What is spiral staircase?
A spiral staircase system constitutes a spiral arrangement of steps or treads. Usually, treads are attached to a central vertical post or column and rotate about it when we move up or down the stairs.
Tread may also be attached to the circular peripheral wall without a central post. Consecutive steps have a constant value of riser and angle of rotation.
This gives the staircase a peculiar, unique design and aesthetics. These staircases may or may not consist of handrails. Spiral staircase can be constructed using materials like glass, steel, concrete, wood, etc.
Where spiral staircase is constructed?
Spiral staircase dimensions should be carefully prepared. Spiral staircases are usually considered as a means of secondary excess.
These are constructed in addition to the primary staircase to enhance the aesthetics and interior of the rooms because of their compactness (occupy smaller space) and availability of various choice of geometric design and handrail types.
Spiral staircases are also preferred to provide excess to loft expansion, terrace, and the area which have limited accessibility and less load.
Because they occupy smaller space than other types of staircases and saves precious spaces. They can be constructed anywhere, inside or outside, providing minimum support structures i.e. a vertical column.
In contrast, they also have been used as means of the primary staircase in various historical monuments like Dharahara. With the implementation of proper design parameters, they can be constructed as a primary staircase as well.
- Spiral staircase occupies much lesser space than that of the straight or L Straight staircase thus they help to save precious spaces.
- They provide better aesthetics due to the availability of a wide variety of geometric design and material choices.
- They don’t require wall support so can be constructed anywhere with help of only vertical column support.
- They can be much economical compare to other staircases.
- The smaller outer diameter of the spiral staircase limits the no. of people that can use the staircase at once and makes the navigation uneasy and congested.
- It seems to be optically steeper than normal staircases with similar risers and number of steps due to acute vertical drop so it can cause the fear problem to some but technically they have a similar number of steps and risers in comparison to that of other staircases.
- In contrast, a spiral staircase with a smaller diameter is usually steeper due to the use of a higher value of the riser.
- Due to limited space transport of furniture is difficult.
Factors effecting design of Spiral staircase
Major factors that influence the design of the spiral staircase are mentioned below. Different Spiral staircase dimensions are also given.
1) Height of the staircase and riser
The height of the spiral staircase is the most important factor affecting the design. It is evaluated by measuring the floor height between consecutive floors i.e from the bottom floor to the consecutive upper floor where the landing of the staircase is placed.
In Nepal normal floor height is about 9’4”. Riser of the steps is determined from the building codes of the respective area. Normally it is taken as 190mm or 179mm i.e. (7.4” or 7”) in residential houses for straight and L shaped staircase.
Since the spiral staircase is for secondary purposes riser can be taken up 9.5”. Riser and floor height is used to calculate the no. of steps which are equivalent to Floor height/riser. The result includes the land platform as a step.
2) Angle of rotation of steps
All steps radiate from the central column and rotate at a certain angle when moving up or downstairs. The angle of rotation is preferred as 30 degrees mostly.
The number of steps and angle of rotation can be used to calculate the number of revolutions of steps in the spiral staircase as (number of steps * 30 degrees) / 360 degrees.
3) Outer diameter of staircase
The diameter of the spiral staircase depends on the opening provided on the ceiling which is determined from the respective code. It may vary from 3’3” (1000mm) to 6’6” (2000mm) depending on load and capacity required.
Other minor factor includes styles of tread and handrails, aesthetics, and functionality.
Recommended dimension of spiral staircase
Table 1 Recommend dimensions of spiral staircase is tabulated in the following table in accordance to British standard
Some general specification for design of spiral staircases
- The minimum headroom of the spiral staircase should be 6’6” which is measured from the bottom of the edge of the platform to the nearest tread below the edge.
- Clear path i.e. tread length excluding handrail portion should not be lesser than 17” wide but the size of not lesser than 23” is recommended excluding portion for the handrail and edge curves (can be up to 3”)
- Tread should have a minimum width of 7.5” at a distance 1’ from the face of the central column
- Minimum railing height is 3’ including handrail.
- It should be designed for a minimum load of 300psf.
- The landing platform should be at least 4” wider than the actual tread length
Steps in design of spiral staircase
- Determine the floor height from top to top of the slab including finishing works Ceiling opening is fixed on the basis of load and purpose of the structure during construction as shown in Table.2
- Measure the ceiling opening in case of the closed internal room where in case of a terrace or open spaces it is not required. And fix the outer diameter of the spiral staircase according to table 1.
- This table also helps to determine the dimension of the vertical column, the clear width of the stairway, actual length, and central width of treads.
Table 2 BS standard for determination of stairway diameter and riser
- Fix the value of riser on the basis of ceiling opening according to table 2 or table 1
- Calculate the number of steps by diving floor height with riser value and determine the number of risers which is equal to the number of steps.
No. of steps = floor height / riser = no. of treads including landing
- Check the number of steps with values defined in local building codes and adjust the riser if the not desirable result obtained.
- The angle of rotation of 30 degrees determined the total angle of rotation and number of revolution which may not exceed more than 1 for normal floor height
- Fix the position and dimension of the platform in exceedance to tread length by 4”.
- The treads or number of steps are counted downstairs from the platform.
- The staircase can be designed clockwise or counterclockwise.
I hope this articles on “Spiral staircase dimensions” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Jenish Shakya