1.4. Structural Considerations

Structural considerations play an important role in the proper design of copper applications. They affect the spacing of cleats and other fasteners, location and design of expansion seams, and the configuration of other joints. The requirements may be calculated with the same formulas used in the structural analysis of other materials, such as steel and wood. Table 1.1A contains information that is useful in these calculations.

Although there are other structural concerns, the primary focus is upon thermal effects. Movement and stresses related to temperature variations must be accommodated. There are two fundamental methods to do this: prevent the movement and resist the cumulative stresses within the copper; or allow movement at predetermined locations, thereby relieving thermal stresses.

In a limited number of applications, such as gravel stops, base flashings at built-up roofs, and flashings around windows and doors, it is possible and often desirable to prevent movement. These tend to involve narrow copper strips that do not cover large areas. The strips should be nailed frequently to effectively transfer stresses to the underlying substrate before causing buckling in the copper. Nails spaced 3 inches maximum on center in a staggered pattern are recommended.

In most other situations it is impractical and undesirable to restrain a copper section so frequently.

In the interest of watertightness, puncturing the copper membrane should be avoided. It is also better, where possible, to allow the copper to move in order to reduce the chance of metal fatigue.

The yield strength of copper is the same for compression as it is for tension. Since buckling is likely to occur when relatively thin sections of sheet copper are in compression, sections should be designed to resist compressive loads. The compressive strength in sheet copper is the product of two factors: the thickness of the copper and the shape of the section. These factors create "columnar strength" which resists accumulated stresses up to a certain length. Beyond this point, expansion joints must be introduced to prevent buckling.

Once an expansion joint is introduced, the section no longer has to resist the cumulative stresses caused by thermal expansion along its whole length. It need only keep its shape and resist the friction between itself, its fasteners, and adjacent materials. The structural design of sheet copper, for the most part, involves the determination of expansion joint spacing. Each section of copper is designed as a "column" capable of transmitting stresses from a fixed point to the free end or expansion joint.

Table 10B shows the maximum allowable column length for "U"-shaped sections. This is the section most frequently used for such applications as gutters, gutter linings, and fascias. The requirements for standing and batten seam roofs can also be determined, since their pans derive their strength from the flanges that form the longitudinal seams.