A guide to earthquake ratings and seismic strengthening

Earthquake percentage ratings

The Canterbury and Kaikōura earthquakes provided many learnings for engineers. However, there are no guarantees as to how a building will eventually perform in a major shake. The NBS rating system is the primary tool currently relied on to help us make decisions regarding obligations in the workplace.

The %NBS rating evaluates the performance of a building compared with a similar new building in terms of protecting life. It’s calculated as part of a seismic assessment.

• A building rating above 67%NBS is considered to be an acceptable seismic risk by the New Zealand Society for Earthquake Engineering.
  
  
• A building rating less than 67%NBS is considered an Earthquake Risk Building (ERB).
  
  
• A building with an earthquake rating less than 34%NBS fulfils one of the requirements for the Territorial Authority to consider it an Earthquake-Prone Building (EPB) in terms of the Building Act 2004.

Important to note: %NBS is measured at the lowest defective point in a building. If there is a particular weakness that generates a lesser percentage, the whole building’s rating is downgraded to this score.

Buildings below 34 percent

There is a common misconception that earthquake-prone buildings can’t be occupied. However, they can be - it’s just a question of safety and managing risks, which is justifiably a key consideration for businesses. The new seismic risk guidance developed by MBIE states that most seismically vulnerable buildings are not imminently dangerous and can remain occupied.

While hundreds of seismic strengthening deadlines are set to expire in Wellington this decade, a multitude of buildings still require work to meet standards. Tenants will not accept tenancies that are not up to code and the accelerating costs being incurred by landlords—from relocating offices to seismic works—are posing challenges. “This makes setting clear communication and expectations from the outset to reduce disruption all the more key,” says Moutos.

What next?

Applying our learnings from the past to the future, there are no certainties, except change. Looking forward, it’s possible that even stricter rules will be introduced as proposed in the government’s Yellow Chapter C5 paper, which reflects the latest engineering knowledge.

“There is rightly a lot of confusion and misunderstanding around seismic building assessment regulations,” says Moutos. “In our view, forward-thinking, proactive landlords should have detailed seismic assessments carried out to account for possible changes in the future.” 

He believes transparency is also crucial. “Be upfront with reporting, which will ultimately save time and accelerate the process in the long-run.”

While new regulations are not being enforced yet, tenants are seeking that extra assurance before committing to a lease, so it’s crucial to have a long-term, future-thinking outlook.

“In Wellington, we are seeing buildings implement innovative seismic works like viscous dampers, which dissipate the kinetic energy of seismic events, in retrofits and new builds, which may prove more cost effective for landlords.” 

It’s this kind of thinking that will prove integral in navigating the changes in seismic standards.