Across-wind responses of an aeroelastic tapered tall building

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Abstract

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.

Introduction

Most modern tall buildings with efficient structural systems use high-strength materials to reduce their weight, and to be more slender and flexible they have lower damping values, which makes them more susceptible to wind-induced excitations that have the potential to reduce their structural safety and cause discomfort to the occupants of the building (Kareem, 1983). Consequently, these structural components can adversely affect the serviceability and safety of the building (Kareem, 1983; Kwok, 1988). Thus, wind-induced excitation could be used as an important design criteria for determining the structural system of tall buildings (Davenport, 1988; Banavalkar, 1990). Many investigations have therefore been conducted to reduce such excitations to an allowable level and to improve the performance of tall buildings. Most of these investigations concentrate on aerodynamic modifications of the cross-sectional shapes of the buildings, such as using slotted and chamfered corners, fins, setbacks, buttresses, horizontal through-building openings and variations of the cross-section with height, that is, tapering. The incorporation of auxiliary damping devices, such as TMD (tuned mass damper), TLD (tuned liquid damper) and TLCD (tuned liquid column damper), can also be incorporated to suppress the across-wind responses, such as the vortex-induced excitations, which occur around the wind speed at which the periodic vortex-shedding frequency coincides with one of the natural frequencies of the building (Vickery et al., 1983; Isyumov et al., 1989; Kareem, 1983; Banavalkar, 1990; Tamura, 1998; Kwok and Bailey, 1987; Kwok, 1988; Kareem et al., 1999).

The use of aerodynamically modified cross-sections, such as slotted corners and chamfered corners, is an effective means to reduce the wake-excited response by up to 30% at the low range of reduced velocities. In addition, the critical reduced velocity at which peak response occurs is lowered to about 8, which suggests that the strength of vortex-shedding is reduced, but that it remains a dominant feature in the across-wind excitations. However, modifications of building corners might not be entirely effective, as adverse effects may also occur (Kareem et al., 1999; Kim and Kawai, 1999). Recently, another aerodynamic modification of a tall building whose cross-sectional shape varies along with the building height through taper was investigated to reduce wind-induced excitations (Tanagi et al., 1999; Nakayama et al., 1992; Fediw et al., 1995). Davenport (1988) has suggested that tall buildings whose cross-sectional shape is tapered along their height might spread the vortex-shedding over a broad range of frequencies, thus reducing the cross-wind responses. Tanagi et al. (1999) investigated the taper effect for reducing across-wind excitations comparing it with the along-wind excitation based on the aeroelastic model tests using a tapered model with and without chamfered corners. They showed that the taper effect is more effective for reducing the across-wind excitations than the along-wind excitations, and that the shape around the peak values for a power-spectral density function of the overturning moment of the across-wind direction in the tapered model seem to have a broad-band shape, which means that the vortex-shedding was spread over a broad range of frequencies. Nakayama et al. (1992) have demonstrated that tapering reduced the across-wind excitation using two types of aeroelastic models: one was a stick model of locking mode shape and the other was a multi-degree-of-freedom model. They found that the first model dominated the total displacement responses and the total displacement of the two models corresponded well. They also showed that the responses by the force-balance test also corresponded well with the aeroelastic responses, even in the range of vortex excitation when the structural damping ratio was as high as 3–4% and the peak value of the power-spectral density function of the overturning moment of the across-wind direction occurred around the reduced velocity of 8 and the strength of the peak weakened due to the tapering. Fediw et al. (1995) and Cooper et al. (1999) investigated the motion-dependent across-wind responses of tapered tall buildings using sectional and model loads, which were determined from pneumatically averaged pressures measured on a rigid model oscillated about a pivot at its base by a hydraulic actuator located below the model. They showed that the taper effect caused an increase in the vortex-shedding frequency over the height of the building and the local Strouhal number decreased with increasing height and did not remain constant. However, their investigation was only based on one kind of building's taper-ratio, which was around 4%. Kim and You (2002) investigated the taper-ratio effect for reducing wind-induced excitations in the along- and across-wind directions based on the force-balance test using rigid models with taper ratios of 5%, 10% and 15%, and one basic model of a square cross-section without taper. Their experiment was conducted under flow conditions which represented the suburban and the urban environments. They discovered that the tapering was more effective in reducing wind-induced excitations in the suburban flow environment than in the urban flow environment. They also showed that the tapering reduces the across-wind responses more than the along-wind responses. And, compared with a basic model without a taper, the tapering was not always effective at reducing the wind-induced excitations of tall buildings. Their investigations assumed that the damping ratios of all the models had a constant value of 2%. However, they could not investigate the effect of the taper ratio on reducing the motion-induced across-wind response which might occur due to the aeroelastic effect, in which the across-wind induced motion may add to or modify the aerodynamic forces and so significantly increase the across-wind response (Kawai, 1992, Kawai, 1993). Such aeroelastic response-forcing mechanisms have proved to be so complex that there is no analytical method available to calculate that response without depending on an aeroelastic wind-tunnel test using an aeroelastic model. In fact, the damping ratios are not the same for models which have different taper ratios, so it is necessary to correctly simulate the damping property, stiffness and mass of each tapered model. Therefore, an aeroelastic wind-tunnel test using an aeroelastic, tapered, tall building model was conducted in this study (ASCE, 1999).

In this paper, to investigate the effects of the taper ratio and the damping ratio on reducing the across-wind excitations of tall buildings by increasingly reduced velocity, a wind-tunnel experiment using aeroelastic models which have ratios of 5%, 10% and 15%, and one without a taper ratio having a square cross-section, with different damping ratios was conducted. The taper effect for reducing the across-wind response appears when the reduced velocity is high and the structural damping ratio has a moderate value of 2–4%.

Section snippets

Wind-tunnel experiment

A wind-tunnel experiment using an aeroelastic tapered model was conducted in a boundary layer wind-tunnel located at the Department of Architecture and Urban Engineering, Chonbuk National University, Chonju, Republic of Korea. It is an open-type wind-tunnel characterized by a test section of 1.5 m width, 1.2 m height and 12 m length. The boundary layer flow condition representing natural wind flow over suburban terrain indicated that the power law exponent of the mean longitudinal wind velocity

Experimental results

The rms across-wind displacement responses at the top of a square cross-sectional model without taper were measured and are presented as a function of model's structural damping ratio and reduced wind velocity, U/nB, where U represents the oncoming mean wind velocity, n represents the natural frequency of the model, and B indicates the width of the model at the middle height in Fig. 4. As shown in Fig. 4, the rms response increased significantly as the damping ratio decreased at the high range

Conclusions

To investigate the effect of the taper ratio on reducing the rms across-wind displacement responses of a tapered tall building including structural damping ratio, aeroelastic tapered models with taper ratios of 5%, 10% and 15%, and without a taper ratio which have different damping ratios were tested in the suburban boundary layer wind-tunnel.

The conclusions obtained from this study are as follows:

  • 1.

    The taper effect for reducing the rms across-wind displacement response appears when the range of

Acknowledgment

This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Program funded by the Ministry of Science and Technology (No. R0A-2005-000-10080-0(2008)).

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