suspension bridge



It includes the design of slabs, tension members, columns and foundation using limit state design.

1.4 METHODOLOGY



Literature survey: Books and codes required for the project are collected.



Relevant data collection: The required soil characteristics are collected.



Traffic survey: Traffic studies are conducted and necessary traffic data collected on the bridge across river Cauvery.



Analysis: Analysis of the bridge is done manually



Preliminary Design: The necessary design data like bridge type, span length, number of spans, type of stiffening girder, cable sag, type of suspenders are arrived at.



Detailed design: Design of members-cables, suspenders, towers are done.



Final report: A detailed report of the project is prepared.

1.5 MAJOR DESIGN EXPERIENCE

The project is a “Structural Design Project”. Design experience in the following areas shall be gained during the course of the project. 

Design of Deck slab.



Design of Cross beams (steel members).



Design of Suspenders (steel cables).



Design of Main steel cables.



Design of Columns.



Design of Anchorages.

2



It includes the design of slabs, tension members, columns and foundation using limit state design.

1.4 METHODOLOGY



Literature survey: Books and codes required for the project are collected.



Relevant data collection: The required soil characteristics are collected.



Traffic survey: Traffic studies are conducted and necessary traffic data collected on the bridge across river Cauvery.



Analysis: Analysis of the bridge is done manually



Preliminary Design: The necessary design data like bridge type, span length, number of spans, type of stiffening girder, cable sag, type of suspenders are arrived at.



Detailed design: Design of members-cables, suspenders, towers are done.



Final report: A detailed report of the project is prepared.

1.5 MAJOR DESIGN EXPERIENCE

The project is a “Structural Design Project”. Design experience in the following areas shall be gained during the course of the project. 

Design of Deck slab.



Design of Cross beams (steel members).



Design of Suspenders (steel cables).



Design of Main steel cables.



Design of Columns.



Design of Anchorages.

2

1.6 REALISTIC DESIGN CONSTRAINTS



Environmental Environmental constraints: During floods, the hydraulic forces exerted on

the supporting columns are expected to be very high in comparison with the light Superstructure load. 

Manufacturing constraints:   Piling foundation can be avoided since good

stratum is available at a greater depth.

1.7 REFERENCE TO CODES AND STANDARDS

The various codes and standards that are used for the completion of the  project are given in Table 1.1 below. Table 1.1 Codes and Standards used CODES

CONTEXT

IS 1835-1977

Design of Steel Wires for Rope

IRC: 6- 2010

Standard Specifications and code of practice for Road Bridges

IS 9282-1979

Specifications for Wire ropes and Strands for Suspension Bridges.

IS 9182-1979

Standard specifications and code of practice for Road Bridge

(part-II)

(section:6 Composite Construction)

IS 456:2000

Plain and Reinforced concrete - Code of practice

IS 800:2007

Code of Practice for general construction in Steel

IRC:24-2010

Standard Specifications and Code of Practice for Road Bridges, Steel Road Bridges (Limit State Method)

IRC:SP-56:2011

Guidelines for Steel Pedestrian Bridges

AASHTO

American code for Pedestrian loading standards

3

1.8 APPLICATION OF EARLIER COURSE WORK

The academic course works that are used in project are shown in Table 1.2  below. Table 1.2 Earlier Course Work used COURSE CODE AND NAME

CE 0201 - Mechanics of Solids

CONTEXT

Evaluation of bending moment and shear forces

CE 0202 - Strength of Materials

Evaluation of slope and deflection

CE 0301, CE 0302 - Structural

Analysis

Analysis

suspension cables

CE 0204 –  Structural Design

Design of steel structures

CE0303,

Design of RCC structures

CE0304

Structural

of

determinate

structures

and

Design CE0104

 –  

Computer

aided

AutoCAD

 building drawing Analysing soil as a medium of water flow, CE0305,CEO311-Soil Mechanics

Structural support and a Primary building material

CE0306- Foundation Engineering

Behaviour of foundations for Engineering structures

1.9 MULTIDISCIPLINARY COMPONENT AND TEAM WORK

This project involves in multidisciplinary team work and helps interacting with the public, builders and Government officials during the selection of layout and plan and also collection of rules and regulations respectivel y.

4

1.10 SOFTWARE/EQUIPMENT USED

The software used in the project is AutoCAD.

1.11 CONCLUSION

A thorough knowledge about suspension bridges, their structural  behavior, their analysis and design are expected to be gained by the end of this  project work.Various technical drawings used for construction and reference have  been drawn using AutoCAD .

1.12 FUTURE SCOPE OF THE PROJECT The analysis and design of this suspension bridge can be extended for bridges with longer spans and to accommodate more traffic over bigger rivers or at any other location.

5

CHAPTER 2

INTRODUCTION

2.1 GENERAL

It is estimated that about 900 million rural people in developing countries do not have reliable year-round access to road networks, and 300 million are without motorized access. All the money invested into infrastructure improvements for paved highways and major vehicular bridges are only serving those with a standard of living appropriating vehicular use and the remaining 300 million rural citizens have unreliable access to even the most basic services or opportunities. Investment in rural transportation improvements would help to reduce poverty through improving access to markets, educational opportunities and medical clinics not currently accessed. Accordingly, a country‟s ability to maximize its economic potential to link is closely linked to efficiency of its transportation system. This type of bridge has cables suspended between towers, plus vertical suspender cables that carry the weight of the deck below, upon which traffic crosses. The arrangement allows the deck to be level or to arc upward for additional clearance. Like other Suspension bridge types, this type often is constructed without false work. Suspension bridges in its simplest forms were originally made from ropes and wood. Modern Suspension bridges use a box section roadway supported by high tensile strength cables. With any bridge project the choice of materials and form usually comes down to cost. Suspension bridges tend to be the most expensive to build. A suspension bridge suspends the roadway from huge main cables, which extend from 6

one end of the bridge to the other. These cables rest on top of high towers and have to be securely anchored into the bank at either end of the bridge. The towers allow the main cables to be draped over long distances. Most of the weight or load of the  bridge is transferred by the cables to the anchorage systems. These are embedded in either solid rock or huge concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free. 2.2 PEDESTRIAN BRIDGES

For nearly 50 percent of world‟s population living in rural isolation, the lack of access reinforces the cycle of poverty. Rural community members spend a great deal of time and effort on transport activities to fulfil their basic needs (Ref. 1). These bridges



For a given capacity, are lighter in weight per foot of bridge.



They can be built to span gaps up to 400 feet with no intermediate s upports.



All bridge parts, with exception of main cables and suspenders, can be built from timber.



Cable and equipment for construction can be divided into light, compact loads. Pedestrian bridge technologies vary vastly in design, cost and function.

From a structural standpoint, pedestrian bridges have taken a number of forms, each with the function of providing safe transport over an otherwise impassible crossing. The arrangement allows the deck to be level or to arc upward for additional clearance. Like other Suspension bridge types, this type often is constructed without false work.

7

2.3 LITERATURE REVIEW

The major components of the suspension type bridge are described  below. Stiffening girders/trusses, Main cables, Main towers in longitudinal and transverse directions, Anchorages and Suspenders are the necessary components of the Suspension type bridges. The below Figure 2.1 shows the major bridge components and  parameters.

Fig 2.1 Structural Components of a Suspension Bridge

The basic structural components of a suspension bridge system are shown in Figure 2.1. I.

Stiffening girders/trusses: Longitudinal structures which support and distribute moving vehicle loads act as chords for the lateral system and secure the aerodynamic stability of the structure. Stiffening girders are typically classified into two-hinge or continuous types. Two hinge stiffening girders are commonly used for highway bridges. For combined highway – 

8

railway bridges, the continuous girder is often adopted to ensure train runnability.

II.

Main cables: A group of parallel-wire bundled cables which support stiffening girders/trusses by hanger ropes and transfer loads to towers. In early suspension bridges, chains, eye-bar chains, or other material was used for the main cables. Wire cables were used for the first time in suspension  bridges in the first half of the 19th century, and parallel-wire cables were adopted for the first time in the Niagara Falls Bridge in 1854. Cold drawn and galvanized steel wires were adopted for the first time in the Brooklyn Bridge in 1883. This type has been used in almost all modern long-span suspension bridges. The types of parallel wire strands and stranded wire ropes that typically comprise cables. As per IRC:24-2010, strands are  bundled into a circle to form one cable. Hanger ropes might be steel bars, steel rods, stranded wire ropes, parallel wire strands, and others. Stranded wire rope is most often used in modern suspension bridges.

III.

Main towers: Intermediate vertical structures which support main cables and transfer bridge loads to foundations. In Longitudinal direction, towers are classified into rigid, flexible, or

locking types. Flexible towers are commonly used in long-span suspension bridges, rigid towers for multi span suspension bridges to provide enough stiffness to the  bridge, and locking towers occasionally for relatively short-span suspension bridges. In Transverse direction, towers are classified into portal or diagonally  braced types. Moreover, the tower shafts can either be vertical or inclined. Typically, the centre axis of inclined shafts coincides with the centre line of the cable at the top of the tower. Careful examination of the tower configuration is important, in that towers dominate the bridge aesthetics.

9

IV.

Anchorages: Massive concrete blocks which anchor main cables and act as end supports of a bridge. In general, anchorage structure includes the foundation, anchor block, bent block, cable anchor frames, and protective housing. Anchorages are classified into gravity or tunnel anchorage system. Gravity anchorage relies on the mass of the anchorage itself to resist the tension of the main cables. This type is commonplace in many suspension  bridges. Tunnel anchorage takes the tension of the main cables directly into the ground. Adequate geotechnical conditions are required.

V.

Suspenders: These are the cables that connect the girders to the main cable. They help in load transfer from the girder to the cable (Ref 2).

2.4 ADVANTAGES OVER BRIDGE TYPES



A Suspension bridge can be made out of simple materials such as wood and common wire rope.



Less material may be required than other bridge types, even at spans they can achieve, leading to a reduced construction cost.



Except for installation of the initial temporary cables,little or no access from  below is required during construction, for example allowing a waterway to remain open while the bridge is built above.



Longer main spans are achievable than with any other type of bridge.



May be better to withstand earthquake movements than heavier and more rigid bridges.

10

2.5 DISADVANTAGES COMPARED WITH OTHER BRIDGE TYPES



Considerable stiffness or aerodynamic profiling may be required to prevent the bridge deck vibrating under high winds



The relatively low deck stiffness compared to other (non-suspension) types of  bridges makes it more difficult to carry heavy rail traffic where high concentrated live loads occur



Some access below may be required during construction, to lift the initial cables or to lift deck units. This access can often be avoided in cable-stayed  bridge construction

2.6 VARIATIONS

2.6.1 Under Spanned Suspension Bridge

In an under spanned suspension bridge, the main cables hang entirely  below the bridge deck, but are still anchored into the ground in a similar way to the conventional type. Very few bridges of this nature have been built, as the deck is inherently less stable than when suspended below the cables. Examples include the Pont des Bergues of 1834 designed by Guillaume Henri Dufour, James Smith‟s Micklewood Bridge and a proposal by Robert Stevenson for a bridge over the river Almond near Edinburgh. Roebling‟s Delaware Aqueduct _begun 1847) consists of three sections supported by cables. The timber structure essentially hides the cables; and from a quick view, it is not immediately apparent that it is even a suspension  bridge (Ref 3).

11

2.6.2 Suspension Cable types

As per IS 1835-1977, the main suspension cable in older bridges was often made fro the chain or linked bars, but modern bridge cables are made from multiple strands of wire. This contributes greater redundancy, a few flawed stands in the hundreds used pose very little threat, whereas a single bad link or eye bar can cause failure of the entire bridge. (The failure of a single eye bar was found to be the cause of the collapse of the Silver Bridge over the River Ohio). Another reason is that as spans increased, engineers were unable to lift larger chains into position, whereas wire strand cables can be largely prepared in mid-air from a temporary walkway. . 2.6.3 Deck structure types

Most suspension bridges have open truss structures to support the road  bed, particularly owing to the favourable effects of using plate girders, discovered from the Tacoma Narrows Bridge collapse (1940). Recent developments in bridge aerodynamics have allowed the reintroduction of plate girders. In the picture of Yichang Bridge, note the very sharp entry and sloping under girders in the suspension bridge shown. This enables this type of construction to be used without the danger of vortex shedding and consequent aero elastic effects, such as those that destroyed the original Tacoma Narrows Bridge. Cable suspension may also be augmented by the inherent stiffness of the structure that has much in common with the tubular bridge light cable suspension may prove less expensive and seem more elegant for a footbridge than strong girder supports.

12

CHAPTER 3

OBJECTIVE AND SCOPE

3.1 Objective

The objective of the project is to analyse and design a suspension bridge with a span of 400 m that caters to pedestrian and two wheeler traffic. The structure that will be designed is a simple conventional suspension bridge structure that involves manual analysis and design of various structural components of the bridge like main cable, suspenders, deck slab, stiffening girder, column and the footing. This project is  proposed to be built on the banks of river Cauvery at Trichy. The primary objective of this project is to divert the pedestrian and two wheeler traffic on to this new structure, thereby streamlining the traffic flow across the river. The suspension  bridge, as mentioned earlier is a simple and a conventional structure owing to reduce the complexity of the manual calculations.

3.2 Scope

The project basically comprises of analysis and design of a suspension bridge of 400 m span as mentioned in the objective of the project. As the structure caters to only two wheeler and pedestrian traffic and not to commercial vehicles it is designed as a simple and a conventional structure. The project envisages the manual analysis and design of the suspension bridge. The simplicity of the structure is also to reduce the complexity in the manual analysis and design of the structure. The manual analysis of the structure is based on the paper presented in ASCE journal by 13

Professor P. Wollman. The analysis is an iterative procedure and hence, involves the use of various mathematical functions and tools. The manual analysis in turn leads to the design of the various components of the suspension bridge. This project involves the design of the main cables, suspender cables, deck slab, stiffening girder, columns and their footing slab. The design of anchorages is beyond the scope of this project.

3.3 MATERIALS AND METHODOLOGY

This entire project is analytical in nature. Literature survey is done with reference to journals, online articles and publishing from previous works. Once literature survey is completed functional layout is prepared. Thus allocating the spaces as per the functional requirement. Plan of the Suspension bridge is prepared  by using AutoCAD software. The deck slab is designed as a conventional rigid  pavement as per AASHTO codes. As per IRC:SP-56:2011, the hangers are continuously distributed along the girder and are inextensible. The girder stiffness is constant for each span.Cross Beams are designed as per IS:800-2007, the Steel members are designed to transfer load to connected Suspenders. As per the reference (3), Gravity Anchorages relies on the mass of the anchorages itself to resist the tension of the main cables.

14

CHAPTER 4

RESULTS AND DISCUSSIONS

4.1 ANALYSIS

Modern suspension bridges are typically analysed using computer  programs with nonlinear analysis capabilities based on finite element formulations . Such models may have many thousands of degree of freedom. For example, a global finite element model with 9,780 degrees of freedom was developed. A global finite- element model with 9,780 degrees of freedom was developed for the final design of the East Bridge across the Great Beltin Denmark (East 1998) . Obviously, there is a need for simpler models that help the designer to understand the behaviour of the structure in a manner not offered by finite element analysis. Such models are useful for preliminary design and for the independent checks of the more complex models. The method of analysis used in this project is from a paper “Preliminary Analysis of Suspension Bridges by Geogor P. Wollman”. In this paper the derivation of the fundamental equations of suspension bridge analysis is based on the deflection theory, and a practical solution method is  presented. This method, while not suitable for hand calculations, can be easily implemented in a mathematical analysis program [e.g. Math=cad(1998)] or for simpler cases in a computer spread sheet program . The derivations in the paper follow the presentation. In Petersen (1993) and Rubin and Vogel (1982) .However, the method has been extended to cover the case where the flexural pylon stiffness is not

15

negligible. The approach is different from that typically presented in the U.S. literature Steinman 1929,1934; Timoshenko and Young 1965) and takes advantage of the analogy between a suspended girder and a beam under tension (Ref:).

4.1.1 Basic equations of Stiffening girder

The following assumptions are made for the derivations of the differential equation describing the suspended stiffening girder as per the Reference (1). a) The dead load (self-weight and super imposed dead load) is uniform and is carried by the suspension cable alone.  b) Under dead load the cable shape is parabolic. c) The hangers are continuously distributed along the girder and are inextensible. d) The hangers are vertical initially and remain vertical under load. e) The girder stiffness is constant for each span.

Fig. 4.1 Cable geometry

Based on the assumptions and the notations used in Fig.4.1, the equations for the geometry of cable under the action of dead load are: y = x tan

 +   x (l-x) ; yꞌ = tan   +   x (l-2x); yꞌꞌ= -    16

(4.1)

Where, y - cable ordinate under dead load y'- cable slope under dead load y''- cable curvature under dead load x - distance from left support l - span length f - sag

y''= -

  

=-2

Hg =

 10  1/m -3

  

(4.2)

Where, g - uniform dead load, including weight of cable; Hg - horizontal cable force component under dead load; Hg =

      

= 48,945 kN

  f  = 

(4.3)

1

=

     

f 1, f 2 = 5.65 m.

  =   = 0.0375   17

  = 0.1  The chord angles of the suspension bridge are:

 = tan = 40/150

tan

α = 14 °55' 53'' The cable parameters are: Lc = l

[  ] 

(4.4)

Where, Lc = Length of cable Side span = 150(

 )   

= 167.89 m Main span = 400(

 )   

= 475.4 m To evaluate the formula, H p must be known. The condition to determine the floors is provided by the compatibility requirement that the horizontal projection of the change in cable length due to live load and temperature equals the change in horizontal distance between the cable end points as illustrated in Figure 4.2.

Fig 4.2 Compatibility conditions for cable 18

The cable stretch due to live load and temperature change is given by

  +  T =      +  T  =     

(4.5)

Where,

  -

Cable stiffness;

T

- temperature change;



- coefficient of thermal expansion;

The flexural stiffness of the pylons is represented by horizontal springs with stiffnesses k  b  and k c  and similarly the stiffness of the anchor block are represented by horizontal springs with stiffnesses k a  and k d. Vertical deflections at anchor blocks and pylons are ignored. If the pylon stiffness can be neglected, then (k  b = k c = 0). In such a case, the compatibility equation has to be written as the sum over all cable segments from anchor block to anchor block, resulting in a single equation for the unknown horizontal cable force component, H p

 ∑    ∑   ∫                

 

(4.6)

Where,

  - Cable stiffness ,   - stiffness of anchor blocks The notations in the above formula have been explained in the earlier article. The integration

∫   is best performed numerically, based on the Simpson rule.

∫    ∫              19

(4.7)

Where, ε - Stiffening girder parameter

 - dimensionless quantity () wo- wn - deflection at various ordinates of the cable

     w (ξ) = [  (    +  ] 

(4.8)

Where,

ε - stiffening girder parameter  =

 

q - transverse load = p + H py'' = p –  H p

;

  

 N - axial tension = Hg + H p As per IS 9282:1979, this method of analysis is completely a mathematical approach. So, the tension in the cable under the action of live load is deduced using an iterative  procedure called the Newton Raphson‟s method. The other quantities used for arriving at the final tension value are also mathematically found out using various mathematical tools like The Simpson‟s integration rule etc.   The Idealized Suspension bridge with necessary symbols displaying each span with its cable tension, lengths and stiffnesses in the below Figure 4.3.

Fig 4.3 Idealized Suspension Bridge

20

The equations used to find out tension are non linear and must be solved iteratively. The steps for the solution of the equation using Newton Raphson‟s method are given below. 

Step 1: Assume an initial value for H p and the step size ΔHp based on the desired levels of accuracy.



Step 2: Calculate deflections w for the given H p as per the equations for w (ξ). Load case  to be considered are the applied live load and the uniformly distributed load directed upward given by -H py''.

∫   using simpson‟s integration rule.



Step 3: Calculate



Step 4: Calculate the new improved H p  using the equation ** given  below

       (()) 

(4.9)

Where,

  

current value for H p New value for H p

Step 5: Repeat steps 2-4 until f(H p) is close to 0 within the desired accuracy.

General assumptions: EcAc = 36×106 kN; EI = 57×10 6 kN/m; K = 8500 kN/m. In order to reduce the monotony, this report contains just the tabulated results of the first and the final iterations. Iteration I:

Assume

 = 6000 kN;

q = 44 kN/m; N = 54945 kN; ε = 12.42;

21

  = =12 kN/m

Table 4.1 Results of deflection due to H p

 

X

X(m)

W due to H p

Sum



0

150

0

0

1

0

0.1

190

5.175

5.175

4

20.7

0.2

230

9.489

9.489

2

18.978

0.3

270

12.643

12.643

4

50.572

0.4

310

14.550

14.550

2

29.10

0.5

350

15.185

15.185

4

60.74

0.6

390

14.539

14.539

2

29.10

Sum

0.7

430

12.603

12.603

4

50.572

0.8

470

9.489

9.489

2

18.978

0.9

510

5.175

5.175

4

20.7

∫   =          ∫   =  (299.44) ∫    3392.53 m  =    ∫    = 0.90   = 6250 kN 22

Σ(Sum×wt)

299.44

Table 4.2 Results for deflection due to H p + ΔHp

X

X(m)

 due to  

Sum



 

Sum

Σ(Sum × wt)

0

150

0

0

1

0

0.1

190

5.094

5.094

4

20.376

0.2

230

9.340

9.340

2

18.68

0.3

270

12.445

12.445

4

49.78

0.4

310

14.323

14.323

2

28.646

0.5

350

14.950

14.950

4

59.80

0.6

390

14.323

14.323

2

28.646

0.7

430

12.445

12.445

4

49.78

0.8

470

9.340

9.340

2

18.68

0.9

510

5.094

5.094

4

20.376

q = 43.5 kN/m  N = 55195 kN ε = 12.44

 = -12.5 kN/m ∫   =          ∫   =  (294.764) ∫   = 3930.13 m 23

294.764

   =     ∫     = 0.8812  )     ( ( )  = 17,968.08 kN The final iteration:

 = 16053.2 kN q = 23.89 kN/m  N = 64998 kN ε = 13.50;

 = -32.10 kN/m ∫   =          ∫   =  (146.80) = 1957.33 m

 =    ∫   q = 43.5 kN/m  N = 55195 kN ε = 12.44

 = -12.5 kN/m

24

Table 4.3 Results for deflection due to



X

X(m)

W due to H p

Sum



(Sum × wt)

0

150

0

0

1

0

0.1

190

2.64

2.64

4

10.56

0.2

230

4.69

4.69

2

9.38

0.3

270

6.168

6.168

4

24.672

0.4

310

7.050

7.050

2

14.1

0.5

350

7.344

7.344

4

29.378

0.6

390

7.050

7.050

2

14.1

0.7

430

6.168

6.168

4

24.672

0.8

470

4.69

4.69

2

9.38

0.9

510

2.64

2.64

4

10.56

 = 0.0745    = 6250 kN; q = 43.5kN/m;

 = -12.5 kN/m ∫   =         ∫   =  (131.08)

 N = 55195 kN; ε = 12.44;

= 1747.82 m

25

Σ(Sum× wt)

146.80

   Sum 

Table 4.4 Results for deflections due to

X

X(m)

W due to

Sum

Σ(Sum × wt)

  0

150

0

0

1

0

0.1

190

2.349

2.349

4

9.396

0.2

230

4.296

4.296

2

8.59

0.3

270

5.715

5.715

4

20.70

0.4

310

6.571

6.571

2

13.142

0.5

350

6.8578

6.8578

4

27.431

0.6

390

6.571

6.571

2

13.142

0.7

430

5.715

5.715

4

20.70

0.8

470

4.296

4.296

2

8.59

0.9

510

2.349

2.349

4

9.396

1.0

550

0

0

1

0

   =     ∫     = 0.493  )     ( ( )  = 16008.2 kN. 26

131.08

Therefore, the tension due to live load analysed by the iterative procedure using deflection theory was found to be 16008.2 kN. The cables and the other structural members of the suspension bridge are designed to withstand the above deduced load.

4.2 DESIGN

4.2.1 Cables

Parallel wires have been exclusively used exclusively as the main cable in suspension bridges around the world. Parallel wires have the advantage of having higher strength and modulus of elasticity when compared with the normal stranded wire ropes. Alignment of the main cable must be decided first. The sag span ratio of the suspension bridge is 1/10. After the structural analysis, the structural area is calculated based on the maximum tension in the cable. High strength steel wire of tensile strength 1770 N/mm 2 is used. Tension in cable = 16008.2 kN Tensile strength = 1770 N/mm 2 Area of cable to be provided = 9044 mm 2 Provide a parallel wire cable of diameter 145 mm as the main cable which has the following properties:  Nominal diameter = 145 mm 2

 Nominal cross-section area = 12080 mm  Nominal axial stiffness = 1873 MN  Nominal metallic mass = 101.5 kg/m Minimum breaking = 19450 kN 27

4.2.2 Suspenders

Suspenders transfer the loads from the deck of the suspension bridge to its main cable. So they are designed for strength of 5880 kN. So for reduced loads of this magnitude smaller wire cables are used. For instance, here, a spiral strand rope of 80 mm diameter is used as suspenders. They are provided at 10 m intervals starting from the left end the cable properties are:  Nominal diameter = 80 mm  Nominal cross section area = 3673 mm2  Nominal metallic mass = 30.3  Nominal axial stiffness = 569 MN Minimum breaking force = 5910 Cable strands and anchored to the cable anchorage frame. Hanger ropes are connected to the main cable with cable bands.

4.2.3 Deck slab

The deck slab is designed as a conventional rigid pavement as per AASHTO codes. According to the design, contraction joints are provided at every 3.5 m intervals and expansion joints at an interval of 60 m.

4.2.4 Stiffening Girder

Plate girders are a deep flexural members used to carry loads that cannot  be carried economically by a rolled beam. Plate girders offer a unique flexibility in fabrication and the cross section can be uniform or non-uniform along the span. Due 28

to the compactness of the plate girders, vibration and impact are not serious  problems. It is a normal practice to fabricate plate girders by welding together three  plates. The plate girder used in this suspension bridge is an example of such a fabrication. The plate girder is designed for a span of 10 m. The girder spans between the suspenders. The design loads for which the girder is desi gned are: Dead load = 42.24 kN/m Live load = 56 kN/m The design of girder is according to the design standards mentioned in IS 800:2007. The design warrants for the provision of web plate of dimension 1200 × 40 mm and flange plates of dimension 350 × 40 mm. Plate girders offer a unique flexibility in fabrication and the cross section can be uniform or non-uniform along the span.

4.2.5 Column

This project envisages the design of a column that takes an axial compressive force equivalent to the vertical component of the tension due to live load H p. In order to withstand the high magnitude of compressive force, a built up laced column is designed.

4.2.5.1 Desi gn of l oad colum n

As per IRC: 6-2010 P = 4124.65 kN L = 12 m

29

Assume a design stress of 125 MPa Required area =

  

 

Use 4 number of 200 200 20 angles Mass = 60 Kg/m Or = 76.4



The dimensions are as follows:





A B = 200 200 Thickness = 20 m

 =15.0,  = 4.8;  = 57.1 mm,  = 5.71 cm  = 2880 ,  = 2880  (max) = 4570  , (min) = 1180  ;  =  = 6.14 cm (max) = 7.73 cm, (min) = 3.39 cm;  = 201  ;  = 201  Area provided = 7640 4 = 30560  For 30560, Req

   = 4124.65 = 134.96 N/ 

Table (11) value of k = 0.65 L for fixed condition

 

Efficient length L = 0.65 12 Required =

 = 97.5 mm 

Moment of Inertia required section = A 30



 = 290.5    = 30560

Equating the required moment of inertia provided 290.5

 = 4+30560̅  ̅ = 75.74 mm 

Spacing of angles(s) = 2 (75.74+57.1) = 265 mm Provide 265 mm spacing  Now,

 =  of built up section   = (4 +30560   r=

   = 216.77 mm  

 =     = 35.98  From table (9c),

 = 35.98; f  = 250  y

f cd = 303.5 N/mm 2 Capacity of the built up column =

  = 9274.96 kN 

9247.96 > 4124 kN Hence the column is safe.

31

4.2.5.2 Connecting systems

Providing a double lacing system with a lacing flat inclined at Both are provided at the centre of the leg.

)  = (265 –  100 –  100) cot  Spacing of Lacing bar, (

 

= 65 mm

  =   

= 1.06 < 50

Should be less than 0.7

 35.98 = 25.18

25.186 > 1.06 Shear force, V = 2.5 % Load (codes) =

  4124.65 

(V) = 103.116 kN

  cosec    cosec  = 

Transverse shear in each panel =

 

= 36457

4.2.6 Section of Lacing flat

Assuming 20 mm bolts According to (clause 7.6.2) w = 3

 20 mm

= 60 mm

32

  .  

Length of lacing = (365 –  100 –  100) cosec



 

= 91.9 mm Minimum thickness of lacing flat =

  (91.9)  = 1.53 m



Providing a flat of 60  6 mm

  =   = 1.73 mm √  √   =   = 37.18 < 145  

Minimum radius of gyration, r =

Hence the flat is safe. For

 = 37.18 and f  = 250  y

f cd = 207.34 N/mm2 Capacity of lacing bar = 270.34

 60  6

= 74640.24 > 36457 Hence the lacing bar is safe.

4.2.7 Connections

Strength of a 20 mm diameter bolt in double shear = 2

 45.3

= 90.6 kN

   = 2.5   120  6   

Strength of bolt in bearing = 2.5 k  b d t

33

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= 305529.629 mm 2 Use a base plate of size 600×600 mm with an area = 360000 mm 2 The column is kept in the centre Therefore there will be a projection of about 335mm on each side

 

W=

= 11.47 N/mm 2

        )

ts =

=

 

= 16.65mm < 20mm Hence provide a base plate 600×600×20 mm Also provide 20 mm diameter and 300 mm lone anchor bolts to connect the  base plate to the foundation concrete. Use a 6 mm fillet weld all around the column section to hold the base plate in place. 4.2.9 Footing

As mentioned in the manufacturing constraints, avoiding the pile foundation due to the availability of good stratum at a greater depth i s a hurdle. This constraint is overcome by designing a massive R.C.C footing slab to overcome the axial compression and high moments. The design of the footing follows design specifications as per IS 456. The design resulted in the provision of a  pedestal of dimension 800x800x900 mm and 10 nos. of 20 mm diameter rods around the pedestal. Also provision of 8 mm tie bars at 300 mm c/c is also mandated. The

 

footing slab with a dimension of 6000 6000 1300 mm is to be provided and 30 nos 35

of 20 mm rods are to be provided at 300 mm spacing. The dimensions of a tie bar is shown in the below Figure 4.4.

Fig 4.4 Dimensions of a tie bar

36

CHAPTER 5

CONCLUSION

5.1 GENERAL CONCLUSION

The various parts of the pedestrian suspension bridge are analyzed with respect to cost, time, availability of skilled labors and ease in construction and designed to arrive at an economical structure which requires low maintenance and thereby providing easier and better access. Various technical drawings used for construction and reference have been drawn using AutoCAD.

5.2 FUTURE SCOPE

The analysis and design of this suspension bridge can be extended for  bridges with longer spans and to accommodate heavier traffic over bigger rivers or at any other location. Investment in rural transportation improvements would help to reduce poverty through improving access to markets, educational opportunities and medical clinics not currently accessed. Accordingly, a country‟s ability to maximize it‟s economic potential to link is closely linked to efficiency of its transportation system. The Suspension bridge can be built up to 400 feet with no intermediate supports. From structural standpoint, Suspension bridges have taken a number of forms, each with the function of providing safe transport to the rural community members who wants to avoid travel by river and to fulfill their basic needs.

37