Case study 5: Deflection of the staircase stringers - structural defects in metal construction

Deflection of staircase stringers - causes, assessment and consequences for stability

Mission and initial situation

The client reported significant vibrations and visible deflections in a newly erected steel stringer staircase in an office building.
After just a short period of use, there were noticeable vibrations when walking on the stairs and cracks formed in the joints of the adjacent wall cladding.
I was commissioned to assess the cause and the structural safety of the construction.

The following case study illustrates the problem of deflection of stair rails.

Damage pattern detected

• Visible deflection of the outer stair stringer by several millimeters
• Vibrations when stepping onto the middle steps
• Cracks in the wall cladding in the connection area
• Contact noise between treads and rail
• Deformation in the support area (tie-bar connection to platform slab)

The staircase thus showed clear deviations from the permissible serviceability.

Root cause analysis

After inspecting the structural calculations and remeasuring on site, several causes emerged:

1. Incorrect static dimensioning
   - The cross-sections of the spars were too small to safely absorb the loads from the dead weight and live load (5 kN/m² according to DIN 1055-3).
2. Insufficient stiffening
   - Missing cross-connections between the spars led to torsional movements.
3. Wrong choice of material
   - Structural steel S235 JR was used instead of the planned S355, which reduced the load-bearing reserves.
4. Incorrect storage
   - The connection to the platform slab was planned to be rigid, but was actually designed to be articulated.

Normative evaluation

The DIN 18065 - Stairs in buildings requires that serviceability must be permanently guaranteed and that no disruptive vibrations may occur.
For the calculation of the load-bearing capacity, the Eurocode 3 (DIN EN 1993-1-1) in conjunction with the National Annex (NA). According to these regulations, the Permissible deflection for stair rails: f ≤ L / 300 (for visible components with passenger traffic).

The measured deflection of 21 mm with a span of 3.8 m clearly exceeded this limit (permissible ≈ 12.6 mm).
This is a material defect within the meaning of VOB/B § 13 (1).

Consequences of the shortage

• Reduced usability (noticeable movements, feeling of insecurity when walking)
• Deformation cracks in the connection area of the walls and floor coverings
• Long-term consequences due to material fatigue at the weld seams
• Loss of trust the load-bearing capacity from the user's point of view

possible Liability claims vis-à-vis the metalworker and structural engineer

Recommended measures

1. Static recalculation of the staircase construction, taking into account the actual geometry and material quality.
2. Reinforcement of the spars through welded-on belt plates or intermediate supports.
3. Installation of cross connections to increase the torsional rigidity.
4. Checking the welded joints according to DIN EN ISO 17638 (magnetic particle testing) for cracking.
5. Proof of suitability for use by measuring the vibration amplitudes in accordance with DIN EN 1990/1993.
6. Documentation and acceptance test before recommissioning.

Summary

Deflections on stair rails are a typical indication of Planning or execution errors in metal construction.
They impair both the Security as well as the Optical quality of the construction.
Error-free dimensioning according to Eurocode 3, a sufficient Torsional rigidity and the Correct choice of material are crucial to avoid this damage.

Note from the expert

As a publicly appointed and sworn expert for the metalworking trade, I assess structural defects, dimensioning and welding deficiencies independently, objectively and in accordance with standards.
Thanks to my training as a Master of Engineering and welding engineer, I can evaluate static and welding faults holistically.

Further information

• Metal construction & trade services (windows, doors, railings, structures)
• Industry & welding technology - Overview of services
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