A Study on Cable Stayed Footbridge

    Cable Stayed Footbridge

    Dr. S.V.Itti, Professor, Department of Civil Engg, K.L.E.S's College of Engg & Technology Belgaum, Ravishankar M. Madagouda, M.Tech Student. In the last decade, several investigations have been directed to condition assessment of cable-stayed bridges and contributed extensively to advances in construction, design, and health monitoring of this type of structures. Results of these investigations have helped toward formation of a unified approach for in-service evaluation and problem solving of these aesthetic structures.

    The holy place Gangambika Temple is submerged under back water of Malaprabha dam for a period more than eight months in a year. Therefore, Government of Karnataka has taken a forward step to restore this site keeping devotees problems in mind. By looking into site and client requirements architectural plans and elevation are prepared. Components involve in this project are dry well of 17 m diameter which is going to construct around the temple keeping temple at the center. The cable stayed bridge having dimensions of 4m width and 90 m overall span, has been proposed for accessibility. The study on the footbridge has been taken for present work.

    In this present work, the analysis and design of cable stayed footbridge has been done by using the programme written in excel sheets. The various components of bridge are designed using the programs written in excel spreadsheets. The study has been carried out for variation in spans (distance between cable spacing) for cable stayed bridge, having single tower and double towers at single side and also at double side. The study shows that the design is economical for double side towers.

    Introduction

    To provide safe and sustainable crossings, those providing technical assistance to local government and communities need simple, easily applied guidelines on the selection and construction of effective water crossings. This paper contains information on water crossings and an introductory chapter on footbridges but within the context of the paper it provide the comprehensive guidelines needed for selecting and constructing footbridge designs for specific applications.

    Before beginning the selection process, it is necessary to confirm that a footbridge is the best option for the water crossing. Other options are:
    Cable Stayed Footbridge
    • For shallow crossings, simple stepping stones may be aquequate.
    • For narrow crossings, a culvert may be a better option
    • For wide crossings, a ferry may be the most practical option
    • For low pedestrian traffic, a cable way may be the cheapest option
    If it is decided that a footbridge is the best option the first step is to carry out a site survey to decide on the alignment of the footbridge and determine its specifications in terms of span (length between supports) and the traffic to be carried. The paper starts from this planning process and works through the process of selecting the most appropriate design of footbridge to meet the specifications.

    Historical Development

    The concept and practical application of the cable-stayed bridge date back to the 1600’s, when a Venetian engineer named Verantius built a bridge with several diagonal chain-stays (Kavanagh, 1973). The modern cable-stayed bridge consists of a superstructure of steel or reinforced concrete members supported at one or more points by cables extending from one or more towers. The concept attracted to engineers and builders for many centuries and experimentation and development continued until its modern-day version evolved in 1950 in Germany. During the past three decades, cable-stayed bridges have found wide applications all over the world, especially in Western Europe and United States. In particular, the cable-stayed girder type of design is fast gaining popularity among the bridge designers, particularly for medium and long spans.

    Cable-stayed bridge stands out as the most recent technological development in bridge construction as demonstrated by several bridges existing all over the world, built of different materials and techniques. The Stromsund Bridge, which was constructed in Sweden in 1955 with a central span of 183 m, is the world’s first cable-stayed highway bridge. Subsequently, a number of cable-stayed bridges were constructed all over the world in many countries. The Second Hooghly Bridge over the river Ganga at Howrah is one of for the longest bridges in the world with a span of 457.2 m, the Tatara Bridge in Japan being the longest with a span of 890 m. Efforts are on to increase the span further beyond 1000 m. For medium spans of 100–300 m, cable-stayed bridges are considered to be the most suitable system.

    Due to their aesthetic appearance, efficient utilization of structural materials and other notable advantages, cable-stayed bridges have gained popularity in recent decades. This fact is due, on one hand, to the relatively small size of the substructures required and on the other hand, to the advent of efficient construction techniques apart from the rapid progress in the analysis and design of this type of bridges. Wide and successful application of cable-stayed systems was realized only recently, with the introduction of high-strength steel, orthotropic type decks, development of welding techniques and progress in structural analysis.

    The recent developments in design technology, material quality, and efficient construction techniques in bridge engineering will enable construction of not only longer but also lighter and slender bridges. Thus, very long span slender cable-stayed bridges are being built, and the aim is to further increase the span length and use shallower and more slender girders for future bridges. To achieve this, accurate procedures need to be developed that can lead to a thorough understanding and a realistic prediction of the structural response to not only wind and earthquake loads but also traffic loads.

    General Features of Cable Stayed Bridge

    Components of a cable stay footbridge:
    1. Tower or Pylons
    2. Deck system
    3. Cable system supporting the deck.

    (a) Towers or Pylons

    The towers are the compression members transmitting the load to the foundations. Towers are of different types to accommodate different cable arrangements, bridge site conditions; design features aesthetic and economical considerations. The various possible types of tower construction are:
    Cable Stayed Footbridge
    1. Trapezoidal portal frames
    2. Twin towers
    3. A-frames
    4. Single towers

    (b) Deck System

    Most cable-stayed bridges have orthotropic decks, which differ from one another only as far as the cross sections of the longitudinal ribs and the spacing of the cross-girders depending upon physical constraints.

    The deck may be made of different materials such as steel, concrete or prestressed concrete.

    (c) Types of Cable Stays

    The cable stays are made up of high tensile steel of different types with an ultimate tensile strength in the range of 1500 to 2000 N/mm2.

    The cable-stays that are usually are of the following types:
    Cable Stayed Footbridge
    1. Parallel-bar cables Fig. 3a
    2. Parallel-wire cables Fig. 3b
    3. Stranded cables Fig.3c
    4. Locked-coil cables Fig. 3d
    The choice of any of these types depends on the mechanical properties required (modulus of elasticity, ultimate tensile strength, durability etc.) as well as on structural and economic criteria (erection and design of the anchorages).

    (d) Longitudinal Cable Arrangement

    Depending on the arrangement of longitudinal stay cables, the cable-stayed bridges can be divided into four basic systems as shown in Figure 4.
    Cable Stayed Footbridge
    1. Harp system
    2. Fan system
    3. Semi-Harp system
    4. Asymmetric system

    Objectives of the Present Work

    • To analyze and design a cable stayed footbridge.
    • The various components of bridge are designed using the programs written in excel spreadsheets.
    • The comparison is made between by providing different types of towers.
    • Also the comparison is made by providing the different spacing for the cables or the stays for different tower types of bridge.
    A comparative study of designs with respect to spacing of cables and tower type variation is done in this paper.

    Analysis and Design

    This type of bridge comprises basically a deck slab, cable system and the tower. Since concrete has a very low tensile strength, the tensile load is transefered by the cables to the towers in the form of compression force, which it transfers to the foundation. The concrete is assumed to carry all the compressive bending stresses.

    Department of Civil Engg, K.L.E. S’s College of Engg and Technology carried out the contour survey work; plan and elevation are fixed as per the client requirement. The distance between the temple and the left side bank of the river is about 90 m and on the right side it is about 75 m to the approach. The RL of the temple is about 624.00 m and during flood, full reservoir level is 634.00 m. According to soil investigation carried out the hard strata is about 18 m from the bed level of the river and its RL is 606.800 m. So we decided to go for dry well construction around the temple to protect it and to provide access for the devotees during flood times. According to that we have decided the diameter of the dry well as 17 m (55 ‘), girder bottom level as 636.00 m and formation road level as 637.500 m according to the approach road. The height of the piers are fixed according to the soil investigation report, it is about 18 m and during festival period there is lot of crowd gathering here that’s why the cross road width is kept 4 m which is more than any other walkways. Since the bridge is only to provide the access to darshana of Devi Gangambika it’s decided to go for provision of footbridge for the pedestrians or walkway.

    Results and Discussions

    The footbridge taken for present study is Cable Stayed type and it is resting on one side abutment and other side resting on dry well wall. It has 90m x 4m overall dimensions in plan.

    The footbridge was taken according to the architectural plan and section. Various dead, live loads were assigned to the structure and the model was analysed and designed using excel spreadsheet.

    From the general L-section and plan the typical deck, girder, cable stays; supporting towers is taken up for study. The bridge deck is analysed and designed for two support conditions one is simply supported and other is cantilever.

    The selected section of footbridge is varied with cable spacing calculation of effective area of cables and steel, and are abulated, & the graphs are plotted for the same.

    Span lengths taken for comparison:
    1. 90 m / 22 equal spans = 4.5m
    2. 90 m / 16 equal spans = 5.6m
    3. 90 m / 15 equal spans = 6m
    4. 90 m / 12equal spans = 7.5m
    5. 90 m / 10 equal spans = 9m
    6. 90 m / 9 equal spans = 10m
    7. 90 m / 8 equal spans = 11.25m
    8. 90 m / 6 equal spans = 15m
    9. 90 m / 4 equal spans = 22.5m

    Deck Slab

    By variation in the span (distance between piers), there is change in long span of deck slab but short span remain same as our footbridge width is fixed. In the case of simply supported deck, slab is designed as two way slab and in the case of cantilever deck the span variation does not make any difference as bending is in shorter span only.

    Simply supported deck is designed as two way slab, reinforcement changes as span changes. In cantilever deck the steel is calculated as 0.2% of gross cross sectional area so it remain constant for all spans.

    Cross Girder

    Cross girder provided in simply supported deck, any variation in the span does not affect on the cross girder reinforcement. Because length of cross girder is fixed, it is equal to width of girder. Also the load coming on cross girder remains same for all spans.

    Main Girder

    By variation in the span (distance between piers) there is change in long span of deck slab due to this change dead weight coming on girder increases. In the case of simply supported deck, there are two longitudinal girders provided and in the case of cantilever deck there is one longitudinal girder provided.

    Volume of Tower Concrete

    By the variation in the cable spacing the concrete in the tower for single tower and for double tower at the single side designed and it is tabulated here and the variation is shown in the form of graph.

    Here SSSC is Single Side Single Column

    SSDC is Single Side Double Column.

    By the variation in the cable spacing the concrete in the tower for single tower and for double tower at the double side designed in the previous chapter is tabulated here and the variation is shown in the form of graph.

    Here DSSC is Double Side Single Column

    DSDC is Double Side Double Column

    Area of Main Steel of Tower

    Cable Stayed Footbridge
    By providing the different cable spacing for the bridge, the area of main steel in the tower for single tower and for double tower at the single side designed. The detailed design procedure is discussed and the same is tabulated here and the variation is shown in the form of graph.

    Here SSSC is Single Side Single Column

    SSDC is Single Side Double Column

    By providing the different cable spacing for the bridge, the area of main steel in the tower for single tower and for double tower at the single side designed.

    The detailed design procedure is discussed and the same is tabulated here and the variation is shown in the form of graph.

    Here DSSC is Double Side Single Column

    DSDC is Double Side Double Column

    Area of Supporting Cables

    As the provision of increase in cable spacing is made in the single tower and double tower at the single side design the calculated area of cables is tabulated here below and represented in the form of graph for both type of towers.

    Here SSSC is Single Side Single Column

    SSDC is Single Side Double Column

    The increase in cable spacing is made in the single tower and double tower at the double side design the calculated area of cables is tabulated here below and represented in the form of graph for both type of towers. For drawing of different type of cable arrangements refer drawing no.4 of appendix A.

    Here DSSC is Double Side Single Column

    DSDC is Double Side Double Column

    Discussion

    Cable Stayed Footbridge
    • The results plotted in figure 5 shows that the short span reinforcement (main) remain constant for all bridge span for both simply supported and cantilever deck, because the short span lengths remain same as our bridge cross width is fix to 4m.
    • The results given in figure 6 shows that, the long span reinforcement (distribution) increases as increase in bridge span for simply supported due to change in cross girder position, for cantilever deck it is constant for all span.
    • From the Figure 7, it is clear that there is no change in reinforcement as cross girder length is fixed as 4m, it is considered only in simply supported case.
    • The results plotted in Figure 8 shows that up to 22.5m span the reinforcement area is almost same in both cases, after that span reinforcement is increases in cantilever support as compare to simply support deck.
    • According to Figure 9, the cable spacing is increasing the concrete in the tower of single sided tower remains the same.
    • From Figure, 10, the concrete qunatity in the single tower is almost same for all spacing, but in the double tower system the concrete quantity is same from 4.5 to 7.5 m, but there is increase in the concrete up to 15 m of cable spacing.
    • According to the Figure 11, the main steel in the tower is same for all cable spacing, since the cross section and the height of the tower is same for all spacing in the double side towers.
    • As per Figure 12, it is seen that there is no much change in the main steel in the single tower but there is a gradual increase in the reinforcement of double tower at double side.
    • From Figure 13, it can be seen that the area of cable is more for less cable spacing. It is for the reason that there are more number of cable and wires are equired for the cable in the single tower. And as the spacing increases the area of cable is decreasing as the less number of wires are required. In the double tower, the cable area is increasing as the cable spacing is increasing at single side tower design.
    • From Figure 14, it can be seen that as the cable spacing is increasing the area of cable is decreasing gradually for the single tower but it is increasing gradually for the double tower at double side.

    Conclusion

    Cable Stayed Footbridge
    With respect to the above discussions, the following conclusions are made.
    • There is a larger variation in cable area in single side tower compare to the double side towers.
    • There is not much change in the reinforcement for single side towers w.r.t. spacing of the cables.
    • Not much change in the main reinforcements of main girder w. r. t. spacing of the cables.
    • There is larger variation in the cross section of tower in single side towers compare to the double side towers.
    • There is no change in the reinforcement of the cross girder in simply supported case.
    • The volume of concrete is more in the case of single side single tower compared to other towers.
    • Double side single tower is economical than other type of towers.

    References

    Cable Stayed Footbridge
    • M.V.Rama Rao, "Analysis of Cable-stayed bridges by fuzzy-finite element modelling", Ph.D thesis, Osmania University, 2005.
    • Paper: ”Static and Stability Analysis of Long-Span Cable-Stayed Steel Bridges” by Shuqing Wang and Chung C. Fu, University of Maryland.
    • “Designing Canopy Walkways” by Willard G. Bouricius, Philip K. Wittman, Bart Bouricius, Canopy Construction Associates, 32 Mountain View Circle, Amherst, MA 01002, FL 32835-5137, MA 01002.
    • “Footbridges”, A Manual for Construction at Community and District Level, I.T. Transport Ltd.Consultants in Transport for Development. June 2004.
    • “Adjustment of the Rion-Antirion Cable-Stayed Bridge: An Innovative Multidisciplinary Response to a Construction Challenge” by M. Marchetti, R. Boudon, J. Monnerie, P. Bouve, D. Dupuis, F.Dadoun, G. Baechler and J. Olsfors, France
    • “Construction Of Nagisa Bridge Hybrid System of Cable- Stayed Pc Bridge And Steel Suspension Bridge” by Yuzuru Sato Shinichi Sasaki, Katsutoshi Morohashi, Nobumasa Suzuki.
    • “Newport City Footbridge For the Bridge Engineering Conference” by A C Fullerton Newport City Footbridge Bridge Engineering 2 Conference 2007 27 April 2007, University of Bath, Bath, UK.
    • “Lateral vibration of footbridges by synchronous walking” by Shun-ichi Nakamura and Toshitsugu Kawasaki in the “Journal of Constructional steel Research” Volume 62, Issue 11, November 2006, pages 1148-1160.
    • “Structural dynamic design of a footbridge under pedestrian loading” by C. Melchor, Blanco1, Ph. Bouillard, E. Bodarwé, L. Ney.
    • “Design of Bridges”, N. Krishna Raju, Oxford & IBH Publishing Co. Pvt. Ltd.
    • “Reinforced Concrete Structures (Vol.1),” Dr. B.C. Punmai, Ashok Kumar Jain and Arun Kumar Jain, Laxmi Publications (P) Ltd.
    • “Advanced Reinforced Concrete Design” N. Krishna Raju, CBS Publishers & Distributors, New Delhi, Bangalore.

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