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Wood-Frame Building Under Construction

Thu, 14 Nov 2013|

Rutherford (NJ) Fire Marshal Paul Dansbach looks at a single-family residence under construction and examines the potential for fire spread and structural instability under fire conditions.



[BLANK_AUDIO] [MUSIC] Welcome to Fire Engineering Training Minutes, my name is Paul Dansbach. Today we are going to look at a wood frame building under construction. The building is a two story structure, that will we occupied as a single family dwelling. When we go inside we're going to look at several construction features. Those construction features include. The potential for vertical fire extension. The type of connections. That are used to hold the building together. The connections between the floor joists and the beams and the floor joists and the bearing walls. We are also going to look at the potential avenues for vertical fire extension within the wood framing of the building. One of the potential avenues for vertical fire extension is on the outside. If the fire were to originate in the garage which is behind us and vent out the overhead garage door, the underside of the soffit, directly above our heads would be exposed. The soffit panels in this building are vinyl sided soffit panels. The fire will very quickly compromise the soffit panels and extend into the wood framing above the soffit panels. We're gonna go inside and follow the void spaces within the wood framing where fire may potentially extend to the upper floors and to the interior of the building. We're in a bathroom on the first floor. And we're gonna have a discussion about the potential for vertical fire extension in this platform frame building. Anywhere building utilities such as drain waste and vent line, water supply lines and HVAC duct work run vertically through the structure, there is some potential for vertical fire extension. They are two drain waste and vent lines. You will also notice there is a double stud wall. We'll come back to the double stud wall. Let's look first at the penetrations created to run the drain waste and vent line. Penetration, number one, is to accommodate the drain waste and vent line to a bathroom sink. There is a penetration in the framing. That penetration extends into the basement. So to fire occur in the basement, the fire is going to find any of the openings created by drain waves through that line,and additional drain waste in that line, that goes to a bathroom on the second floor. Once fire gets into this stud base at this location. The fire will extend vertically, through the two by four top plate, into the floor ceiling assembly above. Once the fire is in this space it's going to run horizontally as far as it can until it reaches a beam or some other form of fire stop. Another construction feature that we mentioned in this bathroom is the double stud wood frame wall. The double stud wall is used to accommodate the drain waste and vent line. Primarily the drain waste and vent line for the bathroom located on the second floor. The larger combustible void space will provide an opportunity for fire to grow in intensity in the combustible void space. When fire compromises the drywall on each side of a double stud wall, fire will extend into the living spaces of this house. We are up in the second floor bedroom, which is located above the garage. If you look through the wood framing, we are looking down into the soffit. Remember our vinyl soffit panels above the garage door. Should fire vent out the garage door, the fire will extend right through the vinyl soffit, into the wood framing, into the two by four stud wall. The fire will then run vertically in the stud wall, the fire will hit the two by four plate at the top of the exterior wall. Once fire compromises the drywall, the fire will extend into the living space of the room. Another avenue for potential vertical fire extension. On the same wall, the same condition exists but we have some HVAC duct work. Should the fire extend into this wall stud space, the fire will run vertically up along the HVAC duct, past the HVAC duct, the HVAC duct penetrates the double top plate in the exterior stud wall, fire will then extend up in the attic space. We're now on the second floor. We're going to follow the path of the drain waste and vent line that penetrates the second floor from the first floor. You will notice that there's penetration in the two by four plate to allow for the installation of the drain waste and vent line. Any opening around that pipe will allow fire to extend into this two by four stud cavity. Continuing up we notice that there's an opening in the double two by four top plate. That opening will allow fire to extend into the floor or the attic space above. As firefighters we have learned. That in platform frame buildings, the fire stopping is inherently built in because of the way the buildings are sequenced. It's penetrations, such as the one we just described, that will allow for vertical fire extension, even though, when the building was constructed, the fire stopping was inherently built in to the structure. Firefighters must understand all the building utilities in the structure, where they run vertically through the building, and to follow those paths for potential vertical fire extension. In this building there are two different types of connections used to connect the floor joists to the walls or a laminated beam. In this example, we have a bearing two by four stud wall. At the top of the stud is the two by four top plate. The floor joist for the second floor, bear on the top of the two by four double plate. The top of the two by four double plate providing the bearing surface. The joists, from either side of the wall, extend beyond one another. They are ten penny nailed together, and the floor joists are toe nailed to the top of the two by four plate. That framing connection, sometimes referred to, as a legacy framing connection creates a very solid connection. As the bottom of a floor joists bear directly on the full width of a two by four top plate. conversely, other connections in this building are not quite as robust as this legacy frame connection. If we look at the floor joist connections to a laminated beam, in the same room, the continuation of the same floor joists, we see a laminated beam has been installed. Architects and designers will use a laminated beam. To bear the weight of the floor above while creating a large open floor area on the living space of the dwelling. What we see are joist hangers. The joist hangers are attached directly to the laminated beam and the joist sits and bears in the joist hanger. Notice the difference between the amount of bearing surface between the joist hanger and the top of the two by four plate as described in the legacy framing connection. Additionally, the joist hanger provides less barring surface for the joist to bear and the joist hanger is steel. It is this similar material used in making the connection. Those factors make the joist hanger more vulnerable in the event of a fire. All fire fighters should be encouraged to arrive in their response area and look at buildings under constructions. Looking at the buildings under construction or renovation will help them understand how the building is constructed and what the fire and collapse potential is for each of those structures. Thank you for watching fire engineering's training minutes.

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