THE STUDY OF AERODYNAMIC STABILIZING FOR TANGENTIAL AND CURVED CABLE-STAYED BRIDGE UNDER CONSTRUCTION

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1 The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan THE STUDY OF AERODYNAMIC STABILIZING FOR TANGENTIAL AND CURVED CABLE-STAYED BRIDGE UNDER CONSTRUCTION ABSTRACT Seong-Ho Kim 1, Joo-Taek Park 2 and Kyoung-Jae Lee 3 1 Daelim Industrial Co. Seoul, Korea, johnpolo@daelim.co.kr 2 Daelim Industrial Co. Seoul, Korea, house@daelim.co.kr 3 Daelim Industrial Co. Seoul, Korea, lkjooo@daelim.co.kr A concrete cable stayed bridge is more economic than steel one, so that there are many cable stayed bridges stiffened with concrete deck. Furthermore recent great advances in concrete technology make it possible for bridge designer to construct somewhat long span bridge. Therefore aerodynamic stability of concrete bridge is also considered seriously. Two kinds of concrete cable stayed bridges in Korea are investigated for comparing. The result shows that box girder is more efficient in the aspects of aerodynamic stability. KEYWORDS: CABLE-STAYED BRIDGE, CONSTRUCTION SEQUENCE, BUFFETING ANALYSIS, BUFFETING CABLE, Introduction Nowadays, cable supported bridges are starting to get longer and longer, the role of dynamic forces, such as seismic force and aerodynamic force, are getting more and more important than any other kinds of bridges due to light weight, flexible and lightly damped structure system. Especially the aerodynamic force of a cable stayed bridge determines, to a great extent, the safety. Flutter, vortex shedding and buffeting analysis are generally discussed and thought to be the most important aspects of aerodynamic stability. Effect of flutter and vortex shedding is well known that a wind tunnel test should be carried out to analyze the exact behavior of a bridge. However it is somewhat easy to analyze the buffeting response by using frequency domain approach. Therefore, not only wind tunnel tests but computational analysis is also used for clarifying the responses of long span bridges(m.s. Pfil, 1995; Praveen Reddy, 1999; Xinzhong Chen, 2000; Kim Ho Kyong, 2004; etc.). There are many studies presenting suggestions to improve the stability of the bridges from aerodynamic forces during construction, especially before side and main span closure(kim Ho Kyong, 2004; Kim Jong Soo, 2005; Choi Seong Won, 2006; etc). This paper will show that what the differences are between buffeting responses of the 2 nd Dolsan and Sepoong cable stayed bridge being constructed by Daelim. Furthermore this paper suggests the methods for improving the wind stability during construction especially wind cable, which is worth comparing the attempts to suggest the optimal positions of wind cables of steel composite cable stayed bridge ( Choi, Sung Won, 2007; etc)

2 Analysis condition Models Two cable stayed bridges Sepoong and the 2 nd Dolsan, adopted for studying the differences between curved and straight cable stayed bridges, are 725 m long having 220m central span and 464 m long having 230 m central span respectively (Figure 1) = (a) Sepoong Stages (b) The 2nd Dolsan bridge Figure 1. Schematic view of adopted bridges Before Main Span (The Dolsan) Before Side Span (The Dolsan) Before Main Span (The Sepoong) Before Side Span (The Sepoong) Figure 2. Construction Stages

3 Buffeting Analysis Result Firstly this paper will compare the response of the two cable stayed bridges and find the reason of showing the differences. Secondly the differences of each stage will be discussed. Finally the methods of reducing the buffeting responses will be suggested. Buffeting Responses at each stages To compare between the responses between two types of cable stayed bridges, maximum deflection of deck and moment are used (Table 2.) The result shows that the vertical deflection of the 2 nd Dolsan with edge girder is larger than Sepoong stiffened with box girder even though the length of the Sepoong is a little bit longer than the 2 nd Dolsan bridge, but larger moment can be seen at the pylon near by the foundation of Sepoong bridge which is the same as what is generally thought to be. Not only be the stiffness of box girder higher than edge girder but it is also fixed to the pylon, so that it is thought that load introduced to deck is transferred to the pylon. Table 2. Buffeting Responses 2nd Dolsan Sepoong Construction Before Side Span Before Main Span Transeverse (m) Deck Deflection Vertical (m) Rotation (rad) Deck Inplane Moment (KN-m) Pylon Moment at the foundation (KN-m) Final Stage Max. Value Before Side Span Before Main Span Final Stage Max. Value Aerodynamic Stabilizing Method Investigated Two kinds of stabilizing methods investigated by this paper are installing vertical and diagonal wind cables at the stage showing the maximum response. The stage before side span closing which means that pylon is standing alone with the longest cantilever arm was chosen to install the wind cables. To find the most effective wind cables, the position of cables was changed from V1 to V4 and from to at the 2 nd Dolsan bridge. There are four vertical cables from V1 to V4 and 2 diagonal cables from to at the Sepoong bridge The 2nd Dolsan The Sepoong V 3 V 4 V 2 V 1 D 2 D 1 D 1 D 2 V 1 V 3 V 2 V 4 V3 V1 V4 V2 V1 V3 V2 V4 Figure 3. The Position of Wind Cables

4 Buffeting Responses in Vertical Wind Cable Table 3 and 4 shows maximum deflection, in-plane moment and reduction ratio of deck and pylon at the 2 nd Dolsan and Sepoong respectively. There is an increase of from 32 percent to 88 percent in the in-plane moment at the Dolsan. Contrary to the Dolsan, vertical cables installed at the Sepoong reduced moment, which caused by the different boundary conditions of deck such as floating type and fixed type. Wind cables established at the Dolsan shows behaviors like constraints. However the cables at the Sepoong reduced the cantilever arm length between pylon and wind cables. Table 3. Buffeting Responses (Vertical Wind Cable at the 2 nd Dolsan) Item N.C V1 V2 V3 V4 Deck Vertical(m) Transeverse(m) Moment(KN-m) 6,267 11,784 8,291 5,740 9,163 Pyl Moment (KN-m) 115,820 48,759 41,662 44,004 53,511 Table 4. Buffeting Responses (Vertical Wind Cable at the Sepoong) Item N.C V1 V2 V3 V4 Deck Vertical(m) Transeverse(m) Moment(KN-m) Pyl Moment (KN-m) Buffeting Responses in Diagonal Wind Cable Table 5 and 6 show the buffeting responses when there are diagonal wind cables. At the Dolsan, diagonal wind cables have the same effect on in-plane moment as vertical effect, namely showing increases. Because of the somewhat short length between pylon and the cables, there are not influences on the buffeting responses, Deck Table 5. Buffeting Responses (Diagonal Wind Cable at the 2 nd Dolsan) Item N.C Value Ratio Value Ratio Vertical(m) % % Transeverse(m) % % Moment(KN-m) 6,267 9, % 9, % Pylon Pylon Moment (KN-m) 115, , % 124, % Table 6. Buffeting Responses (Diagonal Wind Cable at the Sepoong) Item N.C Value Ratio Value Ratio Vertical(m) % % Deck Transeverse(m) % % Moment(KN-m) % % Pylon Moment (KN-m) % % The Most Effective Methods for Improving Aerodynamic Stability V4 vertical cable which can reduce the vertical deflection greatly is the furthest one from the pylon. However it is not the vertical deflection having important meaning to both bridge and construction, but in-plane moments developed in the pylon and decks. Therefore it

5 is worth investigating which cable can reduce the moment introduced to the deck and pylon. As shown at the table7, 8, 9 and 10, V3 and V2 wind cable are the most effective at the 2 nd Dolsan and Sepoong respectively. Vertical cables are more functional than diagonal cables. It is reasonable to say that vertical wind cables such as V3 and V2 should be installed if moments caused by wind force exceed nominal moment. Both types of wind cables vertical, diagonal do not have influence on the horizontal stiffness, which means that cable can not resist to horizontal force. 4. Conclusion This paper estimates the buffeting responses of two cable stayed bridges under construction using computational analysis. Generally fairing and flap are not used for a concrete cable stayed bridge because of heavy weight and high damping ratio. It is however worthwhile to compare the buffeting responses of two types of bridges, which shows that the stiffness of box girder bridge is more stable than edge girder, so that to use one central plane with box girder for curved cable stayed bridge is reasonable. As is well known, one plane cable arrangement is more economic than multi plane cables. It is also shown that two types of wind cables are effective to reduce buffeting response. However the position of wind cables should be carefully investigated. Some wind cables increase the deck and pylon moments. Moreover the furthest wind cable is not the most effective one in respect of moments. In this paper it is suggested that the vertical wind cable located at the one or two segment before the final one should be installed. Reference Simiu, E and Scanlan, R. H. (1996), Wind Effects on Structures, 3rd Edition, John Wiley & Sons. M.S.Pfil (1995), "Aerodynamic Stability Analysis of Cable-Stayed s", Journal of Structural Engineering, Praveen Reddy (1999), "Simulation of Construction of Cable-Stayed s", Journal of Enginerring, Xinzhong Chen (2000), "Aerodynamic Coupling Effects on Flutter And Buffeting of s", Journal of Engineering Mechanics, Kim, Ho Kyong (2004), "Buffeting Analysis of a 700m Steel Cable-stayed for the Design of Pylon Foundation", Journal of Korean Society of Civil Engineering, 24-6A, Kim, Ho Kyong (2004), "Buffeting Analysis of Cable-Stayed for Design of Wind Cables", Journal of Korean Society of Civil Engineering, 24-3A, Kim, Jong Soo (2005), "Wind Stability of Cable-Stayed Considering Construction Sequences", Doctor's Thesis, University of Seoul (in Korea) Choi, Seong Won (2006), "Design Technique of Aerodynamic Stabilizing Cables for a Cable-Stayed in Construction", Master's Thesis, Mokpo National University (in Korea) Jang, Seong Wook (2008), "The Effects of Structural Types on Aerodynamic Stability of Cable-Stayed ", Master's Thesis, Chonbuk National University (in Korea)