How to design a spiral staircase using a BIM model: professional and technical guidelines
Project of a spiral staircase: types, advantages and minimum dimensions. A complete example ready for you to download
Spiral staircases are highly adaptable designs with daring shapes and different configurations that require careful planning attention. They can also be iconic project objects and great space-savers. Here, we examine different styles and dimensions of a spiral staircase design, also providing a 3D model available for you to download that has been produced with a professional software for architects and engineers.
Why chosing a spiral staircase?
The need to optimize spaces is one of the most recurrent requests that technical experts must address in order to deliver creative and at the same time functional solutions. In fact, when designing a staircase, it is crucial to combine aesthetic, functional and structural needs, such as defining the stairs type, shape, material, structure, location and correct layout. They are choices that obviously affect the final quality of a project.
A spiral staircase design guide can help you to better address all these aspects.
For more information on interior stairs planning, see our Focus insight on “How to design a staircase: criteria and examples to download”.
Spiral staircase: types and advantages
The spiral staircase is a particular type of curved staircase that is more commonly built in private and residential buildings, in small rooms or in particular environments.
Spiral staircases can adopt different structural configurations:
The circular spiral staircase is the most common type, as it occupies a smaller area than the other types.
Generally, the structure of the spiral staircases consists of a central support post that develops vertically where cantilevered steps are inserted.
You can use a wide variety of materials when building a spiral staircase, however they are usually built in metal or wood or rarely in reinforced concrete in case of outdoor staircases. Steps can be made with any material.
Opting for a spiral staircase rather than a traditional type of staircase (e.g. L-shaped configured with a rectangular plan, etc.) has obvious advantages, especially in terms of space management. A spiral staircase, in fact, occupies a small surface in plan and can also be positioned in stairwells with a diameter of about 1.20 m.
In addition, a wide range of prefabricated solutions are available on the market that also allow significant savings in terms of cost and installation time.
Spiral staircase dimensions
To determine the correct dimensions of a comfortable and efficient staircase, it is necessary to correctly evaluate the relationship between riser and tread.
Over the centuries, architectural and ergonomic studies have produced various empirical formulas to suggest an ideal rise / tread relationship and ensure safe use of the staircase.
The most commonly used formula to calculate stairs dimensions is attributed to the architect François Blondel, dating back to 1675. The famous formula is based on the fact that the effort made to lift the foot vertically is equal to twice the effort taken to move it horizontally.
He specified that twice the riser height plus the tread depth equals the step length:
2r + t = 62÷64 cm (25.5 in today’s inches)
where r is the value of the riser and t of the tread.
Once the correct relationship between riser and tread has been calculated and they are well proportioned to fit our stride, we need to determine each single value.
For each type of staircase, the tread must not be less than 25 cm. In spiral staircases the triangular shape of the step implies a variable tread from the center of the staircase to the handrail.
The actual tread dimensions for a set of stairs should be calculated considering the path of travel that connects different vertical levels by dividing the height between the levels into manageable steps, after having identified the stairs diameter and 30 cm distance from the handrail. The riser must be calculated keeping in mind the floor-to-floor height, that is the measurement from the top of the pavement floor (extrados) to the ceiling surface (intrados).
In fact, since the spiral staircase wraps around a central post, each step must be at least 2m away from their simmetrical equivalent on the upper module, so as to ensure an adequate level of safety for users passing through.
In small diameter spiral stairs (100/130 cm) where it is not possible to insert more than 12 steps per revolution, otherwise risking reducing the tread depth too much, the value of the riser will tend to be higher than in straight-run stairs.
Spiral staircase design: a practical example
Let’s now take a closer look at a case study that we have prepared to provide a practical example for your spiral staircase design.
Let’s assume that we need to design a circular spiral staircase. Having a floor opening with a diameter of 1.30 m, we need to overcome a height difference of 3.00 m.
It is recommended that the diameter of the floor opening is a few centimetres greater than your stair’s diameter so to easily accommodate body movements. In this case, we have determined the diameter of the stairs to be 1.20 m.
First of all, it is necessary to establish the position and width of the arrival landing and the length and position of the path of travel.
It is advisable that the width of a spiral staircase arrival landing is at least equal to the tread width. For this reason, we have designed an arrival landing with a width equal to the stairs radius (in this case 60 cm). We will thus have an equilateral triangle with internal angles equal to 60°.
For an ideal design, the path of travel should be placed at about 30 cm distance from the handrail. Consequently, for stairs with 1.20 m diameter, we will have a travel path diameter of 0.60 m.
Let’s now calculate the length of the travel path in plan:
0.60 m x π = 1.88 m
to which we need to subtract the space occupied by the arrival landing.
Therefore, the definitive value of the path of travel that can be used for the calculation is 1.88 m – (1.88 / 6) m = 1,57 m.
At this point we can draw an elevation view of the staircase that has been developed in plan view. We can define the path of travel measurement horizontally, that is included between the back and the front landing points (1.57 m) and vertically insert a 2.00m mesurement of the equidistant points on the two staircase modules. By connecting the two ends, we can obtain the actual path of travel equal to 2.54 m (that can be calculated with the Pythagorean theorem).
Next, we can proceed with the calculation of risers and treads.
Finally, we can draw the staircase in plan view to verify a comfortable access and exit from the stairs structure. If the entry point results uncomfortable, we can either rotate the entire staircase, thus also moving the exit point, or change a right-hand to a left-hand stairs direction.
The staircase that we have designed for this case study has a steel structure, with a central post with circular section. The contrasting materials choice, marble and black resin, produces an astonishing final result.
3D BIM model download
Here’s the 3D BIM model of a spiral staircase produced with an architectural BIM design software that you could use as a project example.
Download the 3D BIM model (.edf file) of a spiral staircase project