The steel structure for the 144 Oxford Street project acts as support for a composite roof slab, but more significantly supporting a glass curtain wall to a nine-story atrium.
The architectural brief for the project was to create an atrium space with minimal structural impact, and optimum visibility through the glass curtain wall, which encapsulates the entrance. Only slender structure, specifically box sections and rods, could be used to support the glass.
Why was steel used for this project?
Due to the extreme sensitivity of any deflections, large spans and large point loads as a result of the glass support structure, steel was the only viable choice from conception.
The glass curtain-wall that spans 25m wide by 9 stories high, and 4 meeting pods had no traditional support work due to the brief, and had to be hung from a hidden steel structure spanning over the atrium space. In both the case of the pods and the curtain wall support, high-tension rods were used to connect to the structure above. These rods fitted the aesthetic brief , could be produced in higher grades of steel, and due to the simple one-directional load acting upon them, the basic geometrical shape was fully utilized. The rods connected to the steel roof structure, which consisted of a 25m lattice girder spanning over the atrium onto concrete supports.
As the girder was supporting a long glass facade, minimal deflections were allowed for when under load. The deflection requirement along with the large compressive forces present in the girder was best satisfied with the use of large H-section members in the girder. The curtain wall plane was offset from the girder plane, which resulted in large horizontal tension forces. These forces were resisted by tying the girder vertical members via tension rods to the concrete cores. The tension rods allowed for simplified connections with high loads onto the cores due to the simple rod shape.
From a fabrication point of view, and due to the large section sizes being used in the girder for the high loads it was to carry, the welding at the joints was a critical aspect of the fabrication where pre-heating of the parent material at the joints was necessary and 100% non-destructive testing to confirm suitable welds had been laid down.
Although there were numerous sections of steelwork to be installed on the site the installation of the Atrium girder presented the main challenge since it was not possible to gain access for a crane onto the site and the tower cranes could not lift the weight of the girder which was 12.2 tonnes.
A 500ton heavy lift crane with 105 ton of counterweight was therefore hired in to install the girder from Oxford Street at an installation radius of 52m.
The coordination for this lift was critical since one carriageway of Oxford Street needed to be closed off to traffic to allow the setup of the crane which was planed through from midnight on a Friday and for the balance of the weekend.
The road closure was installed at midnight on the Friday with the heavy-lift crane being set up and ready to lift the girder by 9:00AM on Saturday morning.
The whole operation went to plan and the crane was off the street by 5:00PM on the Saturday afternoon.
What makes this project impressive from a technical perspective?
An extremely large area of glass and structure is hanging from the long-span steel structure above. This required technical expertise to not only understand how the structure will move during loading, but how its performance would translate from design models into the actual fabricated and erected steel structure. Large efforts were made to calculate and refine the expected loading, to accommodate for the expected movement.
What is special/ unusual/ innovative/ aesthetic about the steelwork in this project?
The girder had a camber that would absorb most of the movement, however just as maximum allowable deflection was a concern, so was over cambering. If at any time the girder did not perform and deflect as expected unwanted secondary issues would arise in the glass curtain wall and thus three adjustable deflection phases were built into the installation.
The first phase being in its unloaded state by means of packer plates to adjust the curtain wall supporting beam levels. The second with all permanent loads being applied to it, except the glass, and adjusting deflections by means of shortening the high tension rods with their threaded connections. And the last phase being with jacks used to lift the entire curtain wall into place.
How does this project demonstrate the benefits of Steel as a material?
Steel was the only material that could be manufactured and erected with a tolerance of only a few millimetres, and also perform precisely as modelled. This was aided by the low number of manufacturing variables for steel, as opposed to other materials used in construction.
How did the project team work together?
The project team held several planning meetings, with the contractor included, occurred well before the fabrication of the steelwork even started. This helped the entire team to understand the challenges each member faced and how they would influence the team as a whole. The steel work was modelled and coordinated between consultants via 3d modelling in Revit. Since the steelwork in the Revit model was accurate, it was exported directly to the steel detailer for shop drawings. Inspections of the fabricated steelwork occurred while the steel was still in the factory, due to the extreme time-sensitivity of the erection date.
Project motivation editorials are provided by the project nominator. If any technical details, company names or product names are incorrect, please notify the SAISC so that the error can be corrected.
Physical address of the project
|144 Oxford Street , Rosebank.|
|Google Maps link|
|Completion date of steelwork||August 2019|
|Completion date of full project||November 2019|
|Tonnage and steel profiles used||75 ton of various profiles + 990m2 of Bondek|
|Project Team Role||Company|
|Nominator||TASS Engineering P/L|
|Structural Engineer||Sutherland Engineers|
|Quantity Surveyor||RLB Pentad / Farrow Laing JV|
|Project Manager||Origin Project Managers|
|Main Contractor||WBHO / Tiber JV|
|Steelwork Contractor||TASS Engineering P/L|
|Steel Erector||Van Rensburg Steel|
|Paintwork Contractor||DRAM Industrial Painters|
|Photographer, Photo competition||Sutherland Engineers|
|Photographer, Other submitted images||TASS Engineering P/L|