bow-string girder- design & construction aspects

7/31/2019 Bow-string Girder- Design & Construction Aspects

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BOW-STRING GIRDER- DESIGN &

CONSTRUCTION ASPECTS

By

Anamul Haque Sr DEN/IV/ALD

Sandeep Kumar Sr DEN/II/IZNAnisur Rehman Dy CE/Br/HQ/Hajipur


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Bow string Girder spans of Thane Creek Bridges

The new bridge across Thane creek was required to be constructed parallel to existing

main line bridge which is more than 100 year old constructed with stone masonry for

laying the new corridors between Thane Turbhe Nerul / Vashi commuter line

project. Bridge No.33/1A is located immediately after Thane station and consists of

three clear spans of 30 m each. Bridge No. 33/2A is located across Thane creek and

consists of 5 clear spans of 30 m and one end span of 7.5 m PSC, except for RUB span

of bridge No.33/1A and navigational spans of bridge No.33/2A. Prestressed box girders

of 2.1 m depth resting on neoprene bearing with RCC plate piers have been adopted.

RCC footings rest on good basalt rock was available at shallow depths in creek bed. In

case of RUB span of bridge No.33/1A where sufficient vertical clearance is to be

provided for the passage of light vehicular traffic as per the requirement of Thane

Municipal Corporation and in case of navigational span of bridge No. 33/2A where the

necessary vertical clearance is to be provided as per requirement of inland waterway

authority, a bow string girder type of structure has been considered most suitable, both

from structural and aesthetics point of view.

BOW STRING GIRDER is unique in Construction as it is designed for carrying

the load of two tracks on a single span with MBG loadings and depth restriction of

750 mm, as individual girder spans for both tracks are impossible.

The construction of 3 spans of bridge No. 33/1A and 32/A across Thane Creek is

a part of a commuter line Project from Thane-Turbhe Nerul/Vashi. The construction of

two bridges are on East side of existing arch bridges which are more than 100 year old,

constructed with stone masonry existing bridges are having UCR foundations.

Existing bridge has one navigational clear span of 24.2 m with vertical clearance

6.96m from MHWS (Mean High Water of Spring tides). As indicated by Port

Authorities, that in July 1976, Government of Maharashtra has appointed a Committee

of Technical Experts on the Engineering feasibility ofTHE THANE CREEK-ULHAS

RIVER-BASSEIN CREEK-DHARAMTAR CREEK-ISLAND WATERWAY

PROJECT, MAHARASHTRA PROJECT. They have recommended providing two

navigational spans of 30 m and vertical clearance of 9.0 m at all tide levels. However it


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was possible to provide two spans for navigational purpose but vertical clearance cant

be increased because of near by Thane yard, where gradient of track cannot be increased.

Hence Port Authorities has agreed that same vertical clearance as available in existingbridge can be provided and in due course they will dredge the present bed to a depth of

3.0m.

Existing navigational span of 29 m is semi-through girder bridge. Girder bridges

are causing problems due to non-availability of wooden sleepers and riding quality is

poor due to change of track structures. Hence it was preferred to have ballasted deck

where continuity of the track structure is maintained and riding quality is good with

minimum maintenance cost. An Engineering solution was found by way of BOW

STRING GIRDER where it is possible to restrict the height of rail level to bottom of

girder to 1500 mm, out of which 725 mm is rail level to top of deck for rails, sleepers

and ballast cushion and 775 mm for designing of deck slab and cross beams.

As the hangers are tension members of the Bow String Bridge, these are the most

important and crucial members of the girder. Therefore to protect these hangers from

any damage due to derailment of trains, the fender boxes have been provided at 3.5m

height from Rail Level on each hangers of central arch.

Design Approach

For this bridge, additional vertical clearance was required to be provided in three

spans. Hence design approach was to develop a structural configuration, which will

require minimum depth 1500 mm between rail top to soffit of structure for the long

span of 32 m with MBG loading. Since rail, ballast etc. takes up depth of 750mm,

structural depth of only 750 mm is available. Live load defection permissible as per IRS

Concrete Bridge Code is only span/1000, it was therefore necessary to have

configuration stiff enough to keep live load deformations under check, withoutincreasing dead load & depth below rail. Structure should also have adequate fatigue

strength and durability. Actual design was done as per relevant Railway loading

standards & codes.

Three bowstring girders of 32 m span with limited depth of 750 mm only were

designed to support two tracks whereas normal depth requirement with PSC box girder

was 2.5m. A single-track loading is 50% higher than a two-lane roadway loading, central

bowstring girder is thus carrying equivalent of a 3-lane roadway loading.


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Salient Features

a) Chosen form improves durability as most of the structure is in compression.

b) Permissible stress levels in hangers reduced to minimize corrosion impact and

improve fatigue strength. Hanger being the only tension member, crack width

is limited to 0.1mm.

c) Partial shock absorbing system provided on hangers to reduce the damage due

to accident.

d) Aesthetics, as the bridge is located in the heart of the city.

e) Form chosen was unconventional for railway bridges.

f) Material consumption per sq m of bridge compares very well with

conventional solutions and cost/sq m is only marginally higher.

Top arch being predominantly a compression member requires minimum

reinforcement except at few locations.

Bottom tie being predominantly a tension member can be easily prestressed.

Hangars being axially tensioned members and can either be prestressed as

done for Godavari Bridge at Rajahmundry or if hangars are short, they can be in

RCC as in this case.

In view of short heights of hangers, prestressing of hangars was not possible,

therefore hangars have been designed with reinforcement to take the tensions.

The stress level in the hangar reinforcement is kept substantially lower for

limiting crack width to 0.1 mm and high fatigue strength. Spacing of the hangers

is optimized to ensure to reversal of stresses under any condition of part/fullloading.

Normal precautions like provision of checkrails etc. has been made as safety

against derailment. Additionally on the central hanger, structural steel buffer

boxes have been provided at an appropriate level, which can absorb some of the

impact in case any of the bogies come to rest against the hangars. Spacing of

hangars is such that at least 4-5 hangars will share the load.


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Construction Aspects

In order to reduce the construction joints in beams, slabs and arches it has

been concreted in a single pour of 247 cum covering end cross beams,

longitudinal beams, deck slab and end part of arch (3 m height). Special floating

type of shuttering to cover 3m height of arch was erected. Malamine based

superplasticiser Sikament 500M2 with a dose of 1.2% by weight of cement was

used which gives a slump of 120 mm even after 3 hrs and initial setting time

increased to 6 hrs keeping w/c ratio of 0.37. Concreting sequence was so planned

that new concrete is poured before previously laid concrete achieves initial

setting. Hence cold joints were completely avoided. Hangars were cast withminimum joints by concreting in 2.0 m lifts. All construction joints have been

sealed with epoxy mortar. The reinforcement bars have been given anticorrosive

treatment as per CECRI and hangar bars were treated with fusion-bonded epoxy.

To make the deck slab water proof, 10 mm thick mastic asphalt and 30

mm thick fiber reinforced concrete of M30 grade has been provided. The fiberlon

nylon fibers were used as reinforcement to avoid shrinkage cracks and higher

impact resistance.

To increase the durability of concrete, all surfaces of girders have been

given a coating of IPNet (Anticorrosive Polymer System) in 4 layers with overall

thickness of 300-350 mircon.

To complete in a single pour, concrete of 247 cum was done in 10 hrs, by

deploying two concrete pumps and using ready mix concrete


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Details of Bow String Girder

Effective Span (C/C of Bearings) : 31m

Overall Length of Deck Slab : 32.5m

Overall Length of Soffit Slab : 32.5m

Depth of Girder below Rail Level : 1.5m

Overall Width of Deck Slab (for 2 tracks) : 11.9m

Size of Bottom Tie : 550x1800/1600

Size of Top Arch : 550x1200Size of Hangars : 600x400

Thickness of Deck Slab : 280

Slope of Deck Slab (Cross) : 2.5%

Concrete Grade : M 40

Concrete Grade for Kerbs & Footpath Unit : M20

Prestressing Steel High Tensile Steel : 6 T/Span

Type of Cable : 19 T 13

Inner Diameter of Sheathing for Cable : 85 mm

Thickness of Sheathing : 0.5mm

Ultimate Strength of Each Strand : 185 KN

No. of Cables (Single Stage Stressing) : 12/Span

No. of Dummy Cables : 3

Reinforcement Steel : 56 T

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bow-string girder- design & construction aspects

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