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 · Plans · Proportion · Use
The forum basilica
Make a basilica between a third and a half as broad as it is long; make each aisle a third of the width of the central space, and the columns as high as the aisles are broad.
W = ⅓ to ½ L; aisle = ⅓ nave; column height = aisle width; upper columns ¼ smaller
The Roman basilica was a roofed extension of the forum: a hall for commerce and the law courts, placed on the warmest side of the square so that business could go on in winter. A central space was surrounded by aisles with galleries above, and a clerestory lit the middle. Vitruvius describes the basilica he himself designed at Fano, with columns fifty feet high rising through both storeys.
When Christians needed large meeting halls in the fourth century they adopted this type, with an apse at one end, and turned the civic hall into the church, which is why the basilica figure here is an Early Christian one.
Related rules
Order the church as a single path: a colonnaded atrium, a narthex, the long nave drawn toward the altar, and the apse framed by a triumphal arch at the end of the axis.
Edge the beds before the portico with box, clip box into figures of animals set facing one another, and in the garden beyond cut it into many shapes, even into the letters of the owner’s name.
Make the corner columns thicker than the rest by a fiftieth of their diameter, because, seen against the open sky, they look thinner than they are.
D corner = D × (1 + 1⁄50); Parthenon: 1.95 m against 1.905 m
Divide the height from floor to ceiling into three and a half parts and give two and a half to the opening; make a Doric door five and a half twelfths as wide as it is high (an Ionic one two-fifths), and narrow it slightly at the top.
h = 2½⁄3½ H; Doric w = 5½⁄12 h, Ionic w = h ÷ 2½; top narrowed by ⅓, ¼ or ⅛ of the jamb as h ≤ 16, ≤ 25, ≤ 30 ft
Architecture depends on order (ordinatio), arrangement (dispositio), eurythmy, symmetry, propriety (decor) and economy (distributio).
Cite this page Chris Paton, “The forum basilica”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/basilica-proportions. Text, drawings and models licensed under CC BY-NC 4.0.