Access structures · load and geometry

Aluminium Profile Stairs and Platforms: The Geometry and Load Rules Behind Them

A stair is not a frame with steps on it. It is one of the few aluminium structures where the dimensions are dictated by rule rather than by choice, and where the governing case is a person rather than a machine.

Application: Access stairs, mezzanine, work platform
Governing case: Person, moving and off-centre
Key ratio: 2R + G ≈ 600–650 mm
Read: 10 minutes

Short answer

Aluminium profile stairs and platforms are sized from two things a normal frame never has to consider: the step geometry, which is fixed by a relationship between riser and going rather than by strength, and a live load that arrives as a moving person rather than as a static machine. Typical industrial platform live loads fall in the few kilopascals range, handrails are checked against a horizontal line load at the top rail, and step proportions follow a rise-plus-going formula. The exact figures are set by the code that applies to the installation, so the drawing should state the code and the load class it was designed to.

[01]

Why a stair is the odd one out

Every other aluminium structure on a plant is designed against a load the engineer chooses: a machine of known mass, a conveyor with known product weight, a guard resisting a known impact. A stair is designed against a load that is specified for you, and against dimensions that are constrained before any metal is selected.

That inversion has a practical consequence. You cannot size a stair by picking a section and checking it; you start from the geometry the code permits, derive the spans it forces, and only then choose the stringer. Teams that work the other way round produce stairs that are strong enough and cannot be built to a compliant angle.

The second difference is the load itself. A machine sits still and its weight is known to within a few percent. A person moves, can stop suddenly, can carry something, and can stand at the worst possible place. That is why stair and platform design uses a distributed live load plus a separate concentrated load check, rather than a single number.

On the numbers in this article

The figures given are typical industry ranges used to explain the method. The values that govern a real installation come from the building or workplace code that applies at the site. Specify the code on the drawing; do not take a load class from an article.

[02]

Step geometry: one formula, very little freedom

A stair is described by two dimensions: the riser, which is the vertical height of each step, and the going, which is the horizontal depth you actually stand on. Individually they can vary a little. Together they are constrained by an empirical relationship that has survived because it matches how people walk.

The common form is twice the riser plus the going landing in a band around 600 to 650 millimetres, with the two dimensions in the same units. A steep stair with a small going fails the relationship even if every individual dimension is legal, and it is the sort of stair that feels wrong to use before anyone can say why.

This matters for aluminium because the profile system is modular. The stringer is cut to give the riser and going the code allows, and the tread carriers are positioned on the slot pitch. If the slot pitch and the required going do not divide cleanly, the stair gets an awkward tread position or a carrier that sits between slots — both of which are design problems, not fitting problems, and both are cheaper to resolve on paper.

Dimension	What it controls	Typical range	Why it is constrained
Riser	Vertical effort per step	150–200 mm	Too high is tiring; too low makes the flight long
Going	Foot support depth	250–320 mm	Too shallow and the heel has no support
2R + G	Walking rhythm	600–650 mm	Empirical match to human stride
Flight width	Capacity and escape	Set by use and code	Escape routes have minimum widths
Handrail height	Fall restraint	900–1100 mm	Height where a falling body is arrested

[03]

Two load cases, and the second one governs

A platform is checked twice. The first check is a uniformly distributed live load across the whole deck: a crowd, stored material, or general working load, expressed in kilopascals. The second is a concentrated load applied over a small area: one person, or one heavy item set down at the worst point.

The second case usually governs the deck and the treads, because a concentrated load on a single tread produces a local bending moment that the distributed case does not capture. A deck that passes the distributed check can still fail the point check on one tread, which is why stair treads are specified to their own load requirement rather than inherited from the platform.

The third case, unique to stairs and platforms, is horizontal. A handrail has to resist someone falling against it, and that is a line load applied at the top rail, transferred down the posts and into the deck edge. It is the case most often omitted from an aluminium stair enquiry, and the one that decides the post section and the base fixing.

CASE 1

Distributed live load. Across the whole deck, in kilopascals. Governs the deck spanning members.

CASE 2

Concentrated load. On one tread or one panel. Usually governs the tread and the deck panel itself.

CASE 3

Horizontal line load. At the top rail. Governs the posts and their fixings to the structure.

CASE 4

Self weight plus deflection. A bouncy platform is rejected even when it is safe.

[04]

Slip resistance and the opening between treads

Two detail requirements catch aluminium stairs out, and neither is about strength.

The first is slip resistance. A smooth extrusion surface is slippery, particularly with dust or oil on it, so the tread surface has to be specified rather than assumed. Perforated or serrated tread plate, a bonded grit surface, or a purpose-made tread profile all solve it, and the choice depends on the environment: grit surfaces clog in some processes, open perforations drop small parts in others.

The second is the opening between treads. On an open stair the gap through which a foot or an object can pass is limited, and a stair built from widely spaced treads on a sloping stringer can leave an opening that fails this without anyone noticing until inspection. The fix is either a closed riser or a tread spacing that keeps the opening within the limit.

Tread surface options

Serrated or perforated plate — good grip, drops small items
Bonded grit coating — good grip, can clog and wear
Purpose-made tread profile — consistent, higher unit cost
Smooth plate — only acceptable where kept dry and clean

What decides it

Is the stair indoors or exposed to weather
Is there oil, dust, swarf or product residue
Are small parts carried up and down
Is it an escape route with a cleaning regime
Does it need to be hosed down

[05]

What to send with a stair enquiry

A stair or platform enquiry needs more geometry than any other aluminium structure, because the dimensions are constrained from the outside. Six items cover it; the one most often missing is the code.

RFQ checklist — stairs and platforms

Applicable code and load class — state it first; everything else follows from it.
Total rise and available run — the flight has to fit both, and they set riser and going.
Flight width and whether it is an escape route — escape routes carry extra requirements.
Tread surface specification — and the environment: wet, oily, dusty, hosed down.
Handrail requirement — height, both sides or one, and whether a mid rail or kick plate is needed.
Platform size and its live load — separately from the stair, because they differ.
Base condition — floor type, level, and whether the stair is anchored or freestanding.
Finish — indoor mill or anodised, outdoor a thicker class, plus the fastener material.

Send a sketch with the total rise and the available floor space marked. Those two dimensions decide whether a compliant stair fits at all, and they are the fastest way to find out.

Four ways an aluminium stair goes wrong

Geometry derived from the section rather than from the codeThe stringer is cut to suit a modular length and the riser ends up outside the permitted band. Start from the code, then cut the stringer.

Treads inheriting the platform load caseA concentrated load on one tread is a different case from a distributed load across the deck. Treads need their own requirement.

Handrail horizontal load not checkedThe posts and their fixings are sized by someone falling against the rail. Omit this and the rail is decorative.

Smooth tread surface left unspecifiedAn as-extruded surface with dust or oil on it is a slip hazard. Specify the tread surface, and choose it for the actual environment.

Related on this site

PG40 80 x 40 six-slot heavy profile— a common stringer section for longer flights
PG45 90 x 45 six-slot heavy profile— heavier stringer and post option
Strut profiles category— the structural section range
Aluminium shelving: buckling before bending— the same stability logic applied to uprights

Terms used in this article

Riser: The vertical height of one step.
Going: The horizontal depth of one step, measured nosing to nosing.
2R + G: An empirical relationship between riser and going that matches human stride; used to check a stair feels right to walk.
Concentrated load: A load applied over a small area, checked separately from the distributed load.
Line load: A load distributed along a line, here along the top of a handrail.
Kick plate: A low vertical plate at the edge of a platform that stops objects being pushed off.

Questions buyers ask

Can aluminium profile really be used for a code-compliant stair?

Yes, and it is common for mezzanines and plant access. The difference from a machine frame is that the geometry is constrained by rule and the load case is specified for you. Design to the code first, then select the section.

What section should I use for stringers?

It follows from the flight span and the load class rather than from a habit. Short flights on a standard platform are often served by a 40 series section; longer unsupported flights need a deeper section or an intermediate support.

Do I need a closed riser?

Only if the opening between treads would otherwise exceed what the code allows. Open stairs are common and acceptable where the tread spacing keeps the gap small enough.

Why does my platform feel bouncy when it passes the load check?

Deflection and strength are separate limits. A platform can be safe and still unacceptable because it moves under foot. State a deflection limit with the load case.

Should outdoor stairs be anodised or powder coated?

Both are used outdoors. The decision is about coating thickness and cut-end sealing as much as about the process, and the fastener material has to suit the environment too.

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