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A Transformation Using Steel Framing

(Originally Published 4 April 2016)

A transformation in the heart of Brisbane’s Queen Street Mall was completed recently, with a striking boutique commercial and retail development at 155 Queen Street. The flexibility and long span capability of composite steel framing was a key contributing factor to meeting the program.

Comprising a mix of three retail levels below three commercial floors and an open, roof-top green space, this steel-framed icon will be one of only a few buildings in Australia to offer 20 metre clear spans and a column-free interior on the commercial floors, thus providing outstanding floor plan flexibility for tenants.

The opportunity to attract commercial tenants looking for an unprecedented degree of internal floor flexibility, meant owner ISPT opted to have no internal columns within the 20m x 45m commercial floor plate, offering around 900 sq. m of column free space on each floor. A striking, steel framed and glass lined light-well directs natural light down through the three commercial floors.

The lower office floor has been constructed with a generous four metre floor-to-ceiling height.

Although the lower floors could have been entirely column free, it was agreed to have a single line of internal columns in the retail zone. This option still provides tenants with a 10m x 8m grid, offering a high degree of retail flexibility.

Engineers Bornhorst + Ward approached the building with structural steel in mind. Associate, Eric Ness-Wilson said, “A structural steel framed solution was discussed very early in the design phase with the client. It was determined that steel framing offered the client greater future flexibility options, superior value with the light weight design and improved efficiencies on a tight site.”

“The small site, land locked by the Queen St Mall and existing buildings to the east and west required all construction materials to be delivered via the vacant lot from Elizabeth St", Mr Ness-Wilson said. “Restrictive site conditions combined with the large spans made a composite steel beam with a Bondek slab the obvious choice for the floor framing system.”

He said the development has been designed, “such that connections can be made with the future Regent Tower, a proposed development on the vacant lot currently used for construction access. Bornhorst + Ward’s 3D Revit modelling helped the design team visualise the building connectivity throughout the design process."

The final framing solution is a composite structural steel floor across all six suspended levels. The commercial levels are typically 900WB sections spanning 20m with no internal columns. The retail floors are on a 10m x 8m grid, with OneSteel 610UB sections for primary beams and OneSteel 410UB sections for secondary beams. 

The roof is an open green space that can be utilised by the commercial tenants. Due to the higher loads imposed by the garden areas, the framing is a 20m span, with 1000WB composite steel sections.  

Engineers Bornhorst + Ward utilised OneSteel’s composite design software CompPanel for the design of the floor beams. “The beams were designed as single span composite beams and spaced at 2.7m centres enabling the Bondek to span without the requirement for propping,” Mr Ness-Wilson explained. “OneSteel’s CompPanel software was used very early in the design process enabling floor beams to be sized quickly. This enabled earlier development of the 3D Revit model with accurate beam sizes which was then exported to create the analytical model for lateral stability checks.”

Columns in the retail space are OneSteel 310UC sections. The light weight of the composite structural steel frame enabled columns to be designed as non-composite. They were concrete encased to achieve the required fire protection. Vermiculite fire spray was applied to the floor beams.  

Services are accommodated through penetrations in the steel beams. Future proofing has been achieved through constructing around 100 beam penetrations in each floor, including five in each of the 20m spanning beams. Bornhorst + Ward designed these using OneSteel’s CompPen software. Almost all of these penetrations are unreinforced, with only four of the 100 in each floor requiring stiffeners.

Steel framing enabled services to run through penetrations in the steel beams and meant that floor-to-floor heights could be kept to a minimum. All penetrations were incorporated into the 3D Revit model, enabling clashes with interconnecting beams to be easily identified. The use of OneSteel’s CompPen software enabled penetration sizes through the steel beams to be optimized to achieve the largest possible opening while limiting the need for stiffeners.  

The steel framing was extremely fast to assemble and proved beneficial to the logistics of the build. The reduction in concrete delivery vehicles was particularly favourable while working within the constraints of a small inner-city site.

Unforeseeable problems discovered during excavation resulted in a delay to construction of the core, which subsequently held back construction of the steel frame. To ensure program continued to schedule, Broad Construction initiated an approach which allowed the steel framing to be built ahead of the construction of the core. Whilst excavation at the Elizabeth St (southern) end of the site was still underway, prior to the core construction, the steel frame was erected from the Queen St (northern) end of the site and covered around 70% of the building’s footprint. The steel frame was braced in one direction by the exterior precast walling and in the other direction by temporary steel cross bracing. This allowed the construction of the structural steel frame (including decking, shear studs, slab reinforcement and pouring of the concrete slabs) to proceed independently of the remaining site excavation and core construction. The steel frame was then tied back into the core later on during construction.

Team
Owner              ISPT
Builder             Broad Construction
Architect          John Wardle Architects
Engineer          Bornhorst + Ward
Fabricator        MC Engineering 
Distributor        BlueScope Distribution
Supplier           OneSteel