Light weight in buildings is not necessarily a panacea to the building industry. It can be advantageous when the saving in weight results in some functional or economic gain. When a reduction in weight is required it can be achieved by either one or both of two methods: first, by the use of material in a more efficient manner; and second, by the use of materials with high strength-to-density ratios. Structural sandwiches for buildings can be an example of both of these methods, but usually only the first is used because of the higher cost of high strength materials.

Structural sandwiches are a logical choice when designing lightweight buildings and they

are being use quite extensively in the U.S. at the present time. Several of the sandwiches being used were developed by the aerospace industry. This industry requires very high strength-to-weight ratios in their structural components. Structural sandwiches with honeycomb cores and high strength facings (see fig. 1) provide the highest flexural rigidities with very low weights.

The honeycomb cores for aircraft and space vehicle components are being made from a variety of materials, such as paper, plastic, or even titanium. The facings used on these sandwiches vary from materials such as aluminum to graphite or boron fiber reinforced plastics.

Table 1 is a listing of some of the sandwiches, which are being used or are being proposed for use in buildings at the present time. It should be noted that most of these include paper honeycomb as a core material.

The plastic foam listed here is not used widely in structural sandwiches because of the low shear strength of most of these foams. When used as a core, the plastic foam is relied on only to provide lateral stiffening for the facing and auxiliary structural members are attached to the facings.

Figures 2 and 3 illustrate two structures which are being produced in the U.S. and which utilize sandwich panel material. The building shown in figure 2 was constructed in our laboratory using aluminum-faced, paper honeycomb panels 76mm (3 inches) thick. These panels, which were used in walls, roof, and floors, were connected together by means of sheet metal screws and aluminum extrusions.

The building shown in figure 3 is constructed of sandwich panels using paper honeycomb cores and steel facings. This building is a single-story, single-family house prefabricated by one of the large corporations in the U.S. The complete building is designed to be set up in a matter of hours on concrete piers.

Honeycombs suitable for use as cores in sandwich panels are being produced from many different materials but essentially from those materials which can be formed into thin sheets. Some of the materials now being used for honeycomb are: kraft paper, nylon paper, polyethylene, fiberglass (and other fibers) reinforced plastics, aluminum, stainless steel, and titanium.

With each material, the most frequently used honeycomb has a hexagonal configuration (see fig. 4), but other configurations are being made. Some of these other configurations are designed for applications where special qualities such as double curvature are required.

The structural sandwich panels for buildings are almost always made with kraft paper honeycombs. Kraft paper is available in many different weights and thicknesses and is low in cost in comparison with other materials used for honeycomb.

The kraft paper is usually modified by impregnation with a phenolic resin. The phenolic resin improves both the dry and wet strength, and imparts fungus resistance to the paper. Other treatments and resins are sometimes used to impart special properties to the honeycomb.

The mechanical properties and cost of kraft paper honeycomb vary with the weight of the paper, amount and type of resin impregnation, and cell size. The relative costs of some typical honeycombs are given in table 2. In general, the mechanical properties can be estimated from the unit weight of the expanded honeycomb.

The curves of figures 5 and 6 indicate the effect of the unit weight of some typical honey