T+T Puts Leaning Tower on a Sound Footing

T+T Puts Leaning Tower on a Sound Footing

Italy’s famed Leaning Tower of Pisa, with its 3.99 degree tilt, has nothing on Airways Wellington's striking new air traffic control tower. Already dubbed the “Leaning Tower of Rongotai”, the nine-storey structure leans into the city’s prevailing wind at an impressive 12.5 degrees.

Underpinning the tower’s exceptional, and essential, levels of resilience is Tonkin + Taylor’s geotechnical, coastal engineering and natural hazards expertise – coupled with an eye for value engineering.

As a critical post-disaster facility constructed to Importance Level 4, it’s the first building in New Zealand specifically designed to withstand tsunami inundation, velocity and debris loading.

Wellington sits atop an active seismic zone, so earthquake resilience was also crucial. A raft foundation, 13 base isolators, a concentric braced steel frame and superstructure ductility (for non-engineers, that’s a solid material's ability to flex under strain) give the control tower built-in elasticity to cope with a 2,500-year return earthquake.

T+T was directly appointed by Airways to deploy its geotechnical, coastal engineering and natural hazards assessment and design expertise.

Principal Geotechnical Engineer, Stuart Palmer, and Senior Geotechnical Engineer, Ayoub Riman, collaborated with Holmes Consulting on geotechnical investigations for the project and helped design the foundation.

“From a geotechnical perspective, it was an interesting project,” says Stuart. “It’s quite a tall, slender structure, but we made it work on a shallow foundation.”

Making it work meant taking into account the area’s liquefiable, sandy soils, while incorporating value engineering into the foundation design. The economic solution, which also addressed the potential for potential liquefaction caused by earthquakes was a shallow raft foundation that extends below the groundwater level.

“By going deeper, we got below the looser liquefiable sand onto a denser stratum and the less dependable upper soils were avoided,” Stuart explains.

Richard Reinen-Hamill, T+T’s Sector Director - Natural Hazards Resilience, and Senior Natural Hazards/Disaster Risk Resilience Specialist Dr Bapon Fakhruddin, took on the challenge of helping to design for tsunami inundation and velocity, for which there is no current New Zealand building code or loading standard. The pair turned to the American Society of Civil Engineers standard to determine tsunami wave loads, along with the 2013 GNS Tsunami Hazard Report.

“Finding a method to actually come with forces and loadings used by engineers; to be able to solve the problem in a defensible way without excessive cost was good,” Richard says. “It was about being pragmatic and focused on the outcome.”

“The air traffic control tower critical infrastructure information we had from GNS assumed a 2,500-year, 84th percentile plausible maximum credible event, so we had to treat it with more care than with traditional infrastructure.”

The resulting design incorporates concrete shear walls and concrete-filled steel tubes at the ground floor level to provide the required levels of resistance.

The control tower is likely to be one of the last built in New Zealand, as Airways looks to enter the digital age, minus the need for “bricks and mortar”.