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Circular or curved stairs: This document assists in building or inspecting indoor or outdoor circular or curved stairs, railings, landings.
Circular stairs have been used for centuries to ascend or descend towers or to provide access between floors where there is not enough horizontal space for a conventional straight-run or single-turn stairway. But circular and curved stairs present some special trip and fall hazards that should be carefully addressed. The circular stairs shown at page top were installed by the author in 1978.
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The traditional circular stairway shown in our photo at left was observed in Barcelona, Spain.
This circular stairway accesses four floors in a building and uses stone treads that gave good traction except when they were wet during cleaning. The hand-rail was grasp-able and enclosed. Our opinion was that this was a good stair design.
As we elaborate at HANDRAILS & HANDRAILINGS, while a good handrail is important at every stairway, on curved or circular stairs where treads are not uniform in tread depth, railings are more likely to be grasped by stair users - and are perhaps even more critical.
What are the differences among circular stairs, spiral stairs & winder stairs?
Definition of circular or spiral stairs
Circular stairways shall have a minimum tread depth and a maximum riser height in accordance with Sectionand the smaller radius shall not be less than twice the width of the stairway.
The minimum tread depth measured 12 inches (305 mm) from the narrower end of the tread shall not be less than 11 inches (279 mm). The minimum tread depth at the narrow end shall not be less than 10 inches (254 mm).
Our photo (left) illustrates a circular stairway installed outdoors in San Miguel de Allende, Guanajuato, Mexico. We had a welder double the number balusters to the as-built stair to correct an unsafe (too wide) spacing between those elements.
A benefit of that safety improvement was increased overall rigidity in the entire stair assembly.
Definition & Properties of Spiral Stairs & Curved Stairs
Our spiral stair photograph (left) illustrates this design in a building in Celaya, Guanajuato, Mexico.
1009.9 Spiral stairways. Spiral stairways are permitted to be used as a component in the means of egress only within dwelling units or from a space not more than 250 square feet (23 m2) in area and serving not more than five occupants, or from galleries, catwalks and gridirons in accordance with Section
Not much, but as you can see in our two photographs, the support system for curved stairways must be considered.
Curved stairways may be self-supporting or the stair may be supported by securing a curved stair stringer on the inner or outer radius of the stairs directly to a building wall structure.
At above left the curved stairway is actually a self-supporting structure. A curved stair that is not self-supporting may be secured by connections between its outer radius and a curved building wall.
At above right the stairs curving around the exterior of a heating oil storage tank are secured on the stair's inner radius by welding to the tank sides.
Circular stair kits such as the 1978 kit shown in our page top photo and in our sketch at left are a quick way to add a stairway between floors. that stair was produced by the Iron Shop, a Pennsylvania company in the U.S.
Companies offering stair kits offer an advantage to inexpert stair builders: the stair manufacturer knows a lot about stair codes, dimensions, trip hazards, and tips and tricks for installation.
Which way to face the circular stair: The sketches at left show the basic construction of a typical circular stair kit. The right-hand sketch explains a "right hand" circular stair layout. Notice that for most circular stairwells, the user enters and exits the stairway facing in the same direction.
This "direction facing" detail for circular stairs is important when determining where to place the stairwell, since you don't want your user to reach the top of the stairway and find herself facing a wall.
Install the widest possible circular stairway: When we installed the circular stair shown at page top in the 1970's we had intended to save floor space by ordering the smallest-diameter available circular stair that we could buy. The stair company's expert advised installing the largest circular stairway that would fit in our building. Our own calculations had failed to consider just how small the individual stair tread walking space would be.
After dividing the width of the stair opening in half (since treads have to be placed on either side of the center support post) and after subtracting the diameter of the supporting stairway post, our four-foot diameter circular stair kit was able to provide a triangular stair tread which was about 21" across in its long dimension (post to outer tread edge). The 21" stair tread gave us about ten inches of comfortable walking space near the outer third of the tread, and a tread depth between 12" and 7".
If we had opted for the smallest diameter stair, the comfortable walking area on the triangular treads would have been substantially reduced to just a few inches, increasing the trip hazard of the stairs.
[Our all-steel circular stair photo (left) shows an outdoor metal stairway located in San Miguel de Allende, Mexico after we installed an additional baluster between each of the more widely spaced components, making this stair more child-safe.]
Watch out when choosing the size of circular stairs or when building a structure or area that is accessed only by a circular stair. Ask how you will be able to move large furniture or other objects between floors.
Having inspected a few A-frame cabins whose upper floor was accessible only by a single circular stairway, we had learned that it is difficult to impossible to carry the box-spring for a bed up a circular stairway. (We once had to saw a box spring in half, fold it doubled, and then repair it when we got it upstairs.)
When we built a second floor addition on our laboratory building and knowing that the indoor stairwell would be a 4-foot diameter circular stair, we included a second means of entry to the upper floor: a large outdoor stairway and landing.
Angled or Curved stair treads are a particular trip hazard, especially because of the lack of uniformity and because the tread width at the inside of the curve can be too small for safe walking.
Above we provide a photo and a sketch of a curved stairway with unsafe railing enclosures and of curved stair tread designs. The sketch of curved stair treads is courtesy of Carson Dunlop and shows the minimum tread width and radius for curving stair treads such as the stairs in our photograph.
Our photos illustrates a sample of measurements of the tread on a conventional iron-framed circular stair (below left) and the width of the stair baluster opening (below right).
At about 12-inches from the center post the stair tread provides 9 1/2" of tread width. At 5-inches from the center post, the treads are just 6-inches wide and not safely walk-able. You can observe from the stair tread wear pattern that users step in a space between 8-inches and 16-inches away from the center post.
If you (click to) enlarge the photos you can get an idea of the dimensions of this circular stairway and the spacing between its balusters (an un-safe 12-inches).
Our dog Katie did fall out of these balusters and to the floor below. Retrofit balusters or child guards would be a good safety improvement for this stairwell.
For details about balusters (vertical spindles in railing construction) see details at Railings for a discussion of safety barriers along stairs, and GUARDRAILS on BALCONIES, DECKS, LANDINGS (railings on landings and open hallways, porches, screened porches, balconies that are more than 30" above floors or grade).
As our photo shows, a toddler (in this case very carefully held and supervised) can easily fall through open landing balusters or open balusters on a stairway.
We often see guards and railings enclosed using horizontal members or mesh or link fencing materials. Because a toddler can easily climb these materials, they are not safe for guard or railing enclosures and should not be used.
Examples of unsafe horizontal rail and guard enclosures can be seen at Balusters.
Readers of this material about circular stairs and stair kits should also review the building code and trip hazards discussed at WINDER or ANGLED STAIRS - stairways that include a turn without a landing platform. See these detailed articles on specifications for proper dimensions for stairs, railings, platforms in our list of Related Topics near the top of this page.
Continue reading at STAIRS, RAILINGS, LANDINGS, RAMPS - INSPECTIONS, CODES or select a topic from the More Reading links shown below.
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