Monday, January 04, 2016
Sandwich Panels and the Dubai Hotel Fire
If you were watching TV on New Year's Eve, amid all the spectacular fireworks displays in cities around the world you might have also seen an unplanned spectacle: the blaze climbing up one side of the 63-story Address Hotel in Dubai, the largest city of the United Arab Emirates (UAE). While the Address Hotel is not the tallest skyscraper in the world (that honor goes to the Burj Khalifa, also in Dubai), it's tall enough to attract global attention as it was enveloped in flames during the night. Amazingly, no fatalities were reported, although numerous people suffered smoke inhalation or minor injuries while the hotel was being evacuated. The main reason for the absence of serious casualties was that the fire was confined almost entirely to the "sandwich panels" or cladding on the outside of the building. Why they could catch fire—and why entire buildings are covered with flammable material in the first place—are topics worth pursuing.
As most people know, modern skyscrapers depend on a hidden steel skeleton for mechanical strength, not on the exterior surfaces, which can be chosen for properties other than their ability to support the building. At first, high-rise architects stuck to the traditional stone, concrete, and brick for facades, but in the 1950s, they began to experiment with lighter-weight and cheaper materials, such as glass and aluminum. Properly handled and mounted, aluminum makes a fine, long-lasting sheathing material, and so does glass. Then a couple of decades ago, someone had the idea of sandwiching a few millimeters of plastic—polyethylene or some other heat-softening—between two thin foil-like claddings of aluminum, making something cheaper and lighter but just as good-looking as solid aluminum. Thus the sandwich panel was born.
Now, most heat-softening (thermoplastic) plastics can burn very easily, and some attempts were made to introduce fire-retardant materials into the plastic core of sandwich panels to make them fire-resistant. Apparently, these attempts did not convince U. S. building-code authorities that the new sandwich panels were safe enough to use in high-rises. A comment on an architect's chatroom I found indicates that these types of panels are prohibited in the U. S. for use on buildings taller than about four stories. But other countries either had no such laws, or came to realize the potential for disaster too late.
What can happen is this. If you have a whole building encased in this stuff, and one panel near the bottom happens to catch fire somehow (fireworks seem to be a popular way to do this), you are in big trouble. Aluminum has a low melting point and melts away from the plastic cladding as soon as the flame reaches it, exposing more plastic to air and letting the fire feed on itself. Hot air and flames travel upward to the next panel and so on, and in the case of a 63-story building, there's plenty of upward to travel through. This is what happened, apparently, not only to the Address Hotel, but to several other similarly-clad high-rises in Dubai and elsewhere in the last few years. The architect-chatroom website where this problem was discussed has numerous pictures of burned-over building exteriors in Dubai, China, and elsewhere—all fires in which sandwich panels played a critical role.
Fortunately, the fires that these panels support tend to stick to the outside, and most of the time, people inside the buildings have time to evacuate before anyone gets killed. But nobody wants to leave a building under duress while dodging falling pieces of burning plastic and metal on your way out. And it's very costly to clean up the resulting mess and re-cover the structure with something that won't burn as easily next time.
Both Australia (where such a fire happened in Melbourne in 2014) and the UAE have changed their building codes to require sandwich panels to pass certain fire-retardant tests. There are two problems with this, however. One, it's not clear exactly how fire-retardant a panel has to be in order to resist spreading a fire on a tall building. The only sure way to know is to build such a building and try to set fire to it, and this experiment is beyond the resources of most building-code-writing organizations. Second, such codes generally apply only to new construction, and are not retroactive. So anyone who's already built a skyscraper with flammable cladding doesn't have to take the cladding down and replace it with something better. That is, until it catches fire. Judging by the fact that the Address Hotel fire was the third such conflagration in Dubai in three years, it may be only a matter of time until the others light up too.
Modeling how fires start and spread is still an inexact science, and it is understandable that pressures from the building industry allowed dangerous sandwich panels to be installed in many places around the world, despite the hazards involved. But it takes only one or two fires like this to demonstrate that there's a serious problem. The almost universal tradition of not making building codes retroactive makes sense, because taking stuff out of an existing building to replace it can be more expensive than the original building cost. Better in that case simply to condemn the thing and tear it down, but that's an extreme measure too.
So what's the best that can be done in the present situation? There may be some lower-cost ways to reduce the chances that a fire in existing sandwich panels will spread, possibly by installing some kind of fire-break strip at selected heights. But that would be pretty speculative and might not work. Another proposal has been to install fire sprinklers on balconies near sandwich panels, because many of the buildings are high-rise apartments, and I bet there has been more than one numskull who's tried to light a barbecue grill on his balcony and let the fire get out of hand. If a building has potentially flammable sandwich panels, the owners better make sure that all the fire alarms and protection systems are operational, and conducting regular fire drills might not be a bad idea either. But owners will be reluctant to advertise the fact that their building is a giant firework waiting for someone to light the fuse.
We can also be thankful that U. S. building codes flat-out prohibit the use of sandwich panels in high-rise structures. Yes, it forces builders to use more expensive materials, and drives the cost up compared to construction costs in other countries. But we've had enough towering infernos in this country to last us a long time, and we don't need any more.
Sources: I referred to a Reuters report by Andrew Torchia carried on Jan. 2, 2016 on the Yahoo News website http://news.yahoo.com/dubai-blaze-raises-questions-over-gulf-skyscraper-design-160747983--finance.html#. The architect's chatroom with a comment about the U. S. prohibition of sandwich panels and photos of similar fires in other countries is at http://www.skyscrapercity.com/showthread.php?t=1801571. A report of a sandwich-panel-fueled fire in a Melbourne building in 2014 appeared on the Australian website http://www.architectureanddesign.com.au/news/non-compliant-cladding-fuelled-melbourne-apartment on Apr. 28, 2015. I also referred to the Wikipedia article on sandwich panels.