Post 545 by Gautam Shah
Stairs and escalators are stepped and inclined-vertical transfer systems. Both provide uninterrupted transfer, unlike the modulated transfer by elevators. Traffic on stairs and escalators is restricted by the width of passage whereas the same on elevators is limited by the module size and its speed. Use of stairs requires some orthopaedic proficiency and cautious posturing, but automated escalators allow freedom to see around during the passage. Some flight of stairs can be used for accent and descent, but escalators require different sets. Though reverse escalator for descent may not be provided, as physically it is not very strenuous to climb down.
All movements are essentially directional. A designated unidirectional system like an escalator is more efficient than a mixed movement system like a stair. This factor, however, is relevant for stairs with low to moderate traffic. There are several other transfer systems, such garbage chutes, emergency evacuation tubes, trunks or ramps, and fire-mans’ poles, where movement is unidirectional and generally downwards. The gravity accelerates the down-movement, and inclination retards the rate of passage.
Stairs and escalators are point to point passageways, as there is no mid-way interference, except at landings. Mid-way disturbance occurs in comparatively flatter and very wide stairs over the mountain sides. Here, for ascent or descent, people cross the steps diagonally to increase the ‘tread-depth’ of the steps. Stairs generally have a pitch higher than ramps. Stairs are safer than ramps provided the person is fully mobile and orthopaedically fit, but to ascend or descend stairs are not as easy as the ramps. A ramp can have gradually variable pitch, but a stair has to have a one continuous grade of pitch.
The inclination of steps is defined by the proportion tread versus riser of the steps. This could vary for stairs used for different purposes, ranging from steep ladders to flatter ramps like foot-ways. The dimensions of tread and riser are proportional and can be plotted on a hyperbola. Certain formulas also provide such proportions: 2T + R = 650 to 680 mm or R x T = 43000 to 45000. For steeper pitch the additional effort required to work against the gravity reduces the efficiency.
Stairs have a pitch of not less than 17.30° (5:16), and of not more than of 48.30° (9:8). Below these limits it becomes a ramp or foot-way, and above it a ladder. A ladder is not a comfortable utility. Step-ladders are lower in pitch, less than 75° and require flat treads. Risers may be open or closed, for toe accommodation and handrails may or may not be provided. In the ladders’ class of stairs, some are easier to climb than others. Ladders are used for fire escapes, boiler rooms, fly galleries, attics, decks, etc. Rung ladders are pitched more steeply, above 75°, and have extremely narrow treads or round rungs to accommodate the foot. In certain cases, the space to accommodate the knee between steps may be necessary. Rung ladders usually do not require additional handrails as the side members of the ladder can be used for holding grips. Rung ladders are often caged for safety, though such cages are more useful for ascent then for descent. It is safer to climb down facing the ladder. Swimming pools, water tanks, and sewers have rung ladders. Manhole steps are very narrow in widths, but the width is otherwise compensated by its staggered placement. The same holds true, for climbers for bunk beds, whether in railways, buses, barracks or homes.
Minimum width required for low intensity unidirectional traffic is 600 mm, however most standards specify 900 as minimum width for escape in a hazardous situation. A two-way lane stair should be at least 11200 mm. Sufficient width space for movement is required at torso level, otherwise at feet level a minimum width of 250 mm is required. Where same step is to be used for placing either one of the feet, both, the step and passage widths of minimum width of 500 mm are necessary. Stairs less than 500 mm width are generally emergency stairs rarely to be used, or service stairs to be used by experienced persons. For single lane traffic 750 mm width is an accepted standard. Most of the building bylaws allow minimum 900 mm widths for private buildings. For public buildings a stair width of 1200 mm to 1500 mm is recommended. For pedestrian over bridges and other public thoroughfares, a stair width of 2400 is recommended. On public thoroughfares where traffic is totally segregated or is only one directional, the minimum stair width could be 1800 mm.
Many stepping arrangements are used for emergency and special purposes. Simplest is a knotted rope or a rope ladder secured to a wall or column. In many countries older buildings were required to confirm to new bylaws, open iron stairways on the building’s exterior were placed. Open iron stairs, though are rendered useless by smoke from windows, so must be placed against a blank wall. One of the best fire escape stairs is a fully enclosed stairway in the building itself or in an adjoining tower. Uncoated or unprotected steel is highly hazardous during a fire as it expands and deforms the stairway. Wood though combustible catches fire slowly, and allows more escape time compared to an unprotected steel stair.