Steel Structure·Bridge Steel Truss Bridge

The composition of steel truss

1. Classification: According to the position of the bridge deck, it can be divided into deck truss girder bridge, through truss girder bridge, and double truss girder bridge;

2. Composition: It is composed of main truss, connection system, bridge deck system and bridge deck.

1) The main truss is the main load-bearing structure and bears vertical loads. The main truss is composed of upper and lower chords and web members. Web rods are divided into oblique rods and vertical rods; nodes are divided into large nodes and small nodes; node spacing refers to the distance between nodes.

2） Connection system

*1. Classification: vertical connection system and horizontal connection system.

*2. Function: Connect the main truss to make the bridge span structure a stable space structure that can bear various lateral loads.

*3. The longitudinal connection system is divided into the upper horizontal longitudinal connection system and the lower horizontal longitudinal connection system; the main function is to bear the horizontal horizontal load, the horizontal wind, the horizontal swing force on the vehicle and the centrifugal force acting on the bridge span structure. In addition, the chord is supported laterally to reduce its free length outside the plane.

*4. The horizontal connection system is divided into the bridge portal frame and the middle horizontal connection; the main function is to increase the torsional rigidity of the steel truss girder. Properly adjust the uneven force of the two main girder or two longitudinal joints.

3） Bridge deck system

*1.Composition: the connection system between the longitudinal beams, cross beams and longitudinal beams.

*2. The way of force transmission: the load first acts on the longitudinal beam, then from the longitudinal beam to the beam, and then from the beam to the main truss node.

4） The bridge deck is the part for vehicles and pedestrians to walk on. The form of the bridge deck is similar to that of steel girder bridges and composite girder bridges.

Schematic and characteristics of the main truss

Triangular truss

◆ Vertical web rods are not provided when the pitch is small, but vertical web rods can be set when the pitch is large

◆ The chord has fewer specifications and the number of large nodes, which is suitable for finalized design and is easy to manufacture and install

◆ The standard design of my country's railway medium-span (L=48m~80m) through bolt-welded steel truss girder bridge.

Diagonal truss

◆ The oblique web rod is only under compression or tension

◆ Chords and vertical rods have many specifications, all of which are large nodes.

Double web truss

◆ The diagonal bar only bears half of the internode shear force

◆ The compressed oblique rod is short, which is beneficial to the stability of compression and buckling

◆ Suitable for large-span steel truss girder, such as Wuhan, Nanjing Yangtze River Bridge and my country's railway standard design (L=96m~120m) through simply supported bolt-welded steel truss girder bridge.

 Continuous truss and cantilever truss bridge

Continuous truss beam

◆ Both vertical and lateral stiffness are greater than simply supported beams, and the internal force distribution is more reasonable

◆ For multi-hole bridges with a span greater than 120m, it is more reasonable to use continuous truss beams. Steel materials can save about 8-10% compared with simply supported beams.

Structural layout

◆ The number of spans per joint is two or three spans, rarely more than five spans.

◆ Two-hole continuous beam should be made of equal span.

◆ The reasonable ratio of the span of the three holes is 7:8:7. For the sake of beauty, an equal-span layout is often used.

◆ Beam height: usually 1/7~1/8 of the span, the support can be increased appropriately.

Cantilever truss

◆ Generally, the ratio of outrigger length to anchor span is 1/4~1/3

◆ The ratio of combined span to anchor span should be 1.1~1.4

◆ The height of the mid-span part of the hanging beam and anchor beam is about 1/6.5~1/7 of the span

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