Barrel with Unstiffened Edges


The thin unstiffened edge of a barrel vault is very dramatic but is subject to considerable deflection if made too thin or used on too long a span. Also, more longitudinal steel is required because the downward deflection of the free edge tends to throw more load into the reinforcing. If there are windows along the sides, this deflection could be very troublesome. If it is necessary to use an unstiffened shell with windows at the edge, the mullions should be made structural so the shell becomes an edge supported vault. This type is described later.

An elliptical cross section is superior to a circular section at an unstiffened edge because of the greater curvature of the ellipse, making the shell stiffer at the lower edge. Also, the shell with a complete half circle will be more rigid than one with only a segment of a circle.

The end stiffeners in this example are rigid frames which give a much lighter appearance than the girder shown in the previous example. The columns must be large however, because of the high thrust from the vault. A steel tie disguised as a thin horizontal window mullion will overcome some of this disadvantage.