Is My Earthquake Prone Building Safe to Occupy?

May 1, 2020

Prendos Chartered Structural Engineers take us through what it means to have an Earthquake Prone Building, whether you can stay in it, and why it’s so important to seek guidance from a Structural Engineer.

Following New Zealand’s significant earthquake events of the past decade, building owners and leaseholders have been pushed into obtaining an IEP to give their building a %NBS, despite not really knowing what either of these acronyms mean.

“An IEP is an Initial Evaluation Procedure, or the first step in the seismic evaluation of a building,” an Engineer explains. “It assigns a %NBS rating, which indicates the building’s ability to withstand an earthquake. Buildings with a %NBS of below 34% may be deemed ‘Earthquake Prone’ – implying they will suffer a high level of damage in an earthquake.”

Decisions on (building) occupation should be
made on a case-by-case basis, following guidance from specialist Structural Engineers.

“I’m often asked by clients with earthquake prone buildings (EPBs) whether their building is ‘safe’ – a question to which they expect a black and white answer. What they really want to have explained, in plain English, is ‘what’s going to happen to my building in an earthquake?’. Unfortunately, the honest answer is ‘nobody really knows for sure’. The reality is we’re dealing with events that may happen, but we don’t know when; and with behaviour that’s difficult to predict with any certainty.”

In an attempt to help clients navigate this often murky area, Prendos Structural Engineers have developed guidelines to assist building owners in making a decision around EPB occupancy on a more informed basis.  It starts with a holistic assessment of the building, its surroundings and its use. 


How Risky is an Earthquake Prone Building (EPB)?

“An Earthquake Prone Building is unlikely to withstand an earthquake that has more than one third of the ‘power’ of an earthquake for which new buildings are designed, without causing injury or death”. “An EPB presents a risk to occupants of approximately 10 to 25 times that of an equivalent new building that meets the minimum safety requirements of the New Zealand Building Code.”

But, new buildings are held to a higher standard than existing buildings. The safety target for new buildings in New Zealand results in a 1 in 1,000,000 risk of a fatality – a very low level of risk. Even so, most new buildings will have an expected performance above 100%NBS, if properly designed. Figure 1 shows the expected performance of buildings with differing seismic ratings.

Level of Performance for Seismic Rating Chart
Figure 1: Expected Levels of Performance for different Seismic Ratings

“When it comes to designing an ordinary new building…we’re designing for a 1 in 500 year earthquake. For an EPB, we’d be looking at the potential for collapse in a moderate earthquake, which is nearer a 1 in 50 year event. It should be noted, however, that a 1 in 50 year event could happen tomorrow; as could a 1 in 500 year event – the former is just more likely to happen. High consequence incidents usually have a low probability of occurrence.” 



A building’s %NBS should not be the only deciding factor in determining occupation.

“%NBS is determined by a building’s weakest element. What we must also take into consideration are the consequences of that failure: just because a structural element has exceeded its theoretical ultimate capacity does not mean the building will simply fail catastrophically.”

When determining seismic performance, Structural Engineers must also consider robustness, alternative load paths and the ability of the structure to redistribute loads to other elements in the event of the failure of one element.

“We have to determine whether the building structure acts like a chain, where the failure of one link causes system failure, or more like a rope, where other strands can take on the load in the failure of one strand. If the failure of a particular element does not lead to death or serious injury, then it can be justifiably ignored. If the load it was carrying can be redistributed to other elements without them failing, then again, it can be justifiably ignored. Even when a structural element has exceeded its theoretical ultimate capacity, loads can often be carried in different ways.

“If, on the other hand, failure of one element would lead to the failure of another element (because it cannot take the increase in loading) and so on, we have a lack of robustness and the potential for progressive collapse.”

When the failure of a single element threatens the overall stability of the building and could precipitate a wider building collapse, the recommendation would be that the building remains unoccupied until works are carried out to make it safe. However, it is more likely that an EPB has only local weaknesses, which do not threaten the overall stability of the building but are likely to cause local damage to a limited area. That’s why it’s so important to engage the services of a Chartered Structural Engineer.

“We may determine that local propping or isolation will be sufficient to temporarily address the situation and enable continued partial building occupancy. We would recommend repairs to the affected area/s that, once made, will allow the building to be restored to full use.

“Getting this sort of tailored, individual engineering advice can not only help avoid unnecessary building closure, but ensure the safety of all occupants.”

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