By Kevin A. Gallagher
As I discussed in previous columns, prefabricated or modular construction results in a wood frame residential structure being built in a factory, shipped as modules over the road, and assembled on site. The construction process allows workers to construct the modules—designed by engineers—and have them ready to be shipped within weeks of the process starting. An ample inventory of material, no loss time because of weather concerns, and an assembly line method cuts down on the overall time needed to get the modules out the door and on their way to Anytown, USA.
When the modules, which are transported over the road on trailers, arrive on site, the process moves from construction to assembly. In most cases, the vast majority of construction, plumbing, and electrical work has been completed. It will take the set crew (the group of workers selected to assemble and fine tune) several weeks to complete the project. Soon, the owners are celebrating their first night in their new home.
Now, let’s back up a bit. Before the champagne is poured, something big needs to happen. The separate boxes, once unwrapped following their journey from the factory, need to be placed on the foundation and, if the building is more than one story, placed on top of each other. That job is performed by a crane and crew. Each modular can measure a maximum of 60 to 65 feet in length and a maximum of 16 feet in width and can weigh approximately 22,000 pounds. Moving them from the trailer takes skill and mechanical muscle.
I am aware of two principle methods used to lift the boxes: cable and straps. Both use access points created back at the factory. Prior to shipping, engineers have determined where to drill holes in the rim joist of the boxes (exterior side) and center beam (interior side) to achieve a proper balance point when lifted in the air. These holes are typically ½ inch from the bottom of the joist and between to ¼ inch from each end. If the box is more than 46 feet in length, the holes are placed 10 feet from the end (photo 1).
(1) Photos by author.
Based on the weight of the boxes, lifting from the wrong location could cause the boxes to tip or land in such a way as to damage the structure. These access holes, two sets on opposite sides, accommodate cables or straps, which are run through and then connected to the crane. Spread bars are often used to keep the cables from damaging the structure (photo 2). These holes, if not addressed during the set up, can be a concern for firefighters.
(2)
The lower boxes of a two-story modular are placed on the foundation. Once the marriage walls (the interior walls of the two boxes) are attached, the upper boxes can be lowered into place. It is during this process that the electrical and other wires from the upper boxes, which have been bunched together, are run through conduits in the first-floor box, which exit in the basement. Electricians will eventually connect these wires to the panels.
With the upper boxes now in place, the cables can be removed from the holes. It is important—very important—to note that these holes line up with the void space defined by the floor decking of the second floor and the gypsum board ceiling on the first floor (photo 3). Both membranes of this void are supported by separate and distinct structural members (joists), and the space between the floor and the ceiling can be up to 20 inches. Equally important to note is that inside this void space are combustibles (structural elements) and, often, a highly flammable petroleum based glue. Many modular manufacturers use polyurethane foam structural adhesives to hold the gypsum board to the ceiling joist. I have written in the past on this topic and about what the fire service knows and doesn’t know about how this glue reacts to fire. Suffice it to say that any pathway from the outside into the void could be catastrophic under fire conditions.
(3)
To ensure that this pathway is blocked, the manufacturer sends with the modules materials to be used to cover the holes. Commonly referred to as “belly bands,” this material (typically ½-inch × two feet × four feet) is installed along the point of contact of the corresponding rim joists of the upper and lower boxes. As this area is covered, so is the crane lifting access point (photo 4).
(4)
However, it is my experience that this covering of the holes, which is the responsibility of the set crew, is not always completed. The photo below shows a crane lifting access point that was not covered. This hole allowed open passage for an exterior fire (porch roof) to communicate directly into the void space between the first and second floor (photo 5).
(5)
The sealing of these holes is imperative, especially the holes that line up with the void space. Following a fire in a modular structure in my community in 2008, the Massachusetts Department of Public Safety sent a memorandum to local building officials on this topic (click HERE to download the memorandum).
The bottom line is this: The boxes need to be lifted and set in place, and the use of cranes, slings, and cables adequately perform that task. The access holes, in particular the holes that line up with the void space needs to be sealed and the use of “belly bands” adequately achieves that need. The responsibility to ensure that this important task is performed falls on the set crew. Keep in mind that these holes, if not covered by the belly band, simply get covered with siding and are out of the view of building officials and homeowners.
From this firefighter’s perspective, cutting corners is simply not an option.
Kevin A. Gallagher has served with the Acushnet (MA) Fire & EMS Department since 1986, where he was appointed as chief in 2003. Gallagher has an associate degree in fire science and a bachelor’s degree in political science. He is an adjunct instructor in the Fire Science Program at Bristol Community College. Gallagher serves as the Fire Chiefs Association of Massachusetts representative to the Board of Building Regulations and Standards, the Massachusetts board responsible for overseeing the state’s building code. He has contributed articles to Fire Engineering and has taught several classes at FDIC on the issue of modular construction.
