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Floor Systems and Structural Hierarchy

Mon, 25 Apr 2011|

In this building construction video, Paul Dansbach discusses structural hierarchy and the collapse resistance of different types of floor systems.



[BLANK_AUDIO] [MUSIC] Hello, my name is Paul Dansbach, and welcome to fire engineering's training minutes. In this session of training minutes, we're going to talk about collapse resistance of certain floor systems. We're also going to talk about structural hierarchy. Understanding structural hierarchy will help us identify which structural components are more important than others. In terms of structural hierarchy, building columns are some of the most important structural components of the building. This column runs vertically. It runs from the footing or foundation. It runs up its support, in this case a steel beam. The steel beam in turn runs horizontally and supports the floor joists on top of the steel beam. Back to our column, if our column were to fail, it would result in the loss of support to the beam. The beam may also fail. Dropping the floor joist, which was supported by the beam, resulting in a large area of collapse. Additionally, if this column runs vertically through the building, and supports subsequent floors and the roof, the loss of a column in a basement level or the lower level of a building, will result in a vertical collapse sequence. Vertically through the building, to the floors above and to the roof, resulting in not only a large horizontal collapse zone, but a collapse zone that encompasses all floors of the building above the column which has failed. The building that we're in is an old supermarket. The portion of the building that we're standing in was constructed in the 1970s. You'll notice, the floor joists of this building are large dimensional lumber. They're actually 3x12 floor joists. Additionally, the 3x12 floor joists have been constructed in a manner designed to support the heavy load above. [SOUND] The floor joists are 12 inches on center as compared to the normal framing standard of 16 inches on center. What that does is allows for a greater load. What that means for the Fire Service is that there is more meat in the floor joists. That translates to a greater burn time. The fact that the joints are inches on center builds redundancy into the floor system. In this case we could have a localized area of collapse or damage to several of the joints. But because the joints are redundant one will help take up the load of the adjoining joints should they fail. Another collapse potential in this building is burned through of the floor decking. What we see when we look up between the joists is the underside of the plywood floor decking. Localized burn through of the floor deck creates a collapse potential and an entanglement potential for fire fighters. A fire fighter's leg may fall through, entangling and entrapping the fire fighter in that small area of collapse. [inaudible] As we walk through the basement, we notice this column. This column is unique, in that it is not connected to a beam, at the basement level. This column continues vertically, through the floor, up to a beam, that supports the roof. Therefore, in terms of structural hierarchy,. Any collapse of this column, any failure of this column will result in an area of collapse of the roof. In another part of the basement, we discover a column. This column supports a wood beam, or a wood girder, which in turn supports 2 by 10 floor joists. The 2 by 10 floor joists are spaced 16 inches on center. This part of the floor system differs from the other part of the floor system in that, this dimensional lumber is smaller than the dimensional lumber used in the other floor. Two by tens versus three by twelves. Another difference is that these floor joints are spaced 16 inches on center as compared to the three by twelves being placed 12 inches on center. Obviously, the floor system constructed with the 3 by 12's is designed to carry a much greater load. A common condition which you may encounter is a commercial building is an alteration to the floor system that has resulted in multiple layers of flooring. We have the original floor, a hardwood floor, that was installed in the building. Subsequent to a hardwood floor, a tile floor was installed. After the tile floor was installed, another layer of sub-flooring was added, another layer of tile, a small piece of plywood as a finish coat, and then two more layers of tile. The multiple layers of flooring may help reduce the potential for burn through of the floor decking. [BLANK_AUDIO] On this standing structural hierarchy, and understanding the construction and collapse potential of floor systems, is paramount to firefighter's safety. Thank you for watching this session of Fire Engineering's Training Medics