The ellipse shown in (a) provides a dome with a low rise and a vertical tangent at the support so that a separate thrust ring is not required. This form may have esthetic advantages over the circle but the vertical element requires top forms or placing of the concrete by the shotcrete method.
In (b) a cone is used as a dome. This form does not have as much curvature as the sphere and, therefore, the stresses and deflections may be higher. Also, a heavier thrust ring is necessary for the same height of shell.
The curve in (c) illustrates the principle that a dome of revolution may be formed by the rotation of any curve about a vertical axis. The radius of the curve need not be on that axis.
The last example (d) is a dome formed by a segment of a circle for which the center of the circle is not on the axis of rotation and a dimple is created at the top. A vertical column may be used to support the center of the dome at the low point if the span is very long.