Grade Plane Calculations

In order to qualify the lowest level as a basement, normally in a sloping lot, we have showed on our elevations that at least 50% of the perimeter of the basement to be where the finished grade is 6' from the floor above the basement.

Under the new CBC 2007, do you determine ground plane by averaging the height elevation X each perimeter length for the entire perimeter of the building? i.e., is there one ground plane for each building, or each side?
Original Post
Some definitions may be in order first...

STORY. Although seemingly quite obvious, the definition of a story is that portion of a building from a floor surface to the floor surface or roof above. In the case of the topmost story, the height of a story is measured from the floor surface to the top of the ceiling joists, or to the top of the roof rafters where a ceiling is not present. The critical part of the definition of a story involves the definition of story above grade plane as described in the following discussion.

STORY ABOVE GRADEPLANE. Throughout the code, the number of qualifying stories in a building is a contributing factor to the proper application of the provisions. As an example, a building's allowable types of construction are based partly on the limits in story height placed on various occupancy groups. In this case, the code is limiting construction type based on the number of stories above grade plane. The code defines a story above grade plane as any story having its finished floor surface entirely above grade. However, floor levels partially below the grade at the building's exterior (basements) may also fall under this terminology. The critical part of the definition involves whether or not a basement is to be considered a story above grade plane. There are two criteria that are important to the determination if a given floor level is to be considered a story above grade plane:

1. If the finished floor level above the level under consideration is more than 6 feet above the grade plane as defined in Section 502.1, the level under consideration is a story above grade plane, or

2. If the finished floor level above the level under consideration is more than 12 feet above the finished ground level at any point, the floor level under consideration shall be considered a story above grade plane.

Where either one of these two conditions exists, the level under consideration is to be considered a story above grade plane.
Conversely, if the finished floor level above the level under consideration is 6 feet or less above the grade plane, and does not exceed 12 feet at any point, the floor level under consideration is not considered a story above grade plane.

Although the criteria for establishing the first story above grade plane in Item #2 indicates that such a condition occurs where the 12-foot limitation is exceeded, the application of this provision is not that simple. It is not the intent of the code to classify a story that is completely below grade except for a small entrance ramp or loading dock as a story above grade plane, provided there is no adverse effect on fire department access and staging. An analysis of the impact of such limited elevation differences is necessary to more appropriately apply the code's intended result.

GRADE PLANE. The code indicates that the grade plane is a reference plane representing the average of the finished ground level adjoining the building at its exterior walls. Under conditions where the finished ground level slopes significantly away from the exterior walls, that reference plane is established by the lowest points of elevation of the finished surface of the ground within an area between the building and lot line, or where the lot line is more than 6 feet from the building, between the building and a line 6 feet from the building.

Where the slope away from the building is minimal (typically provided only to drain water away from the exterior wall) the elevation at the exterior wall provides an adequate reference point. The method for calculating grade plane can vary based upon the site conditions. Where the slope is generally consistent as it passes across the building site, it may only require the averaging of a few points along the exterior wall of a rectangular-shaped building( see Figure 1 below). Where the slope is inconsistent or retaining walls are utilized, or where the building footprint is complex, the determination of grade plane can be more complicated. In such cases, a more exacting method for calculating the grade plane must be utilized.

This definition is important in determining the number of stories above grade plane within a building as well as its height in feet.

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For those in California, the amendment to the definition is confusing. The amendment is a piece of legislation that was required to place the measuring point at 5 feet, not the 6 feet as is in the model code.

BSC and HCD edited the legislation and the amendment to its current wording, which confuses something that's already confusing. Its so far led to one official qualifying a basement with a walk-out egress well as the "First Story Above Grade Plane".

Those outside of CA, count your blessings.
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