A conoidal surface, as shown in the sketch above, is formed by drawing straight lines between a curve such as a circle and a straight line. It is a ruled surface because it can be formed by straight lines. A cylinder and a cone also are ruled surfaces but a sphere is not in this category. In the above cases, the shell is supported by a wall or a beam at the left and by an arch at the right.
The appearance of the roof of the typical steel bow string truss building can be reproduced in a concrete thin shell construction by using short shells for the middle bays and conoids for the ends.
This structure is suitable for a large entrance canopy. The horizontal line at the rear can be the second floor level, the curved arch the entrance to the canopy.