Slender Towers Research Articles

Structural capacity and the 20 MW wind turbine

This article addresses a generic and fundamental question: given the specification for a structure, how can one assess or even express its capacity without first designing it? The question is motivated by the consideration of how wind turbines (offshore wind turbines in particular) may scale up further. An underlying premise is that the wind-turbine designs that have evolved from 50 kW to 5 MW may warrant re-examination to check whether scaling effects demand new approaches at larger scales. Since several in the industry are already setting sights on 8 MW and even 10 MW machines as extrapolations of current designs, this article considers the development of a 20 MW (232 m diameter) machine. The conventional three-bladed arrangement on a slender tower is examined for notional 5 and 20 MW machines for similar rated wind conditions. Initially, the article considers the structural capacity requirements for conventional designs at both 5 and 20 MW and how these can be reduced substantially – using a floating framework in place of a tower and by deploying cable bracing on the rotor. When tower bending and the non-productive blade bending moments have thus been addressed, the turbine torque itself emerges as a major cause of cost. The turbine torque directly influences the costs of blades, pitch and yaw bearings, direct-drive generators (if present), gearbox (if present), and main shaft, and it is confirmed that in all cases, the total amounts of material required are at least proportional to the turbine torque for conventional designs. An alternative wind-turbine configuration is examined. In this, gravity is co-opted as an ally rather than an enemy, and power conversion takes place entirely within the rotor by allowing gravity to move bodies within the rotor. The concept is illustrated for a 20 MW machine and shown to be entirely workable in principle. A strong feature of this proposed new machine configuration is that the costs are almost exclusively structural and can be quantified in terms of the uniform currency of structural capacity. The scalability of this concept becomes clear by observing that the mean velocities of the moveable masses relative to the main rotor structure are virtually independent of machine scale.

Investigation of dynamic response of masonry minaret structures

Almost all historical minarets in Turkey were constructed using cut stone, masonry blocks or combination of these two materials. The structural and geometrical properties of each masonry minaret, or slender tower structure, depend on many factors including the structural knowledge and applications at the time of construction, experience of the architect or engineer, seismicity of the region, and availability of construction materials in that area. Recent earthquakes in Turkey have shown that most masonry minarets in high seismic regions are vulnerable to structural damage and collapse. In this study, in order to investigate the dynamic behavior of historical unreinforced masonry minarets, three representative minarets with 20, 25, and 30 m height were modeled and analyzed using two ground motions recorded during the 1999 Kocaeli and Duzce, Turkey earthquakes. The modal analyses of the models have shown that the structural periods and the overall structural response are influenced by the minaret height and spectral characteristics of the input motion. The dynamic displacement and axial stress time histories are computed at the critical points on the minarets. During recent earthquakes, most minaret failures occurred above the base of the structure. Consistent with the observed response, the largest stresses were calculated at the same location.

Dynamic analysis and seismic performance of reinforced concrete minarets

An unusually large number of minarets, which are slender tower structures, collapsed during the 1999 Kocaeli and Duzce, Turkey earthquakes with resulting damage to surrounding buildings and loss of life. The potential effects of the subsequently observed poor reinforcement detailing on the dynamic response is discussed. The probable cause of the extensive damage to reinforced concrete minarets is investigated by studying the observed failure modes and their seismic performance, and through the dynamic analysis of a representative minaret. The effects of spiral stairs, door openings, and balconies on the dynamic behavior are examined. The maximum dynamic internal force demands were compared with the calculated capacities. The locations of the maximum axial, shear, and flexural demands predicted from the finite element analysis of the minaret model were consistent with the earthquake damage observed at those critical locations.

Influence of Stabilizing Cables on Seismic Response of a Multispan Cable‐Stayed Bridge

Abstract: A recent trend in the design of long‐span bridges is multispan cable‐stayed bridges with three or more towers. A critical problem of multispan cable‐stayed bridges is the stabilization of the central tower(s), which has resulted in increasing application of stabilizing cables. The Ting Kau Bridge in Hong Kong is one of a few multispan cable‐stayed bridges adopting stabilizing cables ever built. In this article, the dynamic properties of multispan cable‐stayed bridges with stabilizing cables and the effect of stabilizing cables on bridge seismic response are studied by referring to the Ting Kau Bridge. Based on a validated 3D finite‐element model, modal analysis is conducted which shows that the longitudinal stabilizing cables bring about a number of global modes with strong modal interaction among the deck, towers, and cables. The seismic response and internal force in the central tower are found to be much larger than those in the side towers. The longitudinal stabilizing cables are very effective in reducing the internal force in the central tower generated by longitudinal earthquake excitation, but insignificantly affect the seismic response in the bridge deck and side towers. As a whole, the stabilizing cables act favorably in the reduction of seismic response of the bridge.

Computations of structures formed by the solidification of impinging molten metal drops

Solidification of multiple molten metal drops impacting a cold wall is simulated numerically in axi-symmetric coordinates. The numerical method fully couples fluid flow and heat transfer and is based on a single set of conservation equations written for all phases. Solidification of the melt is modeled by simply assuming that the material freezes when the temperature falls below the prescribed freezing point. Since the shape of each drop depends on the Weber number, Peclet number, and nondimensional superheat, various structures can be built with precise control of these parameters. The deposition of low Weber number drops produce tall, slender towers, while high Weber number drops make short, squat towers. Numerical results also show that sufficiently high cooling rates generally result in final splat diameters independent of Weber number as the drops are not given time to fully deform before solidification. Voids or indentation may develop within the tower of solid drops when the cooling rates are lower than that required for full extensional deformation of the deposited liquid drop, and this phenomenon is characterised by a porosity factor.

Wind tunnel study of the across-wind response of a slender tower with a nonlinear tuned mass damper

The effectiveness of a class of nonlinear tuned mass dampers (TMDs) in suppressing across-wind structure oscillations was examined through a wind tunnel test. The nonlinear TMD employed a wire rope spring to replace both the spring and the damper required in a typical system. First, a single degree of freedom aeroelastic stick model of a slender structure with a square cross-section was tested under different levels of damping. The measurements obtained from the tests were used to determine the root-mean-square (RMS) of the lift coefficient and other aerodynamic parameters. In the second phase of testing, the nonlinear TMD system was attached near the tip of the aeroelastic model. The response reduction achieved by adding the TMD was considerable and was quantitatively expressed in terms of an equivalent viscous damping. A comparison between the nonlinear TMD and an equivalent optimized linear TMD was made. Probability-based procedures were developed to estimate the equivalent damping provided by the nonlinear TMD. The estimated damping was compared with that obtained experimentally to evaluate the accuracy of the prediction method.

Unexpected or Unknown Phenomena

In this discussion of numerical and physical modeling, I would like to mention one advantage of model testing that you do not find to the same extent in numerical analysis: the possibility of detecting unexpected or unknown phenomena. I would like to illustrate this aspect by referring to a couple of examples of test results from my years of experience in model testing of offshore structures. The first example is from the testing of an articulated tower—a type of structure that was extensively used for mooring and transfer of oil from production platforms to shuttle tankers some years back. It consists of a slender tower, fixed to a foundation plate by a universal joint, and the foundation plate finally piled to the seafloor. Our tests included towing the tower horizontally in waves to verify the predicted wave loads during tow-out. When we hoisted the model into the basin, something unexpected happened. The base plate tilted out to an angle way beyond the design capacity of the articulation. If this had happened in prototype, it would have destroyed the articulation, the jumper hoses, etc. The designers had done all kinds of extensive finite element structural and hydrodynamic analysis of the tower and base plate to determine stress levels, but they had forgotten to do a simple check of the hydrostatic stability of the baseplate. Of course, once revealed by the model test, the problem was easily rectified by proper ballasting of the base plate during tow-out. The tower is still operating satisfactorily today. Another example: Many years ago we tested a rather unusual vessel designed for a very specific offshore application. It was more than one km long and consisted of two steel pipes of a couple of meters diameter connected by a narrow deck structure. The main purpose of the tests was to verify the section forces and bending moments in waves. All went well as long as we tested the vessel in waves and current in the longitudinal direction of the vessel. However, as soon as we applied some current in the transverse direction, the vessel flipped over and capsized. The immediate reaction of the designers was surprise, since the vessel had very large metacentric height. (It had been carefully calculated in advance!) However, having seen the vessel capsize in the basin a few times, it was easy to understand the physics of it. The structural design of the vessel resulted in a relatively large bending stiffness in the horizontal plane and a much lower bending stiffness in the vertical plane. Combined with a large length-to-width ratio, this led to instability and capsize even for relatively small lateral forces on the structure. Due to this instability revealed in the model test, this vessel never went further than the model test stage. I could mention many examples where unexpected, unknown, or forgotten phenomena have been discovered in model testing of offshore structures. Sometimes the problem has been easy to rectify, other times it has led to discarding the whole idea of the structure or concept. Safeguarding against such surprises can hardly be done by numerical analysis, since numerical analysis can only deal with those phenomena that are included in the underlying theory of the analysis program. So, my message may sound like advertising for the model basins. My recommendation is to take care when designing offshore structures and to perform careful model testing of the complete structure, system, or marine operation. The model testing will verify that no significant phenomenon has been forgotten in the numerical analysis.

Vortex shedding from a circular cylinder in a smooth and wired configuration: comparison between 3D LES simulation and experimental analysis

Vortex shedding from cylindrical structures with circular section is a widely studied phenomenon since this problem is of interest with respect to many technical applications. By means of experimental tests, many aspects of this very complex topic have been investigated and several devices have been studied in order to reduce the forcing induced by fluid-structure interaction. A particular device, consisting of two wires, having a diameter smaller than that of the cylinder, helically wrapped around the cylinder itself, will be analysed in this paper. Devices like this, whose main idea is to disorganise or modify the flow field near the cylinder surface, are already used in different technical applications such as in civil applications (slender towers, smokestacks) or in maritime applications (pipelines or risers) to take only a few examples. The efficacy of this solution has been investigated, in this paper, both by experimental tests and by numerical simulations. Experimental tests have been carried out in a water channel located at Politecnico di Milano and have produced data for a velocity range that covers Reynolds numbers from 2×10 4 to 4×10 4. Numerical simulations, performed by using a commercial CFD code (FLUENT) with a large eddy simulation (LES) approach, have been feasible only for the lowest values of the Reynolds number, due to the large computational power required for these kinds of applications. Simulations have been performed for a 2D and a 3D configuration and have been compared with experimental results.

SIGNIFICANT GROUND MOTION PARAMETERS FOR EVALUATION OF THE SEISMIC PERFORMANCE OF SLENDER MASONRY TOWERS

The evaluation of seismic risk of masonry monuments requires to study the combination of vulnerability and hazard. In the present work, the global seismic response of slender masonry towers has been studied by means of a specific 3-D fibre model. Accounting for the particular behaviour of such structures, the hazard should also be described by a suitable measure of intensity of the seismic action. A variety of different parameters relating with the ground acceleration recordings have been investigated for what regards their correlation with the damage indicators of the model. The combination of the peak ground velocity of the horizontal component and of the significant duration is an effective measure of intensity. This measure can be improved by considering the accord of the frequency content of the ground motion with the dynamical characteristics of the tower. Since in some cases the effect of the vertical component proved to be important, a further improvement can be obtained by taking into account also the vertical ground motion intensity.

Numerical Simulation of Tidal Current Force on Twin-Tower Pier

Twin-Tower Pier was designed as a new type of pier for a bridge which is constructed at deep sea site. The feature of the Twin-Tower Pier is two slender towers that are enable to reduce the volume of the concrete, resulting the reduction of the construction cost. Tidal current forces acting on the Twin-Tower Pier had been measured by the hydraulic model test in previous paper because the shape of the pier is more complicated than the shape of a conventional pier which is usually a column type. This paper describes the numerical simulation which is modeled to predict the tidal current force on the Twin-Tower Pier without using hydraulic model tests. The tidal current forces predicted by the numerical simulation are compared with those measured by the hydraulic model test. The comparison between them shows good agreement with the error level of less than 10%.

Moment-curvature relationships to estimate deflections and second-order moments in wind-loaded RC chimneys and towers

Second-order moments of considerable magnitude arise in tall and slender RC chimneys and towers subject to along-wind loading, on account of eccentricities in the distributed self-weight of the tower in the deflected profile. An accurate solution to this problem of geometric nonlinearity is rendered difficult by the uncertainties in estimating the flexural rigidity of the tower, due to variable cracking of concrete and the `tension stiffening` effect. This paper presents a rigorous procedure for estimating deflections and second-order moments in wind-loaded RC tubular towers. The procedure is essentially based on a generalised formulation of moment-curvature relationships for RC tubular towers, derived from the experimental and theoretical studies reported by Schlaich et al. 1979 and Menon 1994 respectively. The paper also demonstrates the application of the proposed procedure, and highlights those conditions wherein second-order moments become too significant to be overlooked in design.

Adumbrations of power and the politics of appearances in Medicean Florence

As Nicolai Rubinstein has so eloquently appreciated, no single building in Florence has witnessed as much change or been endowed with as much sym bolic significance as the Palazzo della Signoria, also known as the Palazzo Vec chio.1 It has been a continuous seat of government in Florence since its completion in the early fourteenth century. With its imposing proportions, rusticated stone facade, and slender tower, the Palazzo Vecchio has long been associated with the history and culture of the Florentine Renaissance; within its walls humanists like Coluccio Salutati and Leonardo Bruni delivered their orations in praise of Florence and its republican traditions, and nearly a century later, Niccolo Machiavelli laboured there as secretary to the Second Chancery and to the Died in charge of foreign affairs.2 The Palazzo was originally commissioned in 1298 to be the seat of the guild dominated government of the newly constituted Gonfaloniere di Giustizia, or Standardbearer of Justice, and the Council of Priors. The new government traced its foundations to the victory of the Guelf faction over the Ghibellines and with the successful exile of unruly magnate clans from the city. From its very beginnings the Palazzo made a powerful political statement, for it was built on the site of the ruined medieval towers belonging to exiled magnate families. The Palazzo thus proclaimed a Guelf victory and the power of the city's wealthiest merchant guilds to restore order and govern itself. But by the late fourteenth and early fifteenth centuries, guild government gave way to oligarchy, and first the Albizzi faction and then, in 1434, the Medici faction, led by Cosimo de' Medici, managed the city's affairs, in practice if not in name.

Seismic Retrofit of Vincent Thomas Suspension Bridge

The Vincent Thomas Bridge in Los Angeles, California, is a major suspension bridge with a total length of 1848 m (6,060 ft), including approaches, and a main span bridge 766 m (2,513 ft) long, whose center span is 458 m (1,500 ft). The bridge, built in the 1960s, used the seismic codes of the time. The site conditions and tall, slender towers and bents have resulted in vulnerabilities that are particular to suspended bridges. This paper focuses on the suspended spans. The bridge is located within the Port of Los Angeles in a vicinity that has a legacy of earthworks associated with such projects as dock expansion, fuel lines, and rail lines. Geotechnical conditions include an active fault running between the main towers with a potential for liquefaction, fault rupture, and lateral spreading. A nonlinear time history analysis of the main suspended bridge was performed, using ADINA. The model included the effects of geometric stiffness of the cables and of multiple-support excitation of the bridge. The critical vulnerabilities detected in this study, arising from large predicted longitudinal motions, indicated distress in the towers, in the cable bents near the anchorages, in the short midspan suspenders, and in the stiffening trusses. During the retrofit study the nonlinear behavior was further developed, including tower rocking and uplift, the postyield deformations of the tower, and the optimization of the damper properties. Retrofit measures include releasing of the side-span trusses at the cable bents, creating fused hinges in the stiffening trusses, the installation of dampers at these locations and between the stiffening trusses and the towers of the bridge, stiffening of the bridge towers to prevent plate buckling, and cable and suspender modifications.

Soil-structure-water interaction of intake-outlet towers allowed to uplift

A procedure that can be used for preliminary seismic analysis of intake-outlet towers including soil-structure-water interaction is developed. The formulation also considers the effect of partial soil-foundation separation, a phenomenon of considerable interest for the design of such systems that has not been addressed in the literature. The hydrodynamic pressure of the water is accounted for through added masses given in concise closed-form expressions for easy use in analysis and design. The nonlinear equations of motion, based on foundation-soil bond conditions, are solved numerically. The effects of soil-structure-water interaction are evaluated for a representative intake-outlet tower. Parametric studies are also conducted in the presence and absence of surrounding and contained water for typical cases of soil conditions and tower height-foundation width ratios. The results indicate that hydrodynamic effects are significant and cause an increase in deflections, moments and shears and a decrease in foundation rotation. The study shows that for short towers, foundation uplift is unlikely to occur. On the contrary for slender towers, uplift is more likely to appear, especially for foundations supported by stiff soil, causing significant decrease in moments and deflections.

Seismic analysis of towers including foundation uplift

A seismic analysis procedure including soil-structure interaction and partial foundation uplift for tower structures is developed. The nonlinear equations of motion are derived with the aid of Lagrange's equation. Parametric studies are performed to evaluate the effects of factors such as: soil stiffness, ratio of tower height to foundation width, and partial separation of the foundation from the soil. The studies indicate that the effects of soil stiffness on a short tower are greater than on a slender one. The height-width ratio affects the seismic response of the tower significantly, especially for a tower at a rock-like location. The allowance of foundation uplift may substantially reduce the seismic response of moment and foundation rotation in the case of hard soil and a slender tower, or may greatly increase the seismic response of shear in the case of hard soil and a short tower. The study concludes that uplift is not always beneficial and its effects could be significant for structures under strong seismic motions.

Lösung linearer Differentialgleichungen beliebiger Ordnung mit Polynomkoeffizienten und Anwendung auf ein baustatisches Problem

AbstractAufbauend auf das in [1], [2] angegebene Lösungskonzept für lineare Differentialgleichungen mit konstanten Koeffizienten wird eine Erweiterung auf den Fall nichtkonstanter Koeffizienten (Polynome beliebigen Grades) vorgenommen. Sowohl das allgemeine Integral der homogenen Differentialgleichung als auch das partikuläre Integral wird mit Hilfe der Lösungsfunktionen bj beschrieben, wobei letztere mittels Reihenformeln bestimmt werden. Beliebige Sonderfälle (z. B. einzelne Koeffizienten sind konstant oder Null) werden durch den allgemeinen Fall erfaßt. Anhand des angegebenen Struktogramms kann das Verfahren leicht programmiert werden. Die praktische Anwendung wird schließlich mit der Berechnung eines schlanken Turmes nach der Theorie II. Ordnung gezeigt, wobei sowohl Querschnitt als auch Normalkraft veränderlich sind.

Block gliding and rheological deformation in the Southern Sydney basin

SUMMARY Block gliding caused by low frictional resistance or by the deformation of plastic substrates has been well documented from many parts of the world, but neither of these mechanisms explains the widespread gliding of sandstone blocks away from cliffs in the southern Sydney Basin of south‐eastern Australia. The movement of large blocks over declivities from near zero to a maximum of 5°, high frictional resistance and lack of high porewater pressures rule out a simple sliding mechanism and it is unlikely that slender towers of sandstone could have survived seismic vibration sufficient to overcome frictional resistance to gliding. Highly preferential dip control of the direction of gliding and of the development of benches over which the blocks move, together with mounds, similar to pressure ridges, on the benches, indicate that the block gliding is due to the rheological deformation of the underlying rock, even though it is a sandy siltstone lacking readily deformable beds. Estimated rates of deforma...

Windforces and strains of reinforced concrete chimneys

For the design and construction of slender towers such as chimneys a realistic description of windforces for the reason of security and economy is important. To investigate the loads acting immediately on these types of structures through the natural wind and the resulting structural response in and transverse to the mean wind direction, full scale measurements were done at an reinforced concrete chimney of 230 meters hight. The instrumentation is described and first results will be presented.

SEISMIC RESPONSE OF MODERN SUSPENSION BRIDGES TO ASYNCHRONOUS VERTICAL GROUND MOTION.

Keywords BRISTOL UNIVERSITY SEISMIC, EARTHQUAKES, GROUND, SUPPORTS, SUSPENSION, BRIDGES, LONG SPAN, SPEED, HANGERS, INCLINED, HUMBER BRIDGE, TOWERS, PROPAGATION, METHODS, DYNAMICS, DISPLACEMENT, STRESS, FORCES BOSPORUS, HUMBER TURKEY, UK... Show All

Earthquake response of steel‐framed multistorey buildings with set‐backs

AbstractA study is made of the dynamic behaviour of multistorey steel rigid‐frame buildings with set‐back towers. The effects of set‐backs upon the building frequencies and mode shapes are examined. Then the effects of set‐backs on seismic response are investigated by analysing the response of a series of set‐back building frame models to the El Centro ground motion. Finally, the computed responses to the El Centro earthquake are compared with some code provisions dealing with the seismic design of set‐back buildings.The conclusions derived from the study include the following: The higher modes of vibration of a set‐back building can make a very substantial contribution to its total seismic response; this contribution increases with the slenderness of the tower. Some of the important response parameters for the tower portion of a set‐back building are substantially larger than for a related uniform building. For very slender towers, the transition region between the tower and the base may be subjected to very large storey shears.