Precast Pre Stressed Concrete Bridges
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Transcript of Precast Pre Stressed Concrete Bridges
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.