Guide

Mono stringer staircase engineering, spans, stringer depth, deflection

A mono stringer stair is a structural engineering problem dressed up as an architectural element. The single central beam carries everything, treads, live load, dynamic impact loads, seismic drift, and every visible imperfection downstream traces back to the engineering of that beam. This page is the technical reference we use internally when scoping mono stringer stairs out of our Burnaby shop and when coordinating with the structural engineers of record on Metro Vancouver projects.

The target audience here is architects, engineers, GCs, and technically-inclined clients who want to understand why a mono stringer stair quote differs from project to project. Readers looking for a general overview should start at the mono stringer overview; readers looking for design and material choices should go to the design guide.

Codes and standards that govern

A mono stringer stair in British Columbia is governed by a short list of codes and standards, each of which applies to a different part of the design.

• BC Building Code (BCBC) Part 9, governs residential stairs in small buildings. Defines minimum tread run (250 mm typical), maximum rise (200 mm typical), handrail height (865 to 965 mm from tread nosing), guard height (900 mm minimum interior, 1070 mm minimum exterior), sphere test (100 mm maximum opening in guards), and live load (1.9 kPa).
• BCBC Part 3, governs commercial and assembly occupancies. Same geometric requirements as Part 9 but with higher live loads (4.0 kPa offices, 4.8 kPa assembly areas), more restrictive egress width requirements, and additional fire-resistance and guard load requirements.
• CSA S16, Design of Steel Structures, governs the structural design of the stringer and its connections. Bending capacity, shear, deflection, and connection design are all driven by S16.
• CSA W47.1, Certification of Companies for Fusion Welding of Steel, requires that any fabricator doing structural welding on the stringer be certified. Jeff and Simon Ironworks has held W47.1 Division 2 certification for all structural steel we fabricate.
• CSA W59, Welded Steel Construction, governs the actual welding procedures, qualifications, and acceptance criteria for structural welds.
• CISC Handbook of Steel Construction, the practical reference for section properties, connection design examples, and fabrication standards used by nearly every Canadian steel engineer and fabricator.

HSS tube vs built-up plate stringer

The choice of section type is the first engineering decision after span and load are known. Both HSS and built-up plate are valid for mono stringer stairs; the right choice depends on depth required, visual intent, fabrication complexity, and cost.

HSS tube stringer is the workhorse. A rectangular hollow structural section, typically 8×4, 10×4, or 12×4 inch, with wall thickness 3/8 or 1/2 inch. The section is efficient in bending about its strong axis, the welding to tread brackets is straightforward because the bracket lands on a flat tube face, and the section properties are published in the CISC Handbook. HSS also has an advantage for concealed services, LED wiring, power for tread lights, or pneumatic lines can be routed inside the tube.

Built-up plate stringer is a custom section fabricated from flat plate welded into a C, T, or I profile. Used when the depth or geometry needed is outside standard HSS sizes, when architectural intent calls for a particular cross-section (tapered, asymmetric, with integrated features), or when bracket keyways or LED channels have to be machined into the section. Fabrication labour is meaningfully higher because every weld is developed rather than a closed tube, and weld inspection is more intensive because more welds are load-carrying. On the structural side, the plate section can be optimized precisely for the load, no wasted material in the weak axis if the stair does not need it.

Deflection limits, why L/360 actually matters

Deflection, not strength, governs almost every mono stringer stair design. A stringer with 200 MPa of bending stress under live load will probably feel rock-solid; a stringer with 10 mm of additional midspan deflection under live load will feel bouncy and uncomfortable regardless of stress. The BC Building Code and CSA S16 set L/360 as the general serviceability limit for floors, and engineers have adopted it for stairs as well.

For a 16 ft (4880 mm) mono stringer, L/360 is 13.5 mm of midspan deflection under full live load. L/480 (commercial) is 10 mm. These are small enough to pass the "stand on the stair and feel it" test. L/240, which some codes allow for certain elements, produces roughly 20 mm of deflection on a 16 ft span, which feels alive underfoot even when the bending stress is nowhere near its limit.

We design to L/480 or better on residential stairs as a standard even when L/360 is technically allowed, because perceived quality of the stair is driven by stiffness more than anything else. The marginal cost of a slightly deeper section is negligible compared to the cost of a stair that wasn't stiff enough and has to be retrofitted.

Stringer depth reference table

Indicative starting-point stringer depths for residential mono stringer stairs, HSS tube with 1/2 inch wall, designed to L/480 under 1.9 kPa live load. Commercial loading pushes each of these up by one size class. These are starting points only, final sizing comes out of the engineer's stamped calc.

Tread bracket design

Each tread cantilevers off the stringer through a welded bracket. The bracket is a short cantilever loaded by the live load on the tread; the bracket-to-stringer weld is loaded in shear and bending. Typical bracket is 1/4 to 3/8 inch steel plate, welded perpendicular to the stringer with an 8 mm multi-pass fillet all around, sized per the engineer's detail.

The bracket loading depends on the tread depth. A 12-inch deep hardwood tread with a 90 kg person standing on the nose generates a moment at the stringer weld of roughly 260 N·m. A 15-inch deep tread generates 330 N·m. The welds are sized with a comfortable margin over those numbers because the fatigue performance of a stair tread weld matters over the life of the stair.

Top and bottom connection detailing

The stringer transfers its reactions to the building structure through connections at both ends. These connections are where mono stringer projects most often go wrong, the stringer itself is usually well-engineered, but the detail at the support is afterthought.

Top connection: typically a welded moment connection into a steel beam or embedded plate. The connection has to transfer vertical reaction (roughly half the stair load), horizontal reaction from seismic and live load, and a moment if the stringer is fixed at the top. We detail this with a plate welded to the stringer end and bolted to an embedded plate in the concrete or welded to the structural beam. The connection is designed per CSA S16 and stamped by the structural engineer.

Bottom connection: typically a base plate bolted to a concrete slab with post-installed anchors, or a welded connection to embedded structural steel. The bottom usually carries the other half of the vertical reaction plus the horizontal reaction. In seismic zones like Metro Vancouver, the bottom connection is often detailed as a pin or slotted to allow controlled drift rather than fighting the horizontal earthquake load.

Seismic detailing for Metro Vancouver

Metro Vancouver is in seismic category 4 under the BC Building Code, which means stair connections have to account for the horizontal loads that arise during the design-level earthquake. For a mono stringer stair connecting two floors, the concern is that the two floors move relative to each other (inter-storey drift) and the rigid stair has to accommodate that movement without failing at the connections.

Typical detailing strategies: a fixed connection at one end (top or bottom) and a pinned or slotted connection at the other to allow drift; heavier bolting on both ends with drift calculated through the stringer itself; or, on very long runs, an intermediate expansion joint. The engineer of record makes this call based on the building's lateral system and the expected drift under the governing load case. CSA S16 covers the base design; CSA W47.1 certified welding is required on every structural weld in the load path.

Fabrication tolerances

The engineering calc tells you the stringer will work if it gets built to the detail. Actually building to the detail requires shop discipline. Our internal tolerances on mono stringer fabrication are tighter than CISC shop standard in the places that matter visually, tread bracket spacing to ±2 mm, bracket perpendicularity to ±1°, stringer camber to ±3 mm of nominal, because a stair where every tread is in a slightly different position reads as imprecise even when every dimension is within CISC standard.

Welds are dressed and ground on the visible faces of the stringer because powder coat does not hide a lumpy weld. This adds shop labour that does not show up on an engineering drawing but shows up immediately in the finished stair.

What the engineer's package includes

A stamped mono stringer package from the structural engineer typically includes: loading assumptions and governing code references, stringer sizing calc with deflection check, bracket sizing calc, top and bottom connection calcs, seismic drift check, shop drawings marked up with welding and connection details, and the engineer's seal on each sheet. We coordinate this package for every mono stringer stair we fabricate. For a project-specific quote that includes the engineering, request a quote.

Related reading

Move on to the design guide for tread, nosing, and finish choices, or to the installation guide for on-site mechanics. The cost guide covers how engineering choices affect price.

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