Understanding the Difference Between Arch Bridges and Girder Bridges

Just have a look at a picture of an arch bridge and compare it to a picture of a girder bridge. Do you see the difference? If not, even a written description may not help. The answer is in the question itself. The difference lies between an arch, a curved structure, and a beam, a straight structure.

Let's delve deeper into the structure of each type:

Arch Bridges

Arch bridges, like the Golden Gate Bridge in San Francisco, are supported by arches. These arches carry the compressive forces to their supports and the reactions back onto the abutments (end supports). The shape of an arch allows it to distribute loads laterally, which makes it effective for high spans and heavy loads.

Girder Bridges

Girder bridges, such as steel plate girder bridges, are supported by horizontal beams called girders. Girders carry loads primarily through bending. They are typically composed of a top flange (which carries load into the web), a bottom flange, and a vertical channel or web that distributes the load to the supports.

Truss Bridges

Truss bridges, while they share some similarities with girder bridges, primarily consist of interconnected triangles. Trusses are designed to carry loads in a specific direction (axial) and are less effective when subjected to moments (bending). Each member of a truss bridge is connected at nodes or joints, and ideally, it should be free to rotate. However, if rotation is restrained, secondary stresses are introduced.

Key Differences in Load Distribution

Arch Bridge: In an arch bridge, loads are transferred to the abutments and piers through compressive forces. The shape of the arch ensures that these forces are distributed laterally, reducing the load on the deck or roadway.

Girder Bridge: In a girder bridge, the primary load is carried by the girders, which are subjected to bending moments due to the vertical loads they support. The top flange deals with compression, while the bottom flange handles tension. The web (or vertical members) is responsible for distributing the shear forces.

Truss Bridge: In a truss bridge, the members are arranged to form triangular shapes, primarily acting under axial stresses. The top chords usually carry compression, the bottom chords tension, and the diagonals shear force. This structure is efficient for spanning large distances, but it's important to ensure that the members can handle the axial forces without excessive strain.

Structural Components

Truss Bridge Components: Top Chord: Takes axial compression Bottom Chord: Takes axial tension Vertical and Diagonal Members: Takes shear force in the form of axial tension and compression

The top and bottom chords, along with the vertical and diagonal members, are designed to work in unison to transfer loads along the bridge.

Girder Bridge Components: Top Flange: Takes bending compression Bottom Flange: Takes bending tension Solid Web: Takes shear force

For steel plate girders, web stiffeners are often provided to prevent buckling, especially near the supports.

Conclusion

Both arch and girder bridges play crucial roles in infrastructure. Arch bridges excel in providing aesthetic appeal and spanning large distances, while girder bridges offer versatility and are adaptable to various load conditions. Each bridge type has its unique structural features and advantages.

Explore more detailed diagrams and designs for each type of bridge by following the links for more information.