7/30/13 A Study On Cable Suspension Footbridge

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A Study On Cable Suspension Footbridge

Dr. S.V. Itti, and Amar R. Chougule, Professor, M-

Tech. Student, Dept. of Civil Engg.,KLESCET, Belgaum

The Government of Karnataka has taken to restore

one of the famous Gangambika Temple which is

submerging over eight months in a year under back

water of Malaprabha dam. The temple is situated in

the central part of Malaprabha river basin near MK

Hubli of Belgaum district. To overcome the difficulty for

Devi darshan during its submergence a footbridge is

recommended on either side of the temple. There are

many systems for footbridges construction using such

as Rcc, steel, cable stayed cable suspensions, etc.

Looking at the site conditions and economy, it has

been recommended to provide a cable suspension

footbridge. This also enhances the beauty of

surrounding area. As per the survey details, the cable

suspension footbridge having dimension of 4m width

and 90 m overall span at left and 75 m overall span at

right has been proposed for accessibility. In the

present work the analysis of a left side cable

suspension footbridge has been carried out by

STAAD.Pro 2006 and the various components of

bridges are designed using the programs written in

excel spreadsheets. The parametric study has been

carried out for different spacing of suspenders and

sags of the cable. The study shows that the design is

economical and safe for 1.25m spacing of suspenders

and ratio span/10 as sag.

Introduction

Cable suspension bridges are supported by two main steel cables. The deck is hung from

suspender cables or steel rods. The cables a re anchored into the stream banks and are

supported by intermediate towers Figure 1. Longer span cable suspension bridges are usually

designed with a stiffening truss. The stiffening truss supports and stiffens the deck. Cables are

then added to carry live loads. To reduce the bending moment in the towers anchor cables are

provided. The cables will be having either guided pulley support or roller support like as shown

in Figure 2.

Fixing of Dimensions

Civil Engg. Department of our college has carried out the contour survey work of the

Gangambika site. Plan and elevation have been prepared as per the client requirement. The

size of the bridge is fixed as per the contour map Figure 3 and site conditions. The salient

features of the site are as follows:

Span of the bridge = 90 m

Width of the bridge = 4 m

Height of the tower = 12 m

Thickness of deck slab = 120 mm

Grade of concrete = M 35

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Grade of steel = Fe 415

The 3-D view of the cable suspension footbridge taken up for present study is shown in Figure.

4.

Modeling and Analysis

This type of bridge comprises basically a deck slab reinforced with steel bars, stiffening girder,

cable system and the tower. Since concrete has a very low tensile strength, the tensile load is

transferred by the cables to the anchorage block over the support. The concrete is assumed to

carry all the compressive bending stresses. The loads are considered as per IS: 875 part I, II,

III and their combinations have been taken as per IS: 875 part V which are:

DL + LL

DL + LL + WX

DL + LL + WZ

DL + LL – WX

DL + LL – WZ

The bridge model was analyzed in STAAD.Pro 2006 and designed using excel spreadsheet. The

parametric study has been carried out for different spacing of suspenders and cable sags.

The results shows that the load combination (DL+LL+WL in +Z) is critical, the 3rdview of this

load combination is given in Figure 5.

Results and Discussions

The parametric study is carried out in two folds.

a) Varying the spacing of suspenders and keeping the sag constant.

b) Varying the sags and keeping constant spacing of suspender.

a)Varying the spacing of suspenders and keeping the sag constant.

To decide about the optimum spacing of suspenders,

the bridge has been analyzed for different spacing of

suspenders varies from 1m to 3m and keeping the sag

(span / 10) constant. The results are given in Table 1

as well as in the form of graphs (Figure 6. and

Figrue.7). This study shows that, the minimum

quantity of the bridge is at 2.25m suspenders spacing.

But in the theory, the cable analysis is being done

with udl. Therefore this analysis is more suitable if the

suspenders spacing is closer. Also from the

references, the spacing of suspenders is optimum

when it is between 1.25m to 1.75m, therefore in the

present work 1.25m spacing of suspenders has been

considered and further parametric study on variation

of sag has been carried out.

b) Varying the sags by keeping constant spacing ofsuspender

Once again the bridge has been analyzed and

designed for varying the sags from 7 to 10.5m and

keeping the spacing of suspender 1.25m as constant.

The results are given in Table 2 as well as in the form

of graphs (Figure. 8 and Figure 9 has been left out).

The result shows that ratio the span / 10 are

economical in design.

Conclusions

In all cases, the suspenders diameter is less than the minimum required value which is

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10mm.

The quantity of cable steel has larger variation with respect to the spacing of

suspenders.

1.25m spacing of suspenders and cable sag equals span / 10 gives the economical

design of footbridge.

References

Technical Papers

A dissertation submitted to the SWISS Federal Institute of Technology Zurich for the

degree of Doctor of Sciences ETH” by Pietro Pedrozzi

Steinman, D. B. – “A practical Treatise on Suspension Bridges” by John Wiley and Sons,

Inc., New York.

Willard G. Bouricius, Philip K. Wittman, - “DESIGNING CANOPY WALKWAYS,” 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.

Peters, Tom F., - “Transitions in Engineering: Guillaume Henri Dufour and the Early 19th

Century Cable Suspension Bridges,” Birkhauser, 1987, ISBN 3764319291.

IS Codes

IS 456-2000 “Plain and Reinforced Concrete - Code of Practice (Fourth Revision)

SP 16:1980 “Design Aids for Reinforced Concrete to IS 456- 2000”

Books

Steinman, D. B. – “A practical Treatise on Suspension Bridges” by John Wiley and Sons,

Inc., New York.

Niels J. Gimsing, “Cable Supported Bridges – Concept and Design,” A Wily Interscience

Publication, technical university of Denmark.

Dr. B.C. Punmia, Ashok Kumar Jain and Arun Kumar Jain-2003, “Reinforced Concrete

Structures (Vol.1),” Laxmi Publications (P) Ltd.

Dr. Shah and Karve, “Limit State Theory and Design of Reinforced Concrete,” Structures

Publications.

NBMCW February 2009

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