SPECTRAL CHARACTERISTICS OF FLUCTUATING WIND LOADS ON A SEPARATE TWIN-BOX DECK WITH CENTRAL SLOT

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1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan SPECTRAL CHARACTERISTICS OF FLUCTUATING WIND LOADS ON A SEPARATE TWIN-BOX DEC WITH CENTRAL SLOT Le-Dong Zhu, Shui-Bing Wen and Quan-Shun Ding Professor, Director of Wind Tunnel Testing Division, State ey Laboratory for Disaster Reduction in Civil Engineering, Tongji University / ey Laboratory for Wind Resistance Technology of Bridges (Tongji University), Ministry of Transport, Shanghai 0009, China,ledong@tongji.edu.cn Master Student, Department of Bridge Engineering, Tongji University, Shanghai 0009, China. Associate Professor, State ey Laboratory for Disaster Reduction in Civil Engineering, Tongji University, Shanghai 0009, China,qsding@tongji.edu.cn ABSTRACT Spectral characteristics of fluctuating wind load on a separate twin-box bridge deck with a central slot was investigated via force measurement wind tunnel tests of sectional model in a smooth wind field and various gridgenerated turbulent wind fields. The test results show that for the upwind box, the signature turbulence contribution is significant to all the three components of the fluctuating wind load in smooth flow and the fluctuating drag force in turbulent flow, but it is unremarkable to the fluctuating lift and torque in turbulent flow. And the signature turbulence contribution to the fluctuating drag occurs in several relatively-narrow frequency bands whilst those to the fluctuating lift and torque occur in a single relatively-wide frequency band. However, the signature turbulence contributions to the fluctuation wind loads on the leeward box are much more significant than to those on the upwind box for both the smooth and turbulent flow cases, and mainly concentrate in a single relatively-wide frequency band for all the three force components. Moreover, for both the upwind and leeward boxes, the increase of turbulent intensity of incident wind can raise the spectral values of the fluctuating wind load in the frequency bands with small signature turbulence contribution, and can flat the spectral peaks due to the signature turbulence. The influence of the variation of wind attack angle between the normal values of ± on the spectra of all the three components of fluctuating wind load is generally not remarkable. EYWORDS: SEPARATE TWIN-BOX DEC, CENTRAL SLOT, UPWIND BOX, LEEWARD BOX, FLUCTUATING WIND LOAD, SIGNATURE TURBULENCE Introduction Buffeting is one of important types of wind-induced vibration of long-span bridges, and is resulted in from both the natural turbulence of incoming wind and signature turbulence due to the interaction between the structure and surrounding flow. Although the effect of signature turbulence on buffeting responses was already regarded as one of major challenges in the prediction of long-span bridge response to wind (Jones and Scanlan, 998), the extent of this effect has not been well understood yet and has rarely been investigated in depth because of the difficulty of this issue. Therefore, in most of the current buffeting analyses, fluctuating wind load are normally determined based on the quasi-steady theory, with considering only the natural turbulence of incoming wind (Scanlan, 978; Jain et al., 996; Xu et al., 998; atsuchi et al., 999; Chen et al., 000; Zhu and Xu, 005; Xu and Zhu,

2 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan 005;). However, the increase of span length of modern long-span bridges makes the bridges more and more susceptible to strong wind, requiring more refined and accurate prediction of buffeting responses. It is thus evidently necessary to carry out a thorough investigation on the effect of signature turbulence on the fluctuating wind load and responses. In the wind tunnel test of Deer Isle Bridge, Singh (997) found significant contribution of signature turbulence to fluctuating wind load. It was also seen in the measured force spectra that the incoming turbulence exerted only little influence on the signature turbulence and fluctuating wind load in the higher reduced frequency region, but could significantly disturb the signature turbulence in the lower reduced frequency region. To have an in-depth understanding on the signature turbulence effect on the fluctuating wind loads acting on bluff bridge decks, fluctuating wind load on a separate twin-box deck with central slot was measured and analyzed in spectral form in this study via a wind tunnel test of sectional model in a smooth wind field and various grid-generated turbulence wind fields, and are to be introduced in the following text. Background Bridges Because the signature turbulence is regarded to be much more significant for bridge decks with separate boxes than for other types, Shanghai Bridge over Yangtze River, which is a steel cable-stayed bridge with a main span of 70m and a centrally-slotted twin separate box deck, is thus taken as an engineering back ground in this study. As shown in Figure, the deck height is m, and the widths of the whole deck, the single box and the central slot are 5.5m, 0.75m and 0m, respectively. The ratio of slot width over single box width is about 8.%. Figure. Deck cross section of Shanghai Bridge over Yangtze River Figure Installation of sectional model in wind tunnel Wind Tunnel Test of Sectional Model Sectional Model The sectional model tests of force measurement were carried out in the TJ- Boundary Layer Wind Tunnel at Tongji University. As shown in Figure, the sectional model was made at a length scale of /80, and comprised of a measurement segment made of wood, an upper surrounding segment and an upwind/leeward surrounding segment made of organic

3 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan glass. The two surrounding segments were combined as a whole. A horizontal circular plate with a radius of.5m was mounted at a level of 0cm from wind tunnel floor, just beneath the lower end of the sectional model, to reduce thickness of the boundary layer generated by the wind tunnel floor. This horizontal circular place was also used as a lower D end plate to improve the D performance of the flow around the measurement model in conjunction with the upper surrounding part. An opening with a shape similar to the shape of model cross section and a size a little bit larger than the model cross section was set in the middle region of the horizontal circular plate for the model installation. The measurement part of the model was then vertically fixed on a five-component force balance through the preset opening on the horizontal circular plate while the surrounding segment was fixed on the horizontal beam of a support frame and the horizontal circular plate. To enhance the connection rigidity between the model and the force balance, an aluminum end plate was used for the lower end of the measurement segment. In the test, only the fluctuating wind loads on the measured segment were measured, and by rotating the turntable at 80, the position of the measured segment could be switched between the windward and leeward positions. The mass of the measurement segment was.kg, and the fundamental natural frequencies of the model-balance system were 68Hz, 7Hz, Hz, respectively, in the direction out of the deck plan (simply called vertical with respect to the deck), in the direction within the deck plan (simply called lateral with respect to the deck), and in the torsional direction around the longitudinal axis of the deck. Wind Fields The tests were carried out in a smooth flow with turbulent intensity less than % and in the four types of grid-generated turbulent wind fields (Grid ~) with turbulent intensities of 6.%, 0.%, 5.%and 9.%. The spectra of the turbulent wind are show in Figure. S u () / s u Grid (I u = 6.%) Grid (I u =0.%) Grid (I u =5.%) Grid (I u =9.%) S w () / s w Grid (I u = 6.%) Grid (I u =0.%) Grid (I u =5.%) Grid (I u =9.%) x πn/u x πn/u a) longitudinal component b) vertical component Figure Normalized Spectra of simulated grid turbulence Wind Load Spectra in Smooth and Turbulence Flows Upwind box Figure shows the spectra of fluctuating aerodynamic coefficients of drag, lift and torque acting on the upwind and leeward boxes obtained in the smooth flows and the turbulence flow generated by Grid ( Iu=0.%, Iw=9.7%), where the wind attack angle is 0, and =πb/u, b is the width of the single box. It can be seen that for the upwind box, the fluctuating wind load in smooth flow are significantly smaller than those in turbulent flow, but the contributions of signature turbulence on the fluctuating aerodynamic drag, lift and torque in smooth flow are visible in high frequency region. Furthermore, one can find that the

4 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan turbulence of incident wind can significantly weaken the contributions of signature turbulence to the fluctuating aerodynamic lift and torque. However, the incident wind turbulence doesn t significantly affect the contribution of signature turbulence to the fluctuating aerodynamic drag. It can also be found that the signature turbulence contributes to the fluctuating aerodynamic drag in several relatively-narrow frequency band whilst it contributes to the fluctuating aerodynamic lift and torque in a single relatively wide frequency band smooth flow turbulent flow (Grid, I u =0.% 0 - smooth flow 0 - turbulent flow (Grid, I u =0.% a) drag coefficient of upwind box d) drag coefficient of leeward box smooth flow turbulent flow (Grid, I u =0.% 0 - smooth flow 0 - turbulent flow (Grid, I u =0.% b) lift coefficient of upwind box e) lift coefficient of leeward box 0 - smooth flow 0 - turbulent flow (Grid, I u =0.% smooth flow 0 - turbulent flow (Grid, I u =0.% c) torque coefficient of upwind box f) torque coefficient of leeward box Figure Aerodynamic coefficient spectra in smooth and turbulence flows Leeward Box Form Figure, one can also find the following different situations in this case from those in the case of upwind box. The signature turbulence contributions to the aerodynamic forces on the leeward box are much more significant than to those on the upwind box, and

5 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan mainly concentrate in a single relatively-wide frequency band for all the three force components. Furthermore, for the leeward box, the effects of the signature turbulence on all the three force components in turbulent flow are also very significant like those in the smooth flow, although the incident wind turbulence has more or less weakening effect on the signature turbulence contribution Grid (I u = 6.%, I w = 6.%) 0 - Grid (I u = 6.%, I w = 6.%) Grid (I u =0.%, I w = 9.7%) Grid (I u =0.%, I w = 9.7%) Grid (I u =5.%, I w =.%) 0 - Grid (I u =5.%, I w =.%) Grid (I u =9.%, I w =6.%) Grid (I u =9.%, I w =6.%) a) drag coefficient of upwind box d) drag coefficient of leeward box Grid (I u = 6.%, I w = 6.%) Grid (I u = 6.%, I w = 6.%) Grid (I u =0.%, I w = 9.7%) Grid (I u =0.%, I w = 9.7%) Grid (I u =5.%, I w =.%) 0 - Grid (I u =5.%, I w =.%) Grid (I u =9.%, I w =6.%) Grid (I u =9.%, I w =6.%) b) lift coefficient of upwind box e) lift coefficient of leeward box Grid (I u = 6.%, I w = 6.%) 0 - Grid (I u = 6.%, I w = 6.%) Grid (I u =0.%, I w = 9.7%) Grid (I u =0.%, I w = 9.7%) Grid (I u =5.%, I w =.%) 0 - Grid (I u =5.%, I w =.%) Grid (I u =9.%, I w =6.%) Grid (I u =9.%, I w =6.%) c) torque coefficient of upwind box f) torque coefficient of leeward box Figure 5 Aerodynamic coefficient spectra in various turbulence flows Influence of Turbulence Intensity on Wind Load Spectra Upwind Box Figure 5 shows the spectra of fluctuating aerodynamic coefficients of drag, lift and torque acting on the upwind and leeward boxes obtained in the turbulence flows generated by Grids ~ with various turbulent intensities, where the wind attack angle is 0. It can be seen

6 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan that for the upwind box, with the increase of turbulent intensity of incident wind, the spectral value of the fluctuating aerodynamic coefficient increases in the frequency bands where the signature turbulence contribution is small, for instance, in the whole frequency range (=0~0) for the spectra of lift and torque and in the lower frequency range (=0~) for the drag spectrum. In the frequency range of being to 0, the turbulent intensity of incident wind exerts some influence on the signature turbulence contribution to the drag spectrum, but there is no certain visible rule able to be found. Particularly, in the frequency range of being about 9, there is a peak existing on the spectral curve of drag obtained in the turbulence flow generated by Grid with a peak value significantly larger that those obtained in the other three kinds of turbulent flows. On the other hand, in the frequency range of being about 7 the drag spectrum curve obtained in the turbulent flow of Grid has no peak whilst those obtained in all the other three kinds of turbulent flows have significant peaks. Leward box From Figure 5, one can also find that with the increase of turbulent intensity of incident wind, the spectral value of the fluctuating aerodynamic coefficient of leeward box also increases in the frequency bands where the signature turbulence contribution is small, for instance, the spectra of all three force components in the lower frequency range (=0~.5). This phenomenon is similar to that observed on the upwind box. On the other hand, in the frequency bands where the signature turbulence contribution is significant, for instance, being.5 to 6, the increase of turbulent intensity of incident wind exerts a weakening effect on the signature turbulence contribution, i.e., it can flat the spectral peak due to the signature turbulence, and diminish the difference among the wind load spectra obtained in various turbulent flow. In the high frequency range of being 6 to 0, the differences among the spectra obtained in the turbulent flows of Grids ~ are also unremarkable for all three force components. However, for the turbulent flow of Grid, significant spectral peaks of drag and lift due to the signature turbulence occur again, leading to that the spectral values of drag and lift in this frequency range obtained in the turbulent flow of Grid is evidently larger than those obtained in the other three kinds of turbulent flows. Influence of Wind Attack Angle on Wind Load Spectra Upwind box Figure 6 shows the spectra of fluctuating aerodynamic coefficients of drag, lift and torque acting on the upwind and leeward boxes obtained in the turbulence flows generated by Grid ( I u =0.% I w =9.7%) with different wind attack angles of, 0,. It can be clearly found that, for the upwind box, the variation of wind attack angle between the normal values of ± doesn t significantly affect the spectra of all the three components of fluctuating wind load. Leeward box From Figure 6, it can also be found that, for the leeward box, the influence of the variation of wind attack angle between the normal values of ± on the spectra of all the three components of fluctuating wind load is generally not remarkable. Of course, if strictly speaking, the spectra of drag and torque on the leeward box increase a little bit in lower frequency range of below.5 with the increase of wind attack angle. Moreover, the spectra of all three components of fluctuating aerodynamic coefficients of the leeward box decrease a little bit with the increase of wind attack angle in frequency range near the spectral peaks due to the signature turbulence. Conclusions The spectral behaviors of the fluctuating wind load on the upwind and leeward boxes

7 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan of a twin separate box girder bridge deck with central slot were investigated in this study via wind tunnel force measurement tests of sectional model. The following major conclusions can then be drawn wind attack angle:- o wind attack angle:+ o wind attack angle:- o wind attack angle:+ o a) drag coefficient of upwind box d) drag coefficient of leeward box wind attack angle:- o wind attack angle:+ o wind attack angle:- o wind attack angle:+ o b) lift coefficient of upwind box e) lift coefficient of leeward box wind attack angle:- o wind attack angle:+ o wind attack angle:- o wind attack angle:+ o c) torque coefficient of upwind box f) torque coefficient of leeward box Figure 6 Aerodynamic coefficient spectra for different wind attack angles () For the upwind box, the fluctuating wind loads in smooth flow are significantly smaller than those in turbulent flow. The signature turbulence contribution is significant to all the three components of the fluctuating wind load in smooth flow and the fluctuating drag in turbulent flows, but is unremarkable to the fluctuating lift and torque in turbulent flow. Furthermore, the signature turbulence contributes to the fluctuating drag in several relatively-

8 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-, 009, Taipei, Taiwan narrow frequency bands whilst it contributes to the fluctuating lift and torque in a single relatively wide frequency band. () For the leeward box, the signature turbulence contribution to the fluctuating wind load is much more significant than to that on the upwind box for both the smooth and turbulent flows, and mainly concentrate in a single relatively-wide frequency band for all the three force components. () For the upwind box, with the increase of turbulent intensity of incident wind, the spectral values of the fluctuating aerodynamic coefficients increase in the frequency bands with small signature turbulence contribution. The signature turbulence contribution may change to some extent with the variation of the turbulent intensity of incident wind. () For the leeward box, the spectral value of the fluctuating aerodynamic coefficient also increases with the increase of turbulent intensity of incident wind in the frequency bands with small signature turbulence contribution. And in the frequency bands with significant signature turbulence contribution, the increase of turbulent intensity of incident wind can flat the spectral peak due to the signature turbulence, and reduce the difference among the wind load spectra obtained in various turbulent flows. (5) For both the upwind and leeward boxes, the variation of wind attack angle between the normal values of ± doesn t significantly affect the spectra of all the three components of fluctuating wind load. Acknowledgements The work described here was jointly supported by the National High-tech R&D Program (86 Program) of China (Grant No.: 006AAZ0), the Program for New Century Excellent Talents in University of China (Grant No. NCET-05-08), and the National Natural Science Foundation of China (Grant No ) to which the writers are most grateful. Any opinions and concluding remarks presented here are entirely those of the writers. References Jones, N.P. and Scanlan, R.H. (998), Advances (and Challenges) in the Prediction of Long-span Bridge Response to Wind, Proceedings of International Symposium on Advances in Bridge Aerodynamics: Bridge Aerodynamics, Copenhagen, Denmark, May 0~, -. Scanlan, R.H. (978), The Action of Flexible Bridge under Wind, II: Buffeting Theory, Journal of Sound and Vibration, 60(), 0-. Jain, A., Jones, N.P. and Scanlan, R.H. (996), Coupled Buffeting Analysis of Long-span Bridges, Journal of Structural Engineering, ASCE, (7), Xu, Y.L., Sun, D.., o, J.M. and Lin, J.H. (998), Buffeting Analysis of Long Span Bridges: a New Algorithm, Computers and Structures, 68, 0-. atsuchi, H., Jones, N.P. and Scanlan, R.H. (999), Multimode Coupled Flutter and Buffeting Analysis of the Akashi-aikyo Bridge, Journal of Structural Engineering, 5(), Chen, X., Matsumoto,M. and areem,a. (000), Aerodynamic Coupled Effects on Flutter and Buffeting of Bridges, Journal of Engineering Mechanics, ASCE, 6(), 7-6. Zhu, L.D. and Xu, Y.L. (005). Buffeting Response of Long Cable-supported Bridges under Skew Winds. Part : Theory, Journal of Sound and Vibration, 8(-5), Xu, Y.L. and Zhu, L.D. (005), Buffeting Response of Long Cable-supported Bridges under Skew Winds. Part : Case Study, Journal of Sound and Vibration, 8(-5), Singh, L. (997), Experimental Determination of Aeroelastic and Aerodynamic Parameters of Long-span Bridges, PhD Thesis, Department of Civil Engineering, Johns Hopkins University, Baltimore, MD, USA.