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The seismic rating of buildings has become an important criteria as part of office leasing decisions. However it is easy to become confused as to what types of report are acceptable, what target percentage of new building standard (%NBS) is appropriate and what a tenant can reasonably expect from a building owner. And of course, there is a new set of acronyms to understand.

Colliers leasing specialist, Phil Humphrey, spent time with Hamish McKenzie of Holmes Consulting, an expert in this area. This (Q&A) article is an attempt to de-mystify the subject, and to provide a level of understanding to help the office tenant navigate through this complex subject.

Q 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?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….).

Q 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?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.

Q Can you define the difference between an IEP (Initial Evaluation Procedure) and DSA

(Detailed Seismic Assessment)?An IEP is a desk-top, tick-box process developed as a useful screening tool to enable a large sample of buildings to be ranked according to likely seismic performance. It was developed for use by councils as an initial screening tool and is also useful for screening large building portfolios. From there, more detailed assessments can be undertaken on those that may warrant further investigation.

The IEP was never intended to provide an accurate assessment “score” for an individual building. The IEP is quite subjective and gives broad “tick-box” grades over a range of criteria including building location, age, size, materials and regularity to provide a final output number. It can be undertaken relatively quickly – but doesn’t require any detailed assessment nor calculation to be done in relation to the subject building.

A DSA provides a detailed assessment of a building, including some calculation or computer analysis to more accurately assess the performance of different building components (beams, columns, walls etc) under a range of earthquake loading scenarios. It should assess all key building components, including secondary structural features such as stairs, façade systems, floor systems etc. For small simple buildings, a series

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calculate a “number” (within prescribed standard and guideline parameters), the notion that there is any real difference between a building that has a score of 30% vs 35%, or 65% vs 70%, or 90 vs 100% speaks of a notion that there is a level of certainty, which in reality doesn’t exist.

Furthermore, the “score” that is reported, typically references the point at which a buildings Ultimate

Limit State (ULS) capacity is exceeded. What happens beyond that limit is often not addressed. So a building that has a 70% rating might be a pile of rubble soon after, whilst another building with a 60% rating might have a level of toughness (or resilience), such that it hangs in there when earthquake load levels approach 100% or beyond (albeit with extreme levels of damage). I know which building I’d rather be in!

Q Is the approach of ‘Critical Structure Weakness’ more relevant?

A thorough DSA should capture building components that might have a disproportionate effect on building performance – sometimes known as Critical Structural Weaknesses. Some examples might include; stair detailing, poor precast flooring seating, non-ductile columns, non-ductile diaphragm reinforcing, poorly configured transfer structure etc.

Q In the context of Wellington’s sub strata, should soil stability (rock v reclaimed land)

colour the judgment of a tenant when it comes to site selection? And does an IEP/DSA address the geo-tech issue adequately?Both an IEP and DSA should take into account the site sub-soil flexibility (rock vs reclaimed land) based on current code loading requirements.

Hence the propensity for buildings on poorer/deeper soils to feel higher levels of earthquake shaking than those on rock, is already taken into account.

Q Is the cost of a DSE proving prohibitive for many Landlords?

It may be, however, it’s a real issue in New Zealand and part of any thorough due diligence as part of owning and maintaining commercial property. A typical DSA may fall in the range $20–60k depending on the scale and complexity of a building (cost may be less for smaller two or three level buildings).

Q Anecdotally, Wellington’s buildings are built to the highest seismic criteria/strength of

the country’s main centres.True. The earliest codes of NZ identified central New Zealand as most at risk. However, earthquake load levels and our understanding of their effects

has changed significantly over time and load levels have increased accordingly.

Q Is flexibility the same as ductility? Is flexibility uniformly a good thing?

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.

Q Is there a move back to rigid structures rather than highly flexible ones?

To some extent yes – although more broadly there is a move to consider systems that offer better Low Damage characteristics. Low Damage Design (LDD) incorporates a broad range of structural systems including Base-Isolation, Viscous Damping Devices and more recent systems with articulated joints that seek to dissipate seismic energy without the level of damage seen in more traditional highly-ductile systems. The idea being that LDD may lead to better business continuity after a seismic event.

There is still some way to go in order to ensure certainty of outcomes with some of the low damage systems being proposed. This area would also benefit from greater input form the insurance industry so that the full life-cycle and business continuity benefits are captured.

Q There is considerable concern over the performance of staircases in an event – e.g.

Forsyth Barr in Christchurch. Is this justified and can you explain briefly why they differ from the main structural elements?Stairs would not usually impact the global response of the building – the failure of the stairs would not normally lead to the more global failure of the overall building. However, the need to ensure safe egress from a building after an earthquake is obviously a high priority and one that was highlighted following Christchurch’s events. Some stair systems and configurations used mainly through the 1980’s have been shown to perform poorly in earthquakes, such as the Forsyth Barr stairs. Therefore, the review of stair systems and the need to retrofit some that are likely to perform poorly is justified.

Q As the ‘dust settles’ after recent events locally, do you anticipate any meaningful

changes to the code or its operation? I.e. – is it still adequately protecting occupiers in its current form?When compared internationally NZ has very comprehensive seismic design codes for new

of hand calculations may suffice, but for larger, more complex buildings a comprehensive three dimensional computer analysis may be necessary.

A thorough DSA should consider key building vulnerabilities such as stairs, non-ductile columns, diaphragm connections and foundations to ensure a holistic picture of the building performance is understood – not just the performance of the main lateral system (walls and frames).

Q So, is it really only a DSA which can provide

assurances to tenants?Correct – an IEP may give an initial estimate of a buildings likely performance, but only a DSA will establish a true thorough understanding.

Further, when comparing the seismic performance of several buildings, comparing an IEP score to a DSA score is meaningless. Only a comparison of DSA’s will accurately establish the relative merits of various buildings.

Q Tenants/occupiers appear to have adopted a ‘pass-mark’ criteria of > 67% (New Zealand

Society of Earthquake Engineers ‘NZSEE’ B-Grade) for seismic acceptability. It appears to be arbitrary and almost like a comforting exam result. Is this NBS method appropriate? It is appropriate, provided it’s used as a guideline to reflect the point at which a reasonable level of seismic performance might be expected. It’s not an “absolute” guarantee of “safety”, nor should it be approached with a dogged insistence that nothing less will suffice. The NZSEE 67% mark is a recommended target when strengthening a building with a lower rating. It recognises the fact that strengthening buildings to achieve 100% performance levels may be impractical or uneconomic.

In statistical terms (and when considering a standard 500 year earthquake event), 67% load levels equates to an approximate 20% probability that the building will experience an earthquake of that intensity or greater, over a 50 year life-time. By comparison, at 100% load levels, there is approximately 10% probability over a 50 year period that an earthquake of that intensity (or greater) will be experienced. As load level performance decreases, the chance of that level of seismic shaking occurring over a given time period increases significantly. There is about a 55% chance that 34% earthquake level shaking will be experienced over a 50 year timeframe. So the numbers aren’t arbitrary – they do have statistical meaning.

However, the numbers should be viewed as guidelines and targets when assessing the relative merits of a building, and again the depth and basis for establishing a particular “score” is often far more important than the score itself. There is also a big margin of uncertainty within any calculated score – so whilst as engineers we will invariably

Phil Humphrey

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building design. When applied appropriately, these codes have generally achieved what has been expected of them. However, as we learn more codes will always get tweaked and improved, so whilst I don’t expect to see wholesale changes to the standards we use to design new buildings – incremental improvements over time will still likely occur.

Biggest room for improvement is to address existing building stock (older buildings in particular) and to ensure that they are brought closer to the standard required of new buildings.

This has been the focus of the Canterbury Earthquake Royal Commission recommendations and is a big focus for MBIE. This is already underway with proposed changes to the sections of the Building Act that relate to Earthquake Prone Buildings recently announced.

NZSEE (and others) have also been working on changes to guidelines used for assessing existing buildings.

Q There is a perception that different engineers will assess the same building differently. Is

this true, and if so, can you explain why something which appears to be a factual exercise can be so interpretive?This can be true – much the same way that different doctors may well give different advice regarding a complex medical issue, such that it may take some time and several opinions to gain an accurate diagnosis.

Every building is different. Some are more complex to assess than others and with some possessing archaic materials in applications not well assessed by research. Therefore, accurately assessing the seismic performance of a large and complex building can be a very complex process, requiring considerable expertise and experience and invariably some judgement along the way.

More often, the differences lie within the methodology or approach that has been used – such that an IEP might be compared to a very

thorough DSA, or a DSA that looks at the main seismic system only, might exclude some of the secondary structural features (stairs, floor seating etc) that could bring the overall building performance down.

Ultimately, the level to which an assessment can be accurate will rely on the time, effort and degree of expertise/experience used in undertaking that assessment. So if good advice is sought, consider who is providing that advice, and the methodology they are using as part of that.

Q Will a DSA identify problematic aspects internally within a floor, such as old heavy

ceilings and inappropriately installed services?Not typically. A DSA would normally cover the building structural components. Non-structural items should form the basis of a review by someone more familiar with those items and can often become part of a Building Maintenance project.

In addition, the restraint of fixtures and fittings is another important aspect of earthquake preparedness and is an item that doesn’t require specialist Structural Engineering input to address.

Q In preparing a DSA, will you consider any specific detail for immediately adjacent

buildings?A DSA report should address any issues associated with “pounding” of the subject building into neighbouring buildings, plus any shared structural elements such as party walls. It would otherwise not typically address specific details relating to the adjacent buildings.

Q How well does New Zealand compare in world terms, both for the quality of

specialist knowledge and its buildings? NZ specialist expertise with respect to earthquake engineering is generally very well regarded – although we have quite a small pool

of expertise, so the depth of that knowledge might not be as great as other comparable seismically active areas (e.g. California and Japan).

Perhaps a more meaningful question might be; what is the baseline level of seismic design practice throughout New Zealand and does this meet acceptable minimum standards?

In terms of our existing building stock – I have very limited knowledge of comparable overseas markets (e.g. California and Japan), however I suspect that there may be a greater range of “quality” in New Zealand, in part due to a lack of a consistent national approach to Earthquake Prone Buildings and the lack of traction that has been achieved in dealing with the worst of these buildings.

Q Best advice during an event – stay in the building?

Drop, Cover and Hold – as per advice from Civil Defence; http://www.getthru.govt.nz/web/GetThru

Prior to an event, develop a plan and regularly practice/audit that plan. There are good resources and events through Civil Defence such as the Get Ready, Get Thru campaign and the national Shake Out event.

This planning could involve having Earthquake Wardens carefully assess the buildings egress paths prior to advising staff to leave. Other considerations include, where to congregate, what about tsunami risk, the status of neighbouring buildings, transport routes etc.

Depending on the size and nature of the earthquake and the impact on your particular building, the best advice might be to stay put! This would obviously change if the building was seriously damaged and on the verge of possible collapse. As such, the plan should anticipate a range of scenarios.

Hamish McKenzie is a Chartered Professional Engineering (CPEng) with over 17 years experience in Structural Engineering design, throughout New Zealand and in the UK. He is a partner with Holmes Consulting Group (HCG), who specialise in Structural Design and Seismic Strengthening work. HCG have offices throughout New Zealand and in San Francisco. Hamish has been Business Manager of the Wellington office of HCG since he moved here from the South Island in 2006. He has been involved with the seismic assessment of numerous well known Wellington buildings including Parliament Buildings, Government House, National Library, Majestic Centre, State Insurance Tower, Vodafone on the Quay, Wellington Railway Station and several buildings throughout the Wellington City Council’s Civic Square Campus.