Rules · Structure · Walls · Materials
In a timber-framed house, let the floor joists of an upper storey run out beyond the wall below and carry the upper wall on their ends: the upper storey oversails the lower by a foot or two, gaining room upstairs and sheltering the wall beneath.
The jetty belongs to timber building. The joists, laid across the house, rest on the wall below and run out past it; a beam along their ends (the bressumer) carries the frame of the storey above. The weight of the upper wall on the cantilevered ends helps to stiffen the floor, and on a narrow town plot the jetty added to the floor of the rooms above without taking any of the street.
Jetties were built on one side, on two (with a diagonal dragon beam at the corner) or, in towns, storey upon storey, so that the top floors of a street almost met. In the Wealden house the ends of the house were jettied and the hall between them was not, so that the front stepped back under one long roof.
Build an arch of wedge-shaped stones (voussoirs) whose joints all point towards the centre of the curve: each stone can then fall only by forcing its neighbours apart, and so the ring holds itself up once the last stone, the keystone, is in place.
joints ⟂ intrados (radial to the centre)
An arch pushes outward on its supports as well as down. For the same load and span the horizontal push grows as the rise falls: halve the rise and you roughly double the thrust.
H ≈ w·s² ⁄ 8f (uniform load w per metre, span s, rise f)
A chain hanging freely takes the one shape in which it is in pure tension; turn that shape upside down and it is the shape of an arch in pure compression for the same loads.
free-hanging chain (catenary) ↔ ideal arch under its own weight
Trace the path of the resultant force through the stones from crown to foundations. If a line can be found that stays everywhere within the masonry, the arch will stand; where it touches an edge, a hinge will open, and four hinges make it collapse.
To find how thick the pier of an arch must be, divide the intrados into three equal parts; draw a line from the springing through the first division, and produce it beyond the springing by its own length. A vertical through the end of that line marks the outer face of the pier.
pier ≈ ¼ span (semicircle) · ≈ 0.23 span (equilateral) · less for a lancet
An arch cannot support itself until it is complete. Build it on a timber centring shaped to the intrados, strike the centring only when the keystone is set and the mortar has taken, and reuse it from bay to bay, which is one reason to keep the spans of an arcade equal.
Cite this page Chris Paton, “The jettied storey”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/jettied-storey. Text, drawings and models licensed under CC BY-NC 4.0.