Blast resilient vs Blast Resistant design
Blast Engineering of buildings has a (somewhat) justifiable reputation for being expensive and for producing buildings which can be either fortress like or very significantly more expensive than normal buildings but this does not have to be the case.Where engineers are called upon to make historical buildings safer without compromising their appearance it is true that the expense can still be considerable. However, when planning modern buildings the costs of greatly enhancing performance can be very reasonable and depend upon the type of building, its location and the level of protection required.
When assessing the performance of a building structure against blasts there is a range of performance levels which may be appropriate. In the case of a structure where the risk of an explosive event can be demonstrated and understood then the case for protection and the level of protection is clear. Good examples of this include the control rooms for chemical process facilities and of course bunkers.
The function of all buildings is to protect the inhabitants from the outside environment and for those buildings specifically designed to withstand blasts this can be achieved relatively simply and inexpensively. This usually involves a lack of windows, ductile reinforcement detailing, flexible couplings for services, big heavy and well-sealed doors and the blast engineers favourite: lots and lots of concrete.
However the problem comes when the main purpose of the building is not just to withstand a blast but as a residence, sports stadium, transport hub or a workplace and these would not be fit for purpose if constructed as a fortress like the shelter above.
With much skill and a significant budget this can be engineered around and in the case of buildings like the Europa Hotel in Belfast which has been the subject of 28 bomb attacks it’s clearly a sensible precaution. In this case the engineer would consider the maximum amount of explosives which can be used against it and design it such that people should not suffer unavoidable harm.
When the main purpose of the building is not just to withstand a blast but to be used as a residence, sports stadium, transport hub or a workplace, it would not be fit for purpose if constructed as a fortress.However when we are considering the design of less high profile hotels, offices, schools and sports facilities often we are still asked to design them to this same standard despite the lower level of risk. This has meant that ultimately blast engineering can become unaffordable for projects which do not have this high level of risk due to their business process or iconic nature.
In 1996 the IRA chose to target Manchester city centre, perhaps this was in part because of the efforts made in making London more secure in the wake of the 1993 Bishopsgate bombing and certainly because of the media attention around the Euro 96 football tournament staged there. It was huge device estimated at 1,500 kg and it is only because an extraordinary policing operation that evacuated 75,000 people in an hour that nobody died and only 200 were injured. The £1.2 billion rebuilding costs eclipsed even the £800 million of Bishopsgate in London and led to the demolition of several buildings.
Unfortunately many modern terrorist organisations have no qualms about harming civilians and do not issue warnings prior to an attack. Since budgets are not unlimited and risks can appear away from capital city locations it is important that we must make this money protect the maximum number of people from harm across all of the facilities proportionate to the risks they bear rather than just concentrating ’gold plating’ flagship developments.
Fortunately this type of sensible modification is possible as most modern façade systems are already made from laminated glazing which helps to increase safety for occupants. However, many modern aluminium stick system facades have insufficient capacity within the frames to withstand the forces that the glazing can apply under blast loading. This dangerous condition known as unbalanced design can lead to pull out of the glazing from the frame and contributes to a massive underperformance of the system under blast loading.
Unfortunately many modern terrorist organisations have no qualms about harming civilians and do not issue warnings prior to an attack. Since budgets are not unlimited and risks can appear away from capital city locations it is important that we must make this money protect the maximum number of people from harm across all of the facilities proportionate to the risks they bear rather than just concentrating ‘gold plating’ flagship developments.This can relatively cheaply be addressed at the design stages to create balanced designs in which the glazing hazard is more predictable and is a great opportunity to maximise the performance of the existing building, reducing insurance costs and increasing resilience.
At BB7 we have innovative whole building simulation tools for the rapid assessment of blast damage on complex facades on a panel by panel basis. Hence at a building scale we can show a visual representation of improvements from standard facades to balanced design options to the highest performance designs to allow clients and stakeholders to understand their building’s performance at each level and make informed decisions about how their building should be optimised.