Across-wind Response Research Articles

The acrosswind response of the downwind prism in a twin-prism system with a staggered arrangement

The flow interaction between two identical neighboring twin square prisms in a staggered arrangement in an open terrain was investigated experimentally. The downwind prism was mounted on a rigid-aeroelastic setup in an open-terrain boundary layer flow to measure its acrosswind root-mean-square responses and aerodynamic damping ratios. By varying the relative location of the upwind prism and the Scruton number associated with the downwind prism, the acrosswind aeroelastic behavior of the downwind prism was analyzed and compared to that of an isolated one. Results showed that the acrosswind root-mean-square response of the downwind prism could be either suppressed or enhanced by the wake flow produced by the neighboring upwind prism. Besides the assessment of the wake effect of the downwind prism, finally, regressed relationships were presented to describe the variation of the aerodynamic damping ratio so as to predict its acrosswind fluctuating response numerically.

Wind pressures on tapered and set-back tall buildings

Recent tall buildings tend to have irregular and unconventional shapes as a prevailing but unavoidable trend, which is very effective for suppressing across-wind responses. Suppression of across-wind responses is a major factor in safety and habitability design of tall buildings, and the so-called aerodynamic modification method is comprehensively used. While the effectiveness of aerodynamic modification in reducing wind loads has been widely reported, there have been few detailed investigations of pressure fluctuations. The purpose of the present work is to investigate the spatio-temporal characteristics of pressure fluctuations applied to height-modified tall buildings comprehensively, including differences of vortex formation and shedding mechanism based on the previously reported mechanism of a conventional square tall building. The results show that taper and set-back affect on the bandwidth of power spectra and position of peak frequencies. And through taper and set-back, the height at which the vortex begins to form moves up, and due to the small building dimension, the vortex component formed at that height sheds from the building more frequently before an inverted conical vortex is formed over the whole height.

비정형 초고층 건물의 변동 풍압

Recent tall buildings tend to have unconventional shapes as a prevailing, which is effective for suppressing across-wind responses. Suppression of across-wind responses is a major factor in tall building projects, and the so called aerodynamic modification method is comprehensively used. The purpose of the present study is to investigate the pressure fluctuations on tapered and setback tall buildings, including peak pressures, power spectra and coherences through the synchronous multi-pressure sensing system techniques. And flow measurements around the models were conducted to investigate the condition of vortex shedding. The results show that by tapering and setback, different distributions of mean pressure coefficients at leeward surface were found, which is caused by the geometric characteristics of the models. And the power spectra of wind pressures at sideward surface become wideband and the peak frequencies are different depending on heights, which makes the correlation near the Strouhal component low or even negative. The differences in shedding frequencies were also confirmed by the flow fields around the models.

Design Wind Loads on Reinforced Concrete Chimney – An Experimental Case Study

The present paper is aimed at providing a better understanding of the effect of interference and influence of strakes for wind load on Thermal Power Station (TPS) chimneys. Measurements of across and along vibration have been made on scale model of Panipat power station chimney, India. The aeroelastic model of the chimney has been tested in a simulated atmospheric boundary layer of height 1.0 m in the Boundary Layer Wind Tunnel at IIT Roorkee, India.The chimney model has been fabricated with FRP (fiber-reinforced plastic) at a geometric scale of 1:150 representing a chimney with height of 100 m in prototype. The wind tunnel tests for the present study have been carried out with the objective of obtaining the maximum along-wind and across-wind response of the chimney and this has been expressed in terms of bending moments. In particular, the interference effect between other nearby structures of the TPS has been investigated. Based on dimensional analysis measured values of chimney model have been converted into prototype values.In the present study, particular attention has been given to bending moment due to across-wind vibration, because it has been found that across-wind vibration is more predominant for the case of interference at a 45o angle of wind incidence. Bending moment due to across-wind vibration for interference is found to be approximately double compared to that of stand-alone condition.To control across wind vibration, strakes have been mounted over the top one-third height of the chimney. It is found that there is a significant decrease in the magnitude of across as well as resultant bending moment in presence of strakes.

Characteristics of aerodynamic forces and pressures on square plan buildings with height variations

Many studies on reducing across-wind responses of tall buildings have been investigated, mainly focusing on the effect of corner shape. And it is also known that changing section along the height through tapering or set-back could reduce across-wind responses of tall buildings. In this paper, to investigate the mechanism of aerodynamic force reduction, the wind tunnel tests for fluctuating pressure and fluctuating force were carried out. Two models with different tapering ratio of 5% and 10%, one set-backed model and one prototype square prism with side ratio of unity were employed under two typical boundary layers which represent suburban and urban flow condition. It is concluded that tapering or set-back helps to reduce the mean drag force and the fluctuating lift force. Reduction ratio increases as tapering ratio increases, and the set-backed model is more effective to reduce the fluctuating lift force than the tapered model with identical surface area, reducing the coefficients about 40% in suburban flow condition. And by tapering and set-back, the power spectra of wind pressures at sideward surface become wideband and the peak frequencies depend on height, which makes the correlation near the Strouhal component low or even negative.

A New Model for Estimating Wind-induced Responses of Tall Buildings with Consideration of Cross-term between Background and Resonant Components

In current design practice, the responses of buildings and structures to fluctuating wind loads are usually separated into the background and resonant components, based on the assumption that these two components are not correlated with each other. The present paper proposes a new model for estimating the wind-induced responses of tall buildings, in which the background and resonant components and their cross-term are strictly defined and included. Therefore, the new model is more exact than the conventional ones. Comparisons are made for the along-wind and across-wind responses of tall buildings between this model and the conventional ones, based on a parametric investigation. The results show that the conventional models give results similar to those obtained from the new model for the along-wind responses, but generally underestimate the across-wind responses. Finally, some conclusions and useful suggestions are presented.

Experimental investigation of the aerodynamic stability of the “Endless Column”, Romania

Wind tunnel tests have been performed on several models of the “Endless Column”, a 30 m tall sculpture, created by C. Brancusi in 1938. In spite of its slenderness, the Column, located in Targul Jiu, Romania, has shown a great stability against wind. In order to clarify if the symmetric, original shape has influence upon its stability, we have carried out tests on section models of “Endless Column” shape (EC models) and square shape (SQ models), of various Sc numbers. Across-wind response was determined in smooth flow for wind speeds in wind tunnel of 1–10 m/s ( Re=4000–46,000) for angles of attack 0°, 10° and 45°. Furthermore, an aerolastic full model was created and tests under smooth and turbulent flow conditions were performed for angles of attack between 0° and 45°. For low wind speeds, in the area of vortex-induced vibrations, the EC models had similar response with the SQ models; however, for higher wind speeds the EC models proved to be more stable. Based on measurements of aerodynamic drag and lift coefficients and by a verification of Glauert den Hartog criterion, it could be concluded that is a very low possibility for EC model to encounter galloping; for extremely high wind speeds though, this might not be impossible.

POD analysis of wind velocity field in the wake region behind vibrating three-dimensional square prism

This paper presents a study on systematic structure of wind velocity field in the wake region behind vibrating three-dimensional square prism. Particle image velocimetry (PIV) system was used to visually understand the characteristics of the wake. Focused on the length between prism's leeward wall and identified wake stagnation point, averaged wind velocity field in the wake region was evaluated. The result showed that the length of the wake region was consistent with the across-wind response in the vortex-induced oscillation. Proper orthogonal decomposition (POD) analysis was applied in order to extract the main structure of the overall physical phenomenon from the measured wind velocity field. The obtained eigenvectors of the first and the second modes indicated the Kármán's-type structure of vortex shedding and the result of their reconstruction was able to highlight the dominant structure of prism–wake interaction in the vortex-induced oscillation.

Wind-induced lateral-torsional coupled responses of tall buildings

Based on the empirical formulas for power spectra of generalized modal forces and local fluctuating wind forces in across-wind and torsional directions, the wind-induced lateral-torsional coupled response analysis of a representative rectangular tall building was conducted by setting various parameters such as eccentricities in centers of mass and/or rigidity and considering different torsional to lateral stiffness ratios. The eccentricity effects on the lateral-torsional coupled responses of the tall building were studied comprehensively by structural dynamic analysis. Extensive computational results indicated that the torsional responses at the geometric center of the building may be significantly affected by the eccentricities in the centers of mass and/or rigidity. Covariance responses were found to be in the same order of magnitude as the along-wind or across-wind responses in many eccentricity cases, suggesting that the lateral-torsional coupled effects on the overall wind-induced responses can not be neglected for such situations. The calculated results also demonstrated that the torsional motion contributed significantly to the total responses of rectangular tall buildings with mass and/or rigidity eccentricities. It was shown through this study that the framework presented in this paper provides a useful tool to evaluate the wind-induced lateral-torsional coupled responses of rectangular buildings, which will enable structural engineers in the preliminary design stages to assess the serviceability of tall buildings, potential structural vibration problems and the need for a detailed wind tunnel test.

Interference effects on wind loading of a row of closely spaced tall buildings

Interference effects on a row of square-plan tall buildings arranged in close proximity are investigated with wind tunnel experiments. Wind forces and moments on each building in the row are measured with the base balance under different wind incidence angles and different separation distances between buildings. As a result of sheltering, inner buildings inside the row are found to experience much reduced wind load components acting along direction of the row ( x) at most wind angles, as compared to the isolated building situation. However, these load components may exhibit phenomena of upwind-acting force and even negative drag force. Increase in x-direction wind loads is observed on the upwind edge building when wind blows at an oblique angle to the row. Other interference effects on y-direction wind loads and torsion are described. Pressure measurements on building walls and numerical computation of wind flow are carried out at some flow cases to explore the interference mechanisms. At wind angle around 30° to the row, wind is visualized to flow through the narrow building gaps at high speeds, resulting in highly negative pressure on associated building walls. This negative pressure and the single-wake behavior of flow over the row of buildings provide explanations for the observed interference effects. Interference on fluctuating wind loads is also investigated. Across-wind load fluctuations are much smaller than the isolated building case with the disappearance of vortex shedding peak in the load spectra. Buildings in a row thus do not exhibit resonant across-wind response at reduced velocities around 10 as an isolated square-plan tall building.

Across-wind responses of an aeroelastic tapered tall building

To investigate the effect of tapering on reducing the rms across-wind displacement responses of a tall building, an experiment using an aeroelastic tapered model of a tall building was conducted in a wind tunnel which simulated the suburban environment. Three aeroelastic, tapered, tall building models with taper ratios of 5%, 10% and 15%, and one basic model of a square cross-section without a taper were tested. The tapering effect appeared when the reduced velocity was high and the structural damping ratio had a moderate value of 2–4%. However, the increase in tapering could have an adverse effect, increasing the rms across-wind displacement responses when the structural damping ratio is very low.

THE OPTIMUM LOCATION OF OUTRIGGER IN REDUCING THE ALONG-WIND AND ACROSS-WIND RESPONSES OF TALL BUILDINGS

Outrigger is one of the many tall building structural systems that are used to reduce the building responses to wind. However, it is not known where the outrigger should be placed so that the responses of the tall building due to wind can be minimized. Thus, 64-story reinforced concrete buildings with the ratio of height to the breadth of 6:1 are studied in order to determine the optimum location to construct the outriggers to minimize the along-wind and across-wind responses. Buildings with different location of outriggers are analysed by a structural analysis software in order to determine the natural frequencies and eigenvectors in the along-wind and across-wind direction. The along-wind responses are determined by employing the procedures from the ASCE 7-02 while the across-wind responses of the buildings are calculated based on the procedures and wind tunnel data available in a data base of aerodynamic load. The database is comprised of high-frequency base balance measurements on a host of isolated tall buildings models. Results from the analysis shows that the optimum location to construct the outriggers is between one third to two third of the height of the building.

Influence of Tapered Cross Section on Fatigue Damage of Cladding Fasteners on Side Faces of Tall Buildings

An appropriate choice of building shape and architectural modifications can result in the reduction of motion by altering the flow pattern around a building. Tapering which is one of aerodynamic devices has been known to be effective to reduce acrosswind response [1, 2]. The exterior wind loading patterns on claddings are sensitive to the building shape. When a building has a tapered cross section along the height, the influence of the exterior wind loading patterns on the wind-induced high-cycle fatigue damage of the cladding fastener of the tapered building needs to be found. In this study, the fatigue damage of the cladding fastener of a non-tapered building and two tapered buildings is estimated by using the rainflow cycle counting method and Miner’s rule. The fatigue damage is compared with one another in order to investigate the influence of the tapered cross section on the fatigue damage of cladding fasteners on side faces of tall buildings

Control of Wind-induced Nonlinear Oscillations in Suspension Bridges using a Semi-Active Tuned Mass Damper

In this article, the along-wind and across-wind response of the suspended cables and the bridge deck response of a suspension bridge subjected to wind loading are studied. The mean wind direction is assumed to be perpendicular to the planes of the suspended cables. It is shown that the suspended cable, if constructed alone, will gallop in the across-wind direction when the mean wind speed exceeds a certain critical wind speed. Similarly the bridge deck, if constructed alone, will gallop when a critical wind speed is exceeded. The construction of a suspension bridge, through coupling the bridge deck with the suspended cables via vertical hangers, increases the critical wind speed at which galloping occurs. However, the increase in the critical wind speed of a suspension bridge over a normal bridge is not sufficient to prevent galloping at moderate wind speeds. This article investigates the use of a passive or semi-active tuned mass damper control mechanism to increase critical wind speed for a flexible suspension bridge without changing the shape or size of the bridge deck cross section.

Control of Wind-induced Nonlinear Oscillations in Suspension Bridges using Multiple Semi-active Tuned Mass Dampers

The effectiveness of using Multiple Tuned Mass Dampers (MTMD) to increase the critical wind speed of a flexible long span suspension bridge subjected to wind loading is studied. The mean wind direction is assumed to be perpendicular to the plane of the suspended cable. The along-wind response and across-wind response of the suspended cables and the bridge deck response due to wind are studied, and it is shown that using a single passive or semi-active tuned mass damper (STMD) with a large mass at the mid-span of the bridge raises the critical wind speed of the suspension bridge. As the installation of a single large passive or semi-active tuned mass damper is problematic, we investigate the effectiveness of using multiple tuned mass dampers (MTMD) with small masses located at various positions along the bridge deck. In order to further increase the critical wind speed, the passive MTMD are supplemented by velocity feedback active control forces. The effects of the dampers’ locations, the mass ratio and the damping ratio on the critical wind speed are also investigated.

Control Of Wind-induced NonlinearOscillations In Suspension Bridges

In this paper, the along-wind response and across-wind response of the suspended cables and the bridge deck of a suspension bridge are studied. The mean wind direction is assumed to be perpendicular to the plane of the suspended cable. It is shown that the suspended cable, if constructed alone, gallops in the across-wind direction when the mean wind speed exceeds a critical wind speed. Similarly, the bridge deck if constructed alone will gallop when the wind speed exceeds a certain critical wind speed. The construction of a suspension bridge through coupling the bridge deck with the suspended cables via vertical hangers will increase the critical wind speed at which galloping occurs for the bridge. This paper discusses the means of further increasing the critical wind speed for a suspension bridge using a passive or active control mechanism.

Simplified formulas for evaluation of across-wind dynamic responses of rectangular tall buildings

Tall buildings under wind action usually oscillate simultaneously in the along-wind and across-wind directions as well as in torsional modes. While several procedures have been developed for predicting wind-induced loads and responses in along-wind direction, accurate analytical methods for estimating across-wind and torsional response have not been possible yet. Simplified empirical formulas for estimation of the across-wind dynamic responses of rectangular tall buildings are presented in this paper. Unlike established empirical formulas in codifications, the formulas proposed in this paper are developed based on simultaneous pressure measurements from a series of tall building models with various side and aspect ratios in a boundary layer wind tunnel. Comparisons of the across-wind responses determined by the proposed formulas and the results obtained from the wind tunnel tests as well as those estimated by two well-known wind loading codes are made to examine the applicability and accuracy of the proposed simplified formulas. It is shown through the comparisons that the proposed simplified formulas can be served as an alternative and useful tool for the design and analysis of wind effects on rectangular tall buildings.

Control of Wind-Induced Nonlinear Oscillations in Suspended Cables

In this paper, the along-wind and across-wind responses of suspended cables are studied. The mean wind direction is assumed to be perpendicular to the plane of the suspended cable. It is shown that the cable gallops in the across-wind direction, when the mean wind speed exceeds a critical wind speed. To control the galloping response, a vertical viscous damper, in the vertical plane of the cable, is introduced at a certain location on the cable to a near fixed platform such as a bridge deck. The efficiency of the vertical viscous damper and its location in controlling the galloping of the suspended cable is investigated.

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.

Acrosswind aerodynamic damping of isolated square-shaped buildings

Aeroelastic model tests were performed to study the acrosswind response and aerodynamic damping of isolated square-shaped high-rise buildings. The experimental measurements were compared with response predictions based upon wind loads obtained from stationary pressure model. Results indicate that urban terrain flow field is aerodynamically stable for the square-shaped buildings. In open terrain, the acrosswind responses can be categorized into three regions based on a building’s mass-damping coefficient, M D: (1) region of aerodynamical stable, M D⩾6.28, (2) region of aerodynamical unstable, 2.76⩽ M D⩽5.82, (3) region of aerodynamical divergence, M D⩽2.18. Empirical models for aerodynamic damping are presented. When the aerodynamic damping is incorporated with lift force spectra, the calculated acrosswind responses show good agreement with measurements.