This section is from the "Architectural Iron And Steel, And Its Application In The Construction Of Buildings" book, by WM. H. Birkmire.. Also see Amazon: Architectural Iron And Steel, And Its Application In The Construction Of Buildings.
There is no part of a building which furnishes a better opportunity for ornamentation than the stairway. It is generally a continuation of the entrance-hall, and is the means of communication between the several stories. The design page 72 shows a cast-iron string and cast-iron railing and what are commonly called close-string stairs, the ends of treads and risers not being visible from outside the string.
In straight staircases the height and breadth of steps are very variable quantities, generally subject to the exigencies of the space reserved. As a rule, the broader the tread at the line of traffic, the less should be the riser or height of step. The breadth varies from 9 to 12 inches (not considering the nosing), and the height of riser from 6 to 8 inches - a minimum and maximum that should never be exceeded. It is always a safe rule to make twice the rise plus the tread equal to 25. Within these limits stairs are easy of ascent and not tiring.
An ordinary staircase to be convenient should be pitched at an angle ranging from 24 to 30 degrees. It should consist of steps uniform in height in each flight between the stories as measured from flooring to flooring.
The plan, section, and details, page 73, show stairs of cast-iron with slate treads. The connections of strings are covered entirely by the posts, thus giving them a finished appearance.
The face string extends to the wall for support and forms a risen (See detail B.) It is seldom made less than twelve inches or required more than sixteen inches wide.
The railing height is measured from a point on the tread directly above the face of riser (not the moulding under the nosing of tread), and should be from 2 feet 8 inches to 3 feet. Fig. 2 is the string connection at the top of stairs. The wall string is set 2 to 2 1/2 inches from face of wall to receive the plastering at top and bottom; and is frequently made with a wider top and bottom flange to receive any furring placed on wall. The section of wall string at Fig. 3 shows clearly the manner of receiving the plastering.
To arrange the treads and risers upon the strings, the F distance on Fig. 2 should not be less than 2 inches above nosing of tread, and from 4 to 8 inches at E below riser, depending at this point upon the size of treads, height of risers, and length of string. The F distance need seldom be changed. To have the string show a proper finish at top of stairs, the top moulding should return downward to flooring as shown at C, Fig. 2.
A continuous vertical and horizontal flange, 3/8 by 1 1/4 inches thick, is cast on the string to secure the risers.
The wall string, if of considerable length, can be secured to the wall by lag screws or expansion bolts (see plate of Bolts), and cast with ribs as shown at A, Fig. 3. These ribs perform an important part when the casting is cooling, keeping it straight, also forming separators when the string is drawn up close to wall.
Stairs over four feet in width should have three strings - that is, an outside or face string, a middle string to follow the shape of riser and tread, and a wall string,, If less are used for stairs over four feet in width, the risers should be increased in thickness. The average thickness for an ordinary riser is one fourth (1/4) of an inch.
For area stairs, or stairs to be used for light traffic in any locality, those of wrought iron are the lightest and simplest constructed. The strings are made of flat iron, welded and made the shape of treads and risers, riveted to a plain bar as shown at B. The portion of string at F has angle knee clips for fastening the treads upon. The treads are 3/4 of an inch thick; to give them stiffness a vertical flange is cast on the back. The railing is constructed as shown by the connection D, with the baluster curved to clear the tread at C.
The top rail is composed of a flat bar with a half-round bar of wrought iron riveted on top. These stairs can be adapted for spans from 4 feet to 12 feet long by 3 feet wide. The straight bar G may be from 1 1/4 X 3/8 inch to 2 1/2 X 1/2 inch, the bar F 1 1/8 X 5/16 inch to 1 1/2 X 1/2 inch, the balusters H 3/4 inch square.
The treads to be cast iron, open-diamond pattern. Wooden treads are often used by securing them to the angle knee clippings (A) with carriage bolts.