Prestressed composite truss girder and construction method of the same

1. A prestressed composite truss girder having a truss structure whereon a slab concrete is composed comprises:

a lower-chord member composed of prestressed concrete wherein prestress is introduced to resist against tensile stress and to reduce the displacement generated by external loads and having perpendicular and horizontal cross-section of certain shape and certain length;

web member wherein vertical members and diagonal members composed of a steel beam to resist against the shear stress applied on the composite girder are connected alternately with the upper face of said lower-chord member; and

upper-chord member composed of a steel plate connected with said web member along the longitudinal direction of said lower-chord member to resist against the compressive force generated before said concrete slab is composed,

wherein said lower-chord member comprises a plurality of wire-type strands or tendons placed inside the lower-chord member in the longitudinal direction to introduce the prestress to a certain distribution along the lower-chord member.

2. The prestressed composite truss girder according to claim 1 , wherein said wire-type strands or tendons have different cross-sectional area along the lower-chord member to vary amount of the introduced presetress at the cross section of said lower-chord member in longitudinal direction.

3. A construction method of a prestressed composite truss girder comprises steps of:

(a) forming a prestressed concrete lower chord member having certain length, wherein certain prestress is introduced to the axial direction;

(b) connecting alternately the vertical members and the diagonal members composed of structural rolled steel having certain length on the upper face of said lower-chord member; and

(c) connecting a planar upper chord member with said vertical members and diagonal members along the longitudinal direction of said lower chord member.

4. The construction method of a prestressed composite truss girder according to claim 3 , wherein said (a) step comprises steps of:

(a1) establishing the concrete bed on the ground after smoothing the ground of a certain place;

(b1) establishing the lower formwork in the linear form having a certain width and length on the H-beam after disposing a plurality of the H-beams in the lattice form on said concrete bed;

(c1) establishing spacers between the reinforcing cage and the upper face of the lower formwork to separate the reinforcing cage at a certain interval from the upper face of the lower formwork after placing the reinforcing cage and the connecting unit for the web member at a certain interval along the longitudinal direction of the reinforcing cage;

(d1) anchoring the tendons to the abutments to maintain the tendons at the tense state after inserting and placing a plurality of the wire-type tendons into the reinforcing cage and establishing the abutments at the position separated to a certain interval from the both ends of the lower-formwork and then introducing certain tension to the tendons using oil jack;

(e1) curing the concrete for a certain period after setting the lateral formwork at the lateral side of the reinforcing cage and pouring the concrete into the lateral formwork; and

(f1) cutting the tendons from the abutments to introduce a certain prestress to the cured concrete.

5. The construction method of a prestressed composite truss girder according to claim 3 , wherein said (a) step comprises steps of:

(a2) establishing the concrete bed on the ground after smoothing the ground of a certain place;

(b2) establishing the lower formwork in the linear form having a certain width and length on the H-beam after disposing a plurality of the H-beam in the lattice form on the concrete bed;

(c2) establishing spacers between the reinforcing cage and the upper face of the lower formwork to separate the reinforcing cage at a certain interval from the upper face of the lower formwork after placing the reinforcing cage and the connecting unit for the web member at a certain interval along the longitudinal direction of the reinforcing cage;

(d2) placing a plurality of the sheath tubes wherein anchorage devices are installed at the both ends;

(e2) curing the concrete for a certain period after setting the lateral formwork to the lateral side of the reinforcing cage and pouring the concrete into the lateral formwork; and

(f2) bonding the tendons with surrounding concrete by injecting the cement mortar through the inside of the sheath tube after the curing of the concrete is finished and a plurality of wire-type tendons are placed in the inside of each sheath tube and then the tendons are tensioned using oil jack.