We were asked to produce a design that would utilise an existing owned steel pipe culvert section for the main spine of the staircase. The 'single stringer' was approximately 300 mm in diameter.

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This required us to:

• Survey and detail the existing building structure

• Design using an existing section of material

• Model and analyse a loading scheme to manage deflection in the design using Finite Element Analysis (FEA)

• Design a timber tread for the final installation

• Develop a manufacturing method and information for accurate assembly

• Provide information to assist with installation

• Provide artwork for approvals

• Manage processing of materials

• Incorporate national standards for design

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The staircase was large in size and weight, with a weight of approximately 4 tonnes, a centreline radius of 1780 mm and an overall height of 2725 mm.

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We have experienced many design projects and have been involved in the industry for 14 years. We acknowledge the existence of the iterative process, and we choose to use it to our and our customers advantage. We knew this project would be drastically simplified if we could parameterise the design and reduce our input during the conceptual to detailing design phase, while maintaining a highly-efficient iterative process.

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We broke the design into three major elements:

• The helix of the pipe stringer as a series of tangential elements (numerous identical straight cut pipe sections)

• The overall height and diameter of the helix

• The management of national standards for step tread heights and offset ('going and riser') and tread connection location relative to pipe segments

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This meant that we had to solve a geometric optimisation problem that involved the following parameters:

• Step rise

• Step going

• Pipe section length

• Tread mounting location

• Overall height of the staircase

• Thickness of timber step treads to be used

• Mounting location on the floor (ensure it was not in door way shown)

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As well as a geometric problem to solve, we also had to consider:

• Material choices for the step treads

• The radial offset that a spiral staircase induces in the treads

• Loading and deflection in the system to remove any deflection when being used

• Appearance and manufacturability e.g. Bolted time step tread mounts

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Computer Aided Design (CAD) softwares that we use do incorporate parametric design capabilities, though we decided to approach this in a more thorough and 'bare-bones' type approach. So we built our own software to parameterise the project and output our basic 3D cad model that could be detailed for manufacture.

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The software was written in a raw format making use of the generic drawing exchange format (DXF) that could be input directly into our typical CAD software environment.

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Once the program was written, we could build the designs components to a standard that we knew would be:

• Easily manufactured and assemble

• Consistent, with only three different cut profiles required on the given straight steel pipe culvert and only eight parts in the entire design

• Comply with governing national standards for this type of part

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We significantly reduced the leadtime for conceptualisation, and detailed design. Output information for automated manufacturing processing using CNC profile cutting equipment. And output highly detailed information for the assembly and manufacture of the final part.

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