Spandrel fill is the material used to fill the triangular or irregular space between the curved extrados (outer surface) of an arch and the level surface above it, such as a road deck or floor. It serves both a structural and practical purpose by transmitting loads to the arch while providing a flat surface for use.
Application
Spandrel fill is a critical component in arch bridges, vaulted floors, and historic masonry structures. Its design affects the load distribution, drainage, and long-term performance of the arch below:
- Load distribution: The fill spreads concentrated loads (such as vehicle wheels) over a wider area of the arch. This reduces local stress concentrations and allows the arch to work as intended — primarily in compression.
- Materials used: Traditional spandrel fill consists of rubble stone, compacted earth, gravel, or a combination thereof. In modern construction, lightweight concrete, foamed concrete, or granular fill may be used to reduce dead weight on the arch.
- Drainage: Proper drainage within the spandrel fill is essential to prevent water accumulation, which adds weight and can cause frost damage or deterioration of the arch masonry. Drainage layers or weep holes are incorporated into well-designed spandrel fills.
- Spandrel walls: In masonry arch bridges, the fill is contained laterally by spandrel walls that run along the edges of the bridge. These walls also support the parapet above.
- Assessment and repair: In historic bridge maintenance, the condition of the spandrel fill is a key concern. Voids, settlement, or waterlogged fill can lead to uneven loading and eventual structural distress in the arch.
- Open spandrel alternative: Some bridges use open spandrels — where the space above the arch is bridged by columns or smaller arches rather than filled material — to reduce dead weight on large-span arches.
The depth of spandrel fill varies from zero at the crown of the arch to its maximum at the springings, following the geometry of the curve.
Related terms
- Spandrel wall
- Extrados
- Arch bridge
- Voussoir
- Haunching
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