BOW-STRING GIRDER- DESIGN & CONSTRUCTION ASPECTS
By Anamul Haque Sr DEN/IV/ALD Sandeep Kumar Sr DEN/II/IZN Anisur Rehman Dy CE/Br/HQ/Hajipur
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 of THE 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
was possible to provide two spans for navigational purpose but vertical clearance can’t 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 existing bridge 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, without increasing 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.
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/full loading. 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.
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 with minimum 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
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
: 550x1200
Size 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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