Wednesday, 14 June 2017

Grenfell Tower and Helicopter Rescue From Burning Buildings

Readers who have already read this post once: please scroll down for the updates, especially that for 25/6/2017.

Firstly, the fire at Grenfell Tower in London seems to have progressed extremely rapidly, and proved impossible to fight. Just like several recent fires in high rise buildings in Dubai. In the case of buildings in Dubai (there has been more than one catastrophic fire) the culprit is plastic exterior cladding, intended to insulate the building. (Often as a design afterthought.) Grenfell Tower, reportedly, had similar cladding installed last year. The only difference being that in Dubai the insulation is primarily there against the desert heat and in London it's there against the winter cold. In both cases there is a layer of material stuck onto the outside of the steel and concrete structure, covering the whole building.

The fire containment strategy for tall buildings is compartmentalization, where each flat and floor in the building acts like an independent cell, and there are regular fire doors and other precautions in stairwells, and both stairwells and lift shafts are kept as free of burnable materials as possible. The fire fighting strategy, the only one possible, is to tackle the fire from the inside, with fire-fighting mains built into the structure and always ready to be used by teams of firefighters who enter the building via stairwells. In 2005, this strategy led to a couple of posthumous George Medals for firefighters after a fire in a block of flats in Stevenage, largely because the fire-fighting mains had been compromised by vandalism, so the firefighters were unable to create a mist of water to dampen any flashback before opening a door to rescue the occupants of a flat. (Medawar doesn't believe that anyone was ever caught and punished for that.) Fighting from the inside remains the only possible strategy above the tenth floor, however. In Stevenage, there was no burnable material on the outside of the pre-cast concrete wall panels, so the fire, lethal as it was, remained confined to a relatively small part of the building. The fire containment strategy, in that case, worked as well as could be expected with compromised fire-fighting measures.

Obviously in Dubai, and equally obviously at Grenfell Tower, fire was able to spread through the exterior cladding, which meant the fire by-passed the original design's compartmentalization, engulfing the greater part of the building in a very short while, and it was also impossible to fight such a fire from the inside, and it couldn't be fought from the outside above the tenth floor. Sticking things to the outside of high rise buildings must be banned, until such time as every component of the cladding, including the glue, is completely fireproof and there is no air gap between the cladding and the wall, that could channel flames coming out of a window on one floor into a window on the floor above. (In Stevenage, heat from flames coming out of a window on one floor, broke the glass in the window above and the fire was able to jump floors. That happened because the fire couldn't be fought, because of vandalism.)

The official advice (from 999 operators), that residents were to stay inside their flats, was lethal at Grenfell Tower, because it was only valid in a fire like the one in Stevenage, where the fire was reasonably compartmentalized. With burning cladding all over the building filling flats with toxic smoke, people who obeyed the official advice seem to have died in droves. A refrain common to many survivors is that the fire alarms were not sounding, and that is something that also needs to be investigated, with criminal charges being brought as appropriate.

There are a number of firms offering cladding and even structural panels, made out of compressed, recycled plastic packaging. This is supposed to be "green". These firms need to go into receivership as promptly as possible: these products fatally undermine any attempt to make buildings naturally fire-resistant.

Secondly, bystanders saw harrowing evidence that people were alive and on the roof of Grenfell Tower during the blaze, but nothing could be done to rescue them as the roof was too high for fire brigade hydraulic platforms to reach. As with the terrorist attack on the World Trade Centre in New York City, readers are going to wonder why they couldn't be winched off by helicopter.

Any attempt to do this with the helicopters available to the emergency services during the fire at Grenfell Tower, would have failed. Police and Air Ambulance helicopters are too small to hover accurately in turbulent air above a fire, and military transport helicopters, such as the Merlin and Chinook, are too big and the powerful downdraft from their rotors would have blown bits off the unstable burning structure, to the considerable hazard of fire-fighters and even neighbouring buildings.

The only helicopter in British service that's the right sort of size and has proven acceptable flight characteristics in the vicinity of a fire, is the Bell 412 "Griffin" models that are used (apparently leased?) by the MoD for twin engine training, forest patrol and, with 84 Squadron on Cyprus, for rescue and fire fighting using underslung fire buckets. The Bell 412 is the only UK service type certificated to carry out firefighting duty. (84 Squadron has never been based in the UK in its entire history, and has always been somewhere hot and mostly dry. Hence the Squadron badge, which represents a scorpion.)

The Bell 412 Griffon (with an "o" not an "i") used by the Canadian Armed Forces has acquired a terrible reputation, because it was purchased, as a political gimmick, as a sort of universal helicopter to replace everything from light battlefield observation machines to heavy transports such as the Chinook. This means it is struggling to carry out several missions it is quite unsuited for, and it is also struggling to carry these missions out in Afghanistan, in very hot and high conditions that make any helicopter struggle. It fails in the observation helicopter role, because for that you need the smallest and least obtrusive machine possible, that can disappear behind buildings or trees if shot at.

British forces have always used the Griffin for tasks it was designed to do (it can do most things required of the mid-sized utility helicopters it is the archetypal example of) and there are no really serious gripes.

The Griffin, as it currently stands, isn't going to be enough, though. To stay in the smoke column for long enough to winch people off a roof (and landing on a roof of unknown strength is out of the question) not only will there need to be a really robust breathing system available for the crew and any casualties, but all members of the crew, including the winch-operator and winchman, will need synthetic vision equipment, based on millimetre waves and not infra-red, as is the case with most available (unclassified) equipment. If they cannot see, they cannot do any good.

There are infra-red based synthetic vision systems designed for pilots, but they won't work in the sort of thick black smoke we saw at Grenfell Tower. (And from some burning ships in the Falklands War, when Wessex and Sea King helicopters desperately tried to blow life-rafts away from burning oil on the water with their rotors.) 

There have been research projects to develop passive millimetre wave synthetic vision equipment for military helicopter pilots, because there are often fires on the battlefield (bright flickering flames can jam infrared sensors) and it is desirable to screen helicopter evacuations with artificial smoke to prevent accurate sniping. Any usable products, however, remain classified, assuming they do exist.

The breathing system for casualties could be a chemical cartridge-based mask, like an airliner emergency oxygen mask, albeit with eye protection because of smoke. This could last twenty minutes to half an hour, which should be enough to get them out of the smoke and on the ground. Fresh masks and cartridges could be made available on the ground wherever it was planned to disembark the casualties. But the whole crew, who might make repeated trips into the smoke, would need something with a longer duration, and a mask that was integrated with the necessary synthetic vision equipment. 

Computer flight controls giving some measure of automated gust alleviation would probably be essential, too. 

It could be done, though. But it would need to be done with great deliberation, the right equipment (some of which may have to be invented or adapted from classified military kit) and there would have to be an adequate ongoing training budget for the considerable skills involved.

Footnote: (For those on Twitter who think it's "suspicious" that Grenfell Tower didn't fall down the way the World Trade Centre did.)

The original design of Grenfell Tower has many shortcomings, especially the fact that there is only one stairwell in the central core, but actually falling down is not one of them. Grenfell Tower was built after the Ronan Point disaster, when one of the previous generation of high rise tower blocks did indeed partially collapse after what was, in truth, quite a small gas explosion that didn't even seriously harm the elderly lady who was in the midst of it. There were fatalities in the subsequent collapse, though.

The public inquiry found both that Ronan Point had been improperly assembled, and that the "Large Panel System" buildings in general were not strong enough to resist even small explosions, or even strong winds, or fires. Due to intense public hostility, most LPS buildings in the UK had been demolished by around 1986, which was jolly fortunate because the hurricane of October 1987 would have been a severe test of their structural integrity, even with the reinforcement that was mandated after the public inquiry. A handful remain (with gas mains!), in one of the more psychotically-run London boroughs. Ronan Point (22 stories high) was constructed entirely from large prefabricated concrete panels, which were bolted together on site to form the structure. This was a Danish system and, as it turned out, not very good. At Ronan Point, many of the panels were badly manufactured and didn't fit. The workforce charged with erecting the building, hadn't really been trained in the system -they were used to working with bricks and timber- and they saw no reason why they shouldn't apply force and bend the connecting steel to make the panels fit. This led to a structure that was constantly under load in directions that hadn't been anticipated when the reinforcing steel in the concrete panels was designed, and it wasn't stiff enough in the directions it needed to be stiff in. Once the gas explosion had moved ill-fitted panels outward, a whole corner of the building just unzipped at the joins.

Grenfell Tower was built in the seventies, using both better quality control and workforce training, and an inherently stronger basic design. It is still largely made of (better) prefabricated parts, but in the middle of the prefab structure is a very strong reinforced concrete core, that is shuttered and poured on site, the shuttering being moved up a floor at a time, a couple of floors ahead of the prefabricated structure, until the core is tall enough. The lift shaft and stairwell are in the core, because that's expected to survive most things. Until the advent of the World Trade Centre Twin Towers, most seventies/eighties-era high rise buildings were constructed in this fashion, although there was often more than one separate stairwell and lift shaft in the central core. One drawback is that in an evacuation, people are still in the middle of the building when they reach the ground floor. They are not home and dry, especially if burning aftermarket cladding is cascading down the outside of the building.

The bigger commercial drawback was that, the larger such a building got, the more lift shafts and stairwells were needed in the central core, and the central core gradually displaced the rentable accommodation that the building was meant to provide. One solution was to simply provide less access than strictly needed, as at Grenfell Tower and several similar buildings in London. The solution adopted for the World Trade Centre, was to dispense with the concrete central core entirely and build a bolted steel exoskeleton instead.

(NB: The buildings of London's Canary Wharf are cellular steel structures, like a modern version of the Empire State building, without all the hot riveting and hand-fitted stone exterior, and after a huge IRA bomb attack which blew all the windows right through several buildings, they were all repairable and repaired. The Empire State took a collision with a four-engined bomber aircraft in its stride, in the nineteen forties.)

The exoskeleton of the World Trade Centre was carrying much more concentrated loads that any one part of the cellular Empire State or the bombed Canary Wharf buildings, and the bolted joints only needed to get weaker, for the structure to fail. Regardless of all the conspiracy theories about covertly-planted thermite charges and so on, the bolted joints in the exoskeleton were in danger of failing as soon as their temperature got much over about three hundred and fifty degrees centigrade, because steel softens at a fraction of the temperature required to actually melt it. Once one floor failed, it fell under gravity and hit another, also weakened, floor below it, and that floor failed as well. After that, the falling material was heavy enough and (increasingly) energetic enough to break the supporting joints in the exoskeleton even where there was no heat damage. This cascade was a bit like Ronan Point, but over the whole floor area and not just a corner.

The Alfred P. Murrah Federal Building in Oklahoma City, was a low-rise, layered concrete deck building of the simplest and cheapest kind, with a slick and expensive-looking facade put on it. Layered concrete deck buildings have almost no resistance to lateral forces, and Timothy McVeigh's bomb had been designed to apply as much blast as possible to that glassy facade, in the apparent hope of blowing the facade through the building and producing mass casualties in the Federal Building, with a minimum of casualties in the non-Federal buildings nearby. What he expected, was pretty similar to what actually happened at Canary Wharf. Because of the layered deck construction, though, what happened in Oklahoma City was that the building collapsed along the entire frontage facing the bomb, leaving only the back half of the building still standing. Had the building been built to California's minimum standards for earthquake resistance, for example, it probably wouldn't have fallen down. But the number of casualties from the facade being blown through the building might have been nearly as high. Layered concrete deck buildings are probably a bad idea, anywhere, unless something significant is done to improve their resistance to lateral shocks. There are lots of cheap, layered concrete deck car park buildings in the UK, and although the maximum earthquake strength usually experienced in the UK is only about 3.7 on the Richter scale, this is probably enough to bring a typical multi-storey car park down if the epicentre is nearby.

Update: 18/6/2017. The type of flammable cladding used on Grenfell Tower, was probably already illegal in the UK at the time it was applied. See link. This means that most of the "grave-robbing for votes" condemnation of Theresa May, launched by Labour and the Socialist Workers Party, is misdirected. The local council and property management company are still on the hook, though. As should Tony Blair and Gordon Brown be, who between them introduced two tranches of guidelines, in 1999 and 2010, which effectively meant that local councils had to introduce energy-saving measures at all costs, and which gave them no allowable mechanism for not fitting cladding etc. if they believed it to be dangerous, which is why councils, by and large, didn't ask. The guidelines say that the landlord of a building should check that the materials are fireproof and consult the fire-brigade, but they certainly do not say that not being safe is an acceptable excuse for not fitting the "green" cladding to meet emissions targets. So, you were supposed to "consult the fire brigade" but if they said "God, no!!!" you had to say "sorry, the guidelines say we have to do it anyway!"

The idea behind Blair and Brown's actions was to give local councils no choice except to do what their mate, Al Gore, wanted, and "no choice" is precisely what the councils ended up with. (It would be interesting to know the shareholder history of the company which made the plastic cladding, too.)

All control-freaks are dangerous, but pig-ignorant control-freaks who never bother to think anything through, are very dangerous indeed.

And cladding of this type will never be safe, even if the flammable plastic is piously excluded, because the concrete panels it is fitted to are almost always ribbed to make them stiffer and also nicer to look at, and this means that there will always be a narrow air gap behind the cladding, which will channel flames to windows above. So Andrew Marr's emotional outburst about tearing all the cladding off and replacing it is profoundly stupid: the only sane option is to tear the cladding off and not replace it. These buildings have to be left largely as they were designed, or they risk being orders of magnitude less safe than they were as built.

Update 20/6/17  For those who've been searching for the type of plastic used on the Grenfell Tower cladding, it was reportedly polyethylene (presumably fibres: very flammable) in the actual insulation filling. But there are also two skins involved, of aluminium composite material (ACM), and this probably involves an epoxy-based reinforced plastic, which will be where the toxic fumes and black smoke comes from. Clear polyethylene actually burns fairly cleanly (Medawar remembers burning it in O'level chemistry), but assuming this is recycled in order to be "green" there will be plasticisers and multiple pigments involved.

The cladding is to blame for the fire not being contained in a single flat, because the fire-brigade had just finished extinguishing the flames in the flat where the fire started, when colleagues outside noticed that the first signs of fire in the cladding. If the cladding hadn't been there, the fire would have been contained in the one flat: bad for the tenant, but survivable for everyone. This also shows that the basic fire mains at Grenfell Tower must have been in working order, which wasn't the case at Harrow Court in Stevenage in 2005. One letter-writer to the Daily Mail (sorry, no link as they don't put reader's letters online) said that in his work as a building manager he had attended numerous unattended chip-pan fires in (unclad) tower blocks, and the fires were always confined to the flat where they started. His experience probably didn't include many of the older (sixties) LPS towers, then, because ill-fitting concrete panels mean that there are gaps at the joins of nearly all the supposedly fireproof cells in those.

There is no need for the government to wait for the report from a new public inquiry before getting rid of the remaining LPS towers: there was an inquiry into these in 1968 and it concluded that they would fall down in the case of either: a small explosion; fire; a strong enough wind. Getting rid of the LPS towers immediately would actually make it easier for the government to manage the demand on resources than if they wait, because then they will be having to manage the disposal of the LPS blocks at the same time as rectification work to hundreds of the newer (better built) towers with concrete cores that make them (perhaps) worth salvaging. Experts are still talking of things that can be done to make the 1970s towers safe: they are not talking of ANY solution with regard to the older LPS towers except prompt demolition, which most responsible local authorities had carried out by 1986. The remaining LPS blocks have been retained for another three decades in the face of a consensus that they are unsafe and that there is no rectification work available that would actually succeed in making them safe. 

Update 25/6/2017 So far, sixty of the six hundred UK tower blocks known to the government as having exterior cladding, have failed fire safety tests performed on samples of the cladding. That means that all of the ones tested so far, have failed. However, since local councils were told, by the government, to test the buildings they were most worried about, first, it is inevitable that to begin with, we would see a one hundred percent fail rate. The revealing thing will be: how far into the six hundred towers do we get, before we start to see cladding samples passing the flammability test? The official belief is that cladding panels with polyethylene filling will be flammable and those with rock wool filling will not be. But there are skins of aluminium composite material, which may well burn, there are glues, there are paints. It is probable that most of the cyanide encountered at the Grenfell Tower fire, actually came from the cladding panel skins and not the polyethylene filling. Medawar, in his youth, burned pure polyethylene in a school laboratory and there was almost no smoke and certainly no cyanide. There has to be nitrogen in the hydrocarbon fuel molecule before decomposition gives you cyanide, chemical formula CN. There are plenty of plastics that contain nitrogen: polyethylene is not one of them.

And flammability tests, performed on samples removed from the buildings in question, reveal nothing about the flame channel effect, whereby even a totally non-flammable panel can cause a fire to jump from one floor to the floor above, when the air gap between the original (almost invariably ribbed) concrete wall panel and the after-market cladding panel, acts as a chimney, channeling and accelerating flames coming out of one window to heat and break the glass in a window above.

A panel that tests as non-flammable in a laboratory, can still spread a fire up a building and effectively by-pass the original building design's fire containment strategy.

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