Depressed arch

A depressed arch, basık kemer in Turkish, rises less than half its span, so it looks lower and flatter than a round arch. It may be a segment of a circle smaller than a semicircle, or made of arcs drawn from several centres. It is common over windows and doors, in low passages and wherever storey height is limited.

How the load travels

The flatter an arch, the greater the horizontal thrust it produces as it carries load to either side. A depressed arch therefore needs strong abutments or enough wall mass beside it. Over a window that is usually there anyway, since the wall on each side resists the thrust. Where a row of depressed arches runs side by side, the thrust at the end bays has to be handled with tie rods or heavy corner piers.

• Segmental arch: a single-centred arc smaller than a semicircle
• Three-centred arch: small radii at the sides, a large one in the middle
• Flat arch: wedge-shaped stones forming an apparently level lintel

The depressed arch in Turkish architecture

In Ottoman buildings the lower row of windows is often spanned by a depressed arch, sometimes with a pediment or a relieving arch above. It is also common over doors and windows in traditional houses.

In survey and restoration

Being sensitive to thrust, depressed arches often show damage in historic buildings. A keystone that has slipped downward or joints opening between voussoirs are watched closely in a damage assessment. Measure span, rise and arch depth, and record the number of voussoirs and the direction of their joints. A 3D model built from these data also lets a restoration project describe replacement stones accurately.