation is based on the above 1997 version of seismic design code. It is expected to establish a design process for telecom tower structure, which gives a more brief but comprehensive way for calculating wind and earthquake loads. Finally, a case of 20 meter telecom tower is used as an example to illustrate the design process, along which, the structural steel design is based on AISC-ASD89 structural codes and SAP2000 software is used to calculate and select the members of structure. Hopefully, it would help the designers and researchers of the telecom towers in Taiwan.

碩士<br>朝陽科技大學<br>工業工程與管理系碩士班<br>93<br>A novel pedal is designed, in this study, to facilitate the work on a guyed telecommunication tower by providing extra foot support to a worker. A data logger with six electromyography (EMG) electrodes was utilized to record subjects’ exertions of gastrocnemius, tibialis anterior, and paraspinals muscles while performing antenna mounting tasks on an imitated quadratic tower with or without the pedal. Six experienced and nine unexperienced subjects were tested. Their muscular loads were assessed according to subjects’ ratings of perceived exertion (RPE) and the 50th percentile of amplitude-probability distribution of individual muscle’s normalized EMG. Subjects demonstrated using the pedal have significantly lower RPE in lower back (p < 0.001) and lower limbs (p < 0.001). When pedal was used, gastrocnemius EMG decreased by approximately 40%. The overall and imbalance muscular loads of bilateral gastrocnemius were also reduced significantly by approximately 40% and 52%, respectively, while using the pedal. The results approve that providing adequate pedal can decrease the overall and imbalanced loads in lower limb muscles while workers’ foot placement was restricted by the worksite.

Electromagnetic effects of lightning currents during a direct hit to tall communication towers, other instrumented towers and chimneys can be hazardous to associated cables, as well as, electrical and electronics systems. The standard practice in telecommunication and other related fields is to bond the cable sheath to the tower and ground connection is made before it enters the base station. However, in some specific cases when power, signal and data logging cables are to be supported on the same tower, isolation of power cables is demanded. In a totally different situation, attempts are also made to have a dedicated isolated down conductor. A critical review of the situation demanded a more quantitative answer to the following questions: (i) whether it is possible to electrically isolate a dedicated down conductor, (ii) is it possible to electrically isolate the cables and their terminal equipment both mounted on towers serving as down conductor and if so, what will be the nature of current induced in the cables and (iii) as per the standard practice, if the cable sheaths are connected to the tower/structure, what will be the nature of the current shared by them. Addressing these important issues formed the scope of the present work. For the tall structures considered in this work, for the critical time periods, wave nature of the current dominates. This called for electromagnetic modeling covering Transverse Magnetic (TM) mode of the wave propagation. Owing to the complex geometrical features involved with the problem, both experiments on electromagnetically scaled laboratory models, as well as, theoretical simulation is attempted. An electromagnetically scaled laboratory model is employed for the time domain experimental investigation. This approach, which has been validated earlier, is further scrutinized to ensure its adequacy. In order to achieve generality and noting the fact that the associated parameters are rather difficult to be varied in the experimentation, theoretical investigation is also employed. For this, both NEC-2, as well as, an in-house thin wire time domain code developed for this work is employed. NEC-2 could handle multi-wire multi-radius junctions, while in-house time domain code could handle proximity and non-cylindrical shapes encountered with tower lattice elements. The investigation of induction to isolated cables on simple down conductors and towers is considered first. The induced current is shown to be bipolar oscillatory with the period of oscillation governed by the length of the cable. It is shown that the level of induction for good earth termination is below 5 – 10 % while that with moderate inductance in the earth termination can enhance the induction to higher levels. The level of induction is shown to be not critically dependent on the length of the cable, gap between cable and down conductor/tower. When multiple cables are mounted, they seem to influence each other and individually carry currents of lower amplitude. Also, the effect of shape and proximity of the tower lattice elements on induction is investigated. If the cable is housed inside a metallic tray, the amplitude of induced current is shown to be quite small. Subsequently, the evaluation of electrical stress between the isolated down conductor on tower and simplified representation of the structure is considered. A suitable definition of the electric stress for the wave regime is evolved and then it is shown that, at present, the voltage difference defined by the path integral of electric field across shortest path between the two entities is the best indicator for the stress. The electrical stress in the case of isolated down conductor on tower, as well as, down conductor with isolated cable is shown to reach very dangerous levels. On the other hand, the stress on the isolated cables on towers also serving as down conductors is shown to be relatively moderate. Interestingly, it is shown that the electrical stress and the voltage difference is dependent on the gap and for the critical time period, can be much lower than that calculated as a product of equivalent tower surge impedance and the stroke current, even before the arrival of ground end reflections. Finally, the current shared by cables connected to the down conductor is investigated. For the case of simple cylindrical down conductor with cable connected to it at the top, it is shown that the amount of current shared by the cable is not dependent on its length and the relative radii (cross section) have only a weak influence. For the case with down conductor formed by L and + angles, it is shown that the placement of cable at their interior corner can reduce the initial current shared by the cable. In order to model best possible situation with towers, experiments are conducted with cable inside an aluminum pipe. Even in this case, cable current builds up with successive reflections to become comparable with the current through the pipe itself. Subsequent investigation with 1:40 and 1:20 tower models lead to several interesting observations. Cables running along leg/face of the tower whether placed inside or outside the tower, always shares good amount of current. Further, frequent bonding of the sheath to the tower increases the current shared by the cable. Cable when housed in a metallic tray shares less than 50% of the current shared without the tray. Even though a complete quantification is not to be achieved in this work, it has made a good beginning with some significant contribution towards lightning protection issues pertaining to tall towers and structures.

Electromagnetic effects of lightning currents during a direct hit to tall communication towers, other instrumented towers and chimneys can be hazardous to associated cables, as well as, electrical and electronics systems. The standard practice in telecommunication and other related fields is to bond the cable sheath to the tower and ground connection is made before it enters the base station. However, in some specific cases when power, signal and data logging cables are to be supported on the same tower, isolation of power cables is demanded. In a totally different situation, attempts are also made to have a dedicated isolated down conductor. A critical review of the situation demanded a more quantitative answer to the following questions: (i) whether it is possible to electrically isolate a dedicated down conductor, (ii) is it possible to electrically isolate the cables and their terminal equipment both mounted on towers serving as down conductor and if so, what will be the nature of current induced in the cables and (iii) as per the standard practice, if the cable sheaths are connected to the tower/structure, what will be the nature of the current shared by them. Addressing these important issues formed the scope of the present work. For the tall structures considered in this work, for the critical time periods, wave nature of the current dominates. This called for electromagnetic modeling covering Transverse Magnetic (TM) mode of the wave propagation. Owing to the complex geometrical features involved with the problem, both experiments on electromagnetically scaled laboratory models, as well as, theoretical simulation is attempted. An electromagnetically scaled laboratory model is employed for the time domain experimental investigation. This approach, which has been validated earlier, is further scrutinized to ensure its adequacy. In order to achieve generality and noting the fact that the associated parameters are rather difficult to be varied in the experimentation, theoretical investigation is also employed. For this, both NEC-2, as well as, an in-house thin wire time domain code developed for this work is employed. NEC-2 could handle multi-wire multi-radius junctions, while in-house time domain code could handle proximity and non-cylindrical shapes encountered with tower lattice elements. The investigation of induction to isolated cables on simple down conductors and towers is considered first. The induced current is shown to be bipolar oscillatory with the period of oscillation governed by the length of the cable. It is shown that the level of induction for good earth termination is below 5 – 10 % while that with moderate inductance in the earth termination can enhance the induction to higher levels. The level of induction is shown to be not critically dependent on the length of the cable, gap between cable and down conductor/tower. When multiple cables are mounted, they seem to influence each other and individually carry currents of lower amplitude. Also, the effect of shape and proximity of the tower lattice elements on induction is investigated. If the cable is housed inside a metallic tray, the amplitude of induced current is shown to be quite small. Subsequently, the evaluation of electrical stress between the isolated down conductor on tower and simplified representation of the structure is considered. A suitable definition of the electric stress for the wave regime is evolved and then it is shown that, at present, the voltage difference defined by the path integral of electric field across shortest path between the two entities is the best indicator for the stress. The electrical stress in the case of isolated down conductor on tower, as well as, down conductor with isolated cable is shown to reach very dangerous levels. On the other hand, the stress on the isolated cables on towers also serving as down conductors is shown to be relatively moderate. Interestingly, it is shown that the electrical stress and the voltage difference is dependent on the gap and for the critical time period, can be much lower than that calculated as a product of equivalent tower surge impedance and the stroke current, even before the arrival of ground end reflections. Finally, the current shared by cables connected to the down conductor is investigated. For the case of simple cylindrical down conductor with cable connected to it at the top, it is shown that the amount of current shared by the cable is not dependent on its length and the relative radii (cross section) have only a weak influence. For the case with down conductor formed by L and + angles, it is shown that the placement of cable at their interior corner can reduce the initial current shared by the cable. In order to model best possible situation with towers, experiments are conducted with cable inside an aluminum pipe. Even in this case, cable current builds up with successive reflections to become comparable with the current through the pipe itself. Subsequent investigation with 1:40 and 1:20 tower models lead to several interesting observations. Cables running along leg/face of the tower whether placed inside or outside the tower, always shares good amount of current. Further, frequent bonding of the sheath to the tower increases the current shared by the cable. Cable when housed in a metallic tray shares less than 50% of the current shared without the tray. Even though a complete quantification is not to be achieved in this work, it has made a good beginning with some significant contribution towards lightning protection issues pertaining to tall towers and structures.

A realização deste trabalho tem como objetivo principal analisar as condições de segurança na realização de trabalhos em altura em torres de telecomunicações, mastros, coberturas e fachadas de edificos, onde a falha humana ou a falha material pode originar acidentes muito graves ou mortais. Como objetivos específicos foram definidos elaborar uma análise estatística sobre acidentes de trabalho ocorridos em trabalhos em altura com a especificidade de torres de telecomunicações, mastros, coberturas e fachadas de edifícios; analisar duas situações específicas de trabalho em termos de identificação e avaliação de riscos e elaborar uma proposta de medidas de controlo de segurança no trabalho, afeta aos trabalhos em altura em torres de telecomunicações, mastros, coberturas e fachadas de edifícios. Como pergunta de partida foi definida: É possivel realizar, em segurança, os trabalhos em altura em torres de telecomunicações, mastros, coberturas e fachadas de edificios? A metodologia adotada foi qualitativa e quantitativa recorrendo a pesquisa e análise bibliográfica e documental, aplicação de inquérito por questionário com análise estatística sobre acidentes de trabalho em trabalhos em altura, análise de duas situações de trabalho em altura para sua avaliação de riscos, e por fim a elaboração de um conjunto de medidas de controlo de segurança decorrente de todas as fontes, técnicas de recolha e tratamento de dados e análise de resultados utilizada. Como principais conclusões é possível referir que é possível realizar os trabalhos em altura com um nível de risco aceitável, se forem tomadas medidas de controlo adequadas à especificidade do trabalho em altura, nomeadamente nas torres de telecomunicações, mastros, coberturas e fachadas de edifícios através de planeamento adequado, alocação de recursos adequados de natureza diversa desde humanos a materiais.<br>The main objective of this work is to analyze the safety conditions in carrying out work at heights in telecommunications towers, masts, roofs and building facades, where human error or material failure can lead to very serious or fatal accidents. As specific objectives, it was defined to elaborate a statistical analysis on work accidents occurring in work at heights with the specificity of telecommunications towers, masts, roofs and building facades, analyze two specific work situations in terms of risk assessment and draw up a proposal for safety control measures at work, affecting work at heights on telecommunications towers, masts, roofs and building facades. As a starting question, the following was defined: is it possible to safely carry out work at heights on telecommunications towers, masts, roofs and building facades? The adopted methodology was qualitative and quantitative using research and bibliographic and documentary analysis, application of a questionnaire with statistical analysis on work accidents at work at heights, analysis of two work situations at height for risk assessment, and finally the elaboration of a set of safety control measures arising from all sources, data collection and processing techniques and analysis of results used. As main conclusions, it is possible to mention that it is possible to carry out work at heights with an acceptable level of risk, if adequate control measures are taken to the specificity of work at heights, namely in telecommunications towers, masts, roofs and building facades through proper planning, allocation of adequate resources of diverse nature from human to material.