A relieving arch is an arch built into a wall above an opening, which carries the weight of the masonry above around the opening and down into the piers on either side. This relieves the lintel or frame below and prevents cracking above the opening. It is also called a discharging arch; the Dutch term is ontlastingsboog.
How it works
Masonry is strong in compression but cannot take tension. Above an opening there is no support, so the masonry would try to bend — and therefore crack. An arch resolves this by converting the load into inclined compressive forces:
- The weight above the arch bears on the extrados
- The force travels along the arch through the wedge-shaped units
- At the springing, the force arrives at the masonry at an angle
From which follows the key point: an arch generates thrust. There must be enough masonry either side to resist that sideways force. Immediately beside a window opening in a narrow pier, or close to a building corner, there often is not — and then the arch does not behave as intended.
Where it is used
- In historic masonry the relieving arch is the standard solution above windows and doors, often expressed in the facade and sometimes built as decorative work
- Above a timber lintel or a beam end — the arch takes the load so the timber does not deflect and can be replaced later
- In combination with a lintel: a straight lintel below carries the strip of masonry between arch and opening. The masonry in that triangle is the tympanum or arch panel
- In alterations: where an opening has been enlarged later, or the load path has been disturbed, a steel lintel is often installed beneath the existing arch
Related constructions
- A flat or jack arch has an almost level top face, with the units cut as wedges to a common centre — the same principle, but much flatter and therefore generating far greater thrust
- A soldier course above an opening looks like an arch but only carries load if the units are genuinely cut as wedges
Warning signs
- A vertical crack above the centre of the opening points to insufficient relief
- Stepped cracks running from the springing indicate insufficient resistance to thrust
- A dropped keystone means the arch is behaving as a row of loose units; have this assessed by a structural engineer before any cutting or demolition beneath it
Related terms
- Lintel
- Flat arch
- Arch
- Keystone
- Pier
- Soldier course
- Masonry
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