Partridge proudly takes home the 2021 Award for Excellence in Structural Engineering in the Unusual Building Projects Category!
The sculptural floating stair is a wonderful example of excellence in steel design, engineering, and construction while dealing with the constraints of an existing (strata-titled) building and the many demands of a live fit-out project. The main design feature/element of the project involved the design and documentation of a very ambitious self-supporting, helical structural steel stair which spanned the height of two equivalent storeys and then separated into two independent flights at the upper transition. The stair was wrapped in lathed timber battens, finished with marble treads and required significant strengthening of two existing 1914-era floors and the introduction of tuned massed dampers (TMDs) in order to achieve the required stiffness and vibration limits under footfall.
Engineering design and documentation for the project commenced in May 2018. Construction on site commenced in November 2018 and the project was completed in May 2020. Partridge documented the project using AutoCAD and prepared 13 x A1 drawing sheets (and over 120 hand sketch details/packages during the construction phase).
The project brief and Partridge’s response
Initially, the design had been developed/undertaken by a French architect in conjunction with a fellow French engineer, and as such, Partridge was originally engaged as the local structural engineer to review and/or adjust (as necessary) the agreed/approved design, and/or to ensure that the existing structure was capable of supporting the intended loads. However, upon review of the stair’s design with the construction team it became apparent that the approved stair design was not easily achievable (or possible) using local construction techniques, and as such, Partridge’s scope was expanded to include the analysis, re-design, documentation, and site supervision to support the architect’s design, and deliver a movement-sensitive structure of impeccable standard and with minimal tolerances, within an existing heritage listed building. Partridge’s response to the brief incorporated the following:
- Review of constructability and concept design of an equivalent structural form using the local constraints.
- Remediation and strengthening of the existing 1914 composite steel/concrete floor structures to trim the new stair void and support the proposed stair loads.
- Temporary support of the stair during construction/erection on site to enable the installation of the structural steel grillage and the access floor.
- Design, analysis and detailing of the geometrically challenging helical staircase to account for local building practices and to manage deflection/movement across the various stages of construction and finishing of the slabs.
- Testing/monitoring of vibration response of the stair under construction and upon completion to determine what level of damping may be required to overcome swaying of the stair.
Design & construction
The design process was far from typical given the change in scope from a peer review to full design and documentation of such a challenging staircase, combined with the timelines associated with the delivery of a high-end retail fit-out project. Following an initial briefing meeting with the architect and client, Partridge prepared preliminary concept hand sketches to address the feasibility of removing and re-supporting the stair and to also verify that the works were structurally feasible and would not impact the structural integrity of the building. This allowed the approval process to proceed and for the design of the stair to develop. Once the concept design reports were received from the French design team, it became clear that there had been considerable miscommunication between what they were commissioned to deliver and what Partridge’s scope was originally intended as. Partridge’s scope and role suddenly and significantly grew!
Before establishing who was responsible for the design moving forward, it was decided that there would be benefit in workshopping the concepts with the local architect, contractor and steel fabricator to establish whether the concept was achievable in its current form and whether the current design could be delivered or would need significant modification. As part of the review process, it was determined that the original doubly-curved profile of the trapezoidal box stringer was not able to be constructed locally without adding significant costs to the project, and as such, the structural design would need to change.
The redesign process involved several iterations to ensure that the stair envelope would still be achieved whilst still achieving minimum clearances between flights and also the structural properties of the original stair profile for torsion and vibration purposes. Our initial analysis and design was undertaken in collaboration with the original engineers and on the basis that the original trapezoidal stair profile worked and that all was needed to match the stiffness and strength properties of the original profile.
Once on-site, Partridge worked closely and collaboratively with the builder and his sub-contractors to assist and support their desired construction methodology and the ever-changing demands of a live fit-out project. The timber lathe handrail progress and marble tread installation was closely monitored to ensure that the deflected stair profile was maintained while the self-weight slowly built up and would not crack the treads or pop the partially constructed and interwoven timber handrails. Partridge was consulted and subsequently advised on all manner of issues from construction sequencing, temporary support, to glazed screen fixing details, furniture, and joinery. To achieve a smooth construction process Partridge was on site almost every second day during the erection of the stair to ensure that we responded quickly to site queries and last-minute issues so as to avoid delays to the construction program and stay ahead of the trades.
Sustainability was incorporated in the architectural and engineering design where possible. The stair itself was clad completely in natural timber lathe sections (almost 3 tonne) which also was utilised to assist with the overall bracing and damping of the stair. Timber framing was used wherever the design allowed and was utilised throughout the overall fit-out project itself.
The site is located in a building which was opened in 1914 and has significant heritage value to the area, and as such, any intervention with the existing structure required careful consideration of the construction materials, etc, and how best to connect the proposed staircase whilst maintaining the existing fabric and detailing. Partridge even conducted testing of the carbon content of the original steel columns to make sure that appropriate welding specifications and procedures were adopted and that as much original structure could be retained as possible. Amongst other things, the project involved the refurbishment of the external façade of the building, including but not limited to sandstone wall restoration, refurbishment of all of the external windows/doors to reflect the original construction and also the reinstatement of the original entrance off the corner of the building. The structure now looks as inviting outside as it does inside!
Challenges & resolutions
There were numerous challenges in the design, detailing and construction of this intricate and bespoke staircase. The key challenge from a structural engineering perspective was supporting the large forces that were generated by the staircase itself without entering the floor below. The floor zone available to achieve this was only 150mm, and as such, it was always going to be tricky to design/detail anything that would both fit and achieve the strength/deflection characteristics required. To achieve this Partridge investigated a number of options including carbon fibre strengthening, solid steel plate stiffening and also thickening the original slab by bonding a new layer of concrete and steel onto the original slab. This was anything but straightforward, as all options had to consider the impacts on the various trades, how to get the materials into the building, how the structural access floor could be fixed down. In the end, a structural steel grillage option appeared to meet all of the requirements and was adopted.
Aside from the challenges already presented above (including the redesign of the stair itself), it was observed during the installation of the marble treads that while a person travelling up/down the stairs did not experience significant vibration/activation of the stairs under their own footfall, that people already on the stair, particularly if stationary at one of the two landings (unusual situation for a stair), were particularly sensitive to the movements, and as such, we had to design a tuned mass damper (TMD) system to try and balance out some of the more dominant frequencies that could be experienced by the users upon completion of the stairs.
Other challenges involved site access constraints and/or issues with the sequencing of trades. As the installation of the stair required a full birdscage scaffold to be erected, it limited the number of other trades that could work within the space and, as such, the window for installation was extremely tight. This resulted in the stair stringer/spine being initially measured on site, then fabricated and inspected off site. Obviously it was not fully welded off site as it needed to be dismantled and transported to site and, as such, a considerable amount of time and effort went into getting the structure zipped back together on site whilst still lining up with the intended connection points.