Analysis & Design of Cable Stayed Bridge

Introduction

• Cables stretch diagonally between these pillars or towers and the beam. These cables support the beam.
• A cable-stayed bridge, one of the most modern bridges, consists of a continuous strong beam (girder) with one or more pillars or towers inside the middle.
• The cables are anchored in the tower rather than at the end.

Advantages of Cable-Stayed Bridge

• Much greater stiffness than the suspension bridge, so that deformations of the deck under live roads are reduced
• Can be constructed by cantilevering out from the tower – the cables act both as temporary and permanent supports to the bridge deck.
• For an asymmetrical bridge ( i.e. spans on either side of the tower are the same), the horizontal forces balance and large ground anchorages are not required.

The Difference Between a Cable-Stayed Bridge and a Cable Suspension Bridge

• A multiple-tower cable-stayed bridge may appear similar to a suspension bridge, but in fact, is very different in principle and in the method of construction.
• In the suspension bridge, a large cable hangs between two towers and is fastened at each end to anchorages in the ground or to a massive structure.
• These cables form the primary load-bearing structure for the bridge deck. Before the deck is installed, the cables are under tension from only their own weight.
• Smaller cables or rods are then suspended from the main cable and used to support the load of the bridge deck, which is lifted in sections and attached to the suspender cables.
• The tension on the cables must be transferred to the earth by the anchorages, which are sometimes difficult to construct owing to poor soil conditions.

Components of Cable-Stayed Bridge

Load Transmission

CLASSIFICATIONS

• Based on arrangements of the cables
• Radiating
• Harp
• Fan
• Star
• Based on the shape of the pylon
• A-type
• H-type
• Y-type

Shapes of Pylon

Types of Deck

• Twin I Girder

• Truss Girder

• Box Girder

• Orthotropic Girder

CABLE

• A cable may be composed of one or more structural ropes, structural stands, locked coil stands, locked oil stands, or parallel wire stands.
• A stand is an assembly of wires formed helically around the center wire in one or more symmetrical layers.
• A stand can be used either as an individual load-carrying member, where the radius of curvature is not a major requirement or as a component in the manufacture of the structural rope.
• A rope is composed of a plurality of strands helically laid around a core. In contrast to the stand, a rope provides increased curvature capability and is used where the curvature of the cable becomes an important consideration.

Types of Cable

Properties of Cable

Cables are made of high-strength steel, usually encased in a plastic or steel covering that is filled with grout, a fine-grained form of concrete, for protection against corrosion.

Selection of Cable Configuration

• The selection of cable configuration and number of cables is dependent mainly on the length of the span, type of loadings, number of roadways lanes, the height of towers, and the designer’s individual sense of proportion and aesthetics.
• Cost also plays important role in deciding the selection
• Using fewer number cables increases concentrated load at a single point thereby requiring additional reinforcement for the deck slab as well as a pylon.

Positions of the Cables in Space

• Two plane system
• Two Vertical Planes Systems
• Two Inclined Planes Systems
• The Single Plane System

Two Vertical Planes System

• In this type of system, there are two parallel sets of cables and the tower on either side of the bridge, which lie in the same vertical plane.

• The cable anchorages may be situated outside the deck structure, which is better than the other in terms of space as no deck area of the deck surface is obstructed by the presence of the cables and the towers.
• But this requires substantial cantilevers to be constructed in order to transfer the shear and the bending moment into the deck structure.
• When the cables and tower lie within the cross-section of the bridge, the area taken up cannot be utilized as a part of the roadway and maybe only be partly used for the sidewalk. Thus an area of the deck surface is made non-effective and has to be compensated for by increasing the overall width of the deck.

Two Inclined Planes System

• In this system, the cables run from the edges of the bridge deck to a point above the centreline of the bridge on an A-shaped tower or λ-shaped or diamond-shaped pylon.
• This arrangement can be recommended for very long spans where the tower has to be very high and needs the lateral stiffness given by the triangle and the frame junction.

The Single-Plane System

• This type of system consists of bridges with only one vertical plane of stay cables along the middle longitudinal axis of the superstructure.
• As the cables are located in a single vertical strip thus all the space is utilized by the traffic.
• This system also creates a lane separation as a natural continuation of the highway approaches to the bridge.
• Longitudinal arrangements of the cables used with two plane bridges are also applied to a single-center girder bridge.

Cable Anchorage

Installation of Cables Anchorage

• Unreeling on carriages
• Connecting the tensioning rod
• Pulling into anchorage
• Fixing the anchorage at the tower head
• Installation at girder
• Cable tensioning
• Final anchorage
• Delivery of strands
• Supply PE pipes
• Scaffolding at tower head
• Anchor has at the tower
• Aligning PE pipes bridge deck
• Fusion welding of PE pipes
• PE pipe with lifting collar

Installation of Cables Anchorage

• Strand pulled through the stay pipe

• Temporary connection of the stay pipe at the tower

• Deformations influencing cable lengths

• Mono jack for stressing individual strands

• Stressing of individual strands

• Stay cable without deviator and anti-vandalism pipe

• Protruding strands for re-stressing

• Multi-strand jack for stressing complete cable

• Stressing of complete cable

• Power seating of the wages

• Dampers inside boots and anti-vandalism pipes

• Corrosion-protection paint and strand cap

Bridge Bearings

Functions of Bearings

• Bridge bearings are used to transfer forces from the superstructure to the substructure, allowing the following types of movements of the superstructure:

• Translational movements; and
• Rotational movements
• Until the middle of this century, the bearings used consisted of the following types

• Pin
• Roller
• Rocker
• Metal sliding bearings

Pin Bearing

• A pin bearing is a type of fixed bearing that accommodates rotations through the use of a steel
• Translational movements are not allowed
• The pin at the top is composed of upper and lower semicircularly recessed surfaces with a solid circular pin placed between them.
• Usually, there are caps at both ends of the pin to keep the pin from sliding off the seats and to resist uplift loads if required.
• The upper plate is connected to the sole plate by either bolting or welding. The lower curved plate sits on the masonry plate.

• Rotational Movement is allowed
• Lateral and Translational Movements are Restricted

Roller Type Bearing

Single Roller Bearing

Multiple Roller Bearing

• AASHTO requires that expansion rollers be equipped with “substantial sidebars” and be guided by gearing or other means to prevent lateral movement, skewing, and creeping (AASHTO 10.29.3).
• A general drawback to this type of bearing is its tendency to collect dust and debris

Rocker Type Bearing

• A rocker bearing is a type of expansion bearing that comes in a great variety.
• It typically consists of a pin at the top that facilitates rotations, and a curved surface at the bottom that accommodates the translational movements
• Rocker and pin bearings are primarily used in steel bridges.

Sliding Bearing

• A sliding bearing utilizes one plane metal plate sliding against another to accommodate translations.
• The sliding bearing surface produces a frictional force that is applied to the superstructure, substructure, and the bearing itself.
• To reduce this friction force, PTFE (polytetrafluoroethylene) is often used as a sliding lubricating material. PTFE is sometimes referred to as Teflon, named after a widely used brand of PTFE.
• Sliding Bearings be used alone or more often used as a component in other types of bearings.
• Pure sliding bearings can only be used when the rotations caused by the deflection at the supports are negligible. They are therefore limited to a span length of 15 m or less by ASHTTO [10.29.1.1]

Knuckle Pinned Bearing

• It is a special form of Roller Bearing in which the Knuckle pin is provided for easy rocking. A knuckle pin is inserted between the top and bottom casting. The top casting is attached to the bridge superstructure, while the bottom casting rests on a series of rollers
• Knuckle pin bearing can accommodate large movements and can accommodate sliding as well as rotational movement

Pot Bearings

• A POT BEARINGS consists of a shallow steel cylinder, or a pot, on a vertical axis with a neoprene disk which is slightly thinner than the cylinder and fitted tightly inside.
• A steel piston fits inside the cylinder and bears on the neoprene.
• Flat brass rings are used to seal the rubber between the piston and the pot.
• The rubber behaves like a viscous fluid flowing as rotation may occur.
• Since the bearing will not resist bending moments, it must be provided with an even bridge seat.

Plain Elastomeric Bearings

Laminated Elastomeric Bearings

• Consists of a laminated elastomeric bearing equipped with a lead cylinder at the center of the bearing.
• The function of the rubber-steel laminated portion of the bearing is to carry the weight of the structure and provide post-yield elasticity.
• The lead core is designed to deform plastically, thereby providing damping energy dissipation.
• Lead rubber bearings are used in seismically active areas because of their performance under earthquake loads.

Selection of Bearing Type According to AASHTO

Tests on Cable-Stayed Bridge Model

• Wind Tunnel Test
• Fatigue Test
• TENSILE LOAD TEST OF THE STRAND
• INSPECTION OF ANCHORAGES
• CHECKING THE HARDNESS OF WEDGES
• ROTATIVE FLEXION TEST

Our Project Definition

• We have three spans (120+250+120) m
• The deck at the height of (50) m clearance
• We have (80) Cables arranged in a double plane over the bridge length with (12) m spacing.

Components

1. Pylon

• We have two non-prismatic pylons, each one having (130 m) in height.

2. Deck

• Width = 28 m
• Consists of (6) lanes
• (3) lanes in each direction with a width (of 3.6)m
• An intermediate island (1)m width
• (2) sidewalks each one (2.7)m in width

• Main I girder 2.8 depth of thickness 2 cm and 2 flanges 60 cm width with thickness 5 cm.
• X-girder 2.8 depth of thickness 5 cm.
• Stringers with IPE#600 section.
• Concrete slab of thickness 20 cm Fcu=400 kg/cm2 and 7cm Bitumious asphalt.

3. Cables

• We have (10) cables on each side of the pylon.
• The cables have an initial diameter (of 12)cm.
• The distance between cables in the deck plan equals (12) m.
• The distance between them at their links to the pylon equals (1.5) m