Too often, ramps are added as an afterthought. As the dimension of one becomes greater,
Examples of ideal riser/tread relationships. Figure 11-7
Steps should be designed as integral parts of the overall design composition. 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". 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). First, they should be designed as an integral part of the overall design (Figure 11—7). vehicles such as wheelchairs. 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). Figure 11-8
An example of step risers and treads. Figure 11-5
Minimum and maximum slopes for lawns. The slope should not rise more than 1 vertical foot for every 12 horizontal feet along the ramp. This guideline is especially applicable for entry walks where comfort and safety of people are important. One last dimensional guideline is that ramps should be at least 5 feet wide. Once dimensions are established for a given set of steps, they should not be varied (Figure 11—10). the other becomes smaller. 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. For example, to accommodate 2 feet of elevation change between two levels, a ramp needs to extend 24 horizontal feet. Steps are often the best way to get people from one elevation to another. That is, all the risers should be the same height and all the treads should be the same depth within the flight of steps. The examples in Figure 11—9 demonstrate how the formula can be applied. There are a number of challenges in designing ramps. Figure 11-12
Steps should be oriented 90 degrees to the direction of movement. In addition, steps should have forms that are consistent with the overall design theme, and thus should be studied during form composition. Below 4 inches, the height becomes insignificant and is not easily seen in the outdoors. Consequently, there is sometimes a need to provide ramps on a residential site to allow wheelchairs and other wheeled vehicles to move without limitation. This is awkward and frequently dangerous. The slope or gradient along the ramp should not exceed 8.33 percent (Figure 11-13). There are a number of guidelines for the design of steps. 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. 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. Second, ramps need to conform to proper dimensions. Above 6—1/2 inches, the height of a riser becomes difficult for elderly people, children, and others with walking disabilities to negotiate. Steps also must have appropriate dimensions. However, they do have one major problem: they cannot be negotiated by wheeled
Minimum and maximum dimensions for risers and treads.