- Jerry Schultz, P.E.
Exterior Wall Fires
Initially this article was going to address curtain walls and the recent series of fires in the Middle East. As I researched this article, it occurred to me this is a much bigger issue than just curtain walls and the use of the title “Curtain Walls” would not address the entire problem.
The National Institute of Building Science defines a curtain wall as a thin, usually aluminum-framed wall, containing in-fills of glass, metal panes, or thin stone (source: https://www.wbdg.org/design/env_fenestration_cw.php). The framing is attached to the building structure and does not carry the floor or roof loads of the building. The attachment to the building structure will typically occur at the floor structure, and if you look at the following photograph taken from A/N Blog (Slideshow>Manhattan at the Feet of Four World Trade), one can see a typical curtain wall arrangement.
In the photo, there are two key elements to note. The first element is the silver insulation on the exterior wall while the second element is the gap at the floor. It is this insulation that oftentimes is the contributing factor to vertical fire spread. In certain walls, this insulation may be covered by a thin veneer of metal but look at the photo and note how it runs up the entire height of the building terminating both above and below the windows. If there are no windows present, this insulation will run the entire height of the wall. The gap at the floor provides a means for vertical travel of flames and smoke and is required to be filled and properly sealed. (This is a photo during construction and I am sure that it was sealed.) There is a vertical opening here that may travel the entire height of the building.
The problem is that this is just one issue contributing to the spate of fires being observed. The issue is really a general exterior wall fire issue and not just a curtain wall issue. The use of the term curtain wall fires is a simplistic summary of what is happening. Fires are also occurring in metal composite material (MCM) panels or Exterior Insulation and Finish systems (EIFS) which technically qualify as a curtain wall by definition but present other additional issues. From this point on the discussion will address exterior walls which will include curtain walls, facades, MCM panels, and components & cladding. Recently, there have been a large number of fires in the Middle East where the exterior wall has been involved. One of the best summaries of recent fires (and a compilation of some scary photos) can be found at http://www.fireengineering.com/content/dam/fe/online-articles/documents/2015/Valiulis.pdf. The majority of these fires have occurred outside the United States, so it is important to review what is being done differently in the US than in other countries. I would caution that the US is not without exterior wall fires as well, citing the MGM Monte Carlo Hotel fire in Las Vegas in 2008; although, the use of unapproved materials and procedures were at the center of that fire.
In my last blog I spoke of the need for independent research and fire testing to be conducted by The Fire Protection Research Foundation. A perfect example of the role of the Research Foundation can be found in their publication Fire Hazards of Combustible Wall Assemblies Containing Combustible Components.
The stated goal of this document was "to develop the technical basis for fire mitigation strategies for fires involving exterior wall systems with combustible construction. The first phase was to compile information on typical fire scenarios which involve the exterior wall, compile relevant test methods and listing criteria as well as other approval/regulatory requirements for these systems, and to identify the knowledge gaps and the recommended fire scenarios and testing approach for possible future work."
The report includes extensive material with a lot of great information. Two sections are especially interesting. I found the section on recent fires interesting in that it presents an outstanding summary on exterior wall fires throughout the world. It becomes evident that this is a worldwide problem and is not limited to the Middle East. The second section that I, as a fire protection engineer found interesting, was the outline of various countries test procedures used to evaluate exterior walls. It is obvious that there are numerous ways of testing a combustible wall assembly. So why are we having these issues? Is one test method better than the other? The document does not reach this conclusion but does provide a conclusion section addressing the key mechanisms of fire spread. These key mechanisms are identified in the document as: (note the key mechanisms start as item VI and that my comments are in italics and do not appear in the official document):
Key mechanisms of fire spread after initiating event include:
VI. Fire spread to interior of level above via openings such as windows causing secondary interior fires on levels above resulting in level to level fire spread (leap frogging)—as shown in the photo the wall terminates at a non-rated barrier, the window sill itself.
VII. Fire spread on the external surface of the façade assembly, if combustible
VIII. Flame spread within an interval vertical cavity /air gap or internal insulation layer. This may include possible failure of any fire barriers if present, particularly at the junction of the floor with the external wall.—again, referring to the photo, this is the gap discussed where the floor must be sealed.
IX. Heat flux impacts causing degradation/separation of non-combustible external skin (loss of integrity) resulting in flame spread on internal core—the insulating material is part of the wall assembly itself and is enclosed by finish metal to provide the aesthetics.
X. Secondary external fires to lower (ground) levels arising from falling burning debris or downward fire spread.
XI. Channeling of convective heat and re-radiation between surfaces such as corners or in channels can accelerate flame spread.—many of the tests discussed center on running a corner test too.
Both Chapter 14 of 2015 edition of The International Building Code (used primarily in the United States) and Chapter 37 of NFPA 5000, Building Construction and Safety Code requires walls to comply with the requirements of NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-Load Bearing Wall Assemblies Containing Combustible Components. (Note that the standard does not use the term curtain walls in its title further evidence that this is a bigger issue than just curtain walls). Chapter 10 of this standard provides a detailed series of pass-fail criteria that serves to mitigate the likelihood of these walls behaving as shown in the aforementioned FPRF report (and in the earlier article). NFPA 285 appeared first as a standard in 1998 and it should be obvious that this standard and its acceptance within the building code community is primarily responsible for the lack of exterior wall fires in the US.
Having said that why are we seeing these fires in the other countries. Turns out that in many countries, the wall assembly was not required to be tested to NFPA 285. Some codes were amended to remove the NFPA 285 test and only require the product to be tested to the Steiner Tunnel Test (ASTM E-84 and UL 723) which measures flame spread (http://www.thenational.ae/business/property/revealed-how-the-address-downtown-dubai-hotel-fire-test-was-meaningless#page1) and not the fire propagation. In the article above, the claim is made that this test (E-84) is “meaningless”. For all practical purposes, the test does nothing to challenge the assembly’s mix of metal, combustible components and specific geometries.
Up to this point, this article has centered on one cause for exterior wall fire spread, that of non-tested assemblies. The other issue that must be watched at all times is construction techniques. The product in question consists of combustible materials including HDPE and other synthetics placed tightly together creating a weather resisting barrier. What happens when these assemblies are not installed completely straight and air gaps occur leading to the edges of the insulating combustible material being exposed? Or what happens to the vertical floor separation when it is not sealed at the floor? Several years ago years ago I did an inspection in a Class A high rise building and identified that there was no fire stopping at the floor. The building had a vertical opening running from the first floor to the 40th floor with no stopping anywhere. It should be obvious that we can have the best tests and best products but if the installation is incorrect, the system will fail. These kind of mistakes must be caught during an installation inspection because after a building is buttoned up, they become extremely difficult to inspect and nearly impossible to correct.
In summary, the exterior wall fire is an issue that appears to be addressed by requiring compliance with a test standard such as NFPA 285 however someone must watch construction techniques.
As always, I welcome your comments: j.schultz@thefpi.com
