When you look any building that has open stairs going from one floor to another you will see that the treads are attached to at least two stair stringers, one on each side. Occasionally depending on the style of stairs and how wide they are there is a third stringer in the middle to supply extra support. The stringer is the framework to which all other components of the set of stairs are attached. There are several different styles of stringers in use depending on the materials being used and the location of the stairs.
Many modern buildings use a floating type of stairs that are only attached at the top and bottom. With stairs like this the stair stringers are used to create the shape and direction of the stairs as well as provide a place for the treads to rest. Depending on the choices made by the architect these stringers can be made from metal or wood, although metal is the most common choice for this type of stairway. Wood is usually reserved for homes and stairs that are going to be attached to a wall for extra support and location.
Stair stringers are often matched pairs so that the treads do not have to be different at both ends; however this is not always the case especially in staircases that are curved or go around any kind of a corner.
The stringer is usually L shaped where the tread is supposed to rest; this provides support and a place to fasten the tread down so that it cannot move whenever someone steps on it. Risers are used with some stringers to provide an enclosed look to the stairs, but in buildings that use a floating style of stairway the risers may be left out to give the entire staircase an open look.
If you are building a spiral staircase the use of two stringers that have been twisted into a spiral is the only way to create this look. The inside stringer is usually wound in a much smaller spiral than the outer one and depending on the design feature a pole down through the center and a handrail only on the outside for your safety.
These spirals must be custom bent to exacting calculations is if the "spiral" stairwell is going to allow a person to move properly up or down on the stairs.
While we may actually call them a spiral staircase, in all reality this is not the proper term for them as in mathematical figures a spiral lies on a flat plane. The stair stringers are three dimensional as they rise from one floor to the next, the true mathematical name for this particular shape is a helix. The treads are usually attached to the sides of the stringers instead of created places for them to rest on as you would with a straight set of stairs. If you are looking to have steel stringers custom bent for your next set of spiral or helix stairs, let Albina Pipe Bending create them for you.