PRECAST PRESTRESSED CONCRETE BRIDGES

SAAGAR L. BHATIA 050901001

A bridge is a structure built to span a valley, road, body of water, or

other physical obstacle, for the purpose of providing passage over the

obstacle. Designs of bridges vary depending on the function of the

bridge and the nature of the terrain where the bridge is constructed.

Types of bridges

There are six main types of bridges: 

beam bridges, 

cantilever bridges, 

arch bridges, 

suspension bridges, 

cable-stayed bridges 

truss bridges.

BRIDGE

FORCES

Bridges may be classified by how the forces of 

tension, compression, bending, torsion and shear are distributed

through their structure. Most bridges will employ all of the

principal forces to some degree, but only a few will predominate.

The separation of forces may be quite clear. In a suspension or

cable-stayed span, the elements in tension are distinct in shape

and placement. In other cases the forces may be distributed

among a large number of members, as in a truss, or not clearly

discernible to a casual observer as in a box beam.

PRESTRESSED CONCRETE

Prestressed concrete is a method for overcoming

the concrete's natural weakness in tension. It can

be used to produce beams, floors or bridges with

a longer span than is practical with ordinary

reinforced concrete. Prestressing tendons

(generally of high tensile steel cable or rods) are

used to provide a clamping load which produces

a compressive stress that offsets the tensile

stress that the concrete compression

member would otherwise experience due to a

bending load. Traditional reinforced concrete is

based on the use of steel reinforcement

bars, rebar's, inside poured concrete.

PRECAST CONCRETE

Precast concrete is a form of construction, where concrete is cast in a

reusable mould or "form" which is then cured in a controlled environment,

transported to the construction site and lifted into place. In contrast, standard

concrete is poured into site specific forms and cured on site.

By producing precast concrete in a controlled environment , the precast

concrete is afforded the opportunity to properly cure and be closely

monitored by plant employees. There are many different types of precast

concrete forming systems for architectural applications, differing in size,

function and cost.

Modern uses for precast technology include a variety of architectural and

structural applications featuring parts of or an entire building system.

The advantages of using precast concrete is the increased quality of the

material, when formed in controlled conditions, and the reduced cost of

constructing large forms used with concrete poured on site.

PRECAST PRESTRESSED CONCRETE

Precast and prestressed concrete is now the dominant structural material

for short to medium span bridges. With its inherent durability, low

maintenance and assured quality, precast and prestressed is a natural

product for bridge construction. The ability to quickly erect precast

concrete component in all types of weather with little disruption of traffic

adds to the economy of the job. For short spans(spans to 100 ft), use of

box sections and double tee sections have proven economical. However,

the most common product for short to medium spans in the I-girder.

Spans to 150 to 160ft are not uncommon with I-girders. Spliced girders

allow spans as much as 300ft. Even longer spans can be achieved using

precast box girder segments which are then post-tensioned together in

the field. Using cable stays, the spanning capability of precast and

prestressed concrete has been increased to over 1000ft.

An important innovation in bridge construction has been the use of

precast concrete in horizontally curved bridges.

Another application of precast and prestressed concrete in bridge

construction includes the use of precast deck panels. Used as stay in

place forms, the panels reduce field placement of reinforcing steel and

concrete resulting in considerable savings.

The speed and variety of precast prestressed products and methods

give designers many options.

Benefits to Owner Agencies:Reduction in the duration ofwork zonesReduced traffic handling costsReduced accident exposure risksLess inconvenience to thetraveling publicFewer motorist complaints

Benefits to Contractors:

Reduced exposure to hazardsMore work -- less timeFewer weather delaysLower costsLess skilled laborNo curing time

The Bandra Worli Sea Link  would be an 8-lane , cable-stayed bridge

with pre-stressed concrete viaduct approaches, which links Bandra

and the western suburbs of Mumbai with Worli and central Mumbai,

and is the first phase of the proposed West Island Freeway system. 

The Sea Link is likely to reduce travel time between Bandra and Worli

from 45–60 minutes to 7 minutes. The link has an average daily traffic

of around 25,000 vehicles on weekdays.

The project starts from the intersection of Western Express Highway and SV Road at the Bandra end, and connects it to Khan Abdul Gaffar Khan Road at the Worli end.

BANDRA WORLI SEA LINK

The proposed Link Bridge consists of twin continuous concrete box girder

bridge sections for traffic in each direction. Each bridge section except at

the cable - stayed portion is supported on piers typically spaced at 50

meters. Each section is meant for four lanes of traffic complete with

concrete barriers and service side walks on one side. The bridge alignment

is defined with vertical and horizontal curves. The Link Bridge layout is

categorized into three different parts:

MAIN BRIDGE STRUCTURE

Part 1 - The north end approach structure

mainly with precast (PC) segmental

construction

Part 2 - The Cable Stayed Bridge at Bandra

channel is with 50m - 250m - 250m - 50m

span arrangement and the Cable Stayed

Bridge at Worli channel is with 50m - 50m -

150m - 50m - 50m span arrangement

Part 3 - The south end approach structure

mainly with precast segmental construction

PART - I   NORTH END APPROACH STRUCTURE

The bridge is arranged in units of typically six continuous spans of 50 meters each. Expansion joints are provided at ends of each unit.

Provision for access ramp to connect to Bandstand road below Searock Hotel. Span arrangement for this structure provides for cast in-situ spans.

The superstructure & substructure are designed in accordance with IRC codes. Specifications conform to the IRC standard with supplementary specifications covering special items. The sub - structure consists of 1.5 meters diameter drilled piles with pile caps & some of the piers near Worli end will be directly socketed into the rock.

Bridge is proposed to be built utilizing the concept of precast, post - tensioned, twin segmented concrete box girder sections. An overhead gantry truss crane with self - launching capability is proposed. The PC segments are epoxied together with nominal prestressing. The end segments adjacent to the pier would be short segments "cast - in - situ". Geometrical adjustments are expected to be made by this segment before primary continuous tendons are stressed.

PART- II   CABLE STAYED BRIDGE

The cable - stayed portion of the Bandra channel is 600 meters in overall length

between expansion joints and consists of two 250 meters cable supported main

spans flanked by 50 meters conventional approach spans. A centre tower with an

overall height of 128 meters above pile cap level supports the superstructure by

means of four planes of stay cables in a semi - fan arrangement. The cable - stayed portion of the Worli channel is 350 meters in overall length

between expansion joints and consists of two 150 meters cable supported main

spans flanked by 50 meters conventional approach spans. A centre tower with an

overall height of 55 meters above pile cap level supports the superstructure by

means of four planes of stay cables in a semi - fan arrangement. Balanced cantilever construction is envisioned for erecting the cable supported

superstructure as compared to span - by - span construction for the approaches.

For every 2nd segment, cable anchorages are provided. 

A total of about 264 stay cables will be required for the cable - stayed spans at Bandra channel with cable lengths varying

from approximately 85 meters minimum to nearly 250 meters maximum. The tower is cast - in - situ reinforced concrete

using the climbing form method of construction. The overall tower configuration is an inverted "Y" shape with the inclined

legs oriented along the axis of the bridge. Tower cable anchorage's are achieved by use of formed pockets and transverse

and longitudinal bar post - tensioning is provided in the tower head to resist local cable forces. A total of about 160 stay cables will be required for the cable - stayed spans at Worli channel with cable lengths varying

from approximately 30 meters minimum to nearly 80 meters maximum. The tower is cast - in - situ reinforced concrete

using the climbing form method of construction. The overall tower configuration is "I" shape with the inclined legs. Tower

cable anchorage's are achieved by use of formed pockets and transverse and longitudinal bar post - tensioning is

provided in the tower head to resist local cable forces.

PART - III SOUTH END APPROACH STRUCTURE

This portion of the bridge is similar to the North end approach structure in construction methodology with span by span match cast concrete box girder sections. Similar to the north end approach detailed, access ramps shall be provided for connection to the western freeway

Bangabandhu Bridge, also called the Jamuna Multi-purpose Bridge , is a bridge opened in Bangladeshin June 1998. It is the eleventh longest bridge in the world and the second longest in South Asia. It is amongst the longest bridges in

the world. It was constructed over the Jamuna River. The bridge established a strategic link between the eastern and

western parts

of Bangladesh. It generated various benefits for the people and especially,

promoted inter-regional trade in the country. Apart from quick movement of

goods and passenger traffic by road and rail, it facilitated transmission of

electricity and natural gas, and integration of telecommunication links.

The main bridge is 4.8 km long with 47 main spans of approximately 100 metres

and 2 end spans of approximately 65 metres. Connected to the bridge are East

and West approach viaducts each with 12 spans of 10 metre length

and transition spans of 8 metres. The total width of the bridge deck is 18.5

metres.

BANGABANDHU BRIDGE,BANGALADESH

The crossing has been designed to carry a dual two-lane carriageway,

a dual gauge railway, telecom cables and a 750 mm diameter high

pressure natural gas pipeline. The carriageways are 6.315 metres

wide separated by a 0.57 metre width central barrier; the rail track is

located along the north side of the deck. On the main bridge,

electrical interconnector pylons are positioned on brackets

cantilevered from the north side of

the deck. Telecommunication ducts run through

the box girder deck and the gas pipeline is

located under the south cantilever of the box

section.

SPECIFICATION

Considering the fact that the width of the main channel does not exceed

3.5 km, and after making allowances for floods, a bridge length of 5 km

was considered adequate. In October 1995, one year after the

commencement of physical work of the bridge, a bridge length of

4.8 km, instead of a flood-width of the river at 14 km, was finalised. This

narrowing was essential to keep the overall project cost within economic

viability. It has, however, required considerable river training work to

keep the river under the bridge.

To withstand predicted scourge and possible earthquakes, the bridge is

supported on 80-85 meter long and 2.5 meter and 3.15 meter diameter

steel piles, which were driven by powerful (240-ton) hydraulic hammer.

The superstructure of the bridge is pre-cast segments erected by the

balanced cantilever method. Basic features of the bridge are: length

(main part) - 4.8 km; width - 18.5 metre; spans - 49; deck segments -

1263; piles - 121; piers - 50; road lanes - 4; railway tracks - 1 dual gauge.

The bridge is supported on tubular steel piles, approximately 80

metres in length, driven into the river bed. Sand was removed

from within the piles by airlifting and replaced with concrete. Out

of the 50 piers, 21 piers are supported on groups of 3 piles (2.5

m diameter) and 29 piers on groups of 2 piles (3.15 diameter).

The driving of 121 piles started on October 15, 1995 and was

completed in July 1996. The pier stems are founded on

concrete pile caps, whose shells were

SUB - STRUCTURE

precast and in filled with in-situ reinforced

concrete. The reinforced concrete pier stems

support pier heads which

contain bearings and seismic devices. These

allow movement of the deck under normal

loading conditions but lock in the event of

an earthquake to limit overall seismic

loads through the structure and minimise

damage.

SUPER STRUCTURE

The main bridge deck is a multi-span

precast prestressed

concrete segmental structure, constructed

by the balanced cantilever method.

Each cantilever has 12 segments (each 4 m

long), joined to a pier head unit (2 m long)

at each pier and by an in-situ stitch at mid

span. The deck is internally prestressed and

of single box section. The depth of the box

varies between 6.5 metres at the piers to

3.25 metres at mid-span. An expansion

joint is provided every 7 spans by means of

a hinge segment at approximately quarter

span. The segments were precast and

erected using a two-span erection gantry.