Engineering Variables

The information that a custom ground-mount manufacturer will need includes:

• Maximum design wind speed. This is the highest wind gust speed probable in 50 years, averaged over a 3-second gust at a height of 33 feet. Because wind speeds vary (they are generally higher close to the coast and at high elevations), this is critical information for an engineer. Maximum design wind speeds can be found in the American Society of Civil Engineers (ASCE) Standard 7-10, “Minimum Design Loads for Buildings and Other Structures.” However, your local building authority can provide the wind design speeds they require.

• Snow load. Measured in pounds per square foot, the weight of snow on a structure can stack up, depending on your location. Snow can be five to more than 15 times heavier than a PV module, and the rack must hold the additional weight. ASCE Standard 7-10 includes common snow-load values, but your local building authority will provide their requirements.

• Exposure category. This is related to wind loading and takes into account the turbulence at the site due to surrounding objects (trees, buildings, etc.). There are three main ASCE categories that relate to ground mounts: Category B (lower wind loading)—dense urban and suburban area; Category C (medium wind loading)—open terrain with occasional obstructions; Category D (higher wind loading)—flat and unobstructed terrain. (Category A refers to “large city centers” with at least 50% of the buildings with heights more than 20 meters.)

• Site slope. Again, the engineer will be concerned with the physical dimensions of the rack and the maximum slope the racking can accommodate. You’ll need to provide the average slope and slope direction.

• Soil class. This is necessary for determining specifics for anchors, which behave differently in different soils. Soil classifications are derived from Table 1804.2 of the International Building Code, which classifies five general soil types—type 1: crystalline bedrock; type 2: sedimentary and foliated rock; type 3: sandy gravel and/or gravel; type 4: sand, silt, or clay sand, silty gravel, and clayey gravel; and type 5: clay, sandy clay, silty clay, clayey silt, silt, and sandy silt. If there is a mix of soil types, pick the dominant type. The USDA Natural Resources Conservation Service publishes soil surveys which contain maps and a description of each major soil in the survey area. Your local cooperative extension agent can provide you with maps and help determine your soil type. See www.csrees.usda.gov/Extension/ to find your local office.

• Module type and quantity. Acquire a specification sheet for your modules for the rack manufacturer, who will need at minimum the number of modules, plus module depth, height, width, and hole layout, to size the rack correctly.

• Desired tilt angle. This will be based on your location’s latitude and the seasonal variation in solar gain (see “Specifics to Consider” for more details).

• Ground clearance. This is the height to the lower edge of the first row of modules (2 feet is typical). This basic design decision should be based on site conditions: potential snow accumulation, ground covering, aesthetics, etc.

• Number of modules in a string. This information can help the rack manufacturer design an efficient layout, with strings contained within a row or subarray for less trenching and conduit.