1. Introduction to Concrete Stairs
A staircase is a flight of steps between multiple levels. Simple, right? Well, as it turns out, there’s much more engineering behind choosing a standard size from a manufacturer. Most staircases are specially designed to meet required design standards. Some key design standards include AISC 360/16 or ASIC 318/19 - Specifications for structural steel or concrete buildings, respectively, Eurocode 2 or 3 - Design of Concrete and Steel Structures, respectively, or in Australian Standards - AS 1657.
Safe to say, staircases need to be designed as structural components, which can be built in a variety of styles and finishes to suit any home or commercial building. Staircases are most commonly made from steel or concrete or both, and even then, the options are endless. Regardless of the complexity, engineers should treat staircases as a system of beams and supports that carry the load. Below is a summary of some of the universal key terms and definitions ubiquitous in staircase design. What follows is a step-by-step guide to RC Staircase Design to Australian Standards.
Key Technical Terms
When planning and constructing concrete stairs or staircases, understanding key technical terms is essential for a successful project. Precast concrete refers to concrete elements, such as stair treads or entire flights, that are manufactured off-site and then delivered to the construction site for installation. This method can streamline the construction process and ensure consistent quality. Hollow core concrete is a type of precast concrete that features hollow sections within the slab, reducing the overall weight while maintaining the necessary strength for safety and durability. The stair treads are the horizontal surfaces you step on, while risers are the vertical components between each tread, both of which must meet specific requirements for comfort and safety. Handrails are installed along the sides of stairs to provide support and prevent accidents, and their design and installation are critical for compliance with safety standards.
By familiarizing themselves with these terms, clients and builders can communicate more effectively, ensuring that all requirements are met and that the installation process on site proceeds smoothly. This shared understanding is vital for the construction of safe, functional, and aesthetically pleasing concrete staircases.
2. Staircase Design Steps with AS1657 and Building Code of Australia
Designing a staircase that meets Australian standards involves a series of well-defined steps, each crucial to the success of the project. The process begins with a thorough assessment of the site to determine the most suitable size and design for the staircase, taking into account the specific requirements of the project. Choosing the right materials—whether timber for a warm, natural look or concrete for strength and durability—is a key decision that affects both the weight and long-term performance of the staircase. For commercial buildings, additional considerations such as the inclusion of handrails, appropriate landings, and compliance with safety regulations are essential to ensure the staircase is both functional and safe for everyday use. Throughout the design process, attention to detail and adherence to the Building Code of Australia and AS1657 help guarantee that the finished staircase will not only meet all regulatory requirements but also provide reliable functionality and enhance the overall design of the space.
Steps to Staircase Design
(According to AS1657 and the Building Code of Australia (BCA))
Architects and designers are spoiled for choice when it comes to staircase design. From a feature and floating stairs to transparent stairs, staircases no longer only need to meet a purely functional purpose. They can be the centerpiece of building design. However, local regulations and standards often limit the use of innovative designs and materials. Australian Standards are quite complicated when it comes to leveraging unique staircase designs. So, we’ve provided a step-by-step guide to RC staircase design using Australian standards and regulations. Certain design requirements may vary depending on the specific application or standards, so it is important to review the relevant codes for each project.
Staircases used in habitable rooms or externally must comply with the Building Code of Australia (BCA) Part 3.9.1. All areas not specified in BCA should be referenced in the Australian Standard AS1657 Section 4.
Staircases in storage areas or un-habitable spaces such as attics can be constructed outside the scope of the BCA, but designs must comply with AS 1657.
Three key Australian standards outline requirements for the design, construction, and installation of stairways:
- AS 1428.1: Design For Access and Mobility, General Requirements for Access - New Buildings
- AS 1657: Fixed Platforms, Walkways, Stairways, and Ladders - Design, Construction, Installation
- AS/NZ 4586: Slip Resistance Classification of New Pedestrian Surface Materials
Before we go into a step-by-step guideline for staircase design, we’ll summarise some of the key design requirements and specifications for staircases according to the BCA and the design standards listed above. Stair treads can be custom-made to order to suit the needs of each job, ensuring compliance and adaptability for any project.
2.1 Risers and Stair Treads
The design of risers and stair treads is fundamental to the safety, comfort, and visual appeal of any staircase. AS1657 sets out precise requirements for the maximum height of risers and the minimum depth of treads, ensuring that each step is easy and safe to use.
- A staircase's pitch angle shall be not less than 26.5 degrees and not greater than 45 degrees.
- A staircase must have no more than 18 risers without a 750 \(mm^2\) landing or rest area.
- The staircase should have no more than 36 rises without a change of direction.
- Your staircase must have no less than two risers or no more than 18 risers without a 750 \(mm^2\) landing or rest area.
- Each tread and riser must be the exact measurement within a single flight.
- Tread depth must be greater than or equal to 185\(mm\).
- All treads and top nosing must have a slip-resistant finish or a non-slip strip system near the edge of each tread nosing.
- If a door in your home opens onto a staircase, a landing is required unless the floor-to-floor dimension is less than 570 \(mm\). If the floor-to-floor is less than 570 \(mm\), all that is required is a zero tread.
When working with concrete stair treads, special attention must be paid to the production process, particularly if using precast concrete. This approach allows for high-quality, consistent stair treads that can be efficiently installed on site, making them ideal for both residential and commercial projects. The careful design and installation of these elements not only fulfill regulatory requirements but also contribute to an elegant and inviting grand entrance—whether in a home or a large commercial building. Every detail, from the finish of the concrete to the precise dimensions of each tread and riser, plays a role in creating a staircase that stands out for its quality and functionality.
2.2 Handrails
Handrails are an indispensable part of any staircase, serving as a primary safety feature that helps prevent falls and provides support for users. Both the Building Code of Australia and AS1657 outline strict requirements for the installation and design of handrails, including their height, continuity, and the materials used. For staircases with multiple flights, handrails must be continuous to ensure uninterrupted support, while spiral stairs require special design considerations to maintain safety.
Handrails are required on all exposed sides of a staircase unless there’s a fixed structure within 10\(cm\) of the stairway, and every staircase needs one handrail. A handrail is required on both sides of a stairway if it is wider than 1\(m\).
2.3 Flights
A flight is a continuous series of steps between two landings, and its design is a key factor in the overall functionality and appearance of a staircase. To be compliant, all the steps on the same flight must be identical.
With floors or balconies which have the potential of people falling more than 4000\(mm\) to the surface below, all horizontal elements between 150\(mm\) and 760\(mm\) above the floor or balcony must not facilitate climbing.
In commercial buildings, the design of flights must prioritize safety and efficient movement, while in residential settings, flights can be designed to create an elegant and welcoming grand entrance.
2.4 Loadings
The concept of loadings defines the amount of weight the structure must safely support throughout its lifespan. This includes not only the weight of people using the stairs but also the additional loads from features like handrails, landings, and any applied finishes. When it comes to loads, Australian standards specify that stairs need to have:
- A minimum loading of 2.5\(kPa\), or an equivalent linear load of 2.2\(kN/m\).
- A minimum point load of 1.5\(kN\) is applied on a 10\(cm^2\) area on each step.
Note: loading requirements differ for landings.
2.5 Balustrades
The height of the balustrade on the pitch of the staircase must be not less than 865\(mm\) from the nosing line. This dimension is taken from the nosing line with the rail or balustrade running parallel up the pitch of the staircase.
The height of the balustrade on a finished floor, balcony, landing, or path must be above 1000\(mm\).
2.6 Platforms and Landings
Any stairway with more than 15 steps must incorporate a landing in between flights of stairs.
3. RC Staircase Design to Australian Standards
(A step-by-step guide to RC Staircase Design to Australian Standards)
A staircase must be designed so it fits properly in a proposed building plan; therefore, it is difficult to provide definite dimensions and guidelines without the exact type and orientation of the building. Nevertheless, we've provided some general steps and a cheatsheet for simply supported longitudinally spanning RC stairs:
In the case of longitudinally spanning stairs, the supports to the stair slab are parallel to the riser, commonly in two locations, causing the slab to bend longitudinally between the supports.
3.1. Determine the height of each flight
The height of each flight is determined by dividing the total story height by the number of flights. Remember that according to Australian standards, the minimum number of flights is two, equivalent to two risers or a minimum height of 230\(mm\). The maximum number of risers in a flight before a landing is required is 18.
3.2. Determine flight width
The designer chooses the width of each flight. In residential settings, the minimum codified width of a flight is 600\(mm\); this is generally very narrow for most circumstances, as often furniture or fixtures need to be taken up the stairs. So, ideally, a width of 1000\(mm\) is desirable. Note that stair width is measured from the inside of a handrail to the inside of the other handrail. However, this staircase configuration is more common in commercial settings than residential ones, so care must be taken when measuring flight width.
3.3. Calculate the number of Risers and Treads
The number of risers required is the total height of each flight divided by the height of each riser:
$$\begin{align}\text{Number of risers} = \frac{\textit{Total height of each flight (mm)}}{\textit{Height of individual riser (mm)}}\end{align}$$
The number of treads is calculated as the number of risers minus 1:
$$Number of treads = Number of risers −1$$
3.4. Calculate the Effective Span \(L\)
In a longitudinally spanning staircase and a simply supported arrangement, the effective span \(L\) can be taken as the distance between the centrelines of support:
$$\begin{align}L = \textit{Going}(G) + \frac{\textit{Landing width}}{2}(x) + \frac{\textit{Landing width}}{2}(y)\end{align}$$
Where:
$$\begin{align}x &= X \quad \textit{but} < 1.8\, m \quad (\textit{whichever is less}) \\y &= Y \quad \textit{but} < 1.8\, m \quad (\textit{whichever is less})\end{align}$$
3.5. Assume waist slab thickness
The thickness of a waist slab can be assumed to be between \(L/20\) and \(L/25\) where \(L\) is the effective span calculated in Step 4 or between 40\(mm\) to 50\(mm\) per metre run of horizontal span.
3.6. Calculate the total load on the staircase
As per the standards, stairs should have a loading capacity of 2.5\(kPa\) and 2.2\(kN\) per linear meter whilst also withstanding a point load of 1.5\(kN/10cm^2\) on each step. Total loads \(w\) on stairs include the dead load + live load + floor finishes where:
- Dead Load: self-weight of steps + self-weight of waist slab (common in RC stairs) + self-weight of tread finish
- Live Load: generally taken between 2\(kPa\) to 4\(kPa\) (refer to Table 3.1 of AS 1170.1 structural design actions for reference values for imposed loads). Where a stair tread or landing is structurally independent of the adjoining elements, it should withstand a line load of 2.2\(kN/m\) of the span of the tread or landing.
- Floor finishes: generally taken between 0.8\(kPa\) to 1\(kPa\).
Note: that the total load obtained above will be inclined, so it must be converted into an equivalent horizontal load by multiplying it with a factor that factors in the length of the tread and height of the riser and then finally multiplied by the appropriate factor of safety obtained in AS1170.1:
$$\begin{align}\textit{Factor} = \frac{\sqrt{R2 + T2}}{T}\end{align}$$
Note: on the landing area, the weight of steps is not included.
3.7. Calculate the maximum bending moment
With the maximum \(w\) loading obtained in Step 4, the reactions at each end support can be calculated using equilibrium equations. Given reactions and load values, the maximum bending moment can be calculated where it will occur at a point of zero shears. Suppose the system is treated as a simply supported beam with a UDl. In that case, the max bending moment can be found as:
$$\begin{align}M_{max} = \frac{W L^2}{8}\end{align}$$
Where \(W\) is the factored total uniformly distributed load from Step 5, \(L\) is the effective length of the staircase calculated in Step 4.
Flights with significant end restraint, such as those that are continuous with their supporting beams or slabs, may be designed for a mid-span design moment of:
$$\begin{align}M_{max} = \frac{W L^2}{10}\end{align}$$
Using the maximum bending moment, you can calculate the minimum waist slab depth required using this equation and solving for \(d\):
$$\begin{align}M_u = A_{st} f_{sy} 0.925 d\end{align}$$
Where :
$$\begin{align*}f_{sy} &= \text{yield strength (MPa)} \\A_{st} &= \text{cross-sectional area of longitudinal tensile reinforcement} \\d &= \text{depth of waist slab (mm)}\end{align*}$$
3.8. Reinforcements
The main reinforcement for stairs is calculated as:
$$\begin{align}P_{approx} = \frac{2.54 M^*}{d^2}\end{align}$$
\(P_{approx}\) is the reinforcement for the slab per metre width of the slab and it provides a moderate degree of crack control for flexural strength at first interior support.
The minimum reinforcement for the waist slab to not fail upon cracking is given by:
$$\begin{align}P_{min} = 0.2 \left( \frac{D_s}{d} \right)^2 \left( \frac{f'_{ct,f}}{f'_{sy}} \right)\end{align}$$
Where:
$$\begin{align*}D_s &= \text{overall depth (mm)} \\f'_{ct,f} &= 0.6 f'_c (\text{MPa}) \\f'_{sy} &= \text{characteristic yield strength of reinforcement (MPa)}\end{align*}$$
The minimum reinforcement for crack control (temp and shrinkage):
$$\begin{align}P_{min} = 0.75 \times 0.0035 \times \left( \frac{D_s}{d} \right)\end{align}$$
Finally, the area of steel reinforcement required is:
$$\begin{align} A_{s(required)} = \rho \times b\times d(mm^2)\end{align}$$
Where :
$$\begin{align}\rho &= \text{required reinforcement} \\b &= \text{width of the slab (taken as 1000 mm)} \\d &= \text{depth of slab (mm)}\end{align}$$
In the simply supported case, a positive moment will be induced by the uniform loading, causing tension on the underside of the beam, and requiring reinforcement to be placed on the bottom side of the beam.
3.9. Check for Shear
The slab's design shear strength shall be calculated per Clause 8.2 of AS3600: 2018. Flexural shear is unlikely to be a critical condition; however, your design must satisfy the following condition:
$$\begin{align}\phi V_{uc} \geq V^*\end{align}$$
Where :
$$\begin{align*}V_{uc} &= \textit{ultimate shear strength} = k_v b_o d_o \sqrt{f'_c} \\k_v &= \frac{0.4}{1 + 1500 \varepsilon_z}\end{align*}$$
Where \(x\) is the longitudinal strain factor for concrete, and the formula for this factor is a bit of a doozy, so AS3600-2018 gives allowance for a simplified approach and the process can be found in Clause 8.2.4.3 in AS 3600-2018.
Note that this simplified approach only applies to non-post tensioned sections. If you are confident that your post-tensioning effects are increasing beam shear capacity (most of the time they will), the simplified approach would be safe to use.
bv = effective width of a web for shear
dv = effective shear depth (see clause 8.2.1.9)
3.10. Check for Deflection
Strength requirements are not always critical for stair slabs. It is, therefore, essential that the other limit states are checked, particularly deflection. For concrete longitudinal straight stairs, as before, it is treated as a one-way spanning slab and deflection requirements can be addressed per Clause 9.3.2 or Clause 9.3.3 in AS3600, making appropriate allowances as per the standard.
