The primary force acting within an arch bridge is compression, which is then transferred to the abutments or foundations as both vertical and horizontal forces.
Here's a breakdown of the forces at play:
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Compression: The arch shape is inherently strong under compression. As a load is applied to the arch, it is squeezed together. This compressive force travels along the curve of the arch down to the supports.
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Thrust (Horizontal Force): Because the arch is curved, the compressive force also generates a horizontal force pushing outwards at the base of the arch. This horizontal force is known as thrust. The abutments or foundations of the bridge must be strong enough to resist this thrust and prevent the arch from spreading apart.
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Vertical Force: The weight of the bridge and any load on it is also transferred vertically downwards to the foundations. This vertical force acts in conjunction with the horizontal thrust.
Here's a table summarizing the forces:
| Force | Direction | Description |
|---|---|---|
| Compression | Along the curve of the arch | Squeezing force resulting from the load and the arch's shape. |
| Thrust | Horizontal, outward at the base of the arch | Horizontal component of the compressive force, pushing against the supports. |
| Vertical | Downward at the base of the arch | Vertical component of the compressive force, due to the load and bridge weight. |
Because of the thrust forces, arch bridge foundations require careful design and construction to prevent:
- Vertical Settling: The foundations must be able to bear the vertical load without sinking.
- Horizontal Sliding: The foundations must be able to resist the horizontal thrust and prevent the arch from spreading.
In summary, arch bridges utilize the principle of compression to efficiently carry loads. The compressive forces are transferred to the foundations as both vertical and horizontal forces (thrust), requiring robust abutments to maintain the arch's structural integrity.
