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Aerodynamic stability of narrow decked suspension bridge (Aki-nada Ohashi Bridge)
Journal of Wind Engineering and Industrial Aerodynamics 77&78 (1998) 409—420
Aerodynamic stability of narrow decked suspension bridge (Aki-nada Ohashi Bridge) Akihiro Honda *, Hiroaki Miyata, Hideyuki Shibata Fluid Dynamic Laboratory, Nagasaki Research and Development Center, Mitsubishi Heavy Industries Co. Ltd., 1-1, Akunoura-machi, Nagasaki 850-91 Japan Aki-nada Ohashi Bridge Construction Office, Hiroshima Road Public Corporation, 3-6-21, Nishichuo Kure, Hiroshima 737, Japan Sogo Engineering INC., 3-5-9, Higashi Nakajima, Higashiyodogawa-ku, Osaka 533, Japan
Abstract Aki-nada Ohashi Bridge (tentative name) is now under construction with 750 m in center span length, 1175 m in total bridge length. The girder of the bridge is 19 m in width with two-lanes and pedestrian space, and the configuration is flat hexagonal box shaped. This paper describes the aerodynamic stability of both the completed stage and erection stage of the girder. Its improvement is also described. 1998 Elsevier Science Ltd. All rights reserved.
1. Outline of the bridge Aki-nada Ohashi Bridge is located at the southern part of Hiroshima Prefecture in Japan, and it will connect the main land of Japan and Shimo-Kamagari Island. The suspension bridge is 750 m in its center span length, and 1175 m in total length as shown in Fig. 1. The deck section of the bridge is 19 m in width and 2.5 m in depth, which consists of two traffic lanes and a pedestrian walkway at both sides of the section. It is designed to increase by one traffic lane in the future. Therefore, the pedestrian walkway at one side of the deck will remain, and the cross section will become asymmetric as shown in Fig. 2. The configuration is evaluated through wind tunnel test with section model at PWRI in Japan. Fig. 3 shows the skeleton of the tower, and it is of metal frame structure, because the earthquake load is smaller than that of a concrete structure. The height of the tower is 120 m, and the ratio of height to width is 6.8, which is one of the most slender
* Corresponding author. 0167-6105/98/$ — see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 7 ) 0 0 1 6 0 - 3
410
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Fig. 1. Outline of Aki-nada Ohashi Bridge.
Fig. 2. Cross-section of deck. (a) Deck with two traffic-lanes. (b) Deck with three traffic-lanes.
Fig. 3. Skeleton and cross section of the tower. (a) Skeleton of the tower. (b) Cross-section of the shaft.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
411
Table 1 Dynamic characteristics of Aki-nada Ohashi Bridge Item
Heaving motion
Torsional motion
Natural frequency Mass and inertia moment
0.164 Hz 13.377 ton f/m
0.503 Hz 45.826 ton f s m/m
structures in Japan. A wind tunnel test with aeroelastic model of the tower was also conducted at Kyoto University, and the configuration of the shaft was evaluated. The bridge is now under construction, and is scheduled to be completed in 1999.
2. Aerodynamic stability of completed bridge In this study, wind tunnel test using aeroelastic model of the entire bridge was conducted to confirm the stability with static deformation of the girder by wind. The dynamic characteristics of symmetric mode of the bridge are shown in Table 1. 2.1. Criteria of completed bridge The criteria of wind speed for aerodynamic design were calculated based on the ‘Standard of Honshu-Shikoku Bridge Authority’ as follows: » "37 m/s, » "l » , » "1.2l l » , where » and » are design mean wind speed for gust " " " " response and design wind speed for flutter at girder level, » "10 min mean wind speed at 10 m above ground, l the modification factor of height of (Z/10) "1.25, l the modification factor of gust considering the span length of 1.15, and 1.2 the safety factor for flutter. Namely, » was calculated to be 46.3 m/s, and » determined to be 63.8 m/s. " " Table 2 compares the characteristics of long-spanned suspension bridges with boxgirder, and indicates that Aki-nada Bridge requires the highest reduced criteria of flutter wind speed in Japan. The bridge with narrow deck section requires severe aerodynamic design. 2.2. Model and test procedure The model was 1/120 in scale, and made of aluminum spine which was designed to simulate the bending and torsion rigidity of the girder and piano-wire to simulate the tension rigidity of the cable. Wooden blocks were fitted to the spine to represent the configuration of the girder. After that, concentrated mass and cylindrical tube were wrapped to the piano-wire to simulate the mass and drag force acting on the cable. The model was designed to satisfy ‘Froude’s similarity rule’ as well as ‘Strouhal’s similarity rule’. The test was conducted at the wind tunnel of Nagasaki R&D Center of Mitsubishi Heavy Industries’ Co. Ltd. The wind tunnel cross section is 10 m in width and 3 m in
412
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Table 2 Existing and under construction long-spanned suspension bridges with box-girder No.
Name
Max. Span ¸ (m)
Width B (m)
Depth D (m)
» " (m/s)
Frequency f /f (Hz) 2
» /f B " 2
Nation
1 2 3
Great Belt Humber Jiangyin Yangtze River Tsing ma Ho¨ga Kusten Bosphorus 2 Bosphorus Kurushima 3 Kurushima 2 Severn Ask+y Aki-nada Hakucho
1624 1410
31 22
4.0 4.0
60 37
0.278/0.100 0.300/0.105
7.0 5.6
Denmark UK
1385 1377 1210 1090 1074 1030 1020 987.6 850 750 720
36 41 22 39.4 28 32.3 32.3 22.9 15.52 19 23
3.5 7.6 4.0 3.0 3.0 4.3 4.3 3.1 3.0 2.5 2.5
92 80 50
0.4 /0.139 0.278/0.141 0.388/0.135
6.4 7.0 5.9
70 70 50 52 63.8 67
0.379/0.157 0.377/0.156 0.374/0.143 0.463/0.178 0.503/0.164 0.450/0.116
5.7 5.7 5.8 7.2 6.7 6.5
China China Sweden Turkey Turkey Japan Japan UK Norway Japan Japan
4 5 6 7 8 9 10 11 12 13
Note: f , f : natural frequency of 1st torsion and bending symmetric mode. 2
Fig. 4. Wind tunnel test of completed bridge model.
height. At the mouth of the wind tunnel, rows of blades were installed in order to generate inclined angle of attack and also computer controlled turbulence as shown in Fig. 4. The displacement and vibration of the model were measured by optical sensor.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
413
Fig. 5. Flutter onset velocity of the completed bridge. (a) Flutter wind speed with two traffic-lanes. (b) Flutter wind speed with three traffic-lanes.
Fig. 6. Parapet at the side of the deck (unit: mm). (a) Original design. (b) Improved design.
2.3. Full model test result In uniform flow, vortex-induced excitation of the tower, and flutter of the deck was observed. The critical wind speed of the flutter as a function of the wind angle of attack is shown in Fig. 5. The flutter of the deck with two traffic-lanes was found to be insufficient at the angle of attack of !3°. However, the critical wind speed of the deck with three traffic-lanes was found to be sufficient. 2.4. Aerodynamic improvement of the deck section for flutter From the point of view that additional members (parapet, rail for maintenance cargo, etc.) tend to play an important role for narrow-decked suspension bridge, the effect of the parapet on the deck was investigated for aerodynamic improvement as shown in Fig. 6. The test was conducted using a section model, 1/20 in scale, and the critical wind speed for flutter was found to be higher as shown in Fig. 7. Finally, the
414
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Fig. 7. Aerodynamic improvement of the flutter of the girder (section model). (a) Two traffic-lanes. (b) Three traffic-lanes.
Fig. 8. Aerodynamic improvement of the flutter of the girder (aeroelastic model). (a) Two traffic-lanes. (b) Three traffic-lanes.
stability was also confirmed through wind-tunnel test using the aeroelastic model as shown in Fig. 8. 2.5. Gust response due to turbulent wind Gust response was also tested in the wind tunnel. Computer-controlled pitching motion of the rows of blades generated the turbulence, and its characteristics are shown in Table 3. Fig. 9 shows the peak amplitude of the longitudinal, vertical and torsional random response at the center of mid-span. Among those responses, the longitudinal component was the largest one. However, the amplitude at the mean wind speed of 46.3 m/s (» ) was below the level of allowable stress. "
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
415
Table 3 Turbulent characteristics Component
Intensity
Length scales
Longitudinal Vertical
10% 5%
630 m 120 m
Fig. 9. Response in turbulent flow. (a) Two traffic-lanes. (b) Three traffic-lanes.
3. Aerodynamic stability of girder erection stages The deck of the bridge is scheduled to be constructed from the center of its main span, and Table 4 shows the natural frequency of the lowest mode at each erection stage. The natural frequency of torsional motion is lower at the early stages of erection, and flutter will be of more concern than compared to the completed bridge. From these results, four stages of erection (1,2,3 and 5) were tested in wind tunnel.
416
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Table 4 Procedure and natural frequencies during girder erection stages
Fig. 10. ‘Hinge joint’ of the lifted block of girder.
3.1. Model The girder of the bridge is to be erected using the ‘all-Hinge method’, which releases the bending moment of vertical deflection. A matching piece as shown in Fig. 10, will join each lifted section. In order to simulate the function of the ‘Hinge joint’, an ‘X-shaped spring with thin brass plate’ was manufactured, and connected the spines to each other as shown in Fig. 11.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
417
Fig. 11. Aeroelastic model of girder erection stages.
Fig. 12. Flutter onset velocity of the bridge during girder erection.
3.2. Criteria and test result The criteria of wind speed was calculated based on the ‘Standard of Akashi-Kaikyo Bridge’, which indicates that the wind speed during the erection stage should be 81% of that of the completed bridge. Namely, the erection criteria for 10 min mean wind speed for gust was 37.5 m/s, and the flutter wind speed was 51.7 m/s at the level of girder. Fig. 12 shows the critical wind speed of flutter during the erection stages, and it was found that stages of 1 and 3 are not stable enough compared to the criteria. The solid symbols in the figure mean that the flutter response is coupled with the random response of heaving motion. 3.3. Temporary stiffening system for girder during erection For temporary improvement of the flutter stability during erection stages of the girder, many stiffening systems were considered and certain combinations of these methods, shown in Fig. 13, were tested in wind tunnel.
418
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Fig. 13. Temporary stiffening method of the girder. (a) Strut connecting main-cable and girder. (b) Crosshanger connecting main-cable and hanger. (c) Strand-rope between main-cable and tower.
Fig. 14. Improved flutter onset velocity of the bridge during girder erection: (a) Erection stage-1. (b) Erection stage-3.
Fig. 14 compares the improved and original critical speed of flutter for stages 1 and 3, where the stability was determined to be insufficient. It indicates that a combination of these methods could improve flutter speed beyond the criteria. In the wind tunnel, the gust response due to turbulent wind was also tested. Fig. 15 shows the response depending on the stage of erection. The displacement is evaluated at the mean wind speed of 37.5 m/s, and it was confirmed that no fatal damage is expected during the erection stage.
419
Fig. 15. Response in turbulent flow during girder erection.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
420
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
4. Concluding remarks The following conclusions were reached through a series of aerodynamic investigations: (1) The narrow-decked suspension bridge with box-girder was found to be disadvantageous concerning aerodynamic stability. Reduced criterion of flutter wind speed for ‘Aki-nada Bridge’ is almost the same as that of ‘Great Belt Bridge’ in Denmark. (2) The wind tunnel test using three-dimensional aeroelastic model indicates that wind tunnel test results using the section model may overestimate the critical wind speed of flutter. (3) Aerodynamic characteristics of narrow-decked box-girder seem to be sensitive to additional members (parapet, rail for maintenance cargo, etc.). Minor changes of parapets succeeded in increasing the stability of the girder after the completion of the bridge. (4) During erection stages of the girder, the critical wind speed for flutter decreases, especially at early stages. And, it might be more severe compared with the completed bridge. (5) Some temporary stiffening methods of the girder were proposed, and combination of these methods proved to be effective to stabilize the bridge against flutter in wind tunnel.
Acknowledgements The authors express their gratitude to the members of Technical Committee, Prof. N. Shiraishi, Prof. M. Matsumoto and Dr. K. Yokoyama, for their comment on this wind tunnel test.
Aerodynamic stability of narrow decked suspension bridge (Aki-nada Ohashi Bridge) Akihiro Honda *, Hiroaki Miyata, Hideyuki Shibata Fluid Dynamic Laboratory, Nagasaki Research and Development Center, Mitsubishi Heavy Industries Co. Ltd., 1-1, Akunoura-machi, Nagasaki 850-91 Japan Aki-nada Ohashi Bridge Construction Office, Hiroshima Road Public Corporation, 3-6-21, Nishichuo Kure, Hiroshima 737, Japan Sogo Engineering INC., 3-5-9, Higashi Nakajima, Higashiyodogawa-ku, Osaka 533, Japan
Abstract Aki-nada Ohashi Bridge (tentative name) is now under construction with 750 m in center span length, 1175 m in total bridge length. The girder of the bridge is 19 m in width with two-lanes and pedestrian space, and the configuration is flat hexagonal box shaped. This paper describes the aerodynamic stability of both the completed stage and erection stage of the girder. Its improvement is also described. 1998 Elsevier Science Ltd. All rights reserved.
1. Outline of the bridge Aki-nada Ohashi Bridge is located at the southern part of Hiroshima Prefecture in Japan, and it will connect the main land of Japan and Shimo-Kamagari Island. The suspension bridge is 750 m in its center span length, and 1175 m in total length as shown in Fig. 1. The deck section of the bridge is 19 m in width and 2.5 m in depth, which consists of two traffic lanes and a pedestrian walkway at both sides of the section. It is designed to increase by one traffic lane in the future. Therefore, the pedestrian walkway at one side of the deck will remain, and the cross section will become asymmetric as shown in Fig. 2. The configuration is evaluated through wind tunnel test with section model at PWRI in Japan. Fig. 3 shows the skeleton of the tower, and it is of metal frame structure, because the earthquake load is smaller than that of a concrete structure. The height of the tower is 120 m, and the ratio of height to width is 6.8, which is one of the most slender
* Corresponding author. 0167-6105/98/$ — see front matter 1998 Elsevier Science Ltd. All rights reserved. PII: S 0 1 6 7 - 6 1 0 5 ( 9 7 ) 0 0 1 6 0 - 3
410
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Fig. 1. Outline of Aki-nada Ohashi Bridge.
Fig. 2. Cross-section of deck. (a) Deck with two traffic-lanes. (b) Deck with three traffic-lanes.
Fig. 3. Skeleton and cross section of the tower. (a) Skeleton of the tower. (b) Cross-section of the shaft.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
411
Table 1 Dynamic characteristics of Aki-nada Ohashi Bridge Item
Heaving motion
Torsional motion
Natural frequency Mass and inertia moment
0.164 Hz 13.377 ton f/m
0.503 Hz 45.826 ton f s m/m
structures in Japan. A wind tunnel test with aeroelastic model of the tower was also conducted at Kyoto University, and the configuration of the shaft was evaluated. The bridge is now under construction, and is scheduled to be completed in 1999.
2. Aerodynamic stability of completed bridge In this study, wind tunnel test using aeroelastic model of the entire bridge was conducted to confirm the stability with static deformation of the girder by wind. The dynamic characteristics of symmetric mode of the bridge are shown in Table 1. 2.1. Criteria of completed bridge The criteria of wind speed for aerodynamic design were calculated based on the ‘Standard of Honshu-Shikoku Bridge Authority’ as follows: » "37 m/s, » "l » , » "1.2l l » , where » and » are design mean wind speed for gust " " " " response and design wind speed for flutter at girder level, » "10 min mean wind speed at 10 m above ground, l the modification factor of height of (Z/10) "1.25, l the modification factor of gust considering the span length of 1.15, and 1.2 the safety factor for flutter. Namely, » was calculated to be 46.3 m/s, and » determined to be 63.8 m/s. " " Table 2 compares the characteristics of long-spanned suspension bridges with boxgirder, and indicates that Aki-nada Bridge requires the highest reduced criteria of flutter wind speed in Japan. The bridge with narrow deck section requires severe aerodynamic design. 2.2. Model and test procedure The model was 1/120 in scale, and made of aluminum spine which was designed to simulate the bending and torsion rigidity of the girder and piano-wire to simulate the tension rigidity of the cable. Wooden blocks were fitted to the spine to represent the configuration of the girder. After that, concentrated mass and cylindrical tube were wrapped to the piano-wire to simulate the mass and drag force acting on the cable. The model was designed to satisfy ‘Froude’s similarity rule’ as well as ‘Strouhal’s similarity rule’. The test was conducted at the wind tunnel of Nagasaki R&D Center of Mitsubishi Heavy Industries’ Co. Ltd. The wind tunnel cross section is 10 m in width and 3 m in
412
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Table 2 Existing and under construction long-spanned suspension bridges with box-girder No.
Name
Max. Span ¸ (m)
Width B (m)
Depth D (m)
» " (m/s)
Frequency f /f (Hz) 2
» /f B " 2
Nation
1 2 3
Great Belt Humber Jiangyin Yangtze River Tsing ma Ho¨ga Kusten Bosphorus 2 Bosphorus Kurushima 3 Kurushima 2 Severn Ask+y Aki-nada Hakucho
1624 1410
31 22
4.0 4.0
60 37
0.278/0.100 0.300/0.105
7.0 5.6
Denmark UK
1385 1377 1210 1090 1074 1030 1020 987.6 850 750 720
36 41 22 39.4 28 32.3 32.3 22.9 15.52 19 23
3.5 7.6 4.0 3.0 3.0 4.3 4.3 3.1 3.0 2.5 2.5
92 80 50
0.4 /0.139 0.278/0.141 0.388/0.135
6.4 7.0 5.9
70 70 50 52 63.8 67
0.379/0.157 0.377/0.156 0.374/0.143 0.463/0.178 0.503/0.164 0.450/0.116
5.7 5.7 5.8 7.2 6.7 6.5
China China Sweden Turkey Turkey Japan Japan UK Norway Japan Japan
4 5 6 7 8 9 10 11 12 13
Note: f , f : natural frequency of 1st torsion and bending symmetric mode. 2
Fig. 4. Wind tunnel test of completed bridge model.
height. At the mouth of the wind tunnel, rows of blades were installed in order to generate inclined angle of attack and also computer controlled turbulence as shown in Fig. 4. The displacement and vibration of the model were measured by optical sensor.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
413
Fig. 5. Flutter onset velocity of the completed bridge. (a) Flutter wind speed with two traffic-lanes. (b) Flutter wind speed with three traffic-lanes.
Fig. 6. Parapet at the side of the deck (unit: mm). (a) Original design. (b) Improved design.
2.3. Full model test result In uniform flow, vortex-induced excitation of the tower, and flutter of the deck was observed. The critical wind speed of the flutter as a function of the wind angle of attack is shown in Fig. 5. The flutter of the deck with two traffic-lanes was found to be insufficient at the angle of attack of !3°. However, the critical wind speed of the deck with three traffic-lanes was found to be sufficient. 2.4. Aerodynamic improvement of the deck section for flutter From the point of view that additional members (parapet, rail for maintenance cargo, etc.) tend to play an important role for narrow-decked suspension bridge, the effect of the parapet on the deck was investigated for aerodynamic improvement as shown in Fig. 6. The test was conducted using a section model, 1/20 in scale, and the critical wind speed for flutter was found to be higher as shown in Fig. 7. Finally, the
414
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Fig. 7. Aerodynamic improvement of the flutter of the girder (section model). (a) Two traffic-lanes. (b) Three traffic-lanes.
Fig. 8. Aerodynamic improvement of the flutter of the girder (aeroelastic model). (a) Two traffic-lanes. (b) Three traffic-lanes.
stability was also confirmed through wind-tunnel test using the aeroelastic model as shown in Fig. 8. 2.5. Gust response due to turbulent wind Gust response was also tested in the wind tunnel. Computer-controlled pitching motion of the rows of blades generated the turbulence, and its characteristics are shown in Table 3. Fig. 9 shows the peak amplitude of the longitudinal, vertical and torsional random response at the center of mid-span. Among those responses, the longitudinal component was the largest one. However, the amplitude at the mean wind speed of 46.3 m/s (» ) was below the level of allowable stress. "
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
415
Table 3 Turbulent characteristics Component
Intensity
Length scales
Longitudinal Vertical
10% 5%
630 m 120 m
Fig. 9. Response in turbulent flow. (a) Two traffic-lanes. (b) Three traffic-lanes.
3. Aerodynamic stability of girder erection stages The deck of the bridge is scheduled to be constructed from the center of its main span, and Table 4 shows the natural frequency of the lowest mode at each erection stage. The natural frequency of torsional motion is lower at the early stages of erection, and flutter will be of more concern than compared to the completed bridge. From these results, four stages of erection (1,2,3 and 5) were tested in wind tunnel.
416
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Table 4 Procedure and natural frequencies during girder erection stages
Fig. 10. ‘Hinge joint’ of the lifted block of girder.
3.1. Model The girder of the bridge is to be erected using the ‘all-Hinge method’, which releases the bending moment of vertical deflection. A matching piece as shown in Fig. 10, will join each lifted section. In order to simulate the function of the ‘Hinge joint’, an ‘X-shaped spring with thin brass plate’ was manufactured, and connected the spines to each other as shown in Fig. 11.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
417
Fig. 11. Aeroelastic model of girder erection stages.
Fig. 12. Flutter onset velocity of the bridge during girder erection.
3.2. Criteria and test result The criteria of wind speed was calculated based on the ‘Standard of Akashi-Kaikyo Bridge’, which indicates that the wind speed during the erection stage should be 81% of that of the completed bridge. Namely, the erection criteria for 10 min mean wind speed for gust was 37.5 m/s, and the flutter wind speed was 51.7 m/s at the level of girder. Fig. 12 shows the critical wind speed of flutter during the erection stages, and it was found that stages of 1 and 3 are not stable enough compared to the criteria. The solid symbols in the figure mean that the flutter response is coupled with the random response of heaving motion. 3.3. Temporary stiffening system for girder during erection For temporary improvement of the flutter stability during erection stages of the girder, many stiffening systems were considered and certain combinations of these methods, shown in Fig. 13, were tested in wind tunnel.
418
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
Fig. 13. Temporary stiffening method of the girder. (a) Strut connecting main-cable and girder. (b) Crosshanger connecting main-cable and hanger. (c) Strand-rope between main-cable and tower.
Fig. 14. Improved flutter onset velocity of the bridge during girder erection: (a) Erection stage-1. (b) Erection stage-3.
Fig. 14 compares the improved and original critical speed of flutter for stages 1 and 3, where the stability was determined to be insufficient. It indicates that a combination of these methods could improve flutter speed beyond the criteria. In the wind tunnel, the gust response due to turbulent wind was also tested. Fig. 15 shows the response depending on the stage of erection. The displacement is evaluated at the mean wind speed of 37.5 m/s, and it was confirmed that no fatal damage is expected during the erection stage.
419
Fig. 15. Response in turbulent flow during girder erection.
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
420
A. Honda et al./J. Wind Eng. Ind. Aerodyn. 77&78 (1998) 409–420
4. Concluding remarks The following conclusions were reached through a series of aerodynamic investigations: (1) The narrow-decked suspension bridge with box-girder was found to be disadvantageous concerning aerodynamic stability. Reduced criterion of flutter wind speed for ‘Aki-nada Bridge’ is almost the same as that of ‘Great Belt Bridge’ in Denmark. (2) The wind tunnel test using three-dimensional aeroelastic model indicates that wind tunnel test results using the section model may overestimate the critical wind speed of flutter. (3) Aerodynamic characteristics of narrow-decked box-girder seem to be sensitive to additional members (parapet, rail for maintenance cargo, etc.). Minor changes of parapets succeeded in increasing the stability of the girder after the completion of the bridge. (4) During erection stages of the girder, the critical wind speed for flutter decreases, especially at early stages. And, it might be more severe compared with the completed bridge. (5) Some temporary stiffening methods of the girder were proposed, and combination of these methods proved to be effective to stabilize the bridge against flutter in wind tunnel.
Acknowledgements The authors express their gratitude to the members of Technical Committee, Prof. N. Shiraishi, Prof. M. Matsumoto and Dr. K. Yokoyama, for their comment on this wind tunnel test.