Tower Legs Research Articles

New Carquinez Strait Suspension Bridge, San Francisco, California

The new Carquinez Strait bridge in California with a total length of 1060m and a major span of 728m is the first major suspension bridge to be built in North America in over thirty years. It incorporates the latest in seismic analysis, foundation design and a state-of-the-art steel orthotropic box girder superstructure. The 1927 bridge was replaced mainly because of modern seismic resistance requirements. The bridge is situated in one of the most active seismic regions in the world. In an extreme event, the bridge should suffer only repairable damage. The superstructure is welded continuously from one end to the other, and is isolated from the towers using only rocker links for supports. These rocker links allow the box girder to swing freely between the tower legs, reducing interaction between the towers and the orthotropic box girder. Because of variable geology including soft clays, claystones, shale and fractured claystones and sandstones, the tower foundations were constructed of groups of twelve piles, each consisting of 3-m diameter reinforced concrete piles cast in driven steel shells. Details of the bridge deck are given.

Measurement of aerodynamic coefficients of tower components of Tsing Ma Bridge under yaw winds

Tsing Ma Bridge in Hong Kong is the longest suspension bridge in the world carrying both highway and railway. It has two H-shape concrete towers, each of which is composed of two reinforced concrete legs and four deep transverse prestressed concrete beams. A series of wind tunnel tests have been performed to measure the aerodynamic coefficients of the tower legs and transverse beams in various arrangements. A 1:100 scaled 3D rigid model of the full bridge tower assembled from various tower components has been constructed for different test cases. The aerodynamic coefficients of the lower and upper segments of the windward and leeward tower legs and those of the transverse beams at different levels, with and without the dummy bridge deck model, were measured as a function of yaw wind angle. The effects of wind interference among the tower components and the influence of the bridge deck on the tower aerodynamic coefficients were also investigated. The results achieved can be used as the pertinent data for the comparison of the computed and field-measured fully coupled buffeting responses of the entire bridge under yaw winds.

Characteristics of lightning surges induced in telecommunication center in tropical area

The characteristics of lightning-induced surges in telecommunication equipment due to a direct lightning strike at a telecommunication center building or tower should be investigated to maintain the reliability of advanced telecommunication systems. In particular, investigations in tropical areas are important because these areas have many thunderstorm days. We observed lightning surges induced in a telecommunication building in Kuala Lumpur, Malaysia. The results show that the peak value occurrence frequency and waveforms of direct strike lightning surges were almost the same as those in a temperate area. The peak current relationships between the cables in the building and the tower legs indicate a strong correlation between the current at the tower legs, waveguide, power line, and outer and inner conductors of the coaxial cables. However, the peak value correlations between the tower leg currents and interface cable voltages were not strong. Based on the observation results, we obtained the correlation factors between the peak value at the observation point and the tower legs, and calculated the peak value at the interface cables as a function of the number of thunderstorm days.

MF AM folded monopole characteristics

Folded monopole antennas have been used for many years in medium frequency amplitude modulation broadcast applications. These antennas offer a grounded radiating structure with a height approximating a quarter wavelength. Folded monopoles have been studied exhaustively during a previous effort in an attempt to characterize the input impedance and directivity characteristics as a function of monopole height. As a result it was observed that several impedance regions can be achieved as a function of antenna height and cage diameter: Some of these regions permit a bandwidth operation sufficiently broad to support a high fidelity response for analog or digital transmissions. This type of antenna can be useful at different heights including a self matching region at the standard 50 ohms impedance. By choosing the appropriate antenna physical dimensions, antenna tuners can be avoided and thus it increases efficiency. Measurements have been made on a full scale 72 meter tower with a 6 wire cage in order to validate the calculated antenna input impedance and directivity. The cage and supporting mast make up the equivalent of a transmission line. For this reason different standard triangular towers were analyzed in order to determine their effect on the input impedance. Tower effective radius is determined taking into account the tower width and the tower leg diameter. At the same time, a minimum antenna height with maximum efficiency has been analyzed to solve the hard problem for installations at places where antenna height is a challenge for the designer.

Foundation geology of the River Dee estuary cable-stayed bridge, Flintshire, North Wales

A new highway crossing of the River Dee incorporates a major cable-stayed bridge with a single A-frame tower and a main span of 200 m. Support for the bridge is provided by barrettes that pass through 30 m of superficial Quaternary formations into bedrock strata of the Carboniferous Bettisfield Formation. Lithology of the bedrock stratigraphic sequence varies greatly over short distances and required a flexible response for foundation design. Below the north tower leg, barrettes were socketed some 7 m into massive sandstone that provided relatively good foundation bearing conditions. However, only 50 m away, below the south tower leg, each barrette required individual design to accommodate much weaker and diverse lithologies including siltstone, mudstone, coal, seatearth and disturbed beds. Here, certain barrettes extend as much as 30 m into the mixed coal measures strata. This case history illustrates the need to appreciate the localised nature of the ground conditions when carrying out a site investigation and designing foundations in areas of complex sedimentary geology.

Reradiation (echo) analysis of a tapered tower section supporting a side-mounted DTV broadcast antenna and the corresponding azimuth pattern

Reradiation from a non-vertical obstruction such as a tapered section of a broadcast antenna supporting tower is analyzed. The reradiation takes on the form of a cone, which is symmetrical about the tower leg axis, modified by the reflection coefficient that varies with azimuth. The reradiation is directed either above the horizon where it is lost or below the horizon where it interferes as an echo with the direct radiation. The reradiation has an elevation pattern which is a replica of the transmitting antenna elevation pattern, but is redirected by the nonvertical tower leg. The reradiation combines with the direct radiation to form an azimuth pattern which varies with distance from the broadcast antenna. The reradiation, being an echo, is analyzed for multipath distortion of the VSB pulse. It is shown that the impact on DTV receiver threshold is minimal.

Torsional rigidity of reinforced concrete bars with arbitrary sectional shape

In this paper, the torsional rigidity of arbitrarily shape bar made of different materials is studied on the basis of theory of elasticity and finite element approach. With additional boundary conditions for the common boundaries of different materials from the continuous conditions of deformation and traction across the interior boundary, the torsion function can be solved numerically from the second boundary-value problem of potential theory. The traction jump boundary conditions across the interior surfaces are enforced in the alternate finite element approach. Several examples are shown to check the computational approach proposed, and the approach, at last, is applied to calculate the torsional rigidity of reinforced concrete bar and some multiply connected cross sections such as tower leg section of the Tsing Ma Bridge and other engineering structures.

New Carquinez Bridge, Northeast of San Francisco, California: Technological Design Advancements

The new Carquinez Strait Bridge, northeast of San Francisco, California, will be the first major suspension bridge to be constructed in the United States since the second Chesapeake Bay Bridge in Maryland in 1973. It will replace an existing steel cantilever truss bridge, built in 1927, that was found to be seismically inadequate. The new bridge consists of an orthotropic closed steel box girder superstructure, two main cables 512 mm (20 1/8 in.) in diameter, reinforced concrete towers, and gravity anchorages. The design has set a new standard in modern suspension bridge design in the United States, particularly with respect to seismic safety. Some of the key elements of the design that are discussed are the global design loading criteria for long-span suspension bridges, the design of allowable stresses in main cable wire, the state-of-the-art design detailing of critical welded connections, the finite-element analysis approach for the box girder to determine the actual plate stresses and stress concentrations, and the design of the reinforced concrete tower leg sections for enhanced ductile seismic performance.

Measurement of lightning surges on test transmission line equipped with arresters struck by natural and triggered lightning

Measurement of lightning surges caused by natural or rocket-triggered lightning strokes has been carried out on a 275 kV test transmission line since 1987. In November 1993, lightning currents at the tower top, the ground wires, the tower legs and the transmission line arresters, along with the voltages across the insulator strings were measured simultaneously four times. One of the four flashes was natural lightning, and its peak value was +132 kA. In this case, the arresters operated, and the insulator voltages where the arresters were equipped were controlled properly. The result of a multi-phase analysis by EMTP agreed well with the measured waveforms.

Industrial system grounding for power, static, lightning, and instrumentation, practical applications

Practically every structural steel building erected for industrial use incorporates reinforcing steel bars installed in the concrete footings. This, without any additional effort, constitutes a grounding electrode. The quantity of electrodes is directly related to the building area. When the structural steel framework is erected and interconnected, these electrodes are also interconnected in a parallel arrangement. Similarly, high-voltage metal transmission towers are installed using a metal footing base embedded in a concrete filled hole. The four tower legs are effectively in parallel and offer an adequate low ground resistance such that additional electrodes are generally not required. This brings us to the main point: the reinforced concrete footings, reinforced concrete grade beams, and other reinforced concrete structures provide, with a little work, an effective, reliable, and cost attractive system grounding vehicle.

Microcomputer Analysis of Guyed Towers as Lattices

A discrete field analysis approach was used to develop a solution procedure for the analysis of guyed towers. Three truss tower configurations were considered in which the three tower legs formed the apexes of an equilateral triangle and the truss configuration on the faces of the tower were either: (1) X‐braced; (2) Warren; or (3) Vierendeel. The modeling of the tower was exact in the sense that only the usual assumptions of linear elastic behavior were used. Thus, all forces and moments in each member can be computed. The tower modeling was combined with an algorithm for including the effects of the nonlinear cable/tower interaction. The results were programmed for use on a personal computer with a problem‐oriented language enabling the user to interact as part of the design process. To illustrate the utility of the program, a 700‐ft (214‐m) tower was comparatively analyzed with an X‐braced, Warren, and Vierendeel configuration. It was found that the Warren configuration was superior to the X‐braced in terms of material required, while direct comparisons with the Vierendeel could not be made without completing the design for each.

A note on tower wake/blade interaction noise of a wind turbine

Experimental observations on wind turbine tower wake/blade interaction noise are presented. Results for circular, elliptical and square sections of the support tower legs are compared and quantified with respect to the acoustic frequency spectra produced. The results are preliminary ones obtained in a continuing study aimed at constructing a shape that may yield a lower interaction noise.

Medium Frequency Reradiation from a Steel Tower Power Line with and without a Detuner

Measurements and computations of field strengths and tower base currents are presented for a power line section consisting of 5 towers strung with skywires and illuminated by a nearby monopole antenna. This is done first without detuning stubs, and then with detuning stubs on one tower to reduce the re-radiation. Good agreement is obtained between moment-method computations and full-scale measurements. Computational modelling techniques to represent the power line, the detuning stubs and ground effects are given and validated. The beneficial effect of increased stub spacing from the tower leg is identified.

Articulated Towers for 1000-m Water Depth With Rigid Seabed Connection

Summary Deepwater oil and gas production concepts are reviewed, and a new concept based on a compliant articulated tower is presented. The means of avoiding a seabed articulation are discussed. Alternative outline structural configurations and cost data are given for water depths to 1000 m. Introduction As structures have been designed for ever deeper and more exposed waters to exploit the world's hydrocarbon resources, the cost of such structures has risen dramatically. Reasons for cost increases are closely associated with the increasing size and weight of such structures and with difficulty of installation in harsh offshore environments. It becomes economically unattractive to extrapolate conventional steel jackets to water depths greater than 400 m. There are no proposals for conventional platforms in water depths of 1000 m, although there are alternative structural forms. These second generation offshore structures, although fixed in one geographic location, are compliant and able to move with the waves. Forces thereby are reduced, enabling a reduction in structural weight and a consequential reduction in costs.More than $2 million have been spent by the DSPS (Deep Sea Production Systems) consortium of companies on an R and D program on Production Systems) consortium of companies on an R and D program on seabed hydrocarbon production. This work included the design of a multirole deepwater structure to support power generating equipment, to restrain a shuttle tanker, to support pumping and ancillary equipment, and to support numerous service risers and heavy power cables serving the subsea production plant.Various alternative structures were examined in water depths to 1000 m in an extreme North Sea-type environment with payloads in the range of 3000 to 4000 Mg. Concept of Compliant Structures It is possible to divide fixed deepwater structures into two categories - those with their fundamental natural periods being shorter than the predominant wave periods and those with their fundamental natural periods being longer.Examples of the first category of structures are conventional steel jackets. In 100-m water depth, a conventional steel platform has a fundamental natural period of around 2 seconds. In 300-m water depth, this period has increased to between 4 and 6 seconds, depending on topside load and jacket stiffness. Designers attempt to minimize this period to minimize dynamic excitation of the structure and consequent fatigue damage.An alternative solution in deep water is the other category of structures, which have long natural periods and are generally compliant. For example, a structure to be installed in the Gulf of Mexico in 1983 in 300-m water depth with a natural period of 30 seconds is Exxon Corp.'s guyed tower. The jump to the low-frequency side of the wave spectrum, in the case of the guyed tower, is made by reducing the amount of steel in the structure by reducing its cross section, thereby making it more flexible. Lateral restraint at the top of the guyed tower is provided by guys, each with a 180-Mg clump weight at the seabed before termination at an anchor pile. This is sufficient to prevent overstressing of the tower legs in heavy seas but allows a compliance of the order of 12-m excursion under design wave conditions (without an articulated joint). The guyed tower is, like a conventional platform, bottom supported. JPT P. 430

Currents induced in the human body by high voltage transmission line electric fieldߞMeasurement and calculation of distribution and dose

Electric field measurement provides a basis for calculating the induced human body currents. Distributed body current calculation procedures are presented for an erect human on the ground and on a tower leg in a working position. An electric field dosimeter measurement provided a basis for calculating the time integral of induced body currents. Practical measurements show a much lower electric field exposure than that calculated on the basis of the maximum field in a work area. This work also enables researchers to relate biological research to practical electric field induced currents in humans.

Lightning Investigation on Transmission Lines---III

The field data for 1931 and 1932 are reviewed, with especial reference to lightning stroke currents. When direct strokes take place to towers or ground wires, flashover occurs if the product of current and tower footing resistance is higher than the fiashover strength of the insulators. Flashover seldom occurs if ground wires are present and the towers have low footing resistance. Three years' records on four systems indicate tower currents varying from 1,000 to 150,000 amperes. Tripouts correlate almost exclusively with high tower leg currents of negative polarity, that is, the tower top is negative with respect to the base. This indicates that the severe lightning strokes come from negatively charged clouds. A great amount of experimental data, relating to flashover between electrodes of dissimilar shapes, are shown to be in accord with these field observations.

Questions on the economic value of the overhead grounded wire

Questions on the economic value of the overhead grounded wire