The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

REEVALUATION ON AERODYNAMIC STABILITY OF STEEL BOX GIRDER

Shigeki Kusuhara1, Ikuo Yamada2 and Naoki Toyama3

1 Sub-leader, Long-span Bridge Engineering Center, Honshu-Shikoku Bridge Expressway Co., Ltd., Kobe, Japan, shigeki-kusuhara@jb-honshi.co.jp

2Leader, ditto, Kobe, Japan, ikuo-yamada@jb-honshi.co.jp3 Member, ditto, Kobe, Japan, naoki-toyama@jb-honshi.co.jp

ABSTRACT

Since the serious corrosions on countermeasures for aerodynamic stability were found on the Tozaki viaduct, which was completed approximately 20 years ago, it is necessary to make the repair plan. In order to reduce the future maintenance cost, the reevaluation on aerodynamic stability of this bridge was conducted. As a result, it was clarified that the half of the original countermeasures have little effect on the aerodynamic stability of this bridge and those members were removed. Furthermore, it was also confirmed that there was no particular change of the dynamic characteristics of the girder before and after work the half removal of countermeasure.

KEYWORDS: AERODYNAMIC STABILITY, REEVALUATION, FULL MODEL TEST, BOX GIRDER

Introduction

The Honshu-Shikoku Bridges (HSB) connect Japanese two major islands, Honshu and Shikoku, with three routes and it is a part of the important expressway network (Figure 1). The HSB consist of ten suspension bridges, five cable-stayed bridges, an arch bridge and a truss bridge, including the Akashi Kaikyo Bridge, the world’s longest suspension bridge with the center span of 1991m. Since the aerodynamic stability is one of the most important issues in the wind-resistance design for the long-span bridges, various studies have been carried out. However, countermeasures for aerodynamic stability are not only for long-span bridges but also for short-span bridges in case that the wind condition is very severe.

Figure 1: Outline of HSB

Akashi Kaikyo Bridge HHonshuonshu

ShikokuShikoku

SSaakkaaiiddee

NNaarruuttoo

IImmaabbaarrii

KKoobbee

Ohnaruto Bridge

OOkkaayyaammaa JAPAN OOnnoommiicchhii

HHonshuonshu

Tokyo

Seto Ohashi Bridges

Tatara Bridge ShikokuShikoku

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

In general, countermeasure members for aerodynamic stability are fabricated with thin steel plates, and corrosions tend to occur and progress rapidly. In fact, corrosions were found on the Tozaki viaduct, which was approximately 20 years in use. Therefore, the reevaluation of aerodynamic stability of this bridge, based on the latest knowledge and experience which was obtained after completion, was conducted. Consequently, the half of countermeasures was removed and the future maintenance cost was reduced by half.

Outline of Tozaki viaduct

The Tozaki viaduct (Figure 2,3), which is a steel approach bridge for the Ohnaruto Bridge, consists of 3-span continuous beam bridge (Bridge-A; 108m +108m +108m) and 4-span continuous beam bridge (Bridge-B; 149.6m +190.4m +190.4m +149.6m) with the single-box girder with brackets. Since these bridges were designed against one of the highest wind speed in Japan (V10=50m/s), which is the same as the Ohnaruto Bridge, and were also constructed along the Tozaki cape with steep slopes, many cases of the wind tunnel test were carried out during design and fabrication stage (shown in Table 1 [Ohshima et al.(1982)]). Accordingly, the “double-flap” as the countermeasure against Kármán vortex-induced vibration was installed to each bridge, and “lower-skirt” as the countermeasure against galloping was installed to only Bridge-B. Countermeasures for Bridge-A and Bridge-B were shown in Figure 4.

Figure 2: General view of Tozaki viaduct for provisional 4-lane highways

T1A T2P T3P T4P T5P T6P T7P T8A

Ohnaruto Bridge

324.0

108.0 108.0 108.0 149.6 190.4 190.4 149.6

680.0

3-span continuous beam bridge 4-span continuous beam bridge

(Unit: m)

(Bridge-B)

N

Lower skirt

(Bridge-A)

(Honshu) (Shikoku)

Kobe

Ohnaruto Bridge

Toza

ki v

iadu

ct Tozaki cape

Double-flap

(a) Bird’s-eye view (b) Double flap (c) Lower skirt Figure 3: Tozaki viaduct right after completion

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

Maintenance way

Double-flap Double-flap

Lower skirt Lower skirt Dep

th:

4.5-

8.2m

Width: 18.25m

Double-flap Double-flap

Maintenance way

Dep

th:

4.5m

Width: 18.25m

(a) Bridge-A (b) Bridge-B

Figure 4: Cross section of girder

Table 1: Results of wind tunnel test at construction stage (a) Results of sectional model test for Bridge-A

No countermeasure + Double flap K V K V Attack

angle Galloping Amplitude Wind Speed Galloping Amplitude Wind speed 0 deg. Not onset 34.5 cm 37.9 m/s Not onset 12.8 cm 35.8 m/s 3 deg. 92.3 m/s 40.9 cm 35.4 m/s Not onset 12.0 cm 37.1 m/s 5 deg. - - - Not onset 29.9 cm 33.5 m/s 7 deg. - - - 84.7 m/s 39.6 cm 33.3 m/s

10 deg. 71.0 m/s 91.3 cm 36.8 m/s 82.4 m/s 47.1 cm 32.1 m/s (b) Results of full-model test for Bridge-B Countermeasure No countermeasure + Lower skirt + Double flap Topographic included excluded included included Flow condition smooth turbulent smooth turbulent smooth turbulent smooth turbulentGalloping (m/s) 36 m/s Not onset 37 m/s Not onset 37 m/s Not onset 100m/s Not onset

Amp. 110 cm 40 cm 50 cm 20 cm 121 cm 68 cm 25 cm - 1st

Wind 17 m/s 16 m/s 12 m/s 14 m/s 17 m/s 16 m/s 17 m/s - Amp. 110 cm 30 cm 50 cm 10 cm 102 cm 44 cm 10 cm -

KV

2nd

Wind 27 m/s 25 m/s 19 m/s 23 m/s 24 m/s 23 m/s 26 m/s - KV: Kármán vortex-induced vibration

The corrosion progressed rapidly on the installed countermeasures (Figure 5), because these members had been exposed to the severe natural condition for about 20 years. And it is necessary to replace these members. Therefore, necessity of the countermeasures for aerodynamic stability was reevaluated in order to reduce future maintenance cost. The investigation was conducted by considering the following the latest knowledge and experience which were obtained after completion.

1) The structural damping (δ≥0.05) obtained from the field vibration test [Yoshida (1985)] is larger than the designed value (δ=0.02), where δ means the logarithmic decrement (Table 2).

2) The prevailing wind direction observed from the field measurement is the right angle to bridge axis. Due to the influence of cape shape and location, the wind from sea-side tends to blow up (Figure 6). And the wind from cape-side tends to decrease in speed.

(a) Double flap (b) Lower skirt

Figure 5: Corrosions on aerodynamic countermeasures

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

Table 2: Dynamic characteristics of bridges Bridge-A Bridge-B Mode

Micro tremor Vibration test Micro tremor Vibration test Damping 0.05 ~ 0.10 0.08 0.06 ~ 0.11 0.06 ~ 0.10 1st Frequency 0.96 Hz 0.93 Hz 0.48 Hz 0.48 Hz Damping 0.03 ~ 0.09 0.05 0.04 ~ 0.10 0.04 ~ 0.07 2nd Frequency 1.20 Hz 1.16 Hz 0.65 Hz 0.65 Hz Damping 0.02 ~ 0.06 0.04 0.03 ~ 0.06 0.03 ~ 0.06 3rd Frequency 1.64 Hz 1.62 Hz 0.86 Hz 0.84 Hz Damping 0.02 ~ 0.04 - 0.03 ~ 0.05 0.03 ~ 0.05 4th Frequency 3.26 Hz - 1.00 Hz 0.93 Hz

0.0

10.0

20.0

30.0

0.0 10.0 20.0 30.0Wind speed (m/s)

Win

d in

clin

atio

n (d

eg.)

T3P T7P

Bridge axis

10 20 30 40% E

S

N

W

Win

d in

clin

atio

n (d

eg.)

(a) Occurrence frequency of wind direction (b) Relationship between wind speed and inclination Figure 6: Results of wind observation (1971-1984)

Reevaluation of aerodynamic stability

The reevaluation of aerodynamic stability was carried out by wind tunnel tests, a sectional model test for Bridge-A, and a full aero-elastic test with topographic model for Bridge-B, considering above-mentioned knowledge. The results of wind tunnel tests are shown as follows. [Toyama et al. (2004)]

Aerodynamic stability of Bridge-A The sectional model wind tunnel test for the first vibration mode was carried out under

experimental condition shown in Table 3. The results of wind tunnel test are shown in Figure 7 and 8. As a result, it was confirmed that the cape-side countermeasure does not influence to the aerodynamic stability of Bridge-A (Case A2). And it was also confirmed that the very large amplitude of Kármán vortex-induced vibration appears when the both-side countermeasure was removed (Case A3).

Therefore, it was found that the windward double-flap is indispensable countermeasure for Kármán vortex-induced vibration, and the leeward double-flap could be removed.

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

Table 3: Condition of wind tunnel test Real bridge Required value Sectional model

Scale - 1/48 1/48 Damping ratio 0.05 0.05 0.05 Mass 122.5kN/m 84.57 N/model 83.87 N/model Frequency 0.872 Hz - 3.592Hz

0

20

40

60

80

100

0 5 10 15 20 25Attack angle (deg.)

Am

plitu

de (c

m)

Original(d=0.02)Original(d=0.05)Case1 (d=0.05)Case2 (d=0.05)

[39][36]

[41] [35]] [38]

[38]

[40]

[45]

[53]

[50] [50]

[41][41] [49][51] [53][53][-] [-] [-] [-] [-] [-]

[31

[ ] : Onset wind speed (m/s)Original (δ=0.02) [-] : Not onset CaseA1: with both-side device (δ=0.05)

CaseA2: w/o cape-side device (δ=0.05) CaseA3: w/o both-side device (δ=0.05)

Figure 7: Result of wind tunnel test for Bridge-A (Kármán vortex-induced vibration)

0

50

100

150

0 5 10 15 20 25Attack angle (deg.)

Win

d sp

eed

(m/s

)

Not onset Not onsetNot onset Not onset Not onset

Not onset Not onset

Case

A2

( δ

=0.0

5)

Case

A3

( δ

=0.0

5)

Czas

A1

( δ=0

.05)

Required valueRequired value

Figure 8: Result of wind tunnel test for Bridge-A (Galloping)

Aerodynamic stability of Bridge-B A full aero-elastic model test for Bridge-B was carried out at the Large Boundary

Layer Wind Tunnel [Miyata et al. (1991)] by Honshu-Shikoku Bridge Authority. The scale of full model was 1/100 in consideration of the facility capacity (maximum wind speed: 12m/s). The model is shown in Figure 9.

In order to confirm the influence of countermeasures, 12 cases of wind tunnel tests, shown in Table 4, were carried out and the test results were clarified as follows.

1) The full model test was identified with the previous test in 1981 and the response of real bridge, which does not indicate any significant wind-induced vibrations (Figure 10).

2) According to Case B4 and Case B5, it could not ensure the aerodynamic stability, if the sea-side countermeasure devices are removed (Figure 11).

3) The aerodynamic stability can be ensured against the winds from the sea-side direction (0, ±10, ±20deg.) and the cape-side direction (180, 200deg.), even if the cape-side countermeasures are removed (Figure 12).

4) It was confirmed that the large gust response was generated by the separated vortex from the cape at the wind direction of 180 and 200degrees. However, the amplitude of vibration was less than allowable value (Figure 12, 13).

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

Aero-elastic model (Bridge-B)

Cape-side

T8A

T7P T6P

T5P

Figure 9: Full aero-elastic model test (scale=1/100)

Table 4: Result of wind tunnel test for Bridge-B Double flap Lower skirt Case Wind

direction Damping

ratio Sea-side Cape-side Sea-side Cape-sideKV Galloping

B1 0 deg. δ=0.02 with with with with OK OK B2 0 deg. δ=0.05 with with with with OK OK B3 0 deg. δ=0.05 with without with without OK OK B4 0 deg. δ=0.05 with without without without OK NG B5 0 deg. δ=0.05 without without with without NG - B6 180 deg. δ=0.05 with with with with OK OK B7 180 deg. δ=0.05 with without with without OK OK B8 -20 deg. δ=0.05 with without with without OK OK B9 -10 deg. δ=0.05 with without with without OK OK

B10 10 deg. δ=0.05 with without with without OK OK B11 20 deg. δ=0.05 with without with without OK OK B12 200 deg. δ=0.05 with without with without OK OK

Note: Wind direction was defined 0 degree as the right angle to bridge axis from sea-side KV: Kármán vortex-induced vibration

0.00

0.01

0.02

0.03

0 20 40 60 80 100Wind speed (m/s)

Red

uced

am

plitu

de ( η/

B)

1st mode

2nd mode

4th mode

previous test at 1981 (δ=0.02) Case B1 (δ=0.02) Case B2 (δ=0.05)

0.00

0.01

0.02

0.03

0 20 40 60 80 100Wind speed (m/s)

Red

uced

am

plitu

de ( η

/B) Case B3 (δ=0.05)

Case B4 (δ=0.05) Case B5 (δ=0.05)

Figure 10: Result of reproducibility test Figure 11: Effect of countermeasure devices

Rigid model (Bridge-A)

Sea-side T4P

T3P

Wind (direction=0deg.)

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

0.00

0.01

0.02

0.03

0 20 40 60 80 1000.00

0.01

0.02

0.03

0.04

0.05

0 10 20 30 40 50 60 70Wind speed (m/s)

Red

uced

am

plitu

de ( η

/B)

Wind speed (m/s)

Red

uced

am

plitu

de ( η/

B) Case B8 (δ=0.05)

Case B9 (δ=0.05) Case B3 (δ=0.05) CaseB10 (δ=0.05) CaseB11 (δ=0.05)

Case 6 (δ=0.05, max amp.) Case 7 (δ=0.05, max amp.) Case12 (δ=0.05, max amp.) Case 6 (δ=0.05, RMS) Case 7 (δ=0.05, RMS) Case12 (δ=0.05, RMS)

Allowable value (0.035)

(a) sea-side (b) cape-side

Figure 12: Influence of wind direction

Deck level

Wind

Figure 13: Flow condition of cape-side wind (Measured by particle image velocimetry)

Verification of dynamic characteristics by field measurement of the bridge

Based on the above-mentioned result, the cape-side countermeasures were removed from the real bridge in 2004-2008. In order to observe the changes of dynamic characteristics, a field measurement, installing accelerometers and anemometers to each bridge, was carried out before and after removal, as shown in Figure 14. The field measurement resulted that there was no particular change of vibration characteristics and no harmful vibration during strong wind, as shown in Figure 15.

Bridge-A Bridge-B

T1A T2P T3P T4P T5P T6P T7P T8A

: accelerometer : anemometer (ultrasonic)

Figure 14: Sensor arrangement for field measurement

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0Wind speed (m/s)

SD o

f acc

eler

atio

n (c

m/s

2 )

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0Wind speed (m/s)

SD o

f acc

eler

atio

n (c

m/s

2 ) x : before removal of device o : after removal of device

x : before removal of device o : after removal of device

Bridge-A Bridge-B

Figure 15: Relationship between wind speed and standard deviation of acceleration

Conclusions

The reevaluation results on aerodynamic stability of the Tozaki viaduct were summarized as follows.

1) The half of the original countermeasure could be removed, based on the latest knowledge and experience which was obtained after completion.

2) There was no particular change of the dynamic characteristics of the girder before and after the half removal of countermeasure.

3) It can reduce the future maintenance cost by half. It is necessary to design and fabricate the bridge under the limited time or the

insufficient information at construction stage. Therefore, it is inevitable that the result is not necessarily efficient and economical. However, a reevaluation is needed in order to conduct the more efficient maintenance, because the bridges will be used for an extended period.

Acknowledgment

The authors would like to express the acknowledgement to Dr. Toshio Miyata (professor emeritus at the Yokohama National University, chairman of technical committee for the wind resistance design of Honshu-Shikoku Bridges) for the encouragement and suggestions of this work.

References

Miyata T., Yamada H., Yokoyama K., Kanazaki T., Iijima T. and Tatsumi M. (1991), “Construction of Boundary Layer Wind Tunnel for Long-span Bridges”, Eighth International Conference On Wind Engineering, Ontario, Canada, July 8~12

Ohshima H., Miyashita C. and Ohashi H. (1982), “Wind Tunnel Test of Tozaki Viaduct”, Honshi Technical Report, No. 22, October 1982, p.p.14-21 (in Japanese)

Toyama N., Hata K., Kusuhara S. (2004), “The Reevaluation of the Aerodynamic Stability of the Tozaki Viaduct”, Proceedings of 18th National Symposium on Wind Engineering, Tokyo, Japan, December 1~3, p.p.491-495 (in Japanese)

Yoshida O. (1985), “Compulsory Vibration Test & Measurement of Aerodynamic Deformation on Tozaki Bridge”, Honshi Technical Report, No. 36, December 1985, p.p.61-66 (in Japanese)