It is frequently claimed that corrugated galvanised iron (CGI) sheets are ‘flying razorblades’ in storms, and that they shouldn’t be used in reconstruction. This has caused their complete prohibition, for example after the 1991 Cyclone in Bangladesh. However, they are durable, relatively cheap, and usually very popular with affected people. So what is the truth, and should they be used in recovery and reconstruction projects?
Having looked for evidence, I have found absolutely no evidence that CGI is any more dangerous than any other flying debris in windstorms. In fact the leading causes of death in windstorms are drowning (due to storm surges and wind-induced flooding), falling trees and falling debris (walls falling over, tiles falling off roofs, etc). Where data is available there are cases of injury and death (more injuries than deaths) from windborne debris, but not one that cites windborne CGI as the cause.
There is no reason to avoid the use of CGI in post-disaster reconstruction. Rather than banning CGI, reconstruction efforts should focus on effective evacuation of flood-prone areas and on stronger, safer buildings in general.
Details of the evidence I found are below:
Wind-borne debris is extremely hazardous for buildings
It is well established that wind-borne debris is a serious hazard for buildings. Objects carried by the wind can puncture the walls and windows of buildings, allowing the wind to get inside, potentially destroying the building. However, none of the literature or evidence states that CGI is any more dangerous than anything else.
“Wind-borne debris generated from loose-laid ground materials and/or from dislodged building components can fly an unbelievable distance, up to several hundred feet, and impact neighboring buildings. Almost all building envelope materials, particularly windows, doors and skylights, as well as most wall sidings and roof coverings used for homes, are at risk for impact from such high-energy projectiles. Breaches to the building envelope by such projectiles will allow rainwater intrusion and subsequently damage the building interior. It may also pressurize the interior of the building, increasing the uplift on the roof and the outward forces on the walls, making them more likely to fail. In high category hurricanes (e.g. category 3 or higher), wind-borne debris damage occurs essentially in parallel with the damage due to differential pressures.”
Damage to buildings from windborne debris (Tamura 1991)
Structural design codes have special provision to ensure that fixtures, fittings and cladding of buildings is well enough attached so it does not become detached and form flying debris (see right, for example, from IBC 2009).
Is is well established that in built-up areas windborne debris from one building can hit the next, and so on, causing what is know as the damage chain.
Damage to roofs, and the damage chain (Tamura 1991)
- Wind Issues in the Design of Buildings by American Society of Civil Engineers.
- ASCE 7-10
- IBC 2009
- FEMA P361 Safe Rooms for Tornadoes and Hurricanes
But does wind-borne debris kill lots of people?
The description of the effects of different category hurricanes mentions wind-borne debris injuring or killing people, and common sense says that any solid object flying at 60+mph that hits people will severely damage them. But what’s the actual evidence?
- Category One Hurricane: Loose outdoor items will become projectiles, causing additional damage. Persons struck by windborne debris risk injury and possible death.
- Category Two Hurricane: Loose outdoor items will become projectiles, causing additional damage. Persons struck by windborne debris risk injury and possible death.
- Category Three Hurricane: Persons struck by windborne debris risk injury and possible death.
- Category Four Hurricane: Windborne debris will cause extensive damage and persons struck by the wind-blown debris will be injured or killed.
- Category Five Hurricane: Severe injury or death is likely for persons struck by wind-blown debris.
Most of the studies I can find focus on disasters in rich countries, as it is only richer countries that have systematic collection of mortality and morbidity information.
- One paper, notes that 23% of the 63 fatalities from Cyclone Mireille, which struck Japan in 1991, were due to flying debris (Tamura 1991). The paper also gives a good overview of the damage chain. The paper does not mention CGI.
- A systematic review into the health impacts of windstorms (Goldman 2013) in the UK concludes that becoming airborne, being struck by flying debris or falling trees and road traffic accidents are the main dangers, with a number of secondary effects like electrocution also being significant. The review also finds that flying debris causes far more injuries (mainly fractures and blunt trauma) than it does fatalities. Most of the fatalities from flying debris are due to head injuries. The review does not mention CGI, but largely only references research from the UK and USA.
- A 1984 paper reviewed the multiple casualties caused by a gale in the UK: “Four patients were injured by falling slates and twelve by other falling or flying objects, including window glass, a tree, a milk-crate and a dust-bin.” This was out of a total 116 injuries treated. (Illingworth 1984)
- Another similar paper found 36% of injuries were due to flying debris and falling masonry (Cugnoni 1992)
- Medical examiner reports in the US report that 9 deaths were caused by blunt or penetrating trauma in Hurricane Andrew (1992) and 2 of 18 deaths were caused by flying debris in Hurricane Charley (2004).
- Hurricane Katrina killed over 1,200 people in the USA in 2005. One study, looking at 971 deaths in Louisiana, found that drowning (40%), injury and trauma (25%), and heart conditions (11%) were the major causes of death. The mechanism of injury was unspecified for 226 trauma or injury deaths; specified injury-related causes of death included heat exposure (n 6), unintentional firearms death (n 4), homicide (n 2), suicide (n 4), gas poisoning (n 3), and electrocution (n 1), but numbers due to flying debris are unfortunately not mentioned. (Brunkard 2008). Interestingly the paper strongly links likelihood of death to age, poverty and social exclusion.
- Hurricane Sandy in 2012 killed around 160 people. The CDC examined the causes of 117 deaths captured by American Red Cross Mortality Tracking. Of the 117 deaths, 67 (57.3%) were classified as directly related deaths, and 38 (32.5%) were indirectly related to the storm. Of the directly related deaths, the most common mechanism was drowning (40 [59.7%]), followed by trauma from being crushed, cut, or struck (19 [28.4%]). Poisoning was the most common indirectly related cause of death; of the 10 poisonings, nine were caused by carbon monoxide. Most directly related deaths occurred during the first few days of the storm, whereas indirectly related deaths continued from the day before the storm into the middle of November. More information is provided in an Earthsky article, based on data from the New York Times. Flying debris was not one of the top 7 causes of death:
So what happens when you look at the most deadly windstorms, which don’t tend to happen in rich countries, but for which the data is much harder to find:
- The 1970 Bhola Cyclone in Bangladesh killed up to 500,000 people. Sommer and Mosley undertook two surveys after the cyclone looking mainly at mortality, but also at morbidity. The primary causes of death were drowning, although they did note a ‘cyclone syndrome’ where trauma was caused by hanging on to trees and being buffeted by the floodwaters. (Sommer, 1972). In fact six of the top ten deadliest windstorms of all time have hit Bangladesh, with the lead cause of death drowning. (Washington Post, 2013)
- Typhoon Haiyan/Yolanda hit the Philippines in 2013 and killed 6,300 people. The National Disaster Risk Reduction and Management Agency final report states simply that most deaths were caused by drowning and trauma. It includes an extract, but not the entirety, of the deaths reporting, containing details of 163 deaths. The cause of over half of these is unknown and the leading known cause of death is drowning. Falling debris and blunt or penetrating trauma represent only 6 of the known deaths:
Windborne debris is dangerous, but it is not the leading cause of death or injury in windstorms. Drowning, falling trees and collapsing walls/roofs are more significant causes of death.
CGI, rather than general windborne debris, is not cited as the cause of injury in a single one of the papers or reviews of storm mortality and morbidity I have found. There is no evidence that CGI is any more dangerous than any other windborne debris. The only way to limit wind-borne debris, especially important in built-up areas to avoid subsequent damage to adjacent buildings, is to ensure all parts of a building are tied down and robustly attached.
There is no reason to limit use of CGI in post-disaster reconstruction, but there is every reason to ensure it is used correctly and buildings are well-built.
Thanks to the ‘community of shelter geeks’ for the discussion that started this off!