A tall steel story: The mezzanine that came through the window


Now here is an amazing way to convert a “problem” inside space into an outstanding new office floor.


The brief may have seemed like “Mission Impossible” but all involved in the project found a solution with steel that sounds like a tall story. But it’s not — it’s an ingenious one!


The AMP Society building in Customhouse Quay, Wellington, is one of the city’s most significant and distinctive historic buildings, and carries a Category One rating with the New Zealand Historic Places Trust.


Architects Stephenson & Turner had already designed an award-winning entrance to the building and refurbished the upper floors, terrazzo stairwells and lifts. The next challenge was a practical one: the need to create additional floor space for office accommodation.


This would help the building’s inspiring 1920s classical features meet the future by catering inventively for modern office needs.


The problem to counter was the fact that the existing ground floor was dominated by a double-height former reception area — so it was too imposing to be used as office space and too vast to be effectively air conditioned (you cannot blow air down six and a half metres).


The solution was creating a new mezzanine level —one which has turned out to be visually and structurally stunning, appearing light and airy as it hovers in space yet with the extraordinary tensile strength of steel.


The mezzanine solution may seem an obvious one, but the limitations of a heritage site posed huge logistical challenges. How do you execute the heavy construction of a mezzanine floor when you cannot risk damage to heritage features, you cannot attach anything to ornate marble pillars, or spill or rip anything on original heritage carpet below?


Whilst the only solution is to hang a floor off the floor above using high tensile stainless steel rods, how do you bring core structures in when the front door is out of bounds because the outside façade and the central space is heritage rated?


The expertise required to solve these challenges was considerable. For example, the main feature, the bridge between the floors (an all-steel, three-dimensional truss) could not be adjusted on site.


As Troy O’Donoghue of Stevensons Structural Engineers Ltd pointed out, the only alternative was bringing it through a double-height window then erecting the mezzanine floor inside like giant meccano.


“Between Fletchers Interiors, the architects, Aurecon (previously Romulus Consulting Group) and ourselves, the whole team worked everything out exactly.


“The bridge between two floors just sneaked in the window with millimetres to spare”, Mr O’Donoghue says. “We suspended it off the HIAB crane and poked it through at right angles to avoid hitting the tram wires.”


Structurally, only steel could achieve the design objectives as Murray Robertson, architect from Stephenson & Turner, pointed out.


“There are many different grades of steel. It has great strength, particularly when used in tension as we have on this project.


“We were actually able to create a lighter structure than we even thought possible when we started the project. The balustrade posts are only 50mm x 10mm and are at 1500mm spaces.”
Steel also provided opportunities to cleverly conceal services, leaving a cleaner ceiling with only light and sprinklers visible.


There is a lot going through the trusses and, unusually for a New Zealand office space, the new ceiling uses perforated steel tiles.


Integrated active chilled beams were also used. These contain the air supply, cooling and lights — all in one unit — and threaded through the trusses.


From an aesthetic point of view, steel was a brilliant modern contrast. The architects put something very new in a 1920s classical space — and it worked beautifully.


“The light elegant glass and steel structure was designed to provide a contemporary contrast to the solid robustness of the marble stone and bronze surrounding space.


“While the new structure contrasted with the existing, it also had a rigid order that related to the classical columns and reflected the classical nature of the space,” Mr Robertson says.


“Mission accomplished”, with not a scratch on any heritage spot, or a spot on any heritage carpet.


Few would realise the lengths gone to achieve it — a testament to the expertise of all involved — and the versatility of steel.