How Steel Stairs Are Designed: Loads and Sizing

Steel stairs are designed in a set order: first the geometry to meet the Ontario Building Code (rise, run, width, headroom and landings), then the loads (1.9 kPa for stairs serving one dwelling unit and 4.8 kPa for other stairs, plus the stair's own weight), then the treads, stringers, landings and connections, and finally stiffness, vibration and the guard and handrail loads. On most stairs, strength is the easy part. What usually sets the member sizes is stiffness, how the treads attach and how the stair connects to the building. This guide walks through each step and includes a simple worked load example, so you know what your designer or fabricator is checking.

Step 1: Fix the geometry

Every steel stair design starts with layout, because the code dimensions control everything that follows:

• Total rise, measured from finished floor to finished floor
• Number of risers and treads, within the limits for private or public stairs
• Stair width and the clear height (headroom) along the whole flight
• Landings wherever the code requires them, including where a flight would rise more than 3.7 m
• Plan space for the run, landings and door swings

The rules themselves are in Ontario Building Code rules for steel stairs, and the arithmetic for rise, run and stringer length is in the steel stair calculator guide. If the stair has an intermediate platform, see steel stair landings.

Get the finished floor build-up at the top and bottom right at this stage. A stair designed to bare slab that ends up under 20 mm of tile fails the riser uniformity rules, and that can't be fixed with a calculation.

Step 2: Work out the loads

Live load

The 2024 Ontario Building Code (Article 9.8.9.1.) requires stairs to be designed for strength and rigidity under uniform specified loads of:

Larger buildings designed under Part 4 use the live load table there, which lists 1.9 kPa for stairs within dwelling units and 4.8 kPa for exits and fire escapes. An industrial stair or platform may need more if equipment or materials are carried on it; the designer decides.

Dead load

Dead load is the weight of the stair itself: stringers, treads, risers, landing frames, concrete fill in pan treads, wood or stone tread finishes and the railing. Concrete-filled pan stairs and stone treads are heavy, and that weight has to go into the stringer and connection design and into the structure that supports the stair.

Load factors

For strength, Part 4 combines loads with factors. The usual governing case for a stair is 1.25 times the dead load plus 1.5 times the live load (Table 4.1.3.2.-A, load case 2). Stiffness and vibration checks use the unfactored (specified) loads.

Step 3: A worked stringer load example

Here is a simplified hand calculation for one straight flight. It shows how the numbers flow; it is not a design for any real stair.

Assumptions: a stair serving a single house, 900 mm wide, two steel stringers, a horizontal span of 3.0 m between supports, and an assumed dead load of 0.5 kPa for the stair and its treads.

1. Load per stringer. Each stringer carries half the width, 0.45 m.
   • Live: 1.9 kPa x 0.45 m = 0.86 kN/m
   • Dead: 0.5 kPa x 0.45 m = 0.23 kN/m
2. Factored load. 1.25 x 0.23 + 1.5 x 0.86 = about 1.6 kN/m
3. Factored bending moment for a simply supported span, using the horizontal span: 1.6 x 3.0² / 8 = about 1.8 kN·m

Stair loads are applied on the horizontal projection of the flight, which is why the horizontal span is used.

What the example shows: the bending moment on a house stair stringer is small. A steel channel or plate deep enough to receive the treads usually has far more bending strength than this needs. That's why the real sizing questions are elsewhere:

• Depth to fit the treads. The stringer has to be deep enough to take the tread connection, the nosing and any riser plate.
• Stiffness and vibration. A thin plate or a single mono stringer can be strong enough and still move noticeably underfoot.
• Lateral stability. A tall, thin plate stringer can twist or buckle sideways unless the treads brace it.
• Torsion. A mono stringer with cantilevered treads twists when someone stands on one side.
• Connections. The ends have to carry the reactions into the floor, landing or wall.

With 4.8 kPa on a wider commercial stair, or heavy concrete-filled pans, the loads rise several times over and strength checks matter more. For the section types and sizes commonly used, see steel stair stringer sizes.

Step 4: Design the treads

Each tread is a small beam spanning between stringers (or cantilevering from a mono stringer). The designer checks:

• Bending and deflection across the stair width under the uniform load and a concentrated load from a person standing mid-tread.
• The tread type: checker plate needs stiffening angles or a bent edge to span; bar grating is chosen from the maker's span data; pan treads gain stiffness once filled with concrete; wood and stone treads need their own span checks.
• Nosing details to meet the code's rounded or bevelled edge requirement and slip resistance.
• The tread-to-stringer connection, whether welded, bolted to clip angles, or screwed from below for wood treads.

Our guide to types of steel stair treads compares the options.

Step 5: Check stiffness and vibration

The National and Ontario codes treat deflection and vibration as serviceability limit states: the structure must be stiff enough for comfortable use, not just strong enough not to fail. For stairs, that matters because people notice movement on stairs more than on floors, and walking down a stair excites it rhythmically.

Stairs most at risk of feeling lively:

• Slender feature stairs with thin plate stringers
• Mono stringer and cantilevered tread stairs
• Long flights with no intermediate support
• Stairs hung from flexible framing or supported by long-span floor beams

Engineers check stiffness against a deflection limit chosen for the use, and on sensitive stairs they estimate the natural frequency and the response to footfall using published vibration design guidance. Fixes include deeper or closed sections, a post under a landing, stiffer tread connections, or tying the stair to a wall.

Step 6: Design the supports and connections

A stair is only as good as the structure it hangs on. The design covers:

• Top connection: clip angles or an end plate bolted to a header, landing beam or slab edge, or welded to an embed plate.
• Bottom connection: a base plate anchored to the slab or footing, often with slotted holes for adjustment.
• Landings: posts, hangers or wall ledgers.
• The supporting structure: the floor beam, joist header or wall that receives the stair has to carry its reactions. On renovations, check it rather than assume.
• Anchors: type, embedment and edge distance into concrete.

Shop welds and field bolts are the usual split, so the stair can be fabricated in large pieces and bolted together on site. Bolted vs welded trade-offs are similar to those for beams; see bolted vs welded steel connections.

Step 7: Guards and handrails

Guards and handrails have their own loads. Handrails and their supports must resist a concentrated load of at least 0.9 kN at any point and in any direction, and, outside single dwelling units, a uniform load of 0.7 kN/m. Guards must resist the loads in the code's guard load table. Those forces end up in the stringer or landing frame, so guard post locations and base connections are part of the stair design, not an afterthought. Details are in Ontario guard and handrail rules for steel railings.

Who designs a steel stair

• Houses (Part 9): the code gives dimensional rules and loads, but no prescriptive sizes for steel stringers. The stair is sized by design, usually on the fabricator's shop drawings, and the building department may ask for an engineer's stamp.
• Commercial, industrial and larger residential buildings (Part 4 design): structural steel is designed to CSA S16, "Design of steel structures," and stairs are designed or reviewed by an engineer, often as a delegated design by the fabricator's engineer.

Either way, the shop drawings are where the design becomes buildable: every member size, weld, bolt, tread and anchor, checked against the architectural layout. Review them carefully before fabrication.

Getting a steel stair designed and fabricated

We fabricate custom steel stairs, landings and railings with shop drawings prepared in-house and CWB certified welding, and engineering services are available for stairs that need design. Send your floor-to-floor height, opening size, stair type and any drawings, and we'll prepare a quote. We fabricate and deliver across the GTA and Ontario; installation is by your installer or contractor.

Request a steel stair quote, or see our services for shop drawings, engineering and fabrication.