Open steel mono-stringer with plate and gusset or full tread plate

The open tread mono-stringer is now an industry standard, due in no small part to Arden Architectural Staircases’ optimising this design solution.
Design

The open tread mono-stringer is now an industry standard, due in no small part to Arden Architectural Staircases’ optimising this design solution. The M2 open mono-stringer will maximise the open lines of sight and light penetration inherent in an open tread design.

The obvious features of this staircase style include a dramatic structural appearance with treads appearing to float over the central spine of the stairs. This effect is increased by the use of a “plate and gusset” tread support system rather than the more usual “folded tread plate” support system.

The choice of treads for the M2 is of particular importance. The mono-stringer's position underneath and away from the edges of the treads makes it retreat from view, and allows the impression of the staircase to be dominated by the treads themselves from all angles. The impression of the stringer is often further played down, through the choice of a dark or neutral pre-finish coat. Alternatively, one may choose to highlight its characteristics of strength and purity of line by choosing a more dramatic colour for its coating. Arden's customers frequently choose to complement the M2 with a glass balustrade to maintain the translucent character of this staircase.

As with all open staircases, the BCA rule on a gap of less than 125mm being required between the top of a tread and the underside of the tread above. Most modern staircases have a rise per tread of between 170mm and 190mm, thereby requiring a functional tread thickness of between 45mm and 65mm. The tread solutions for open staircases applicable to the M2 are provided in Arden technical data sheet ‘A.3 Treads’. The details illustrated draw upon these tread solutions.

Technical

The core member of the M2 is constructed from SHS or RHS mild steel sections of a size suitable for the required loads and spans. These sections are mitred and welded at the angle points into a continuous solid member. Support plates for the treads and landings are welded to the core. Mounting plates are also usually welded where required at this stage. On occasion instead of using mounting plates the stringer is welded directly to pre-existing steel structure on site.

While a mono-stringer generally only needs two support points per flight, it must be kept in mind that the supporting structure must provide rotational support as well as lateral and vertical support. On exceptionally long spans a central column support is a common solution to maintain a slim metal stringer line.

There are also two alternative tread support plate designs. Light duty stairs would use the folded tread plate design. Where high design loads or a wide flight is specified, a heavier gauge steel tread plate is called for. This would make a folded design impractical, and we would therefore recommend the use of the two piece orthogonal welded design.

Compliance

Arden Architectural is a BSA licensed contractor for carpentry, joinery, glass, glazing and aluminium as well as structural metal fabrication and erection. Arden supplies a Form 16 (Licensed Contractor) on all projects. In design and construct contracts, a Form 15 (Design Engineer) certification is supplied upon request. For products and services incorporating this design element, the table below details compliance with relevant codes and standards.

Code
Title Applicability
BCA The Building Code of Australia Full compliance
AS NZS 1170.1-2002 Structural Design Actions – Permanent, imposed and other actions Full compliance
AS 1288-2006 Glass in Buildings. Selection and installation Not applicable
AS NZS 1554.1-2004 Structural steel welding - Welding of steel structures Full compliance
AS 1554.6-1994 Welding stainless steels for structural purposes Full compliance
AS NZS 4586-2004 Slip resistance classification of new pedestrian surface materials Full compliance
AS 1428.1-2001 Design for access and mobility Conditional*
AS 1657-1992 Fixed platforms, walkways, stairways and ladders. Design,
construction and installation
Full compliance

* Requires opaque risers for compliance