Classification of Stairs
Metal stairs are classified according to both Type and Class. The Type designation identifies the physical configuration or geometry of the stair, while the Class designation refers to its construction characteristics the degree of refinement of fabrication and finish and to the general nature of its usage. Obviously, the Type designations are applicable to stairs made by any material, but the various Classes, as here described, apply more particularly to metal stairs.
Types of Stairs
The four types of stairs are Straight Stairs, Circular Stairs, Curved Stairs, Spiral Stairs, Winder Stairs, Alternating Tread Stairs, and Ship's Ladder. This listing of types is not necessarily all- inclusive but it represents the great majority of stairs. It is not uncommon to find two or more types, represented in the same stair, and in rare cases there may be found a stair which properly falls in none of these type categories.
Straight Stairs
Straight Stairs are by far the most common type, representing the bulk of the stair market. Though the term "straight" is self-explanatory, for purposes of classification a straight stair is defined as one in which the stringers are straight members. Straight stairs, unlike stairs of the other three types, may be arranged in several different ways:
a) Straight run:
either a single flight extending between floors, or a series of two or more flights in the same line, with intermediate platforms between them
b) Parallel:
successive flights which parallel each other and are separated only by one or more intermediate platforms
c) Angled:
successive flights placed at an angle of other than 180° to each other (often 90°), with an intermediate platform between them
d) Scissor:
a pair of straight run flights paralleling each other in plan and running in opposite directions on opposite sides of a dividing wall
Circular Stairs
Circular Stairs are stairs which, in plan view, have an open circular form, with a single center of curvature. They may or may not have intermediate platforms between floors.
Curved Stairs
Curved Stairs are stairs which, in plan view, have two or more centers of curvature, being oval, elliptical or some other compound curved form. They also may or may not have one or more intermediate platforms between floors.
Spiral Stairs
Spiral Stairs are stairs with a closed circular form, having uniform sector shaped treads and a supporting center column.
Winder Stairs
Winder Stairs in plan view are parallel or angled. However, unlike straight stairs, no platforms are used where the 180° angle occurs for the successive flights of parallel stairs, nor are platforms used for successive flights placed at angles other than 180° (often 90°). Instead, the stairs continue to rise through the angled areas with sector shaped treads having the same riser heights as the straight part of the stair.
Alternating Tread Stairs
Alternating Tread Stairs are an exception to the upper pitch limitation for stairs. In this type of stair the treads are alternately mounted on the left and right side of a center stringer. Because of this tread construction and the use of handrails on each side, these stairs permit safe descent facing outward from the stair. Generally, pitch angles used in these stairs will range from 56° to 68°.
Ship's Ladders
Ship's Ladders generally have pitch angles ranging from 59° to 75°. They require flat treads and handrails on at least one side, de- pending on stair width.
Pre-assembled and Pre-erected Stairs
Pre-assembled and Pre-erected Stairs are stairs whose components are assembled in the plant to make up units of varying sizes and degrees of complexity. These may be platform units, flight units, combination platform and flight units, or larger units comprising the complete floor-to-floor story-height stair. Pre-assembled stairs may be of the architectural class, but pre-assembly methods are commonly applied to stairs of the commercial and service classes because of the repetitive use of identical units. Pre-assembled units for multi-storied buildings may be designed to be self-supporting so that they may be pre-erected on the building site. Such units can be stacked one upon the other and field-connected to form stair towers. Stair towers can be erected and ready to use prior to erection of the surrounding building structure. The use of pre-assembled units and pre-erected stair systems usually effects considerable savings and expedites the construction of buildings.
Classes of Stairs
The Class designation of stairs, is a key to the type of construction, the quality of materials, details and finish and, in most cases, the relative cost. As stairs of all classes are built to meet the same standards of performance in respect to load carrying capacity and safety, these class distinctions do not represent differences in functional value, but in character and appearance. It is important to recognize that where function is the prime concern, and aesthetics are of minor importance, significant economies can be achieved by specifying one of the less expensive classes.
The following descriptions indicate the general construction characteristics of each class, but it should be recognized that because each manufacturer has his own preferred methods of fabrication, the details of construction vary somewhat throughout the industry. The four classes of stairs, listed in the order of increasing cost (as a general rule), are described as follows:
Industrial Class
Industrial Class: Stairs of this class are purely functional in character and consequently they are generally the most economical. They are designed for either interior or exterior use in industrial buildings such as factories and warehouses, or as fire escapes or emergency exit ways. They do not include stairs which are integral parts of industrial equipment.
Industrial class stairs are similar in nature to any light steel construction. Hex head bolts are used for most connections, and welds, where used, are not ground. Stringers may be either flat plate or open channels; treads and platforms are usually made of grating or formed of floor plate, and risers are usually open, though in some cases filled pan type treads and steel risers may be used. Railings are usually of either pipe, tubing, or steel bar construction.
When used for exterior fire escapes the details of construction are similar, except that treads and platforms are of open design, usually grating or perforated floor plate. Also, the dimensions, methods of support and other details are usually dictated by governing code regulations.
Service Class
Service Class: This class of stairs serves chiefly functional purposes, but is not unattractive in appearance. Service stairs are usually located in enclosed stairwells and provide a secondary or emergency means of travel between floors. In multi-storied buildings they are commonly used as egress stairs. They may serve employees, tenants, or the public, and are generally used where economy is a consideration.
Stringers of service stairs are generally the same types as those used on stairs of the industrial class. Treads may be one of several standard types, either filled or formed of floor or tread plate, and risers are either exposed steel or open construction. Railings are typically of pipe construction or a simple bar type with tubular newels, and soffits are usually left exposed. Connections on the under side of the stairs are made with hex head bolts, and only those welds in the travel area are smooth.
Commercial Class
Commercial Class: Stairs of this class are usually for public use and are of more attractive design than those of the service class. They may be placed in open locations or may be located in closed stairwells, in public, institutional or commercial buildings.
Stringers for this class of stairs are usually exposed open channel or plate sections. Treads may be any of a number of standard types, and risers are usually exposed steel. Railings vary from ornamental bar or tube construction with metal handrails to simple pipe construction, and soffits may or may not be covered. Exposed bolted connections in areas where appearance is critical are made with countersunk flat or oval head bolts; otherwise hex head bolts are used. Welds in conspicuous locations are smooth, and all joints are closely fitted.
Architectural Class
Architectural Class: This classification applies to any of the more elaborate, and usually more expensive stairs; those which are designed to be architectural features in a building. They may be wholly custom designed or may represent a combination of standard parts with specially designed elements such as stringers, railings, treads or platforms. Usually this class of stair has a comparatively low pitch, with relatively low risers and correspondingly wider treads. Architectural metal stairs may be located either in the open or in enclosed stairwells in public, institutional, commercial or monumental buildings.
The materials, fabrication details and finishes used in architectural class stairs vary widely, as dictated by the architect's design and specifications. As a general rule, construction joints are made as inconspicuous as possible, exposed welds are smooth and soffits are covered with some surfacing material. Stringers may be special sections exposed, or may be structural members enclosed in other materials. Railings are of an ornamental type and, like the treads and risers, may be of any construction desired.
General Requirements, All Classes of Stairs
All fixed metal stairs, regardless of class, are of fire resistant construction and are designed and constructed to carry a minimum live load of 100 pounds per square foot of projected plan area or an alternative concentrated load of 300 pounds applied at the center of any tread span. Railings and handrails are designed and constructed to withstand a minimum force of 200 pounds applied vertically downward and horizontally in a perpendicular direction at any point on the top rail.