By Salvatore Ancona
For many years, buildings of ordinary construction have been at the forefront of mixed occupancies used for commercial-residential applications. Main Street USA often presents firefighters with a building with a storefront as its ground level occupancies and multiple dwellings in the succeeding floors above with combustible interior structural elements. In recent years the explosion of podium wood-frame multiple dwellings has given the industry the ability to build combustible buildings in a mid-rise application that are compliant with the International Building Code (IBC). The nature of these structures does not change for us, except for the fact that they have taken combustible multiple dwellings and put them in the air.
Podium wood-frame multiple dwellings are composite structures utilizing two types of building construction classifications. The podium or pedestal of the building uses Type I (fire resistive) construction of reinforced concrete. The podium can be up to three stories in height. This can be used as commercial, parking space, or both. The podium is required to have at least a three-hour fire-resistance rating. With respect to the aforementioned construction classification, the podium will act under fire conditions under the same conditions as fire-resistive buildings do: the contents will be destroyed and the structure, without prolonged burning, will recover. Under the 2015 IBC, the structure can be up to eight stories or 85 feet high with multiple podiums. For example, if a double podium is used, then six stories of wood-frame occupancy can be built above. This surpasses the normal limited height of an ordinary constructed multiple dwelling of seven stories. The floors above the podium use combustible (Type V) construction. The wood-frame occupancies for residential use are constructed with platform design and use tie rods and compression studs at shear panels for connection to the podium. While peaked roofs are used, you will often find flat roofs with resident usable space. These structures can have upwards of 180 units.
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The problems firefighters face are often not with the podium space but the floors above using combustible construction. At FDIC International 2019, a roundtable panel discussion appropriately named “Fire in the Sky: A Panel Discussion on Podium Wood-Frame Multiple Dwellings” went over the issues concerning podium wood-frame multiple dwellings. Panel members identified that fire travel in these structures could be disastrous due to both the heavy fire load of the structure itself and the void spaces, which enable both vertical and horizontal fire spread. Pipe chases and utility passageways are unprotected combustible void spaces that enable fire to travel vertically unimpeded to the roof. As with most private dwelling apartment buildings, the units’ kitchen and restroom spaces are butted against each other using common utility passages. Another issue is the joist channels of the ascending combustible floors. Whether it be I-joist, wood truss, or dimensional lumber, fire can travel horizontally in the joist channels and truss lofts unimpeded. Panelist Bill Gustin described this as “horizontal balloon frame.” When it comes to the roof, is it is not uncommon to see us lose the building when fire makes its way into the cockloft or into the attic space. Fire can span the building once it makes its way into the attic space.
The main issue is the life hazard associated with these multiple dwellings. Hundreds of occupants can become trapped in combustible units. While escape stairwells will be of fire-resistive construction, the hallways and units will not. A nighttime conflagration can become a serious rescue effort. It should be noted that these buildings will have sprinkler requirements based on the occupancy types, which, love them or hate them, will certainly work to our advantage. The issue faced is unprotected combustible void spaces, which will most likely not be sprinkler protected. Something else to consider is a car fire within the fire resistive podium, where the products of combustion alone can move into staircases and elevator shafts and overcome victims on the floors above. Although fire spread from podium to residential units is uncommon, this can become a significant issue requiring a lot of manpower. As builders continue to push the envelope advocating for combustible materials in multiple dwellings, fire service professionals must keep up identifying the risks and formulating preplans. As environmentally friendly businesses continue to push back on concrete and steel and grasp for renewable materials that are often combustible, it is important to keep an eye out for the next trend in building construction. podium wood-frame multiple dwellings or so-called “Toothpick Towers” represent extreme conflagration potential that firefighters must be aware of.
SALVATORE ANCONA is a deputy chief fire instructor at the Nassau County (NY) Fire Service Academy; a former captain and training officer for the Bellmore (NY) Fire Department; and a paramedic supervisor in Queens, New York. He has an A.S. degree in fire science from Nassau Community College and is in the emergency services administration undergraduate program at John Jay College of Criminal Justice. Ancona is the author of the building construction page “The Sons of Brannigan” on Facebook and was a recipient of the 2019 FDIC International Honeywell DuPont Scholarship.