(First published in 2014)
The benefits of choosing structural steel framing for multinational property developers and construction businesses were highlighted at a presentation in Adelaide organised by the Australian Steel Institute and presented by the team who worked on the Ibis Hotel. Providing insights on the project were Principal Structural Engineer at PT Design Andre Vreugdenburg; State Manager at WATPAC, Tim Tape, and Director at SA Structural Michael Mangos.
Main factors contributing to the success of the project were:
- Working through potential Request for Information with the design and build team before the start of construction allowed for a smooth build program.
- An innovative use of inverted “T” beams concealed in the slabs with deck support on the bottom flange of the “T” beam.
- Lightweight tubular SHS columns and “I” beams enclosed in party walls provided fire protection.
- The use of a spider crane for steel erection relieved pressure on tower crane usage.
- Excellent focus on logistics and the build program by the fabricator.
- Close cooperation between the builder, the design team and the steel contractor.
The 17 storey premium economy hotel has 311 hotel rooms, conference and meeting facilities, a guest lounge, food and beverage retail and a fitness centre. The hotel’s complex arrangement of functions, each require a high level of building service requirements. The ground floor consists of retail space, the lobby, public space, hotel back of house facilities and a driveway off Twin Street. Level 1 consists of hotel dining facilities, bar, balconies, kitchen, and conference room facilities. Level 2 consists of plant rooms, hotel gymnasium, outdoor space, and the start of the guest hotel rooms.
Level 3 and the 14 floors above consist of 311 guest rooms (approx. 3.5m wide by 6.5m long) with two types of room.
The overall dimensions of the site were limited, roughly 30m x 30m. The requirement for the rooms to have natural light meant the total site area was not developed. The building is ‘L’ shaped above Level 3 with site dimensions and the required number of rooms leading to a unique footprint.
Principal Structural Engineer at PT Design Andre Vreugdenburg said PT Design initially explored the use of various structural systems and tested them against the architectural and client design requirements, trade availability and competitiveness.
Andre said two key elements in making a steel framed building work for hotels in general were:
- meeting the floor to floor heights of a flat slab system
- costs associated with the requirement to fire rate elements of the steel structure.
When these conditions are not required, the steel framed solution can be competitive against concrete framed alternatives he said.
“In the IBIS Hotel design we recognised we could do a legitimate steel option that would incorporate steelwork into the structure and which did not introduce cost and height penalties to the project. “Our contract on this project was to provide 80% documentation for a D&C tender. Having recognised the potential to do a steel frame for this project, the decision was made to progress documentation in structural steel.”
Andre said the stability of the structure was provided by the insitu concrete core and insitu precast shear walls, which dealt with the lateral force requirements. Whilst insitu concrete columns and floors were used for the lower two floors to deal with the transfer of loads from above, on a structural column grid this suited the complex hotel functional requirements.
A major component of the structure from level 2 to 16 is steel framed, as an alternative to a conventional flat plate concrete design.
Andre went on to say the most efficient slab system (the slab was 140mm thick) was to span across the rooms. This was a good outcome for seismic mass considerations and footing loads.
“The party walls between the units are the support lines, which could have been load bearing walls. For seismic weight considerations we opted to leave these as lightweight and provide steel beams to support the slab.”
The normal disadvantages associated with the use of a steel frame were negated by the use of small steel sections i.e. OneSteel’s 300PLUS 250UB beams and C450PLUS 100 - 150SHS columns, hidden within fire rated walls. Because the corridor zone is a service intense zone, the party wall beams could not extend over the corridor to maintain the support grid. Instead, inverted T beams cast into the slab support the corridor slab and provide a clear soffit, free of obstructions, effectively equivalent to a flat plate design.
The design also resulted in a minimum weight solution in regard to earthquake mass, with resulting savings in shear forces and foundation loads. The tender process did not preclude the use of alternative solutions and none were offered.
The design of the steel frame system above level 2 was a catalyst for the builder Watpac, and fabricator SA Structural to add their creative input to solve logistical issues and incorporate other cost effective measures into the project.
Watpac’s Tim Tape said the site’s location in the heart of the city meant site access was very tight. “Demand for the tower crane was high with a single loading point. Early engagement and close coordination with the fabricator SA Structural and PT Design changed the construction strategy to improve the program”.
The solution was the use of a “spider crane” to erect the steelwork from a working platform. The tower crane was no longer required to pick and place off the truck; instead it unloaded the steel members on floor which were then erected by crawler crane (spider crane) and scissor lift. This reduced the reliance on the tower crane which was able to erect two floors at a time. During erection of the steel frame, temporary steel columns and bracing members (which were re-used at higher levels) were incorporated into the design to brace the structural frame prior to the slab being poured.
The fire rating requirement of the structural steel members can be a major cost for projects. In this case the party walls provided the fire rating requirement under the Building Code of Australia and structural steel members were accommodated within the party wall with no additional passive fire protection. Only the exposed flanges of the small ‘T’ beams needed to be fire protected at a relatively low cost.
The post tender and construction process from the builder’s, engineer’s and fabricator’s perspective has been highly integrative and rewarding.
Tim Tape of Watpac concluded:
- We would do it again
- Keep steel design simple and repetitive
- Spider crane reduces Tower crane load
- No coating to internal steelwork
- Set design early potentially eliminating RFI’s
- Use collective skills of all
- Contract signed March 2013
- Doors open July 2014