Last reviewed 16 Sept 2026 · 10 min read
Arches
An arch is a structure built of wedge-shaped units (or monolithic concrete) spanning an opening and supporting loads above it mainly by compression, transferring them as inclined thrust to the supports (abutments or piers).
Terms
| Term | Meaning |
|---|---|
| Intrados | Inner curve (lower boundary) of the arch |
| Soffit | Inner surface of the arch (the underside surface) |
| Extrados | Outer curve (upper boundary) of the arch |
| Voussoirs | Wedge-shaped units forming the arch ring |
| Keystone | The central (topmost) voussoir at the crown — placed last |
| Springers | The lowest voussoirs at the springing points, resting on the skewback |
| Springing line | Imaginary line joining the springing points |
| Skewback (springer bed) | Inclined surface of the abutment or pier on which the arch rests (radial to the arch) |
| Crown | Highest point of the extrados |
| Span | Clear horizontal distance between supports |
| Rise | Vertical distance between the springing line and the highest point of the intrados |
| Depth (ring thickness) | Perpendicular distance between intrados and extrados |
| Haunch | Lower half of the arch between crown and springing |
| Spandrel | Space between the extrados and a horizontal line through the crown (or between adjacent arches) |
| Abutment | End support of an arch (in a wall) |
| Pier | Intermediate support between adjacent arches |
| Impost | Projecting course at the top of a pier or abutment from which the arch springs |
| Bed joints | Radial joints between voussoirs |
Classification by shape
| Arch | Features |
|---|---|
| Flat arch (jack arch) | Intrados nearly flat (often with a slight camber); extrados horizontal; skewbacks inclined about 60° forming an equilateral triangle; spans limited (small openings) |
| Segmental arch | Centre below the springing line — curve is a segment of a circle; exerts horizontal thrust; common for doors, windows |
| Semi-circular arch | Centre on the springing line; rise = half the span; thrust is nearly vertical at supports |
| Horseshoe arch | Curve more than a semicircle — horseshoe shape; architectural (Islamic architecture) |
| Pointed (Gothic) arch | Two arcs meeting at a point at the crown — equilateral (centres at springing points, radius = span), lancet (radius > span, sharp point), drop (radius < span) |
| Venetian arch | Pointed arch with greater depth at crown than at springings; four centres on the springing line |
| Florentine arch | Intrados semicircular and extrados pointed (three centres) |
| Elliptical / semi-elliptical arch | Three- or five-centred approximation of an ellipse |
| Stilted arch | Semicircular arch raised on vertical portions above the springing line |
| Relieving arch | Arch built above a lintel or flat arch to relieve it of the load of masonry above |
| Parabolic arch | Parabolic shape — efficient for uniformly distributed loads |
Classification by number of centres
One-centred (segmental, semi-circular, horseshoe, flat), two-centred (pointed), three-centred (elliptical, Florentine), four-centred (Venetian, Tudor), five-centred (semi-elliptical).
Classification by material and workmanship
- Stone arches — rubble arches (roughly dressed stones, rough work) and ashlar arches (finely dressed voussoirs).
- Brick arches — rough brick arches (ordinary bricks, wedge-shaped joints — thicker at extrados), axed/gauged brick arches (bricks cut or rubbed to wedge shape — fine joints, better appearance), purpose-made brick arches (moulded wedge bricks).
- Concrete arches — plain, precast or reinforced concrete (monolithic) arches — larger spans, bridges, shells.
Stability of arches
An arch carries loads by compression along its ring and exerts horizontal thrust at the supports.
Modes of failure
| Failure | Cause / prevention |
|---|---|
| Crushing of voussoirs or mortar | Excessive compressive stress — use strong materials and adequate ring thickness |
| Sliding of voussoirs along joints | Line of thrust inclined too much to joints — keep joints radial, use proper mortar |
| Rotation (hinging / overturning) of voussoirs about their edges | Line of thrust passing outside the middle third — adjust shape, add load (spandrel filling) |
| Uneven settlement or yielding of abutments | Weak or insufficient abutments cannot resist horizontal thrust — design adequate abutments and piers, ties |
Line of thrust
- The line of thrust (line of pressure) should lie within the middle third of the arch ring at every section so that no tension develops (the middle third rule), and within the kern of abutments and piers.
- For a parabolic arch under uniformly distributed load, the line of thrust follows the arch axis.
FormulaHorizontal thrust (parabolic/segmental arch approximation, UDL)
= uniformly distributed load per unit horizontal length; = span; = rise. Flatter arches (smaller rise) produce larger thrust.
FormulaRadius of a segmental arch
Construction of arches
- Centering — temporary formwork (timber or steel) supporting the arch until it becomes self-supporting; consists of ribs (templates of intrados shape), laggings (boards) and props with folding wedges (for easy striking).
- Laying voussoirs — begin from both springings simultaneously, working towards the crown, keeping joints radial and fully filled with mortar.
- Keystone placed last, tightly fitted (wedging action).
- Spandrel filling and walls above are built symmetrically.
- Striking (easing) the centering — after the mortar has gained sufficient strength (commonly after one to two weeks for ordinary work, depending on mortar and span), by gradually loosening the wedges so that the arch settles uniformly; for relieving arches, the lintel or core is removed after the arch is effective.
Lintels
A lintel is a horizontal beam placed over an opening (door, window, cupboard) to support the masonry and loads above.
Types
| Lintel | Features |
|---|---|
| Timber lintel | Easy to fix; used in hilly/timber-rich areas and small spans; susceptible to fire, decay and termites; may be strengthened by steel |
| Stone lintel | Single stone slabs; limited to small spans (stone is weak in tension); heavy; common where good stone is available |
| Brick lintel | Bricks laid on end or edge (soldier course), sometimes with a key; only for small openings (commonly up to about 1 m) |
| Reinforced brick lintel | Bricks with reinforcement in mortar joints (vertical joints) — moderate spans |
| Steel lintel | Rolled steel angles, channels or I-sections (singly or in pairs, sometimes encased in concrete) — large openings, shopfronts; protect from corrosion |
| Reinforced cement concrete (RCC) lintel | Most common — cast in situ or precast; fire resistant, durable, any span; can be combined with chajja (sunshade) and lintel bands |
Bearing: lintels should have adequate bearing on the supporting walls at each end — commonly at least 150 mm or the depth of the lintel, whichever is greater (more for larger spans).
Load on lintels (arching action)
When there is sufficient height of masonry above the opening, the masonry forms a natural arch, and the lintel carries only the weight of masonry within an equilateral triangle with its base equal to the effective span (60° dispersion).
FormulaLoad on a lintel (triangular load)
- Area of equilateral triangle on span :
- Total load ( = wall thickness, = unit weight of masonry)
- Maximum bending moment for this triangular load (peak at mid-span):
- Self-weight of lintel (UDL ):
If the masonry height above is insufficient, or there are openings, floor loads or concentrated loads within the triangle, the full loads are considered.
Lintel bands and chajjas
- Lintel band — a continuous RCC band at lintel level along all walls — ties walls together, improves earthquake resistance (IS 4326) and acts as a lintel over openings.
- Chajja (sunshade) — cantilever slab projecting from the lintel over windows and doors to protect from sun and rain; designed as a cantilever with reinforcement at the top, anchored into the lintel/band.