Innovation Sees Old Transformed into New
(originally published 1 August 2014)
20 Martin Place, Sydney CBD
Situated in a historic location in the Sydney CBD, on the corner of Pitt St and Martin Place, the original 1974 building was unsuited to the demands of the modern office market. Faced with demolition of the entire structure, an innovative solution was developed which instead drove a completely new building, with a 25 per cent increase in net lettable area, while retaining the existing building frame.
Extensive remodelling of the building was achieved by completely removing the existing façade, non-structural masonry lift shaft and concrete floor slabs. As the existing masonry lift shaft was located in the centre of the building, positioning the new structural core on the northern edge increased the amount of usable floor area and the efficiency of the floor layout. The new 20 Martin Place achieved the maximum floor space ratio permitted by the City of Sydney without increasing the height of the building.
“Retaining the existing steel frame was important in delivering the building with a 5 Star GBCA Green Star Rating”, said Alex York, Associate at Crone Partners, architects for 20 Martin Place. The original building did not have a Green Star Rating, but Mr York explained that “In achieving a 5 Star Green Star Rating, points were available through the ‘Building Reuse’ criteria, which was achieved by adaptation of the existing structural frame.” Other Green Star points were also delivered via upgraded plant equipment, an innovative façade system and an enhanced workplace environment.
“The original building sat as an isolated structure, a ‘building in the round’ as it were, which was set back from the adjacent building on its northern boundary. This offered us the opportunity to relocate the core and increase the floor plate, without impinging on any of the constraints imposed by building in such a historic area. Through the relocation of the core to the northern edge, the new floor plate now delivers a far more efficient and enhanced solution, with a layout that suits the modern commercial office space.
“Due to the inherent flexibility of steel structures we have found they lend themselves well to adaptive re-use.” As the core was to be relocated, the existing lift motor rooms and plant rooms were also removed, necessitating the removal of the top three levels. The highest remaining floor, Level 20, was converted from a plant room level to an office level.
Structural adequacy of the structural steel frame during the construction phase required particular attention by the structural engineers Taylor Thomson Whitting.
Wystan Alexander, Associate at Taylor Thomson Whitting, explained: “As the floor slabs were being removed, bracing of each floor was provided through the addition of in-plane cross bracing, via a series of 24mm diameter rods, providing the diaphragm action necessary to lock each floor together and transfer loads to the temporary vertical bracing elements. Temporary vertical bracing was installed in two locations, which involved a series of OneSteel 300PLUS PFC sections in a cross brace configuration, ranging from 150PFC sections at the top to 250PFC sections in the basements. Additional loads due to the tower crane, Alimak and scaffolding system were also accounted for in our temporary works design.
“New floor areas are provided where the original core was once positioned,” Alexander said. “The floor plate was extended 3.5m on the northern edge of the new core to the northern perimeter precast wall. The framing for the new floor areas incorporated composite structural steel beams, supporting a concrete slab on metal decking. Only one new column was required to be added, with the majority of the new steel beams able to be supported off the existing steel frame or the new concrete core. Structural design of the floor beams was done using the OneSteel software CompPanel, which checks both the strength and serviceability criteria of the composite beams.”
Building upon the simplicity and classic modernity of the existing building, 20 Martin Place incorporates a variety of different cladding systems that respond to the various environmental and urban conditions while creating a cohesive design expression. Along Martin Place, the design features a simple and elegantly detailed glass curtain wall consisting of a series of floor-to-floor glass panels that are sloped outward at the sill level and overlap the row of glass panels below to create a ‘shingling’ effect on the façade. A sill of luminous coloured dichroic glass provides a distinct brightness when seen from below, adding richness to the façade. The tower glass fulfils the required balance of low outward reflectivity by using a high-performance glass with a low emissive coating that transmits a high amount of visible light while keeping out the sun’s heat.
The lower podium shop front utilises glass that maximises visibility in and out of the retail spaces, set between columns clad in cast glass, in homage to the masonry context of Martin Place. The east and west end façades include vertical aluminium blades that extend beyond the façade. The mullions have a nickel coloured anodic coating that will capture glints of light throughout the day while being particularly responsive to the sun’s rise and fall. They provide a subtle texture on these end elevations. Although this façade solution was heavier than the existing façade, there was no need to strengthen the existing steel frame to account for the increase in loads.