What is a jack arch floor?
A jack arch floor is a floor made of a series of shallow, segmental brick (or later concrete) arches spanning between parallel beams of timber, cast iron, wrought iron or steel. The tops are filled and levelled to form the floor surface, while from below you see a ribbed, gently curved ceiling with the beam flanges in between. In Dutch it is called a troggewelf, in German a Preußische Kappendecke. Note that the term “trough vault” usually refers to a different form (see below).
How it works
Each little arch carries its load in compression to the beams on either side. The rise is small compared with the span, which keeps the floor shallow but produces a significant horizontal thrust.
Between two arches the thrusts largely cancel out. In the end bays, however, the last arch pushes outwards against the external wall. That is why tie rods are often fitted across the beams to hold the bays together. Without them, cracks can open along the crown.
Jack arch floor or trough vault?
A trough vault is a single large vault over a rectangular room, curving up from all four sides to a straight ridge. A jack arch spans only between two parallel supports and is repeated in a row. A jack arch in a wall opening is something else again: a flat arch over a door or window.
Uses
- Basements and cellars in 19th and early 20th century buildings
- Mills, warehouses and factories, where brick arches between iron beams were used as “fireproof” construction instead of timber floors
- Stables and farm buildings
Practical tips
- Get a structural assessment before adding heavy loads (screeds, bathrooms) or cutting an opening.
- Check the beams for corrosion: rusting flanges expand and can push the brickwork apart. Beams can often be cleaned, coated or supplemented.
- Never remove tie rods without engineering advice.
- Fire protection: exposed beam flanges need adequate protection, for example plaster.
Related terms
- Trough vault
- Barrel vault
- Segmental arch
- Tension rod
- Rolled steel beam
- Basement
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