This Project was entered into the 2016 LSAA Design Awards (5047)
Entrant: Taylor Thomson Whitting (Engineer)
Location: 50 Martin Place, Sydney. Completed: Unknown Client: Macquarie
Team: Johnson Pilton Walker, Taylor Thomson Whitting, Brookfield Multiplex, Sharvain Pacific Steel
Application: Glazed Roof over an Atrium.
Description:
50 Martin Place was refurbished to provide a communal office from a very important heritage building. To increase light into the Atrium, the Atrium was increased in size by removing the perimeter slab, leaving the beams and columns. To provide maximum light, the roof was fabricated from glass. The final design incorporated a dome on steel trapezoid section suspending triple layer glazed panels, some of which were adjustable.
Design Brief
The client brief was to provide a unique glazed roof over an enlarged Atrium with minimum support system and dome roof that houses a range of staff and client functions and facilities over two additional levels, high speed glass lifts for client use from the banking chamber to the roof top facilities and a large light-filled central atrium incorporating interconnecting stairs between floors.
The project faced many challenge with regards to heritage requirements - building height limit, maintenance of Banking Chamber and Vault operations, a tight program, overall budget, structural framing and detailing to meet high quality visual and architectural requirements, fabrication processes and systems and safe construction techniques. These issues led to a unique and ambitious structural design, ultimately resolved through close collaboration between the engineer, architect, fabricator and head contractor.
Structural Systems
Roof Structure – Aesthetics and Originality
A fine and light steel structure constructed of fabricated sections is an expressed and celebrated feature of the 50 Martin Place rooftop addition and one of its central and significant architectural characteristics.
The design brief demanded a long span glass roof on an existing support structure that avoided any transfer structure beams, together with a requirement for a fine and unobtrusive glass support structure to maximize natural light. The building height restriction also required minimization of structural depth.
All structural elements were designed to efficient minimize structural sizes and for their visual impact. Each exposed steel connection was creatively designed and detailed to a high level of quality.
A range of roof options were considered with the architect during concept design taking into account aesthetics, structural scheme, heritage fabric, ESD, ventilation and consistency with the client brief.
The Beaux-Arts tradition of steel and glass domes and canopies formed the basis for an expressed steel structure solution being adopted. Visual lightness and the potential to celebrate the craft for fabrication and connections also confirmed steel’s suitability over other materials.
Roof Structure – Innovative Design
The curvature of the rook in both directions provides structural efficiency in terms of strength, serviceability and stability. The curved rafters allow axial compression to develop rather than pure bending, leading to design efficiency.
The smooth curves of the arched roof solution also resulted in much lower wind pressures, compared with the sharp edges of a traditional roof, with a wind tunnel test providing accurate wind pressures that also increased design efficiencies.
Generally, the steel arched rafters are spaced at 2.3m centres, aligning with the existing building’s module and grids. These rafters are supported by ring beams at the top and Level 11 with loads transmitted back to the existing building column grid at Level 10 via the internal raking columns and the external tri columns, eliminating transfer structure. The lateral force at the bottom of the raking column is supported by existing Level 10 diaphragm floor.
Visual lightness is enhanced with a tapering trapezoidal rafter cross section, and by eliminating a hierarchy of primary and secondary members with a 2.3 structural intervals. Tri-columns collect groups of rafters to transfer loads to existing structure.
The introduction of the internal raking columns reduced the span and provided a better balance to the spans. These break the spans into 12.4m, 11.6m and 12.4m spans, By introducing pins at the top of the raking column and a bolted joint at the natural inflection point, bending moments in the rafters and deflections are significantly reduced.
Lightness of the structure was particularly important in the longer central span, the appearance of such a structure ruling out traditional techniques for restraining long spans against buckling, such as “fly-bracing”.
Showcasing the craftsmanship of the steel had been vigorously pursued by the entire design team. Overcoming the architectural implications, passive fire protection to the steelwork was dealt with in the following ways:
- Structural analysis – In demonstrating structural principles and fire engineering we were able to reduce the number of elements requiring protection, Furthermore, we were able to reduce the fire rating to 60 minutes which enabled a wider variety of products to be utilized.
- Intumescent coatings in lieu of overcladding – in consultation with our coatings specialist and the market, we concluded that a site-applied, water based intumescent coating was the most appropriate product for the project.
We utilized a Promat acrylic intumescent which was able to be site-applied due its low toxicity and fumes and ability to be sanded and prepared for application of the finished top cat.
Roof Structure – Efficient Use of Materials
A total of 200 tonnes of Australia-made steel that was utilized for the whole project with the roof compromising of 27 tonnes of exposed steelwork. All rafters were fabricated into box sections using laser-cut profile plates.
A collaborative and holistic design process produced a highly efficient structural design. The arched and domed form capitalizes on the strength of steel and its ability to minimize and efficiently resolve loads.
The use of high strength steelwork and innovative structural designs allowed the total steel quantity to be reduced by over 30% in comparison to a conventional design.
Comfort of roof structure
The roof comprises a triple glazed unit with a metalized mesh with micro shades inbuilt into the glazing cavity. This arrangement is the first of its kind in Australia. The curved roof was triple glazed mesh ensured that a uniform appearance was achieved for views while also responding to sun movement. Micro shades create the visual effect of high transparency, while also being a highly effective shaded element – a portion of the roof is always blocking solar radiation coming into the building.
Extensive thermal modelling was conducted to inform the roof and ensure appropriate material performance for the roof elements. The design achieves optimal thermal comfort and code compliance for energy efficiency. Thermal comfort modelling demonstrated that under worst case conditions a 90% level of satisfaction (Predicted Mean Vote – 0.5 to 0.5) would be achieved. The thermal comfort conditions were successfully achieved in design, despite the initial challenges of having a fully glazed structure highly exposed to solar radiation.
Environmentally Sustainable Design, Engineering and Construction
A key requirement of Macquarie’s brief was to achieve a 6 star Green rating, representing ‘World Leadership’ in sustainability. The structural design met this requirement with a maximum Green Star Rating awarded for the steelwork including dematerialization points for the efficient design of the new roof.
Innovative sustainability is evident throughout the project with many initiatives including building services and façade. The historic building has been truly transformed as a completely modernized building which will support the building for decades to come.
Materials
The two basic materials were structural steel and glazing, chosen to provide maximum light into the enlarged atrium. The steelwork was fabricated and erected by Pacific Steel and the glazing was supplied by Sharvain,
Fabrication
The curved roof structure was designed to be prefabricated as much as possible. This is not only improved the program and safety, but reduced the extent of site connections improving the aesthetics of the exposed structural elements. All site connections are bolted connections using high strength pins where posed and standard bolts were hidden.
Due to tight tolerance in the design, maximizing offsite fabrication not only provided a high level of accuracy for the shape and curvature required for the roof but it also ensured all exposed connections and welds could be carried out in a controlled environment.
Collaboration, Construction and Maintenance
Engagement of Design and Construction Teams
Early involvement of the builder, and engagement of a structural steel draftsman to commence work, mitigated delay and flushed out all set out and construction issues. Section lengths, fabrication sizes, transport and erection sequencing were discussed and carefully managed to minimize joints in the exposed steelwork, to cope with tight tolerances, and ensure the curvature of the roof was maintained. The erection sequence of the glass over the steelwork was also carefully managed to minimize deformation of the dome shape.
Effective use of BIM
3D surveying of the existing building was carried out at the early stage of the project to enable much needed accuracy in the 3D modelling, fabrication and to ensure a tidy and clean connection at the interface with the heritage fabric.
The roof dome was analyzed using Strand7, a 3D finite element program. Applied loads from glass and wind were applied in patches to suit the wind tunnel modelling and installation sequencing. The structural steel was modelled and detailed using Revit.
The overall design used physical modelling, CAD modelling and documentation incorporating the ASI AESS Guide.
Working with the Fabricator/Erector
From the onset, a collaborative approach was adopted by all team members, including the structural engineer, architect, head contractor, draftsman and fabricator.
Whilst the detailing discussions could be robust, it was the passion for the Project amongst all members that made the communication so effective. Weekly face-to-face workshops were held with all team members throughout the design development.
With the in-depth understanding of the building, its requirements and the design intent there were far fewer RFIs issued than would normally be expected for a building of this nature.
Ref: sliders/DA2016/Cat5/5047 DP ID 242
