Regen‐Wind‐induzierte Schwingungen – ein State‐of‐the‐Art Report

Rain‐wind induced vibrations can occur, when rain and wind simultaneously act, for instance, on the cables of cable‐stayed bridges, on inclined steel hangers of arch bridges or on the backstays of guyed masts. As a result of wind and gravity, the rainwater forms small rivulets, which flow down on the surface of these structural members. The rivulets disturb the wind flow around the dry cross section and cause a modified unsymmetrical distribution of the surrounding wind pressure, which leads to periodic exciting forces. Due to the movement of the cable and the wind forces, the position of the rivulets varies on the cable surface. This constellation can lead to aeroelastic excitation with large amplitudes. Rain‐wind induced vibrations can reduce the life cycle of a structure seriously, because the initialising wind velocity of rain‐wind induced vibrations is significantly lower than the design wind velocity and thus has a high probability of occurrence. In this paper, the further researches and investigations up to now dealing with rain‐wind induced vibrations are summed and commented.     

Regen‐Wind‐induzierte Schwingungen – Ein Berechnungsmodell auf der Grundlage der neuesten Erkenntnisse

Rain‐wind induced vibrations – a calculation model based on recent investigations. Rain‐wind induced vibrations of slender structural members with circular cross sections, as of bridge‐cables or hangers of tied arch bridges may occur when these members are exposed to both wind and rain. This vibration phenomenon is known to bridge‐engineers after the spectacular closing of the Erasmus Bridge shortly after inauguration and the damages at hangers of the Dömitz Bridge across the river Elbe. For the onset of rain‐wind induced vibration various parameters as the intensity of rain, the wind velocity and the wind direction in relation to the slope of the member play an important role. Vibrations once die away, when the intensity of rain or the wind velocity change significantly or the wind direction varies. Rain‐wind induced vibrations are self‐induced vibrations where the inducement mechanism from the variation of the position of the rain rivulets interacts with the movements of the cables or hangers. This type of vibration is not novel, however the mechanism has so far not yet been sufficiently investigated to produce a calculation model that allows predicting the magnitude of the vibration amplitudes.     

3D‐Tragwerkssimulationen zur Erfassung der aerodynamischen Beanspruchungen bei der Planung von Brückenstrukturen

On the Modeling of Aerodynamic Effects in the Design Phase of Bridges via 3D Structural Simulations. The use of innovative building materials has led to a trend towards increasingly light and slender structures. Especially for cable‐stayed and suspension bridges, but also for girder bridges, there has been observed a growing dynamic vulnerability. The vibrations are caused, besides pedestrian and traffic‐induced vibrations, mainly by wind action. Apart from local vibrations of cables, the global dynamic behaviour of the bridge deck is of special interest. This paper gives an overview of the analysis methods currently used in practice. Load models which are based on a two‐dimensional treatment of the flow around the cross‐sections and their numerical treatment within the framework of a 3D structural simulation are presented. The experimental measurement of stationary and instationary cross section parameters is briefly recapitulated. Application in engineering practice in both preliminary and final design phases is demonstrated by four examples.     

Numerische Untersuchungen zur Ermüdungsbeanspruchung vorgespannter Ringflanschstöße mit Imperfektionen

Numerical investigations on the fatigue‐relevant bolt stresses in preloaded ring flange connections with imperfections. For the structural design of preloaded bolted ring flange connections in tower‐like steel structures (e.g. chimneys or wind turbine tubular towers), the fatigue assessment of the bolts is very important. Gaps between the contact surfaces before preloading (so‐called flange imperfections) may have a negative influence on the fatigue‐relevant bolt stresses. A FE‐model for imperfect ring flanges was built up and validated by means of the results of large‐scale tubular bending tests on flange connections. With the validated FE‐model, a parametric study on imperfectly simulated L‐flange connections having realistic dimensions of typical wind turbine towers was performed. The results enable differentiating the various gaping forms with regard to their negative influence on the bolt fatigue. This influence may not be neglected when assessing the fatigue safety of the bolts.     

Einsparpotentiale beim Bau von Kraftwerken am Beispiel von Windeinwirkungen auf Kühltürme

Financial safety potential of power plant constructions by example of wind loads on cooling towers Wind loads are one of the most dominant loads in power plants. In this paper, the financial benefit of windtechnological analyses on large‐scale power plants is examined exemplarily by means of cooling towers in the context of the new German wind code DIN 1055‐4:2005‐03. The results of the investigations clarify the amount of financially quantifiable savings as a result of windtechnological analyses.     

Mitigation of wind‐induced motion of Milad Tower by tuned mass damper

With a height of 435 m, Milad Tower, situated in north‐west of Tehran, Iran, would be the fourth highest telecommunication tower of the world. This tower has the largest head structure among its counterparts. Preliminary studies demonstrate that the upper part of the tower has excessive wind‐induced acceleration‐related vibrations beyond human comfort limit during wind events. In this paper, the effectiveness of tuned mass damper (TMD) on the suppression of wind‐induced motion of Milad Tower is examined through mathematical analyses. The tower is modelled as a vertical linear cantilever beam, with 57 degrees of freedom. The fluctuating wind speed is assumed to be a stochastic process which is identified by an appropriate power spectral density function. Random vibration analyses were carried out to determine response statistics. The possible application of TMDs in suppressing wind‐induced motions of the tower was investigated, and a TMD was designed to be installed at the sky dome. The results clearly show that the designed TMD has a considerable influence on the suppression of the structural response of the tower below human comfort limits. The authors believe that Milad Tower can be considered as a benchmark control problem for television and telecommunication towers by the structural control community. Copyright © 2008 John Wiley & Sons, Ltd.     

来流湍流度对斜拉索风雨激振影响的理论研究

斜拉索风雨激振严重地影响斜拉桥的安全运营,而来流湍流度是影响风雨激振的一个重要因素。采用理论分析方法,研究来流湍流对斜拉索风雨激振响应的影响。引入2种类型的风场,首先,假设来流为简谐脉动风场,然后,采用谐波合成方法,将Kaimal风谱作为目标谱,模拟来流随机脉动风场。采用上述2种来流风场,在已建立的两质量三自由度理论模型的基础上,研究各种湍流度下拉索和水线的风雨激振振动规律。计算结果表明,当湍流度达到15%时,水线不能在拉索表面稳定地存在,风雨激振能得到有效的抑制;水线运动的脉动成分主要受来流风脉动成分的影响,拉索运动对水线运动有一定的影响,但影响较小;随着紊流度的增加,起振风速也增大。     

Improving the wind‐induced human comfort of the Beijing Olympic Tower by a double‐stage pendulum tuned mass damper

With five sub towers and a maximum height of 246.8 m, the Beijing Olympic Tower (BOT) is a landmark of Beijing. The complex structural properties and slenderness of the BOT render it prone to wind loading. As far as the wind‐induced performance of this structure is concerned, this paper thus aims at a tuned mass damper‐based mitigation system for controlling the wind‐induced acceleration response of the BOT. To this end, the three‐dimensional wind loading of various wind directions are simulated based on the fluctuating wind force obtained by the wind tunnel test, by which the wind‐induced vibration is evaluated in the time domain by using the finite element model. A double‐stage pendulum tuned mass damper (DPTMD), which is capable of controlling the long period dynamic response and requires only a limited space of installation, is optimally designed at the upper part of the tower. Finally, the wind‐induced response of the structure with and without DPTMD is compared with respect to various wind directions and in both the time and frequency domains. The comparative results show that the wind‐induced accelerations atop the tower with the wind directions of 45, 135, 225, and 315° are larger than those with the other directions. The DPTMD significantly reduces the wind‐induced response by the maximum acceleration reduction ratio of 30.05%. Moreover, it is revealed that the control effect varies noticeably for the five sub towers, depending on the connection rigidity between Tower1 and each sub tower.     

Praktisches Verfahren zur Bemessung von Brückenseilen und ‐hängern zur Vermeidung Regen‐Wind‐induzierter Schwingungen

A semi‐empirical method for the design of bridge cables and bridge hangers in order to avoid rain‐wind induced vibrations. Rain‐wind induced vibrations have been widely studied during the last decades. Today it is desirable to use the experience gathered in this filed in order to develop a practical design procedure offering both security and economy. After a restricted review of some analytical models, this paper points out the need for a semi‐empirical method if the complexity of this design procedure has to be kept at its minimum. Based on measured data collected during field tests and wind tunnel experiments, a new proposal is suggested.     

Wind effect of a twin‐tower super high‐rise building with weak connection

An analysis and estimation method of multibalance synchronous test is established to study the wind effect of a complex super high‐rise building with weak connection. First, the frequency domain method is applied to deduce the calculation process of the wind effect of the multitower structure on the basis of the high frequency force balance (HFFB) technique. Then, the synchronous force test of HFFB is conducted on a twin‐tower super high‐rise building connected by a bridge. The wind‐induced response and loads and the interference effect between the two towers are analyzed based on the wind tunnel test data. The displacement correlation between the towers and the relative displacement of the multitower structure are investigated. Results show that the maximum and minimum relative displacements in the along‐bridge direction are 0.26 m in the along‐wind direction and ?0.26 m in the crosswind direction, respectively. The channeling effect formed by the surrounding buildings is the main cause of the maximum cross‐bridge displacement. The influence of the correlation between the two towers can be ignored for the along‐bridge relative displacement. The results of the HFFB and high‐frequency pressure integral test agree with each other, thereby indicating the reliability and effectiveness of the proposed method.     

Wind tunnel: a fundamental tool for long-span bridge design

An overview of wind tunnel activities and methodologies to support the design of long-span suspension bridges is proposed. The most important aspects of the wind-bridge interaction are investigated considering the aerodynamic phenomena affecting the different parts of the bridge (mainly deck and towers). The experimental activities and results are proposed in the framework of a synergic approach between numerical and experimental methodologies that represent the common practice in defining the full scale aeroelastic behaviour of the bridge starting from scaled reproduction of the wind-bridge interaction. Static and dynamic wind loads, aeroelastic stability, vortex-induced vibrations will be investigated.     

Vorgespannte Betontürme für Windenergieanlagen: Einflüsse nichtlinearer Materialeigenschaften auf die Eigenfrequenzen als Entwurfskriterium

Pre‐stressed Concrete Towers for Wind Energy Turbines: Influences of non‐linear Material Properties on the Eigenfrequencies as a Design criterion The fast increase of the wind energy market in Germany promoted a dynamic development of wind turbine technology in the last decades. New concepts in the fields of construction, control and generation resulted in improved quality and efficiency of the wind turbines. With the growing rotor size and tower height new questions arise for the structural engineers and the building companies. In order to distribute the large forces at the tower‐head down to the soil and to balance the eigenfrequency of the structure with the excitation of the wind‐turbine, in general pre‐stressed concrete towers are necessary in case of tower heights beyond 100 m. The present paper will summarize the main design criteria for such towers and will explore the influences which are relevant for a reliable dynamic design.     

遗传算法和人工神经网络在斜拉桥可靠度分析中的应用

将遗传算法(GAs)和人工神经网络(ANN)这一类智能方法引入斜拉桥可靠度分析领域,分别对斜拉桥主梁和索塔在多种失效模式下的可靠度进行计算和分析.实例数值结果表明,遗传算法和人工神经网络为结构可靠度研究提供了新的有效的思路和手段.     

Bemessungskonzept für fußgängerinduzierte Brückenschwingungen

Design concept for the analysis of pedestrian‐induced bridge vibration. Unexpected vibrations of footbridges, e.g. during inauguration of Millennium Bridge in London and Passerelle Solferino in Paris, have drawn engineers' attention to more accurate forecast models to determine pedestrian‐induced vibrations and especially pedestrian‐structure‐interaction of light and slightly damped footbridges in the design phase. This contribution presents a programme to simulate bridge vibrations with realistic ‘numerical pedestrians’, which is calibrated against measured response under real pedestrian loading. By means of this simulation tool observed vibrations and pedestrian‐structure‐interaction phenomena can be analysed. In a second step an engineering model based on a spectral approach is derived, which enables to determine characteristic pedestrian‐induced structural response. Hence, if necessary the design of the footbridge can be adjusted or damping measures can be planned to fulfill serviceability requirements. A design concept for the dynamic analysis of the reversible serviceability limit state, which fulfills the Eurocode requirements, is described and an innovative consideration and definition of comfort criteria are proposed.     

A combined tuned damper and an optimal design method for wind‐induced vibration control for super tall buildings

The wind‐induced vibrations of super tall buildings become excessive due to strong wind loads, super building height and high flexibility. Tuned mass dampers (TMDs) and tuned liquid column dampers (TLCDs) have been widely used to control vibrations for actual super tall buildings for decades. To fully use both the economic advantage of the TLCD system and the high efficiency of the TMD system, an innovative supplemental damping system including both TLCD and TMD and called combined tuned damper (CTD), which can substantially decrease the cost of the damper, was proposed to control the wind‐induced vibrations of tall buildings. The governing equations are generated for the motion of both the primary structure and the CTD and solved to anticipate the dynamic response of the CTD‐structure system. Moreover, an optimal design method of human comfort performance is proposed, in which the life cycle cost of the damper‐structure system is considered as the quantitative index of the performance. The life cycle cost includes the initial cost, the maintenance cost and the failure cost. The failure cost can be calculated using the vibration‐sensation rate model, which is based on the Japanese code AIJES‐V001‐2004. Copyright © 2016 John Wiley & Sons, Ltd.     

Design of viscous dampers targeting multiple cable modes

Rain–wind induced stay cable vibrations may occur at different cable eigenfrequencies. Therefore, external transverse dampers have to be designed for several target cable modes. The resulting modal damping ratios have to fulfil Irwin’s criterion for minimum Scruton number such that rain–wind induced vibrations can be excluded. For this situation, this paper presents a systematic and easy applicable design procedure for linear viscous dampers that respects Irwin’s criterion, minimizes the damper position and leads to almost minimum variance of the target modal damping ratios. Minimum damper position is preferable from the aesthetic point of view, and it minimizes the installation costs, reduces the damper support flexibility and thereby increases the damper efficiency. Minimum variance of the target modal damping ratios maximizes the safety against large amplitude vibrations due to the unpredictability of the predominant mode.     

风荷载作用下钢塔架的疲劳寿命预估

结构的风致振动使得结构产生围绕平均应力的脉动应力 ,它能导致结构的疲劳累积损伤 ,可能造成结构在低于设计风荷载的各种水平风荷载往复作用下而失效。由于风荷载是钢塔架等钢结构的主要荷载 ,风荷载作用下上述结构的疲劳寿命预估因而是至关重要的。在作了一些简化假设后 ,得到了钢塔架疲劳寿命。由于塔架结构的顺风向响应为宽带过程 ,在考虑了窄带共振响应和宽带背景响应后 ,用等效窄带法得到了其疲劳寿命的计算公式。最后给出了一个疲劳寿命计算的工程实例。     

Non‐Gaussian characteristics and extreme distribution of fluctuating wind pressures on large cylindrical–conical steel cooling towers

Large cylindrical–conical steel cooling tower (SCT) represents a new configuration of cooling tower, and its wind load distribution pattern and forming mechanism are very different from those of the traditional hyperbolic cooling towers. Large eddy simulation was used for the numerical simulation on a superlarge cylindrical–conical SCT that exceeds the specification limit, which is also the highest (189 m) SCT under construction in Asia. The surface flow field and time history of 3‐D aerodynamic force were obtained for the cylindrical and conical parts, respectively. Comparison with the measurements of other large cooling towers and the results of wind tunnel test confirmed the validity of the numerical simulation. Then, based on the probability density distribution and spatial correlation of representative measuring points, regions of non‐Gaussian distribution were identified. The forming mechanism of non‐Gaussian wind pressure distribution was revealed from the perspective of the correlation of non‐Gaussian distribution versus flow separation and eddy motion. The criteria for classifying the region of non‐Gaussian distribution for the cylindrical and conical parts were analyzed, respectively. Research shows that the wind pressures in the windward regions of conical and cylindrical parts obey Gaussian distribution; however, the wind pressures from the region of extreme negative pressure to the region of flow separation are largely non‐Gaussian and the wind pressures of the conical part are generally non‐Gaussian in the leeward region. Finally, the three algorithms for calculating the extreme values of wind pressure were used, namely, peak factor method, improved peak factor method, and Sadek–Simiu method. The distribution patterns of peak factors and extreme values of wind pressure in SCT towers were analyzed comparatively. The 2‐D formulae for fitting the extreme values of wind values for the cylindrical and conical parts were derived by nonlinear least square method. Moreover, strategy for value determination was also presented. The present research aimed to strengthen the understanding of the fluctuating wind pressure distribution and its forming mechanism for large cylindrical–conical SCT towers.     

Sicherheit und Restlebensdauer altersgeschädigter Naturzugkühltürme

Safety and residual service life of aging natural draft cooling towers Since over 40 years, large natural draft cooling towers dominate thermal power stations in Europe. With actual heights up to 200 m, they balance thermal plant efficiencies closely to 50% with environmentally soft releases of residual process‐heat. Between 1965 and 1984, a series of cooling tower collapses hit the international power technology, from which Germany remained unaffected. But some of the German towers demonstrate, after 35 and more years of service life, severe aging effects due to survived storms and service conditions. The article describes, after preliminary remarks on design characteristics of natural draft cooling towers, typical damage phenomena which may affect structural safety as well as residual service life. Then the paper turns to computer simulations of degrading towers to determine reductions of safety and residual life, modeling aging effects of gales and service deterioration.