Another purpose for grading is to accommodate circulation on sloped ground or between spaces of different ground elevations. As stated in the previous section, walks should not exceed a slope of 5 percent (or a 1-foot vertical change for every 20 horizontal feet; Figure 11—6). This guideline is especially applicable for entry walks where comfort and safety of people are important.
Where the ground is too steep to provide a properly sloped surface, steps may be necessary to take up the elevation change between two spaces. There are a number of guidelines for the design of steps. First, they should be designed as an integral part of the overall design (Figure 11—7). Steps should not be designed as an afterthought to other aspects of the design and made to appear as an “add-on” (see the top portion of Figure 11—7). In addition, steps should have forms that are consistent with the overall design theme, and thus should be studied during form composition.
Steps also must have appropriate dimensions. Both the tread, the horizontal portion of the step on which the foot is placed, and the riser, the vertical portion of the step (Figure 11—8), must have the correct depth and height to be safe and feel comfortable. A guideline that is commonly used to establish the tread and riser dimensions is the following formula:
Twice the riser height plus the tread depth should equal 26", or 2R + T = 26".
The examples in Figure 11—9 demonstrate how the formula can be applied. If the riser (R) is to be 6 inches high, then the formula is used to determine the proper tread depth (T) as follows:
Step 1: 2(6") + T = 26"
Step 2: 12"+ T = 26"
Step 3: T = 26"- 12" = 14"
Or, if each tread (T) is to be 15 inches, the riser height (R) is found as follows:
Step 1: 2R + 15" = 26"
Step 2: 2R = 26"- 15"= 11"
Step 3: R = 5.5"
As can be seen from this formula, the dimensions of the treads and risers in a flight of outdoor steps are interdependently related. As the dimension of one becomes greater,
the other becomes smaller. Once dimensions are established for a given set of steps, they should not be varied (Figure 11—10). That is, all the risers should be the same height and all the treads should be the same depth within the flight of steps. If these dimensions vary, they are apt to catch people by surprise and cause them to trip or fall.
There are several limitations on minimum and maximum dimensions for risers and treads (Figure 11—11). Each tread should be at least 12 inches deep. A tread that is smaller than this is too shallow for the average foot. The height of each riser should be at least 4 inches but no more than 6—1/2 inches. Below 4 inches, the height becomes insignificant and is not easily seen in the outdoors. This short dimension also creates the need for too many risers in a set of steps. Above 6—1/2 inches, the height of a riser becomes difficult for elderly people, children, and others with walking disabilities to negotiate.
Steps function best when they are oriented 90 degrees or at a right angle to the prime direction of movement (Figure 11—12). It is easier to walk up a flight of steps “head-on.” The designer should avoid placing steps so that people have to walk up or down them across a sharp corner (right side of Figure 11—12). This is awkward and frequently dangerous.
Steps are often the best way to get people from one elevation to another. However, they do have one major problem: they cannot be negotiated by wheeled
Figure 11-11
Minimum and maximum dimensions for risers and treads.
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Figure 11-12 Steps should be oriented 90 degrees to the direction of movement. |
vehicles such as wheelchairs. Steps act like barriers in the landscape to free movement. Consequently, there is sometimes a need to provide ramps on a residential site to allow wheelchairs and other wheeled vehicles to move without limitation.
There are a number of challenges in designing ramps. First, they need to be located and designed along with every other element in the design so that they appear as an integral element. Too often, ramps are added as an afterthought. When this happens, ramps usually look poorly related and out of place. Second, ramps need to conform to proper dimensions. The slope or gradient along the ramp should not exceed 8.33 percent (Figure 11-13). The slope should not rise more than 1 vertical foot for every 12 horizontal feet along the ramp. The result of this is that most ramps take up a large horizontal distance on a site, especially when compared to steps. For example, to accommodate 2 feet of elevation change between two levels, a ramp needs to extend 24 horizontal feet. This is extensive compared to only a few feet needed for a set of steps for the same elevation change. One last dimensional guideline is that ramps should be at least 5 feet wide.
