Memoirs 8. Reflections

by Milo Ketchum
circa 1990

In reviewing the descriptions of structures that I had listed, I am struck by the diversity of the types of shells we designed, and the list does not include many others equally as interesting, particularly small structures with special problems, as well as the consulting we did on other structures. Some examples are:

  1. In 1960 we designed for a school in Iowa, a typical short shell with a span of 165 feet, with 6 bays of 21 ft. 4 in.
  2. A folded plate that was completely edge supported on the sides, with ties at the ends so that it could be free of internal ties.
  3. A series of precast barrel shells, 8 feet wide with spans of from 30 to 40 feet. These were used as a cover for a water collection conduit in the mountains, and the length was in the order of 4 miles. The shells were stacked so that only one form need be constructed. Many other structures were constructed using this method.
  4. A lobby for a school building that was a cone dome with a skylight in the center, with a span in the order of 60 feet.
  5. A similar folded plate dome for a school building.
  6. A automated chicken farm with many units consisting of domes of revolution about 60 feet in diameter
  7. A storage facility made with tilt up walls, and a folded plate roof, a complete box.
  8. A small cylindrical intersection shell rectangular dome, cast on an earth form, into place, and used for a small storage shed.

It seems to me that what we had to contribute, was the willingness to tackle almost any structure, and come up with a design that was satisfactory to the architectural client. At the same time we were able to influence the architectural concept and the details. It is interesting to see shell structures designed by different engineers. They all bear the mark of the engineer and his capabilities.

I have included a copy, Fig 8.1, of a review of the book by Faber on Candela's structures which gives some of my thoughts on problems of the structural engineers working for architects.

I have often been asked why shell roofs are no longer being built. First on the industrial building side, in the 1960's this type of construction was being industrialized. For example the rise of precast, prestressed products could be ordered from a catalog and the costs were well defined. Steel roof systems were being developed with long span joists that made cost estimates and design easy. They may not have been cheaper but they were not as financially experimental. Also these systems were being heavily promoted. Shells had no such promotion, and were being designed by individuals. First cost became the only criteria for selection of systems.

On the architectural side, shells were built in a time where structural expression was in vogue. The leading architects were designing on this basis. Also it took a good architect to design a shell properly. Too much structural expression will become monotonous. The real beauty of shell structures is on the inside, as any one knows who has seen them built.

Engineers were not properly trained or had enough imagination to design shell structures, and there were enough failures to make them suspect by owners. Truly I selected the proper time to get involved in a very fascinating phase of structural engineering.

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