Calculations for Building Exterior Loads

Calculation Factors

• Exterior load-bearing calculations take many forms and rely on a number of factors. In general, builders need to know how much weight a given material or structure can support. This is combined with a weight estimate for higher parts of the building, the eventual building contents and pressure from weather that will place a load on the lower structure to determine whether it is adequate for the application. For example, a home's foundation must be strong enough to bear a load comprising the weight of the home itself, the weight of a family's possessions, appliances and fixtures, and the weight or pressure placed on the exterior of the home by snow, rain and wind.

Distributed and Point Loads

• Engineers measure a building's exterior load in a number of ways. Two major categories for loads are distributed loads and point loads. A distributed load refers to the weight of a load on a large surface, such as a poured concrete foundation or the square footage of a roof. A point load refers to the specific load on a single element, such as a front porch column. Both types of loads require calculations involving the strength and pliability of the material that bears the load.

Load Transference

• Exterior load transference refers to the fact that a home's structure transfers weight to lower structural components. Eventually, all weight comes to bear on the foundation. Portions of the home's structure that are higher, such as roof trusses and attic rafters, need to bear less weight than lower structures. However, they must also be designed in such a way as to allow them to transfer their weight to lower structures through carefully chosen and reinforced points of contact.

Spans and Pitch

• A home's roof is its highest load-bearing element. Roof trusses, which provide support for the roofing material and bear additional loads from snow, wind and earthquakes, have special load calculations. Load tables allow builders to determine which type of trusses to install, and how many are needed to support a roof. Computer models create truss designs that can accommodate specific load requirements for homes that use custom designs or face special risks due to the environment.