Cullen Grummitt & Roe was commissioned by Drivas Property Group, a well known commercial property developer in Australia to be the structural and stormwater drainage engineering consultant for the expansion and structural up-grade of one of their existing commercial buildings at the corner of Liverpool Street and Kent Street in Sydney, Australia.
The existing building was a three storey structure with masonry walls/piers and timber floors built in late 1950s. The local council approved the expansion and up-grade of the building into a five storey modern commercial building. As the approval was for an up-grade only demolition of the existing building was prohibited.
To conform to the conditions of the approval, the existing masonry walls on the boundaries were to be maintained and all alternations to the structure were to be carried out from the inside of the building. The major challenges faced on the project during the design and construction stages and the engineering solutions to overcome them are summarised below:
- To maintain the three storey high free standing existing masonry walls in structurally stable and safe condition during demolition of existing timber floors and until the newly constructed reinforced concrete floors were built structurally incorporating the existing walls with floors; this challenge was tackled by removing floor planks and rafters but maintaining and strengthening the existing timber columns and beams into the walls.
- Design the connections between the existing walls and new floor without affecting the strength and stability of the walls above e.g., at the time of first floor construction, the height of the walls to be maintained above was two storeys; the solution was achieved by erecting the formwork for the concrete floor attached to the walls and notching the walls in hit-and-miss pattern for concrete slabs and beams. The positive connection between the walls and the new floors was created by using additional reinforcing bars into the `hit` notches and chemically anchored bars into the `miss` lengths of the walls.
- To overcome the calculated additional loads due to replacement of timber floors with reinforced concrete floors and addition of two new floors; two new internal columns and a vertical core to house a new lift and a new stair well were added thus reducing loads on the existing walls and foundations on the boundaries.
- The approved elevation of the building required creation of a large opening on the ground floor. This proposition had two structural aspects. One - to support the wall panels above the proposed opening and transfer loads safely to the piers; Back to back structural steel channels with top and bottom flanges strapped to allow the walls to be physically supported, were designed to carry the loads from the walls above, the straps were provided after erection of the channels and fixing in position and two - to calculate the loads on and check the structural capacity of the existing brick piers and the effective footings existing underneath the piers; Calculations demonstrated that available pier sizes and effective foundations were structurally sufficient to carry the design loads at all locations except one. A new structural steel column with a new footing was designed at that location.
All the structural works and the external and internal finishes along with the installation and commissioning of new lift have been completed.