Steel bridges

STEEL BRIDGES

Steel Bridges are provided with steel beam cross sections for longer spans and heavier loads. Deeper and stronger sections in steel can be built up by riveting or welding together a combination of steel plates and steel angles. This is known as Plate Girder. Steel bridges are used for a wide range of spans. They are constructed in highways, railways, etc. Various types of steel bridges are:

1. Steel Truss Bridges

2. Steel Arch Bridges

3. Steel Rigid Frame Bridges

4. Steel Bow String Girder Bridges

5. Suspension Bridges

6. Movable Steel Bridges

 

1. STEEL TRUSS BRIDGES

A Steel Truss Bridge consists of two trusses – one on either side of a road way with floor members placed between them. These type of bridges are used economically in the span range of 100 to 200 m.  Advantages are:

(i) Simple Steel Sections: Primary forces in its members are axial forces. Therefore, simple steel sections are enough.

(ii) Light Weight Members: The construction and erection of a steel truss bridge are considerably simplified due to the light weight of its members.

(iii) Wind Pressure: These bridges are not affected by wind pressure. Therefore, they are used for long railway bridges, highway bridges, water pipes, gas or oil pipes, etc.

Types of Steel Truss Bridges

See Fig. 9. Steel truss bridges are supported on Abutments on either side by means of bearings. The chord on the top surface of the truss is known as Top Chord. The chord on the bottom surface is known as Bottom Chord. Various types of steel truss bridges are:

(i) N-Truss Bridge [Fig. (i)]

(ii) K-Truss Bridge [Fig. (ii)]

(iii) Through Warren Truss Bridge (Fig. (iii)]

(iv) Warren Low Truss Bridge with Stringers [Fig. (iv)]

(v) Curved Chord Warren Truss Bridge [Fig. (v)]

(vi) Dick Warren Truss Bridge [Fig. (vi)]

(vii) Pratt Low Truss Bridge [Fig. (vii)]

(viii) Curved Chord Pratt Truss Bridge [Fig. (viii)]

 

2. STEEL ARCH BRIDGES

Steel Arch Bridges are used for very long single spans to a maximum of 150 meters. These are constructed in places where it is not possible to construct intermediate piers. Steel arches may be either of the Trusses Arch Type or Spandrel Brazed Arch Type. They consist of trusses or plate girders used in the form of curved beams, called Arch Ribs. They may be fixed, two hinged or three hinged.

See Fig. 10. Types of steel arch bridges are: (i) Truss Type Arch, (ii) Two Hinged Steel Arch (iii) & (iv) Open Spandrel Steel Arches

 

3. STEEL RIGID FRAME BRIDGE

See Fig. 11. In this type, Steel Rigid Portal Frames are used for the bridge construction and hence the name Steel Rigid Frame Bridge. Both the superstructure and substructure are built together to form a monolithic Steel Portal Frame.

The portals may be single or multiple of plate girders. These bridges have a number of parallel girders connected to the supporting Piers and Abutments. They are spaced along the width of the bridge. Flooring (Road Way) is supported over the portal frame.

Advantages

•  This type of bridge is well suited where the distance between the two road ways is restricted and where the cost of excavation for the abutments is high.

•  Bearings are not required.

• This bridge has more clearance below it due to thin steel frames.

• Abutments are thin in cross section. Being a steel structure, heavy abutments are not required. Hence, they cause the least obstruction and more clearance below the bridge.

[Compare the construction of Rigid Frame R.C.C. Bridge (Fig. 8 in Section 3.7) with the above Steel Rigid frame bridge.]

 

4. STEEL BOW-STRING GIRDER BRIDGE

See Fig. 12. In this type, Hangers or Suspenders are provided from the Arch Rib to carry Road Way.

A Steel Girder Tie is provided, joining the two ends of the arch. Tie resists the horizontal thrust developed. Major portion of the load is supported by the tie. This results in less thrust on the Abutments. Therefore, thin abutments are sufficient.

[Compare the construction of R.C.C. Bow-String Girder Bridge (Fig. 4 in Section 3.3) with the above Steel Bow-String Girder Bridge.]

 

5. SUSPENSION BRIDGE

See Fig. 13. The suspension form of a bridge is like an arch turned upside down. A flexible Cable is freely hung from two high Towers. The Load (Road Way) is suspended from two main cables on either side of the road way by means of Suspenders. This represents a Suspension Bridge.

Bridge builders discovered that a level-way could be obtained by suspending a stiffening girder from the cable on either side of it. To do this, hangers or suspenders are used.

The suspension bridge has the following components: Two flexible cables, Back stay, Towers or piers, Suspenders, Stiffening girder, Cable anchors, etc. The cable should be made of high tensile steel. The tower is built of steel and concrete.

Suspension Bridge is suspended on two Cables on each side of the Road Way. The cables are anchored at the ends by means of Cable Anchors as shown.

For heavy loads, stiffening is required to prevent vertical distortions. For this, Stiffening Girder is provided at the road way. Stiffening Girder is an all-welded steel stiffened box girder. Sometimes, two Side Spans are also added in addition to the main central span. The side spans are suspended from the Back Stays.

Advantages

•  Suspension Bridges are used in places where it is difficult to adopt other types of bridges.

•  They are economically used for long spans of more than 500 meters. Generally, they are single span bridges. The world's longest span bridge at present is the Akashi Kaikyo Bridge in Japan. It is a suspension bridge with a main span of 1990 meters.

• Suspension bridge is much lighter than any other bridge.

 

6. MOVABLE STEEL BRIDGES

A Movable Bridge is one that can be changed in position so as to open or clear the passage for ships, boats, vessels, streams, etc.

Movable steel bridges are used over the navigation channels where permanent and sufficient clear water way cannot be provided. They provide a passage of necessary width and clearance for the ships, vessels, steamers or boats.

See Fig. 14. It shows a movable steel bridge, called Bascule Bridge. Two bascules are provided just below the road way. They are supported by thin Abutments.

The bascules swing vertically upwards on a horizontal axis, just as a trap door. Thus, necessary width is provided for the ships etc., to pass through.

Movable steel bridges may have a Single Bascule or Double Bascule (as shown above). The rail track between Rameshwaram Island and Mandapam are connected by a movable double bascule bridge.