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)
Rules · Doors & windows · Ornament · Proportion
The architrave a sixth of the light
Make the architrave round a door or window as wide as a sixth part of the opening’s breadth, and dress the openings of the principal floor more richly than those of the storeys above and below, with a frieze and cornice, and on the grandest a pediment.
architrave = ⅙ × width of the light
Palladio gives the ornaments of doors and windows after the orders: an architrave round the opening, then, where there is room, a frieze and a cornice over it, the members in the proportions of the entablature they imitate. He makes the architrave as thick as the jambs, which are to be no less than a sixth of the breadth of the opening and no more than a fifth (I.25–26); the designer takes the sixth.
The rank of the storey decides how much of this a window is given: the principal floor, where the best rooms are, has the full dress, the storeys above and below a plain architrave or none.
Related rules
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
To unify a façade of two or more storeys, run a single order of pilasters or columns through all of them, from the base to the main cornice.
Divide the inside of a dome into coffers that shrink in step with the meridians as they rise, so that their perspective deepens the dome and draws the eye to the light of the lantern.
Proportion every step so that twice its height plus its depth equals the length of a natural pace on the level: two feet, or 24 pouces.
2 r + g = 24 pouces ≈ 65 cm (modern practice: 60–66 cm)
Hold that the beauty of the orders comes from fixed proportions that please by nature, as consonances do in music; where the ancients differ, the differences answer to circumstances such as distance and height.
Cite this page Chris Paton, “The architrave a sixth of the light”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/palladio-window-ornaments. Text, drawings and models licensed under CC BY-NC 4.0.