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Wood-Frame Townhome Construction

Mon, 23 May 2011|

Paul Dansbach takes a look at some of the features and hazards associated with townhomes that makes use of parallel chord and peaked roof truss construction.



[MUSIC] Hi, my name is Paul Dansbach. Welcome to Fire Engineering's Training Minutes. Today we are at a construction site to look at the construction of some wood framed townhomes. These townhomes that are being constructed, use parallel cord and peak roof trusses in the building. We're going to look at some of the construction features, methods, techniques and hazards associated with this type of construction. These buildings are being constructed in a fashion similar to platform framing. Specifically we have a bottom plate. The bottom plate sits on top of the subflooring, which sits on top of the power cord wood trusses. The feature that we're interested in here is the method of platform framing in that there is no vertical plywood station potential through the stud bed. Furthermore, as we run vertically through the wall. There is no vertical fire extinction potential at the top of the stud bay were we see our double 2x4 top plate. The wood frame walls that separate the town house dwelling units are constructed with a technique of using a double studded wood frame wall. The double studded wood frame wall creates a combustible void space. One two by four stud an air space equal to another two by four stud and then another two by four wall stud. This space is a combustible void space that is fire stopped at every floor level. There is fire stopping in the floor system below,. And fire stopping at the top plate to prevent the vertical extension of fire within a double stud wall. One of the greatest concerns of fire fighters in this type of construction is the use of parallel cord for light weight wood trusses. As we see in this floor system. These trusses are spaced 24 inches on center. Lightweight cord trusses lack surface to mass ratio. Surface to mass ratio is key in determining the collapse potential of any structural element, particular floor system. The lightweight parallel floor trusses there on the top plate of our wood frame wall. What you notice here is fire stopping. The fire stopping is placed in between the floor trusses. The fires stopping is designed to prevent the horizontal spread of fire. Into the double stud wall cavity and vertically through the building and horizontally into the next townhouse unit. The lack of fire stopping or the removal of fire stopping during construction will result in rapid vertical or horizontal fire spread through the truss space. The use of parallel cord white weight wood trusses greatly increases the fire spread potential within the dwelling unit. Our trusses, as we mentioned, are 24 inches on center. The lack of solid [SOUND] material, or solid wood joists, would allow any fire that extends beyond the interior finish. Or originates in the combustible void space to spread anywhere within the limit of the dwelling unit up to any fire-stopping that might be present. If the building were constructed with 2 by 10 floor joists installed 16 inches on center, we would have a space between the joists 14 and a half inches wide that would allow for, for horizontal fire extension. In the direction parallel to the floor joints. The difference between jointed construction and parallel cord word trusts construction is the potential for fire extension in that combustible void space. In the trust system, the fire has almost no limits, up to it reaches the dwelling unit line and code required fire stopping. Another potential for fire extension for this type of construction is when plumbing lines are run though the combustible void space created by the parallel cord lightweight floor tresses. What we see here is a plumbing line penetrating the floor. Any fire in the truss woft or the combustible void space below. Will extend up around the opening in the floor that may not be properly fire stopped. Resulting in fire extension to the floor above. We're on the top floor of the building and we're gonna look at the roof structure as mentioned the roof structure is constructed with peak light weight roof trusses. Let's look up and we can see, we see our bottom cord of the truss. We see web members and we see the top cord of the truss. Also notice the large combustible void space, that will remain after the drywall has been installed on the underside of the bottom cord of the truss. If we look at the area of the peak, we will notice that a combustible void space between the bottom cord of the truss to the peak of the roof will be between 12 and 15 feet. The use of peak lightweight wood trusses create large combustible void spaces for fire to grow, extend. And overwhelm that truss loft. This view through the floor truss system clearly details the potential for horizontal fire extinction throughout this area. Also, notice the electric utility installed in the truss base. Any mechanical or electrical equipment installed in the combustible void space. Brings the potential for ignition and fire to occur within the combustible void space created by the floor trusses. The use of lightweight peak roof trusses create a large combustible void space. Should the fire originate or extend into the combustible void space created by the lightweight peak roof trusses. The fire will grow in intensity, unnoticed to the firefighter operating below. The use of light weight, floor trusses, and roof trusses, in this type of construction, constitutes an extreme hazard to fire fighters, should the fire involve a combustible void space of either the floor or the roof truss. Thank you for watching this section of Fire Engineering's Training Minutes.

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