Design Key Points and Difficulties of Steel Structure Spiral Stairs

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I. Design Key Points

1. Structural Safety

    - Load Calculation: It is necessary to consider static loads (self-weight), live loads (usage by people, furniture handling, etc.) and dynamic loads (vibration, wind loads, etc.), and it should meet the requirements of the *Code for Loads on Building Structures*.

    - Support System: The selection of the central column type, cantilever type or double-helix structure directly affects the stability. The bending and torsional resistance capabilities need to be verified through finite element analysis (FEA).

    - Node Design: The connection nodes between the tread plate and the central column/side beam need to avoid stress concentration. Welding or high-strength bolt connections should be adopted, and fatigue checking should be carried out.

2. Optimization of Geometric Parameters

Marsabit     - Spiral Parameters: Determine the relationship among the rotation radius (R), rotation angle (θ) and pitch (H), and avoid the tread height (h) and depth (d) exceeding the comfortable range for the human body (it is recommended that h = 15 - 20 cm, d ≥ 25 cm).

Marsabit     - Slope Control: The slope of the spiral staircase is usually relatively steep (45° - 60°). Anti-slip tread plates need to be set or intermediate rest platforms need to be added (suitable for large-scale spiral staircases).

Marsabit 3. Coordination between Space and Function

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    - Passage Width: The difference between the inner diameter and the outer diameter needs to meet the minimum passage width (≥ 60 cm for a single person, ≥ 90 cm for two people) to avoid the risk of collisions when going up and down.

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Marsabit     - Clear Height: The vertical clear height inside the spiral needs to be ≥ 2 m to avoid users hitting their heads.

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Marsabit 4. Selection of Materials and Processes

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Marsabit     - Selection of Steel: It is recommended to use Q355B or higher-strength steel. For highly corrosive environments, hot-dip galvanized steel or weathering steel should be used.

    - Surface Treatment: The anti-slip texture (such as checkered steel plates, anti-slip coatings) and the anti-rust treatment of the railing (spraying or electrophoresis process) need to be designed simultaneously.

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Marsabit II. Design Difficulties

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Marsabit 1. Complex Stress Analysis

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    - Torque and Lateral Force: The spiral structure will generate torque and lateral thrust when loaded. Nonlinear analysis needs to be carried out through 3D modeling software (such as ANSYS or Rhino + Karamba).

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    - Resonance Risk: The natural frequency of the steel structure needs to avoid the human walking frequency range (1.6 - 2.4 Hz) to prevent potential safety hazards caused by resonance.

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2. Precise Machining and Installation

    - 3D Curved Surface Molding: The bending accuracy of the spiral beam needs to be controlled within ±2 mm. A numerically controlled plate rolling machine or grinding after segmented welding should be used.

    - On-site Assembly Error: The positioning deviation of the embedded parts will lead to overall deviation. Virtual pre-assembly needs to be carried out through BIM technology, and adjustment devices (such as adjustable supports) should be reserved.

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Marsabit 3. Conflict between Norms and Humanization

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    - Tread Uniformity: The tread height of the spiral staircase may vary due to the change of the radius. It is necessary to maintain uniformity by using tapered treads or adjusting the rotation angle.

    - Railing Continuity: The smooth transition of the curved railing and the handrail needs to take into account both aesthetics and the comfort of gripping. It is recommended to use parametric modeling to optimize the curvature.

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Marsabit 4. Adaptability to Special Scenarios

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    - Fire Evacuation: It needs to meet the requirements of the *Code for Fire Protection Design of Buildings* for the width and fire resistance limit of the evacuation staircase (for example, the outdoor staircase needs to have a fire resistance of more than 1.5 hours).

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Marsabit     - Barrier-free Design: If barrier-free passage is required, it is necessary to locally add platforms or adopt a gentle slope spiral structure, but this will significantly increase the floor area.

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III. Examples of Solutions

Case 1: Cantilevered Spiral Staircase

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Use box-section steel beams as cantilever supports, and share the torque through diagonal tie rods to reduce the space occupied by the central column.

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Case 2: Modular Prefabrication

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Marsabit Split the staircase into standardized tread units, and connect them with bolts on site after prefabrication in the factory, reducing the construction difficulty and cost.

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IV. Summary

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The core of the design of the spiral staircase lies in balancing the mechanical performance and the space experience, and it is necessary to achieve the unity of function and aesthetics through multi-disciplinary collaboration (structure, architecture, construction). It is recommended to give priority to the following work in the scheme stage:

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Marsabit 1. Establish a parametric model to verify the geometric feasibility;

Marsabit 2. Conduct a 1:10 local node physical test;

3. Communicate with the construction party in advance about the limitations of the processing technology. 

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