The usual form for domes is that of the sphere; the base circular. When the interior dome does not rise too high, a horizontal tie may be thrown across, by which any degree of strength required may be obtained.
* The lengths and bevils of rafters for roof-valleys can also be found by the above process.
† See also Art. 68.
Fig. 93 shows a section, and Fig. 94 the plan, of a dome of this kind, a b being the tie-beam in both. Two trusses of this kind (Fig. 93), parallel to each other, are to be placed one on each side of the opening in the top of the dome. Upon these the whole framework is to depend for support, and their strength must be calculated accordingly. (See Arts. 70 to 80 and 214 to 222.) If the dome is large and of importance, two other trusses may be introduced at right angles to the foregoing, the tie-beams being preserved in one continuous length by framing them high enough to pass over the others.