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 · Cities · Proportion · Plans
The forum: two parts broad to three long
Size the forum to the population, neither cramped nor deserted, and make its breadth two-thirds of its length, so that the oblong suits the shows held in it.
breadth : length = 2 : 3
Vitruvius contrasts the square Greek agora, ringed with double colonnades, with the Italian forum, which was also the arena for gladiatorial shows: hence its oblong shape, the wide intercolumniations of its porticoes, and the bankers’ offices and upper balconies he wants round it. In a coastal town the forum stands by the harbour; inland, at the centre, where the two main streets cross.
Related rules
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).
Lay out a city’s streets so that they do not line up with the directions of the principal winds, which otherwise rush down them cold, violent and unhealthy.
Cite this page Chris Paton, “The forum: two parts broad to three long”, The Textbook of Architecture, https://textbookofarchitecture.com/rule/vitruvius-forum. Text, drawings and models licensed under CC BY-NC 4.0.