Arches, Lintels, Stairs and Roofing
Arches, Lintels, Stairs and Roofing tests arch terminology and centering, lintel bearing, stair geometry, and roof types including RCC slabs, trusses and sheet roofing.
Core rules
- Arch parts include springing line, skewback, voussoir, keystone, crown and intrados; centering supports the arch until the mortar hardens.
- Semicircular arches spring from half the span; segmental arches have a rise less than half the span.
- Lintels need adequate bearing and should not be used as a substitute for a designed arch or beam.
- Stair comfort commonly follows 2R + T = 600–640 mm, with equal risers and treads in a flight.
- Roof slopes and drainage must be built in; sheet roofing needs correct purlin spacing and end laps.
Worked example
A floor-to-floor height of 3.0 m uses 150 mm risers. Number of risers = 3,000 ÷ 150 = 20. A flight has one fewer tread than risers, so 19 treads. With a 280 mm tread, going = 19 × 280 = 5,320 mm. Check comfort: 2 × 150 + 280 = 580 mm, slightly below the usual 600–640 mm guide, so a 300 mm tread gives 600 mm and a 5.70 m going.
Traps that cost marks
Unequal risers, removing arch centering too early, insufficient lintel bearing and ignoring roof slope are common. Do not design a stair with a different riser at the last step; it causes trips.
Key points
- Arch centering stays until the arch can carry itself.