Composite rcc deck and prestressed parabolic bottom chord underslung open web steel girder bridge superstructure

Abstract

Composite decks increase bridge strength and stiffness. Prestressed composite open web steel girder has added advantage of high strength cable support. Results of typical 125 m span bridges having heights of 9.0 m, 10.0 m and 12.5 m, and another 50.0 m span and 2.5 m height are given. Member stresses and bridge deflections during erection remained safe. Average steel off take for the 125 m bridge is 2.65 t/m and for the 50 m span bridge it is 1.77 t/m for limiting live load deflection of Span/800. Its reserve strength is 3.2 times service condition live load. The girders are panel wise workshop fabricated, assembled at site, jacked up or crane lifted to secure over bearings. Connection of the cross members, and onsite deck casting in parts with stage wise bottom chord prestressing is carried out. Short to long span bridges for single or multiple lanes in road, rail, metro rail, and coastal link projects are feasible.

I claim: 
     
         1 . A method for prestressed open web steel girder composite bridge superstructure construction, the method comprising:
 connecting a composite top chord ( 1 ) to a prestressed parabolic (polygon) bottom chord ( 2 ) using a plurality of open web members ( 3 );   aligning cables ( 4 ) and anchorages ( 5 ) within and along the prestressed parabolic bottom chord;   supporting cross girder ( 7 ), stringer beams, and composite deck slab using a plurality of shear connectors ( 6 );   connecting end cross girder ( 7 ) with at least two main girders, wherein the girders at the ends are supported over bearings ( 8 );   obtaining prestressed parabolic (polygon) bottom chord ( 2 ) for an underslung bridge and connected to the top chord ( 1 ) for nearly uniform tension under uniformly distributed load to attain a composite prestressed underslung bridge which is conducive to prestress;   the composite prestressed underslung bridge up to a predefined span between 10 m to 200 m; fabricating a plurality of superstructure panels in a workshop, their assembly and connection at site, girder wise launching, followed by on site composite RCC deck slab concreting in symmetrical parts with designed stage wise prestressing; and   stage wise prestressing of the bottom chord, from girder lifting stage to bridge commissioning stage, facilitating member stress and bridge deflection management within threshold limits during construction and service life of the bridge.   
     
     
         2 . The method as claimed in  claim 1 , wherein stiffness of the composite deck slab connected to the top chord and strength of high tensile strength cables being provided to the composite prestressed underslung bridge, results in its low deflection, high strength and stiffness, and about three times reserve strength in elastic limit. 
     
     
         3 . The method as claimed in  claim 2 , comprises prestressing of high tensile strength cables laid within the parabolic bottom chord. 
     
     
         4 . The method as claimed in  claim 1 , wherein prestressing of the bottom chord ( 2 ) counters tension due to the applied loads and the prestressing of the bottom chord exerts load balancing upward thrust. 
     
     
         5 . The method as claimed in  claim 1 , wherein prestressing of the bottom chord ( 2 ) causes longitudinal pre-compression in the deck slab, which may render its design on no-crack basis possible, which is highly desirable for its better fatigue performance. 
     
     
         6 . The method as claimed in  claim 1 , wherein prestress is applied for countering half the live load with impact, reduces curvature and girder flexural stress effect due to live load in the deck slab to half. 
     
     
         7 . The method as claimed in  claim 1 , wherein the bottom chord ( 2 ) profile of the bridge, when made parabolic, results in its uniform tension under uniformly distributed load due to self-weight or live load, which facilitates the bottom chord prestress. 
     
     
         8 . The method as claimed in  claim 1 , wherein the predefined span is between 10 m to 200 m of the composite prestressed underslung bridge for single or multiple lanes of road, rail, metro rail, and projects like fly over and sea link. 
     
     
         9 . The method as claimed in  claim 1 , wherein axial member stresses during erection and concreting of the composite RCC deck slab are checked with prestressing applied at different stages as required, to be low and safe. 
     
     
         10 . The method as claimed in  claim 1 , wherein preventing premature buckling failure and increasing its strength and stiffness, using the plurality of shear connectors, whereby RCC deck slab is made composite with the top chord of the bridge. 
     
     
         11 . The method as claimed in  claim 1 , wherein expansive concrete grouting of the box sections, converting these to CFST composite, increases strength and stiffness of the superstructure, apart from corrosion prevention. 
     
     
         12 . A prestressed open web steel girder composite bridge comprising:
 a top chord ( 1 ) connected to a prestressed parabolic (polygon) bottom chord ( 2 ) using a plurality of open web box or CFST members ( 3 );   
       a plurality of cables ( 4 ) and a plurality of anchorages ( 5 ) housed and aligned along the prestressed parabolic bottom chord ( 2 ); 
       a plurality of shear connectors ( 6 ) adapted to support cross girders and stringer beams; 
       a plurality of bearings ( 8 ) configured to connect end cross girder with at least two main girders; and 
       a composite RCC deck slab ( 7 ) cast over the top chord ( 1 ), cross girder and stringer beam, and longitudinally pre-compressed due to the prestressing allowing its design on no crack basis for its better fatigue performance.