What is a rib in construction?
A rib is a long, relatively narrow member of square or rectangular cross-section that stiffens or carries a structure. In building the word has three common meanings, and you have to read the context to know which one is meant.
1. The rib as a timber size
In the timber trade a rib is a sawn section smaller than a beam and heavier than a batten. Common sizes run roughly from 38 x 38 mm up to 75 x 150 mm. Ribs are used for studwork, formwork bearers, decking sub-frames and building up timber-framed walls.
Practical tip: for external work buy timber of a suitable durability class or pressure-treated, and always re-treat cut ends. For structural work check the strength class stamped on the timber, such as C16 or C24.
2. The rib in a floor or roof structure
In a ribbed floor or ribbed cassette floor, the ribs are the thickened, load-bearing strips of concrete between thinner panels. The same principle appears in profiled steel decking: material sits where the bending moment is greatest, which keeps the structure light over a long span.
3. The rib in a vault
In vaulting, a rib (vault rib) is a projecting, arched band of stone or brick running along the intersection lines of a vault. It springs from a column, a corbel or the wall itself and meets at a boss or keystone at the crown. The ribs form the skeleton; the thinner surfaces between them are the webs or cells.
4. One more meaning
In heating, an individual section of a column radiator is also called a rib.
Related terms
- Vault rib
- Beam
- Coffered ceiling
- Formwork
- Reinforcement
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