Assessing earthquake risk in the UAE
On 16 April this year, for the second time in a week, Dubai experienced ground tremors following a significant earthquake in Iran, hundreds of kilometres away. These shakes, though relatively minor, caused significant alarm across the country; many people stayed outside for several hours after the tremors had ceased (although this isn’t the safest of places to be) and several queries on seismic resistance have subsequently been received from anxious property owners.
Following these tremors I was prompted, by the enquiries we (BuroHappold) received and several press articles questioning the risk to older UAE buildings (built before seismic requirements were imposed), to do some research on the UAE’s level of seismic hazard. It should be pointed out that structural design and risk are inseparable; risks are associated with the forces we design for, the materials we use, and the accuracy of the construction process; none of which can be determined with 100% accuracy at the design stage.
To design buildings with a minimal risk of collapse, we assume statistically low estimated values for material properties (which can be highly variable) and design events (leading to design forces) that have a statistically low chance of being exceeded during a building’s design life (typically between 2% to 10% chance in a 50 year design life). As a result of this approach, it is understandable that buildings designed and constructed correctly (according to relevant codes of practice) using well specified materials and built by experienced contractors, will demonstrate a capacity to withstand forces beyond which were assumed at the design stage (which themselves have a relatively low possibility of occurring); indeed studies have shown that such buildings could conceivably resist forces more than 50% higher than they were originally designed for, particularly when a good standard of detailing is adopted (though this doesn’t necessarily mean that they can withstand significant earthquakes).
The second part of my research looked at the UAE’s seismic hazard (specifically Dubai); several independent UAE-specific technical papers and reports strongly suggest that the level of seismicity in Dubai and the Southern Emirates is low; far lower than we were, at the time, designing for (something also backed up by the many site specific seismic hazard assessments carried out for high profile buildings in Dubai). Some studies do argue that the level of seismic hazard is higher, but often very onerous assumptions are made to support this.
I had convinced myself that the UAE’s seismic hazard was low, and that even buildings not designed for seismic resistance could still resist higher loads than designed for so I thought I would share the good news in a blog.
At this point, Dubai Municipality increased the seismic hazard for design (with a corresponding 30% increase in the forces we design for). Somewhat surprised I began to prepare a new angle to my blog outlining why I found it difficult to reconcile with this decision, but subsequently stopped to think it through. Don’t get me wrong, I still believe that the risk of strong earthquakes in the UAE is low, but what I can understand is that this decision is inextricably linked to the risk approach to design and specifically the risk from two angles: the human factor and the multi-dimensional aspect of risk itself.
Scientists and engineers were taught a very sobering lesson in risk and personal culpability in October 2012, when six Italian scientists were convicted of manslaughter following “falsely reassuring statements” about tremors in L’Aquila, Italy, prior to a major earthquake that tragically killed more than 300 people in 2009. Their convictions will have made anyone responsible for making risk based decisions think very carefully, and only the bravest would not err on the conservative side.
Secondly, the multi-dimensional aspect of risk can sometimes provide a case for reconsidering seismic forces to be used for design purposes. Risk in the greater context is related to several factors: hazard, exposure and resilience. With increasing hazard or exposure, overall risk also increases unless there is a comparative increase in resilience. Dubai today is a very different place to twenty or even ten years ago; a rapidly expanding building stock (urbanised area has increased by more than 60% in 20 years) and population (doubled between 2000 and 2010) has led to a marked increase in the emirate’s exposure to the risk of major events. It’s not unreasonable for local authorities to attempt to mitigate against this increasing cost exposure (human and financial) of a major event occurring, so long as it’s part of a consistent, strategized approach. Whether the new requirements are the result of such a risk approach is unknown.
So what are the implications? The updated requirement is that all buildings above ten storeys need to be designed for a higher level of seismic hazard, including buildings under construction. There’s a logic to the height limit, as the seismic events we are likely to be subjected to will almost certainly originate in Iran. As a result, the characteristics of the shaking felt in Dubai will be filtered by hundreds of kilometres of rock and soil, resulting in the ground accelerations being dominated by low frequency, long wavelength motion; the type of motion that affects tall buildings to a greater degree than shorter ones. Tall buildings in the region of 150m plus in height will still be strongly governed by wind effects and the cost implications of the new requirements for such buildings could be relatively low; the main impact will likely be seen in buildings between 10 storeys up to around 40 storeys. Depending on the building system adopted, the structural costs of the increased design requirements for new designs could be up to 10% of structural cost (2% of construction cost) while for buildings already under construction, additional penalties may apply due to the complications of retrofit requirements and delays in re-approvals etc. Away from structure, the cost related to the loss of saleable floor area and the increased cost of other building components should also be considered. MEP systems and particularly facade systems would likely need to be upgraded in future designs to ensure that they are robust enough to meet with the new requirements and these would bring with them additional cost. These additional costs however are unlikely to dampen the enthusiasm for construction in Dubai which is currently going through a strong upturn.
Only one thing’s for sure – the increased requirements can only be positive for the average person in Dubai…
Listen to Mark discuss seismic risk in Dubai on the BBC Business News