wind tunnel study on aerodynamic optimization of suspension ...

The Seventh Asia-Pacific Conference on
Wind Engineering, November 8-12, 2009,
Taipei, Taiwan
WIND TUNNEL STUDY ON AERODYNAMIC OPTIMIZATION OF
SUSPENSION BRIDGE DECK BASED ON FLUTTER STABILITY
Qi Wang 1 Hai-li Liao 2 and Ming-shui Li 3 Rong Xian 4
1 PhD Student, Research Centre for Wind Engineering, Southwest Jiaotong University
Chengdu Sichuan 610031, PR China, wangchee_<strong>wind</strong>@swjtu.edu.cn
2 Professor, Civil engineering school, Southwest Jiaotong University
Chengdu Sichuan 610031, PR China, hlliao@swjtu.edu.cn
3 Professor, Research Centre for Wind Engineering, Southwest Jiaotong University
Chengdu Sichuan 610031, PR China, lms_rcwe@126.com
4 PhD Student, Research Centre for Wind Engineering, Southwest Jiaotong University
Chengdu Sichuan 610031, PR China, rxianok@163.com
ABSTRACT
Nanjing the 4th bridge of Yangtze River is a three span suspension bridge of main span lengths 1418m, and the
flutter checking <strong>wind</strong> speed is up to 60.8m/s for the completed bridge. Through a 1:50 scale section model, more
than 40 model cases were tested in order to obtain an optimized aerodynamic configuration of the girder. The
influence of modifications of the accessory components and the geometry on the aerodynamic stability has been
established through this <strong>study</strong>, which has been beneficial to the final design of the bridge. At last, this paper tries
to discuss the mechanism of the flutter <strong>wind</strong> speed increasing.
KEYWORDS: NANJING 4 TH BRIDGE OF YANGTZE RIVER, TRAPEZOIDAL BOX SECTION,
AERODYNAMIC CONFIGURATION, OPTIMIZATION, WIND TUNNE TEST,
Introduction
Nanjing 4th bridge of Yangtze river in Jiangsu Province of southeast of China is a
three span suspension bridge with main span 1418m. The original design of bridge deck is a
trapezoidal steel box girder with overall width of 37.7m and a height of 3.4m, see Fig.1.
According to the <strong>wind</strong> statistic data and the Chinese Code of Bridge Wind Resistance,
the flutter checking <strong>wind</strong> speed of the bridge is up to 60.8m/s, the aerodynamic stability
becomes a governing factor in the design. Unfortunately the flutter critical <strong>wind</strong> speed of the
original girder is less than 45m/s, which was found by intensive <strong>wind</strong> <strong>tunnel</strong> testing of section
model. Because of its intrinsic limit in the aspect of flutter instability, it is necessary to adopt
some countermeasures to improve aerodynamic performance to meet the requirements of
<strong>wind</strong> resistance code. Aerodynamic optimization of the deck configuration is hence definitely
required to ensure the safety of the bridge.
Through a 1:50 scale section model, more than 40 configuration cases of deck were
tested in order to establish the influence of the accessory components and the geometrical
modifications on the aerodynamic stability, such as the porosity of railing in the sideway, the
position of inspection rail, the guide wing, the edge configuration of the section, the steepness
of side wall slopes. Those countermeasures have been proved to be effective to improve the
aerodynamic performance particular flutter instability of bridge by other researchers and
engineering practices (A. Larsen, 1993; K. Wilde et al., 2001; B. Luca et al., 2002; Song
Jinzhong et al., 2002; T. Miyata, 2003; Liu Cijun et al., 2008; Yongxin Yang et al., 2008 ). In
The work described in this paper was supported by the grants from the National Natural Science Foundation of China
(Project No. 90815016)

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
this paper their sensitivity relative to aerodynamic stability were investigated through a series
of section model <strong>wind</strong> <strong>tunnel</strong> testing. The results has benefited for the final design of the
bridge, and other long span bridges.
Figure 1: Outline of Nanjing Yangtze 4th Bridge
The Influence of Railing on Flutter Critical Wind Speed
In order to examine the influence of the different railing and its position on the critical
<strong>wind</strong> speed, two kinds of railing with porosity 90% and 60%, three locations of railing on
deck, one is on the edge (the original design), the others are 5mm (prototype 250mm) and
10mm (prototype 500mm) from the sideway edge, are chosen to be tested. The results are
given in table 1. It is found that higher aerodynamic stability for the railing with higher
porosity comparing to the lower one. This similar finding was also reported by Allan Larsen
in the aerodynamic design of Great belt East Bridge (A. Larsen, 1993), and other researchers
(B. Luca et al., 2002; T. Miyata, 2003; Liu Cijun et al., 2008). Comparing to the section
configuration of old Tacoma Bridge deck, see fig.2, the low porosity of railing has made the
deck configuration close to I-shape, which may lead to flow separation, and a vortex shedding
in a rhythmic fashion will generate separation bubbles above the deck (see fig3). The vortex
creation and drift process will dramatic weaken the stability of the girder. However, the
critical <strong>wind</strong> speed of girder is not sensitive to the change of the position of railing.
Table 1: The Critical Wind Speed Varying With The Different Railing
case porosity position
Flutter critical <strong>wind</strong> speed(m/s)
-3° 0° +3°
1 90% Initial position >74.6 73.0 61.3
2 60% Initial position >75.3 57.6 44.8
3 60% 5mm inside >74.8 58.6 42.4
4 60% 10mm inside >76.2 57.9 44.0
Figure2: Vortex Movement of Tacoma Deck
Figure3: Vortex Movement of Box Girder

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
The Influence of Inspection Rail on Flutter Critical Wind Speed
The aerodynamic stability of the girder may be sensitive to the distance between the
rail top and the girder bottom (Yongxin Yang et al., 2008). Three different kind of adjustment
were carried out to investigate the potential influence. The distance was increased by 1mm
and 3mm (prototype 5cm and 15cm) in the tests (the porosity of railing is 90%). Flutter
critical speeds of these sections were measured and are summarized in table 2. It is found that
the increase of distance is benefit to the critical <strong>wind</strong> speed at 0°and -3°attack angle, but
disadvantage to the aerodynamic stability of the girder at +3°attack angle. Because the
dominant factor of aerodynamic stability is the minimum value among three critical <strong>wind</strong>
speeds, the increasing of distance is not a good choice to strengthen the aerodynamic stability
of the girder.
Table 2: The Critical Wind Speed Varying With The Change of Railway
case
distance
Flutter critical <strong>wind</strong> speed(m/s)
-3° 0° +3°
Initial design 5mm from the bottom >79.0 73 61.3
Increment 1mm 6mm from the bottom >81 >80 59.6
Increment 3mm 8mm from the bottom >84 >82.5 58.1
The Influence of Guide Wing on Flutter Critical Wind Speed
The guide wing on the edge of sideway can smooth airflow while passing through the
section. Hence the aerodynamic stability maybe strengthened (K. Wilde et al., 2001; Song
Jinzhong et al., 2002; T. Miyata, 2003). Because of the particularity of deck configuration, the
optimized guide wing should be selected by intensive <strong>wind</strong> <strong>tunnel</strong> tests. Total nine different
types of guide wings are applied in the tests, with different width and obliquity. The railing
porosity is 60% for the section model in the tests. Flutter <strong>wind</strong> speeds were obtained in the
tests for the section model with attack angle +3 deg and 0 deg, and the results are shown in
Table 3.
It is observed that the wider and the positive obliquity guide wing can improve the
aerodynamic stability distinctly although the railing has low porosity. On the other hand, the
guide wing will increase the complexity of the structure design and construction, particularly
in the location of rostra, and the maintenance cost will increase correspondingly. The guide
wing of the deck is not recommended in the design unless there is no alternative means to
improve the aerodynamic performance.
Figure 4: Outline of Guide Wing

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
Table 3: The Critical Wind Speed Varying With Different Guide Wings
case
Guide wing Flutter critical <strong>wind</strong> speed(m/s)
width obliquity 0° 3°
1 50cm +15 52.2 44.9
2 50cm 0 51.47 41.98
3 50cm -42 54.75 42.34
4 100cm +15 54.96 52.05
5 100cm 0 52.78 50.23
6 100cm -42 58.77 45.63
7 125cm +15 60.23 63.88
8 125cm 0 56.94 55.48
9 125cm -42 62.78 49.64
The Influence of Section Rostra on Flutter Critical Wind Speed
Because the critical <strong>wind</strong> speed is sensitive to shape of section rostra (A. Larsen, 1993;
Song Jinzhong et al., 2002; T. Miyata, 2003), rostra with different width and acutance is taken
into account in the tests. The acutance varies from 57 deg to 25 deg, correspondingly the
width varying from 1.9m to 3.3m. Total twenty-one model cases were tested. The results are
shown in table 4. It is noted that the flutter critical <strong>wind</strong> speed increases along with the
increasing of rostra width and acutance. However, the flutter critical <strong>wind</strong> speed comes down
distinctly when the width of rostra exceeds 3m. The mechanism for rostra with a width more
than 3m may weaken the stability will be discussed in the later chapter.
Table 4: The Critical Wind Speed Varying With Different Section Rostra
Flutter critical <strong>wind</strong> speed(m/s)
case Type of the section rostra
+3° -3° -3°
1 width:1.9m 51.5 >69.6 >70.6
2 width:2.1m 53.7 >70.7 >70.3
3 width:2.3m 54.6 >71.2 >70.5
4 width:2.5m 56.7 >70.8 >71.2
5 width:2.7m 58.6 >71.5 >75.2
6 width:3.0m 63.4 >72.5 >70.9
7 width:3.3m 59.3 >71.3 >72.8

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
64
3m width
62
flutter critical <strong>wind</strong> speed (m/s)
60
58
56
54
52
3.3m width
2.7m width
2.5m width
2.3m width
2.1m width
1.9m width
50
20 25 30 35 40 45 50 55 60
the acutance of rostra (degree)
Figure 5: Flutter Wind Speed Varying With The Acutance of Rostra
The Influence of Steepness of Lower Inclined Web Slope on Flutter Critical Wind Speed
The wider and acuminate section rostra are more difficult to be fabricated and fixed,
implying more cost in design and construction, although it can strengthen the aerodynamic
stability of the girder distinctly. Alternate way is to decrease the steepness of side wall slope,
which can make the deck cross section more streamlined. When the slope decreases to 15°,
see fig6, the flutter critical <strong>wind</strong> speed is up to 67m/s, and it directly increases the flutter
performance by 10%, and the width of section rostra is only 2.4m. The detailed test results are
shown in table 5. The results also lead to an optimized section of girder: shorter rostra,
without guide wing, low porosity railing. It also satisfies different kinds of requirements: high
security, low cost, and more convenience.
case
Table 5: The Critical Wind Speed (Slope Is 15°)
Flutter critical <strong>wind</strong> speed(m/s)
Section rostra
-3° -3° -3°
1 width:2.4m 67.1 >71.3 >73.5
2 width:2.6m 62.0 >70.8 >72.7
3 width:2.8m 61.1 >71.5 >72.4.
Figure 6: The Lower Inclined Web Slope Decreased To 15°

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
The Discussion on Mechanism of Aerodynamic Improvement
According to the research results of airfoil (Abbott I. H., 1958), all classical airfoils
are stalled at 16 deg. The lift is increasing with increasing attack angle, and once exceeding
16 deg, the lift decreases, see figure 7. The flow detaches from the suction side and forms a
vortex while the airfoils are stalled. Larsen based on the experience from vortex shedding
tests (Larsen A. et al., 2000, 2008 ), found that the trapezoidal box sections was similar to the
airfoils, and gave a conclusion that the flow along the bottom plate would stay mainly
attached if lower inclined web angle α is less than approximately 16 deg. He also gave an
example about the design <strong>study</strong> for a two span suspension bridge in Chile, whose lower
inclined web angle is 14.8 deg, and no vortex-induced vibration was observed in the <strong>wind</strong>
<strong>tunnel</strong> testing. Another example was a new 1345 m suspension bridge in northern Norway,
with 15.8 deg of lower side web angle, and the <strong>wind</strong> <strong>tunnel</strong> tests demonstrated that vortex
induced vibration were absent. The contrary examples were box girders of the Great Belt East
Bridge and Osterøy Bridge, lower inclined web slope angles are 26.6° and 29.5°, respectively.
Explicit vortex induced vibrations were observed for these bridges in full scale.
Figure 7: The Lift Coefficient of Wings Varying With Attack Angle
Similarly to a box section in the vortex shedding vibration status, in the flutter critical
status, two vortices with counter direction have also been observed at the both sides of the
nose tail line of Great Belt East Bridge section through the PIV (Particle Image Velocimetry)
technique(Zhang et al., 2009), which can give the girder enough momentum to increase its
amplitude in a shot time and finally make the girder instable. When the <strong>wind</strong> speed is low,
and under the flutter critical speed, the positive vortex blow the nose-tail line is more
powerful than the above negative one, the aerodynamic force is just a static lift force, see
figure 8. When the <strong>wind</strong> speed is increasing, and close to the flutter critical speed, the
negative vortex above the nose-tail line has been strengthen as powerful as the blow positive
one, and the aerodynamic force becomes to fluctuated force, see figure 9. If the frequency and
the phase of the fluctuated force are close to bridge’s, the flutter of girder will be occur soon.
In this paper, when the slope of the lower inclined web decreases to 15 deg(also less
than 16 deg), there is a smaller dead air wake region below the nose-tail and the flow along
the bottom plate will stay mainly attached the web, which making more difficult for formation
of a large vortex. When the <strong>wind</strong> speed is increasing to the original flutter critical <strong>wind</strong> speed
(former test girder with low critical <strong>wind</strong> speed), because the little room gives the restrain to
the forming of vortex, the counter vortices can’t give the girder powerful and efficient

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
excitation. When the <strong>wind</strong> speed increasing to a higher one, the equalization counter vortices
will form again at the similar position and the rhythmic excitation is be back, which bring on
aerodynamic instability to the girder again.
This explains can also be extended to interpret the results show in table4. When the
slope of the lower inclined web is less than 16 deg, varying with the increasing of rostra width,
there is a lager room for formation of a large vortex, which is not propitious to the girder
stability. In the tests, when the width of rostra exceeded 3m, there would be a larger room for
formation of a larger coherent vortex which made the flutter critical <strong>wind</strong> speed descend. Of
course, this explains are also applicable to the interpretation of results in table 5.
Needing to point out is that the explain about the mechanism is based on the Larsen’s
research on the vortex shedding vibration of Great Belt East Bridge, and Zhang’s research on
the flutter critical status of the same bridge by PIV technique, it only a assumption and a
deduction of aerodynamic improvement. Further more, it needs to be verified through the
further <strong>study</strong> by the <strong>wind</strong> <strong>tunnel</strong> tests and CFD method in the future researches.
Figure 8: Vortex Moment At The Low Wind Speed (Blow The Critical Speed)
Figure 9: Vortex Moment At The High Wind Speed (Closing To Critical Speed)
Conclusions
The girders with high porosity railings have higher flutter critical <strong>wind</strong> speed than the
ones with low porosity railings. The wider and acutance section rostra can strengthen the
aerodynamic stability of the girder, but width can’t exceed 3m. The flutter critical <strong>wind</strong> speed
is also sensitive to the steepness of blow inclined web slope. The lower is the slope, the higher

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan
is <strong>wind</strong> speed. When the slope of lower inclined web is less than 16 deg, the flutter critical
<strong>wind</strong> speed <strong>wind</strong> will increasing dramatically.
Through the <strong>wind</strong> <strong>tunnel</strong> tests, the final section has a good aerodynamic stability, and
the flutter critical <strong>wind</strong> speed is up to 67.1m/s, and 79m/s in the later full scale aerodynamic
model tests. Of course, there is no vortex shedding vibration observed in the 1:50 and 1:20
section model testing.
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