Masonry Magazine April 1995 Page. 21
Step Riser - Tread Relationships.
Riser-tread relationships have been studied for many years. Most steps are constrained to fit into a set elevation at the top and the bottom of the steps. For these steps, riser-tread relationships have been developed for safe and efficient use. In other areas, such as plazas, the tread dimension may not be constrained, which allows freedom of design. In these areas, riser and tread dimensions will be dictated by human dimensions and appearance.
Most local building codes mandate the minimum and maximum riser dimension and minimum tread dimension. The 7-11 rule is used most frequently; that is, maximum riser height is 7 in. (180 mm), while the minimum tread depth is 11 in. (280 mm). The riser must also be greater than 4 in. (100 mm) in height. Due to normal walking and gait, optimum riser-tread dimensions do exist. One formula for determining this relationship has been recommended for use [1]. It provides a general guideline for riser and tread dimensions.
where:
TXR = 77.5 (TXR = 500, for SI units)
T = tread width, in. (cm)
R = riser height, in. (cm)
In this equation, the riser is restricted between 4 in. (10 cm) and 7 in. (18 cm). Since brick is a small element, there are a variety of bonding patterns for the tread and riser. Figure 5 shows several examples of bonding arrangements with modular brick.
Landings may be included in steps, especially when the cumulative height of steps is great. Building codes set the maximum height between landings at 12 ft (3.7 m); however, it is usually more desirable to limit the maximum height to 5 ft (1.5 m) between landings. The length of the landing should be long enough to allow easy cadence, which is about 5 ft (1.5 m) or a multiple of 5 ft (1.5 m).
Edge Details.
For safety reasons, several issues relating to the edge of the steps should be considered. In most pedestrian applications, it may be beneficial to highlight the edge of the step by varying the color of the tread and riser brick, changing the bond pattern of the tread or by extending the tread slightly over the riser. These distinctions of the edge allow easy visual indication that a change is coming, which helps to avoid tripping. Extending the tread over the riser creates a shadow line highlighting the stairs and also allows the treads to be slightly deeper than if they were squared off. However, the maximum projection should be 1% in. (40 mm) to avoid catching the foot while stepping up. If a rounded tread is used, the leading edge should be a maximum % in. (13 mm) radius.
Ramps.
The model building codes and other accessibility codes [2] usually limit the slope of ramps to no steeper than 1:12 in most applications. Greater slopes are allowed only if the total rise is less than 6 in. (150 mm). Slopes greater than those allowed by codes make it difficult for persons in wheelchairs to negotiate the ramp. The width of a ramp should be at least 3 ft (0.9 m) wide for one-way traffic and 5 ft (1.5 m) wide for two way traffic. In addition, landings should be provided every 30 ft (9 m) horizontally.
Support and Bonding.
Brick steps and ramps are usually supported by a concrete base, but any material capable of supporting the brick properly could be used, if designed properly. Deflections or settlement of the support must be minimized to avoid cracking in the brickwork. Figure 6 shows a concrete support system for a step and ramp. Brick should be adequately bonded to the support or restrained around its perimeter to avoid loosening of units. Mortar is usually used to bond the brick to the concrete. This paving system is very effective when proper materials and installation are used. Dowels or ties into the mortar joints are not necessary since the mortar provides adequate bond. Newer types of adhesives are now being used to bond the brick directly to the concrete. These adhesives must be durable to withstand the severity of its environment. Adhesives can only be used when the concrete surface is fairly even and free of contaminants. Caulks and sealants are not appropriate for this purpose.
Adequate footings should be designed for the step or ramp support. The depth of the footings should extend below the frost line. Since the paving assembly is supported on its own footing, an isolation joint should be used between the pavement and building and between the pavement and ramps or steps.
Safety.
In addition to the required physical dimensions of the element, the slip resistance of the surface should also be considered. The static coefficient of friction is usually used to determine if a surface is considered slippery. There is no consensus minimum value for coefficient of friction; however, some codes are promoting minimum static coefficient of friction values of 0.6 for pavements and 0.8 for ramps [2]. Limiting the static coefficient of friction to these values is believed to make the surfaces safe and passable by both able-bodied pedestrians and the physically impaired. It is usually excessive water ponding or the contamination of the pavement by other substances which cause most