86. These are usually formed of a number of ribs or semi-trusses resting on a circular base and connected to a circular ring, plate or pole at the top. Each rib must possess sufficient stiffness to resist the load upon it, including its own weight, without suffering distortion. The outward thrust of the rib, which is comparatively small in a hemispherical dome, is taken up by a plate forming a ring about the base of the dome, and to which the ribs are attached. The sheathing of the dome, if the frame is of wood, or the angle purlins of a steel dome, also assist in taking up the outward thrust and in preventing the ribs from bulging outward. Occasionally domed roofs are supported by interior vertical supports, in addition to the outer support; under such conditions the ribs are merely curved rafters, supported by curved purlins, and the mechanical principles involved are very simple.