By Dale R. Ellickson, copyrighted 2017
It happened again!
Another high-rise fire occurred in a Dubai tower that was clad with an aluminum/polyurethane sandwich panels. The fire started when one of the residents lit a barbeque grill on an outside balcony. Fortunately, the fire was limited to the exterior causing damage to one or so stories of the building, but all residents escaped unharmed. This was the second such fire in less than a week.
Previously, on August 4, 2017, Dubai’s Marina Torch Tower – the highest residential building in the world – became a vision to match its name when a fire spread up multiple stories consuming the exterior aluminum/polyurethane sandwich panels. Dubai’s Civil Defense, which is responsible for fire rescue from high-rise buildings, has apparently become quite practiced at handling such fires as it soon suppressed the exterior fire and prevented it from traveling into the interior. Again, thanks to its quick action, the residents escaped unharmed.
Thousands of miles away in London on June 14, 2017, a similar fire consumed a 24-story residential high-rise known as Grenfell Tower. Some of the exterior cladding was a sandwich panel of aluminum/polyurethane. This time the fire spread to the interior apartments killing 79 people and leaving about 50 people missing.
The public media quickly called for rethinking the system of checks in those countries to prevent any further loss of life.
Why is this happening?
Designers of tall buildings have always had a common problem with material strength balanced against material weight. That is, what the structural engineers call “dead load.” As the building rises higher the dead load increases – sometimes dramatically depending upon the materials used. For instance, the Monadnock Building in Chicago’s Loop became the world’s tallest office building when completed in 1892. Up to that time, most buildings were supported by their outside walls. The north half of the building is the tallest ever built that is supported by exterior load-bearing brick walls. “At ground level, those walls are six feet thick” - according to the Monadnock’s website. (Yes, it is still used today as an office building!) Initially designed by John Wellborn Root, the building represents, according to the website, “a historic transition in the development of structural methods” because of a south half that was added to meet heightened demand. Since Root died during the construction of the north half, the south portion was designed by the firm of Holabird &Roche in similar color and profile, but they abandoned load-bearing brick and substituted a steel frame for structural support, hanging the exterior glass and brick walls from that frame at each and every floor level. This is called “curtain wall” construction where the façade no longer supports the building but is used as a “curtain” to keep out the elements.
The use of steel and concrete structures, in increasingly sophisticated configurations and formulas, along with elevators has enabled ever higher skyscrapers. Root’s original Monadock building has 16 stories. The Holabird & Roche addition has 17 stories – at essentially the same profile and total building height. Thus, the landlord got one extra floor of leasable space when a steel frame was substituted for load-bearing exterior brick walls. The economies of the addition’s design were obvious. Ever since, architects and engineers have searched for inventive ways to lighten the dead loads, including the loads from the curtain wall.
Now, we come to the aluminum/polyurethane panels. The panels can be fabricated in almost unlimited configurations and sizes. The aluminum sheets wrap the insulation protecting it from the elements – thus the sandwich configuration is created. Increasing the thickness of the polyurethane results in a proportionate increase in a panel’s insulating value. The sandwich panels can also be clad in stainless steel, zinc, copper or titanium instead of aluminum. Generically, such panels are referred to as MCM (Metal Composite Material). The dead loads of MCM are significantly less than other curtain wall materials such as precast concrete or even thermal glass/aluminum frame systems. As a result, architects can give their clients much taller buildings like Dubia’s Marina Torch Tower.
MCMs are a relatively new technology – having been first developed about 40 years ago. Some manufacturers even developed their own in-house designers with expertise in MCM to jump start market demand. MCM has some compelling advantages – its relatively light weight reduces the dead load allowing for taller buildings and its insulating qualities boost the energy efficiencies of buildings. For the office developer for instance, those advantages mean more leasable space and a higher LEED rating that in turn garners a higher rent per square-foot for that space. Architects and engineers were in the forefront of considering MCM’s ramifications and applying it to buildings. As such, they were the first to recognize the risks inherent in such panel systems.
In 1987, the American Institute of Architects (AIA) added a new provision in its key-stone document, A201 – General Conditions of the Contract for Construction recognizing that architects needed to rely upon design certification from the contractor for specified materials, systems or equipment such as MCM. By the 1997 edition of A201, the AIA went further and required such services to be provided by a “licensed design professional.” The professional engineers and contractors soon followed with similar provisions in their own published contract forms.
Many of the construction industry’s regulators were initially slower to adapt. In fact, the International Building Code did not adopt a section to deal with MCM as a component in curtain wall construction until 2000. That was preceded by a testing standard for fire and flame spread published only two years earlier by National Fire Protection Association as NFPA 285 Standard. Up until that time, designers and manufacturers were on their own except for the requirements in their contracts such as AIA Document A201. Eventually, the building codes, using NFPA 285 as a testing standard, limited the aluminum/polyurethane panels to applications no higher than 40 feet above grade, except when special fire-prevention methods are used increasing the height limit to 75 feet.
Returning to London’s Grenfell Tower fire, the large loss of life can be attributed to numerous compounding factors including:
- Panel design by the subcontractor/installer who selected a flammable panel over a fire-resistant alternative to save 2 £ per square foot on the multi-story remodeling.
- Lack of building code restrictions in London on MCM panels, such as the restrictions currently found in the International Building Code that is linked to NFPA 285.
- No fire sprinklers on the interior of the apartments to prevent the fire from spreading.
- Inadequate fire-warning system and procedures. In fact, residents were told to shelter-in-place rather than to evacuate.
Not since 1911, when the Triangle Shirtwaist Factory fire in New York City caused the deaths of 146 workers due to inadequate fire escapes, have policy-makers been so highly focused on building fires. It is very likely that governments around the world will adopt new building regulations and codes to deal with MCM.
To see the original article, click here.