Engineering the glamour

They’re the engineers behind the glamour - the masters of the possible who made an acclaimed luxury vehicle showroom become a reality.

And the Babbage Consultants engineers have done it before.

The Giltrap Group head office in Auckland’s Great North Road, known by its address as “119GNR”, has been a magnet for attention since it opened in July as the city’s sales home for Aston Martin, Lamborghini and Bentley luxury cars.

From the signature double-storey concrete truss that allows unencumbered ground floor and mezzanine front windows, to the curved internal vehicle ramps and Green Star rating, the building has been hailed for its design and performance.

It’s a building where each vehicle brand has its own showroom, there’s a hand-over room where buyers can collect their cars, and basement workshops are sited below glass-enclosed areas where owners can watch the mechanics at work.

But behind the design is the engineering that makes the building work.

Babbage deployed a suite of its engineering expertise on the project including structural, building services, civil and geotechnical.

Structural engineering principal Dr Victor Lam calls the building “one of a kind” and says it was “challenging” in terms of engineering design.

And the challenges came thick and fast, including

•The excavation for the four basement floors while protecting the brick building next door and stopping Great North Road falling into the site

•The drainage

•The signature main truss

• The form work for the “sine-curve” vehicle ramps

• The lighting, ventilation and other building service systems - including provision for electric cars of the future, and

• The calculations required to achieve the Green Star rating

“We had to have a very tight control on quality,” Lam said.

“We even needed to use laser-cut steel plates to guide the formwork, especially for the ramps - everything needed to be perfect.

“Even the underground workshop is in perfect condition so any of the {vehicle] owners can go there to watch their cars being worked on.

“Looking at the ground level, in order to make architect Warren and Mahoney’s vision possible we incorporated the structural design [into the main concrete truss] that’s a feature in itself. And the mezzanine floor above the entry - we’ve hung that from the floor from above.

“This is innovation in the project.”

It’s not just digging a hole
Excavating the site was a challenge.

Babbage engineers designed a top-down construction methodology to avoid potentially difficult negotiation with neighbours and the council to have ground anchors installed for the retaining wall.

Top-down excavation involves drilling the perimeter piles then excavating down to first-basement level and laying the concrete floor, which braces the sides and lets you dig down to the next level.

But the Giltrap site was on a slope so for the first five metres there were only three sides, requiring complex bracing and excavation.

Malcolm Stapleton, principal of Babbage Geotechnical, an environmental engineer and Babbage board chairman, said water on the site was also a problem.

“You’re balancing the inflow of water with the rate at which you can let it move off site,” he said.

“You had to manage things so water can flow slowly through a drainage system we built, but not slow enough to build up a water pressure that’s going to cause your basement slab to pop up.”

Babbage building services group manager Matthew Foskin said the client wanted gravity to handle the drainage rather than pumping stations. Because of the building’s depth, achieving a gravity discharge required directional drilling to the council storm water system.

“There were a lot of challenges because of the deep basement and the requirement to enable ground drainage and storm water drainage without pumping,” he said.

“But at the end of the day we engineered that.”

Trussing up the building
For the building’s signature concrete truss Babbage had to come up with a system that allowed it to be cast on site, but with a degree of finesse that fitted in with the rest of the exacting standards the building was designed to meet.

Babbage structural engineer Ivy Lo said two key elements were required - the structure of the truss itself, and the perfect surface finish to the concrete that encased it.

“We designed a reinforcing cage that did not require spacers to support it so it was completely free-standing, and so there were no marks on the steel mould’s perfect surface finish,” she said.

“To achieve the smooth [concrete] finish, we specified self-compacting concrete. It’s a special concrete we don’t normally use - it doesn’t require compaction and it fills every void.”

Cutting to the shape
When you’ve got expensive cars with just a few centimetres of ground clearance, the last thing you want is for them to leave their under-floor systems on the concrete as they go down the ramps.

To get around the problem, the architects designed the ramps as a sine curve - a flowing wave that allowed the cars to go from floor to floor trouble-free.

Having the shape was one thing - achieving the shape was another, so the Babbage engineers had steel plates cut by lasers to create a profile for the contractor to screed the concrete against.

“The architects designed the curve of the ramp and we worked with them to make that happen,” Lo said.

Shedding the light
But while getting the structure exact is essential, it’s building service engineering that brings it to life - the lighting, ventilation, power, communications and fit-out.

Babbage electrical engineer Tommy Lee said two elements stood out - the lighting and its sophisticated control system, and the calculations required for 119GNR to achieve the coveted Green Star rating.

The focus of the building is the three luxury brands of motor vehicle. Each had its own show room and presentation requirements, right down to the lighting.

“Each brand had different requirements to show its identity,” Lee said.

“We had to integrate all our services design into these layouts, which in turn were driven by the different branding requirements for each space. There was a lot of co-ordination.”

The Babbage team designed the layout of the lighting arrangements, co-ordinated the power, emergency lighting and fire sprinklers in the lighting extrusions, and worked with supplier Targetti to implement them. Exposed ducting and other services had to be elegant and efficient.

For the Green Star rating, Babbage was responsible for the calculations that proved the building met the requirements.

For the building services team that meant precise fresh-air rates, vehicle exhaust from the basement workshops had to be extracted, the solar gain from the glass and the amount of daylight had to be calculated, and computer models had to be developed to show how much energy was used in the air conditioning.

“The client was quite passionate about sustainability and getting the Green Star rating,” he said.

“It was important to them, so they were really keen to have a sustainable building.”

Plugging into the future
And with an eye to the future, the building also looks to the age of electric cars.

“We put in quite a lot of infrastructure and flexibility to allow for the future car charging,” Lee said.

“There are some [electric cars] now but they know for the future there will be a lot more, so we had to put in quite a lot-bigger switchboard and transformers so they could move to electric [cars] in the future.

“We did a lot of research around different aspects of charging so we got the capacity right.”

Been there done that - sort of…
But while 119GNR features cutting-edge design and engineering, another Giltrap building just up the road served as something of a template when it was opened six years ago.

The distinctive Audi building at 150 Great North Road features a dramatic racetrack-curve in the main showroom with opposed sloping walls, and a distinctive exterior.

But for Babbage engineers many of the problems were the same - the top-down excavation and water so deep in the basement Lam described it as a “swimming pool”.

It too has basement workshops with exhaust extraction systems, below-ground vehicle storage, and gentle access ramps.

Support of the excavation was driven by the need to protect a major underground fibre optic network on the site boundary, and the design that dealt with that was also challenged by buried electrical mains found inside the boundary.

While the top-down excavation wasn’t complicated by sloping land, dealing with the water on the narrow constricted site posed major problems for the engineers and the contractors.

Stapleton said not all the props the engineers wanted to have could be used because they restricted access for the heavy equipment needed for the work. To get around the problem the engineers used temporary berms.


Having got down to 9m the next challenge was the issue of water entry and the water pressure that would be applied to the basement slab. 

A drainage solution was developed that allowed the hydraulic forces and pressures being applied to the walls and the slabs to be controlled, and a high-quality waterproof system that could be progressively installed without compromising watertightness or drainage controls.

The result is a system of sub-basement tanks and pumps that keep the water under control.  

Babbage building service engineers were also responsible for the design of the air conditioning and the overall electrical systems.

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