SPIRAL STAIR LAYOUT WITHOUT REGRETS
Just Beam Me Up: Vertical Access in Limited Space.
By John M. Corbett
STAIR THE LOAD: When planning support, note that the load in a modular cast iron spiral is carried by the stairs themselves, not the centerpole, as is the case in other spiral systems.
Requiring only about half the floor area required for conventional
staircases, spiral stairs sometimes furnish the only legal means of access to
upper levels, roofs and lofts in areas where space is limited. Additionally,
these prefabricated, freestanding assemblies add a dynamic design element,
save labor and relieve project schedules. The labor and scheduling advantages of prefabricated elements come at a cost, however. By their nature, these stairs
are tightly configured to the space allowed and once fabricated, they have little
capacity to accommodate additional issues as they arise. The step most critical
to the success of the project thus becomes laying out and ordering the stair itself.
To find out how to get it right, we talked to a man who consults on these issues
every day, Rick Harrington, the manager of stair sales for Steptoe and Wife, LLC,
of Toronto, Ontario. Steptoe builds a period cast iron stair, but the principles
will apply roughly to all spiral systems, whether fabricated from wood, steel
Spiral stairs compress a number of issues into a compact package. Mr. Harrington
recommends that his customers get to know the particular stair before committing
to it. By preventing misunderstandings, he helps his customers avoid unhappy surprises. For example, he recently spoke to an architect whose San Francisco
client wanted to run a staircase from a first floor exterior deck to the
second floor and proposed supporting its weight by extending the staircase center
pole, by itself, down through the deck to bear upon a concrete footing below.
In many spirals, this center pole does indeed support the weight of the steps,
but not in the Steptoe version. Here, the load is instead transmitted to the
floor through the stairs themselves, much as in a conventional stair system.
The pole cannot be used to support the load. By asking the question, this
designer avoided surprising his client with additional project costs.
Regarding the structural needs of spirals in general, Mr. Harrington reminds
us that they are free standing structures lacking the lateral stability of
built in stairs. Not only should clients be made to anticipate movement when
climbing spiral stairs, it's all part of the fun, they can be told, but
spirals over twelve feet tall will require a degree of sway bracing.
For a recent spiral built to pass through a twenty-foot space, Steptoe provided
this stability by furnishing a landing area halfway up which tied into the
walls and provided a small living area. By understanding the structural
requirements of the spiral installation, the designer was able to incorporate
them into the overall design.
Once paid for, delivered and sitting in the yard, the staircase belongs to the purchaser, mistakes and all. The fabricator will rightly disclaim responsibility for confirming dimensions in the field or for conformity to local codes. Nevertheless, Rick Harrington makes every effort to smoke out misunderstandings by requiring the purchaser to furnish such drawings as are necessary to define the space. The errors he most commonly sees are normal, tape rule type miscommunications about how to measure rough openings, whether they were made finished floor to finished floor, for example, as required, or if they were instead made rough to rough, finished floor to subfloor or something else entirely. Once Mr. Harrington has confidence in these dimensions and in his understanding of the space, he will produce drawings of the spiral stair installed, for the purchaser's review and approval. This is a final opportunity for the purchaser to verify all dimensions in the field. The fabricator's drawing will also prove useful in obtaining approval for the spiral design from local building officials. Local codes place restrictions on the permitted uses of spirals and the only way to be sure if your design conforms is to ask your inspector. Spirals seem to fall into a gray area for some officials and it may not always be clear that a design will pass until it does. The permit application process gives them a chance to make their contribution to the design process and sometimes that is all that they need. Certainly, no order should be placed until their approval has been secured.
Layout for a spiral stair can be as simple as the project permits. Spirals offer a great deal of design flexibility both in that they can be rotated on their footprint to accept traffic from any point of the circle and in that traffic can be made to ascend either clockwise or counter clockwise through the shaft. These two options usually allow the designer sufficient leeway to configure standard stair assemblies to fit any space. To orient the spiral, traffic patterns at one end or the other of the spiral will be deemed dominant and allowed to govern. For example, where traffic is to be conveyed from a main living area to a loft, the requirements of the main area would likely rule. In such a case, the bottom entrance to the stair passage would be oriented to best expedite traffic flows and victory is declared. All that remains to do is to provide a landing deck or plate connecting the uppermost step tread with the main loft floor area, taking care only that the landing material is not so thick as to impinge on the minimum height required by code for the stair passage way below.
But what if the project requires a specified orientation of the passage at both top and bottom of the shaft, whether because of code, clearances or customer preference? Is there any way to adjust the relationship between the orientation of the top stair and that of the bottom? There is, at some cost and with a little tinkering. Steptoe's stairs perform one complete rotation every thirteen steps, winding twenty-seven degrees around the center pole at each step. A climber's orientation when reaching the uppermost step will be determined by the number of steps it took to get there. For every twenty-seven degrees that we need to advance or rewind the orientation of that topmost step, we must add to or remove one step in the series while maintaining the same overall stair height. Steptoe accomplishes this by introducing shims between all the assemblies to make up a step or by milling down the cast iron tread assemblies to make room for one. These are the same methods Steptoe uses to make the incremental adjustments to tread heights necessary to fit a stair into any given vertical opening, but adjustments for this purpose are minor, a quarter inch or less. The milling of the iron step assemblies necessary to create space for a whole additional step will be much more substantial and will constitute a significant additional expense. Also, with the addition of a step, the overall length and shape of the spiral will be altered. A standard tubular handrail will no longer follow the pattern so the stair must be fitted with a custom handrail at additional expense. Deviations from the standard staircase pattern will entail additional cost, but given the full range of design variables available, a spiral configuration can be developed to accommodate almost any design requirement.