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Home>Topics>>'Hidden' Bowstring Truss Roof

'Hidden' Bowstring Truss Roof

Mon, 21 Feb 2011|

Paul Dansbach shows us another commercial building in which the presence of a heavy timber bowstring truss roof can't be detected from the exterior.



[MUSIC] Hello my name is Paul Dansbach and welcome to fire engineering's training minutes. This session of training minutes will be the third session that we've done on truss roofs. In the first session we looked at a building that had a heavy timber wood truss roof system. It was a large manufacturing facility and the ceiling on the first floor concealed the wood trusses above. In the second video, we looked at a mercantile occupancy. An occupancy were we might not suspect a truss roof. The truss roof was a steel frame parallel cord truss system. The appearance on top of the roof. Provided a flat roof surface, which may confuse firefighters, or lead firefighters to believe that the building was not constructed with a truss. In our third building, we're again looking at a small mercantile occupancy. From the exterior of the structure, from the A-side, looking from the street. We miss some size up cues. Our size up cues are actually misleading. We look at the building and do not see any form of a wood truss. If we move around to the b-side of the building, again we look up, we see a parapet wall, but what we see appears to be a flat roof. We don't see the large, rounded-over roof surface that you would expect to find with a bowl string truss. Yet, this building is constructed with a heavy timber, bowl string truss. We're gonna walk inside and take a look at that roof system. As we enter through the front door of the occupancy, we notice a suspended ceiling. An aluminium grid ceiling, with tiles, are in place. At this point we still have no idea that this building has a heavy timber wood truss roof. We need to look further into the building to identify the truss roof on this building. As we walk through this occupancy the heavy timber truss roof system is clearly obvious. The lack of a ceiling will allow firefighters to readily identify the fires location and to be able to apply a hose stream. Once again incident commanders need to consider the area of involvement. What's burning the building? Is it a content fire? Is it a structure fire? Are there other parts of the structure, which in the case of this occupancy, there are other parts of the building that are not constructed with trusses. Is the fire involving the trusses? If the fire is in the trusses, do we have the opportunity to apply a heavy caliber hose stream, with good reach and penetration. In a few short minutes to see obvious results of that hose string, or must we back out and take a defensive posture because the fire has evolved and taken hold in the truss space. As we walk through the occupancy we discover sky lights. The sky light provide an opportunity. For quick ventilation, as they are natural openings that are easily opened by firefighters. Again, we need to consider where the fire is and what's burning. Is it a content fire that hasn't affected the integrity of the trusses? Or, has the fire started out as a content fire, and gained significant headway and is now involving the structure. Those factors need to be considered before we commit firefighters to interior fire fighting operations and exterior roof ventilation operations. As we walk through the structure, we notice there are other rooms and spaces. We must understand the type of roof that's on each one of these rooms and spaces. The room behind us. Has a wood joisted roof, a flat roof, independent of the heavy timber bowstring truss. Therefore, a fire in this room or space can be fought just like any other structure fire that does not involve a truss roof system. As we enter the structure through the front door we encountered a ceiling. Here is another door that leads directly into the working area of this building. Firefighters can access the building via this door. Through this door with a hose stream, they have direct access to the trusses. Should the fire be up in the trusses. Placement of a large caliber hose stream with good reach and penetration operated from this doorway may darken the fire down. Should that be the case firefighters can continue to operate from a safe distance while continuing to know down the main body of fire. Should firefighters place a line in service in such a manner. And the hose stream have no impact on the fire up in the truss space, firefighters must withdraw to a defensive position because the fire has gained significant hold in the truss space and interior operations are not an option. As the trusses are heavily involved with fire and in danger of collapse. We're back outside, and to reinforce our size up lesson, we're again looking at the b side of the structure. And what we don't see from the outside, looking around the building, particularly at the b side, is the truss roof. We must always do a size up, we must always look at the building from the exterior for size up queues to identify the type of construction and the roof structure. Unfortunately that will not always give us the correct information. Preplan information, reports from the roof, reports from interior crews are all part of a constant and ongoing size up at any fire station. Thanks for watching this session of Fire Engineering's training minutes

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