Highway Research Record

IV. BRIDGE ENGINEERING
COMPLETED PROJECTS
Studies for Aerodynamic Stability of Cable Stayed Bridge Decks

Duration: December 1999 to June 2004
(i) Central Road Research Institute, New Delhi (R)
(ii) Ministry of Shipping Road Transport and Highways (S)

Present Status and Progress
The project has been completed after achieving the targets as stated in the technical note of MORTH. The results of the project were compiled in two reports and submitted to MORTH in June 2004.

Findings/Conclusions
The wind effects on cable stayed bridge decks and the methods to evaluate these effects with the help of wind tunnel studies especially using sectional model investigation has been studied in the project. The flutter theory and techniques used in the determination of flutter derivatives were studied with an emphasis on free oscillation method which has been used for wind tunnel investigation in this project. Qualitative use of flutter derivatives in assessing the aerodynamic stability of cable stayed bridges has been brought out. The flutter equivalences derived in this project are useful in expressing the unsteady force coefficients to flutter derivatives as in free oscillation technique.

The experimental setup for wind tunnel testing has been described and illustrated with the testing of a cable stayed bridge under construction on the river Yamuna in Allahabad.

After three dimensional modeling, free vibration analysis of the bridge was carried out. Experimental set up for conducting static and dynamic sectional model tests were created in CRRI. Sectional model of 1:100 scale, in perspex was built with appropriately modeling the railing and crash barrier. Static wind tunnel tests were conducted to obtain the steady state force coefficients – drag, lift and moment coefficients at various angle of attack. A three component balance designed, fabricated and calibrated was used to measure the forces developed on model during wind tunnel studies. The angle of attack was varied by rotating the bridge deck model in clockwise and anticlockwise direction during the wind tunnel investigation. The wind tunnel tests to extract the aerodynamic force coefficients are carried out at 1.81m/s and 2.55 m/s corresponding to 18.1m/sec and 25.5 m/s at the bridge site. The drag coefficient CD for the bridge deck at zero angle of attack is obtained as 1.414. The lift coefficient CL is 0.3155 and the moment coefficient is –0.037 at an angle of attack of zero degree. The slope of drag coefficient curve is computed as 2.215. The slope of lift coefficient curve is computed as 5.33. The slope of the moment coeffients curve computed as 1.366. The slope has been computed using the coefficients at angle of attack -3o + 3o. The slope of lift curve and moment coefficients are close to the slope of lift and moment curve of flat plate, i.e., =6.28 and =1.57. The strouhal number of bridge deck is estimated as 0.11.

The wind tunnel testing of sectional model of bridge deck using free oscillation technique by allowing the model to oscillate in vertical direction only has been conducted by varying the wind speed from zero to 6.0m/s. The time history of displacement has been recorded using instrumentation tape recorder in a video cassette (tape speed 1.2 cm/s, sampling rate 320 ms) at zero, 1.81 m/s, 3.13 m/s, 4.04 m/s, 5.11 m/s and 6.00 m/s. Similarly, the wind tunnel testing of bridge deck model using free oscillation technique by allowing the model to oscillate in torsional direction only, has been conducted by varying the wind speed from zero to 6.99 m/s. The time history of displacement has been recorded at zero, 1.81 m/s, 3.13 m/s, 4.04 m/s, 5.11 m/s, 6.00 m/s, 6.51 m/s and 6.99 m/s. From the time history of displacement of bridge deck model in torsional direction, spectra were obtained after analyzing the data using FFT analyzer. From the time histories and spectra it is observed that there is change in frequency and damping with variation in wind speed.

During wind tunnel investigation it is observed that with increase in wind speed, the oscillatory amplitude in vertical direction of spring mounted sectional model tends to increase. However, the oscillation damps out rapidly due to the effect of aerodynamic damping. During the wind tunnel studies, at wind speeds of 6.76 m/s to 7.25 m/s (corresponding to reduced velocity of 8.32 to 8.9) the amplitude of oscillation in vertical direction is observed to increase considerably. This corresponds to a wind velocity at bridge site equal to 93.2 m/s. When the test is conducted with model allowed oscillating in torsional direction, the amplitude oscillation in torsional direction observed to increase rapidly at 7.0m/sec, equivalent reduced wind speed is 7.179. The corresponding wind speed at bridge deck location is estimated as 102.65 m/s. However, these wind speeds are much beyond the hourly wind speed at site ie. 40 m/s or design gust speed at the level of deck of 52 m/s. Therefore, from the wind tunnel studies, it is observed that the Yamuna Bridge is aerodynamically stable at design wind speed.

Mathematical tools and software have been developed to convert the wind tunnel data to flutter derivatives.The methodology for estimation of buffeting response and vortex induced oscillatory response of cable stayed bridge are presented based on aerodynamic coefficients obtained from wind tunnel studies. Software has been developed for the prediction of aerodynamic stability of cable stayed bridge using the flutter derivatives obtained from wind tunnel investigations. The validation of software is illustrated with the example of Tsurumi Bridge –an existing bridge in Japan. Also, the aerodynamic stability of Yamuna Bridge have been predicted.

Based on the work, guidelines for the aerodynamic design of cable stayed bridges have been prepared.

Limitations
The project work reported is limited to aerodynamic stability of cable stayed bridge decks.

Recommendations
More efforts need to be made to investigate the aerodynamics of other type of bridge decks used in long span cable stayed bridges. Further wind tunnel studies are to be carried out to understand the aerodynamic effects on pylons of different configuration and cables of different diameters.

Reports/Publications
1. State-of–the–Art Report on Studies for Aerodynamic Stability of cable Stayed Bridge Decks, Report No:     CRRI/BIE/MORTH/B-25/SSP-0014/1, June 2004.
2. Report on Studies for Aerodynamic Stability of Cable Stayed Bridge Decks, Report No: CRRI/BIE/MORTH/B-25/SSP-0014/2,     June 2004
3. A technical presentation on “Wind Effects on Long Span Cable Stayed Bridges”was made at the 51st HRB meeting held     during IRC Annual Session in January 2005 at Bangalore.