Aluminium Profile Stair Stringer 45 x 90 mm for Access Platforms

01What actually limits the span of a profile stair stringer?

Deflection, almost every time. An aluminium 45 x 90 mm stair stringer at about 4.85 kg/m is strong enough for a person on a tread by a wide margin, and it is still the wrong section if it bends enough to feel like a spring. Access stairs are judged by how they feel underfoot, and a stair that deflects visibly gets rejected regardless of its factor of safety.

The usual working limit is span divided by 300. For a 2.4 m stringer that is 8 mm, and 8 mm is roughly where a stair stops feeling solid. That number, not the yield strength, is what sets the section.

02How do you size the stringer for a span?

Start from the tread load, split it between two stringers, and solve for the second moment of area that keeps deflection inside the limit. For a simply supported beam with a central point load, delta = P x L^3 / (48 x E x I), and setting delta to L/300 gives a direct expression for the required I:

Formula

I required = 6.25 x P x L^2 / E

Assumed tread load

1.5 kN, shared between two stringers

P per stringer

750 N

E, aluminium

70 000 N/mm2

Simplified

I required = 0.06696 x L^2, with L in mm

Two things about that table are worth spelling out. First, the requirement grows with the square of the span, so the last row is not twice the third row, it is more than twice. Second, a 40 x 40 section is in the region of 8 cm4, which is why a 40 x 40 stringer only works up to about a metre — and why a 40 x 40 stair that “seems fine” at 1.2 m is a stair that bounces at 1.8 m.

For this section, a rough estimate from the mass puts it well above every requirement in the table, which is consistent with the practical range of about 1.5 to 2.4 m per stringer. Check the actual I against the section data sheet rather than trusting the estimate — the estimate assumes an average wall and ignores the slot grooves.

03Why does a stair fail at the connection before it fails at the beam?

Because a stringer is not a simply supported beam in reality. It is bolted to a platform at the top and to a floor plate at the bottom, and those two connections decide whether the calculated deflection is what you get.

The last row is the one that surprises people. A guard rail post is a cantilever, and a person leaning on it applies a load at the top of a lever arm of about a metre. A single bolt into the stringer web will move visibly. The post needs either a backing plate that spreads the load along the slot, or its own frame.

04How are the treads fixed?

Two M10 T-nuts per tread end, in the stringer slot, is the normal arrangement. The slot gives adjustment, which matters because the tread pitch has to be right for the whole flight, not just the first step.

• Tread pitch: 200 to 250 mm rise with a 250 to 300 mm going is comfortable for most industrial access. Consistency matters more than the exact figure — an inconsistent rise is the most common cause of trips on a new stair.
• Two fixings per end, not one. A single fixing lets the tread rotate under an eccentric foot load, and it will creak.
• Tread surface: perforated or serrated for wet or oily areas, and open enough to drain. A solid tread in an outdoor location holds water and becomes the slip hazard it was meant to prevent.
• Keep the tread clear of the slot mouth where possible, so the T-nut seats on the flat of the groove rather than on the edge.
• Check the first tread after a week of use. New stairs settle at the connections, and the first tread takes the most traffic.

05What is our scope, and what is not?

This matters more on a stair than on any other page, so it is stated plainly rather than in a footnote.

Access stairs to machinery are covered by standards such as EN ISO 14122-3, which sets the rail heights, the load cases and the geometry. Those requirements are the designer’s to apply and to certify. What we can do is supply extrusion that meets the section tolerance and the alloy you specify, and cut and machine it accurately — which is the part we are actually good at.

06What about the platform itself?

The same section works for platform framing, with one difference: a platform beam is loaded by a distributed load rather than a point load, so the deflection calculation is different and generally kinder at the same span. Use delta = 5 x w x L^4 / (384 x E x I) for a uniformly distributed load, with the same L/300 limit.

Two practical notes on platforms. Decking adds stiffness if it is fixed to the beams properly — a screwed deck acts as a diaphragm and stiffens the frame in plane, which is why the deck fixing pattern matters and not just the deck thickness. And a platform is a frame, not a set of beams: bracing in the horizontal plane is what stops it parallelogramming, and it is the first thing to be left out.

07Questions from fabricators and project engineers

Related sections and categories

Browse Stairway and Platform Systems, 45 Series Aluminium Profiles, or see the 45 x 90 mm six slot heavy section for the full section data, and the 45 x 180 mm ten slot section when the span needs more depth than 90 mm gives.