Combinations of shells are useful and lend variety to the other shapes and forms. The number of combinations is practically unlimited so that only a few may be shown here. The first step will be to list all of the combinations previously described and indicate which types can be combined. The basic types of shells were previously classified as: 1) folded plates and domes, 2) barrel shells, 3) short shells, 4) domes of revolu@on, and 5) warped surfaces. The intersection shell has been omitted since it is really a combination. Warped surfaces do not combine very well with other types, particularly the folded plate so they will not be considered in combinations.
The combina@ons possible from the above list are: 1) barrel shells and folded plates, 2) barrel shells and short shells, 3) barrel shells and domes of revolu@on, 4) barrel shells and conoids, 5) folded plates and short shells, 6) folded plates and domes of revolu@on, 7) folded plates and conoids, 8) short shells and domes of revolu@on, 9) short shells and conoids, and 10) domes of revolu@on and conoids.
DOME AND BARREL VAULT
In the structure shown above the side of the square dome suggests the shape of a barrel vault. These are really independent structures since the structural elements are all formed before the attachment has been made and could be cut apart without destroying the structure.
Because of the form of the plan, this structure suggests a church or other auditorium with a large central area and adjacent seating.
Vaults can be attached to any of the four sides to produce a T shape or a cross shaped building and the wings may be of various lengths to suit the seating arrangement. The ties across the sides of the dome can be eliminated by L shaped walls acting as thrust abutments.
FOLDED PLATE AND BARREL VAULT
In this structure, a folded plate structure is combined with a barrel vault. For the same width of element, the transverse bending moments in the folded plates are usually larger than the barrel vault so it is important to keep the width of the plates so the slab will not be thick. The form is not especially suitable for long spans since the structural efficiency of the folded plate is not very great. However, it does provide a chance to develop an unusual form. A very interesting combination is the folded plate Z shell with the north light shell by making the upper end of the Z a smooth curve and the lower end a folded plate.
FOLDED PLATE AND CONE
The cone on the ends of the folded plate finishes off an otherwise angular structure with a curved façade and might have some architectural applications where a bay window effect is required. The cone can hardly be said to have enough stiffness to replace the rib that is otherwise required at the columns. This cone acts like a curved folded plate rather than as a dome.
A similar structure is a dome of revolution used on the end of a curved barrel shell to provide a circular rotunda.
BARREL SHELL - DOMED ENDS
It was a little difficult to say exactly where the above type should be placed in this classification of shells. Finally it was included in this chapter on the theory that it was really a combination of types. The folded plate with tapered ends was included for the same reason. Tapered ends have the advantage that the eave line can be kept the same all around the building. Less material is required because the stiffener is in domed ends. The amount of formwork is probably less but the forms cannot be moved lengthwise of the shell and must be lowered the full depth of the barrel.
A plane surface can be used instead of the curve at the end. The thickness would be greater because the bending moments are larger in the plate than in the domed shell. However, it would probably not be thicker than the vertical wall type stiffener often used at the ends of barrel shells.
FOLDED PLATE - TAPERED ENDS
This structure is a combination of a folded plate and a folded plate dome. The taper acts as a stiffener and transfers the thrust of the inclined plates to the columns. It has many of the same advantages and disadvantages of the barrel shell with dome ends shown on the previous page.
The outer form of this structure is similar to the hipped roof used for house construction and it is conceivable that folded plates might be used for an inexpensive structure for mass produced houses.
SPACE ACTION OF ARCHES AND VAULTS
This structure consists of two intersection shell domes made from parts of cones. The edges of the domes are supported by ribs acting as arches. The thrusts from these arches are transferred to horizontal girders which, in turn, carry the load to horizontal ties at each end of the building. The horizontal girders also serve as slabs over the side aisles. Two rows of columns are required to support each of these slabs.
This type of space structure is very useful for church buildings because the arches can spring from a higher level and cross ties are not needed except at the ends of the building.
SHELL SLAB
This structure is a combination of a dome of revolution and a plate. The low rise dome in the center of an otherwise orthodox flat plate will have shell action at the center of the pane; and plate action along the column lines. This should result in some savings in both steel and concrete. The flat plate elements should be prestressed so that the thrust of the dome can be fully utilized. The reduction in concrete should result in smaller column sizes for multistory buildings. The thickness of the plate at the columns can be made greater than for the ordinary flat plate structure.
BARREL AND SLAB
This shell is a combination of a barrel shell and a slab. The slab must be of sufficient thickness to carry the required bending moment and shear at the end of the span. The small columns shown on the outside edges are for support of the edges only; otherwise, there are no inside columns for this structure. The barrels also must be of sufficient depth at the center of the span for the required bending moment. Since this is half of the depth at the ends, it is evident that the spans possible with this construction are not very great. This structure would be very suitable when high windows were required to the north and a cantilever slab for sun protection to the south. Then the structural characteristics would be fully utilized.
FOLDED PLATE AND SLA
It is intended in this structure to have a center row of columns so that the greatest depth is at the point of maximum moment and the shear and bending moment at the outside edges are low. A stiffener will be required at the center row of columns. However, the slab acts as its own stiffener. The slab thickness might be somewhat thicker than the shell but as soon as the total depth of the shell became sufficient to resist the stresses, the thickness could be reduced.