The Seismic Question

Wellington-Moving-Times-The-Seismic-Question-1Holmes Consulting Group engineer Hamish McKenzie recently spent time with Colliers leasing specialist Phil Humphrey to help landlords get a handle on the new rules on seismic ratings, engineer reports and the question of %NBS. The result was published in the Colliers magazine Moving Times Wellington edition.

Question: Wellington has always had to design for seismic activity. In the wake of Christchurch and following recent events, the building stock has come under detailed scrutiny. In assessing numerous buildings, should occupiers be heartened by the performance of Wellington’s office stock generally?

Answer: Yes and no. It has been heartening that damage was generally quite limited and that the industry’s response has meant that we were able to get the city up and running quickly. However, the recent seismic activity in Wellington has been relatively moderate, both in terms of earthquake intensity and duration – hence, buildings were not tested to anything close to a modern design level earthquake.The recent events have been a further wake-up call. They have highlighted that issues associated with key building vulnerabilities are real; and that we need to continue to evaluate and improve building performance to address issues like; stairs, precast flooring, unreinforced masonry, pounding, foundations and liquefaction, cladding systems and building services restraints (to name a few).

Question: Tenants are clearly interested to understand the seismic resilience of the buildings they (currently or potentially) occupy. What advice do you have around a realistic approach they should take in satisfactorily answering the question of building safety?

Answer: Tenants need to understand that there are a range of different methodologies used to reach an understanding of building performance.

What is more important than a % NBS “score”, is the methodology and thoroughness used in reaching that “score”.

For example there is a big difference between an Initial Evaluation Procedure (IEP) and a Detailed Seismic Assessment (DSA). Ultimately, if you want to be completely informed, only make decisions on a thorough DSA not an IEP. It may be more reassuring to be in a building with a 50-60% DSA rating where there is a thorough understanding of likely building performance up to and beyond that load level, than a building with a much higher IEP rating.

Question: Is flexibility the same as ductility? Is flexibility uniformly a good thing?

Answer: No, flexibility and ductility are different. Flexibility is a measure of how much a building will move around (wobble) up to the point at which yielding (or structural damage) starts to occur.

Ductility is a measure of how much more displacement the building can sustain after the onset of yielding (structural damage). Whilst a flexible building will tend to mean the building is subjected to lower seismic forces than a more rigid one, it will need to move a lot in order for that to happen.

Hence, it may be prone to more non-structural damage of building components.There is often a trade-off between increased flexibility, therefore the ability to have less structure, and enough stiffness to limit non-structural damage.

Read the interview in its entirety: Colliers Moving Times – Wellington – Q4 (pdf 1.3MB)