EDGE MEMBERS
The above sketch is an inside view of a multiple barrel with edge beams at the junctions of the cylinders. The reason for using edge beams is that if a long span is required, say 100 feet or more, the depth of the shell would have to be at least 10 feet, and for a normal column spacing of 20 feet, a full half cylinder having vertical slopes at the edges of the shell would be necessary. Concrete is difficult to place, and, therefore, edge beams should be provided so the radius of the shell is greater and the slope at the top of the edge is not more than 40 degrees. For example, for the 20 foot wide shell, the depth of the edge members would be about 6 ft to keep the slope to 40 degrees.
Barrel shells with edge members lend themselves to prestressing by the post tensioning method. The cables are placed in the edge members and are tightened on the ends of the building. If the barrel is given camber and the prestressing cables are straight instead, the distribution of stresses is more satisfactory.