Mark Emery discusses why it's time for the contemporary fire service to adapt strategically and tactically. Why It's Time For the Contemporary Fire Service to Adapt Strategically and Tactically Characteristics of the structural fireground began to evolve 50 or 60 years ago. Collectively...
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The term "lightweight" does not infer that contemporary structural components are weak or unimportant, in fact, pound for pound, lightweight structural members are incredibly strong and each component very important. Lightweight construction refers to structures that feature structural components with the following characteristics: math (precision engineering), reliance on tension, exponential connections and less time before failure.
- Math - One hundred years ago, sophisticated mathematics and precision engineering were not used when designing a building to resist loads. If you wanted a large open span in a warehouse, you had two choices: span the opening with a gigantic heavy timber (configured as a simple beam) or insert a compressive member (such as a column or bearing wall) that would divide the girder into shorter segments, thus distributing the load along the length of the beam. A third choice was to support 25-foot floor and roof joists with a load-bearing masonry wall at each end of the joists. This 25-foot simple beam span did not require heavy timber and is representative of the unreinforced masonry, Main Street U.S.A., ordinary-construction "taxpayer." (Note: A fourth option would be suspension, which is 100% tension.)
Reliance on math and precision engineering has significantly reduced the mass (dead load) of a building and can be assembled at a faster pace. In addition, reducing the mass means that you can reduce the number of compressive members such as columns. Developers love lightweight building construction.
Contemporary structural components are precision engineered to have the least amount of material (mass) necessary to support the designed load. A century ago, structural "redundancy" was achieved by making things bigger (mass, dead load) and distributing this load with compressive members (columns, bearing walls); today, structural redundancy is achieved by multiplying the lightweight components - nothing gets bigger, you just add more pieces and more geometry (triangles). This is an important strategic consideration for fire officers: Because of precision engineering and geometry, a building can support more load by adding more pieces, more geometry - or both. In general, it is not necessary to increase the mass (size) of each individual piece of the assembly - the webs and the chords.
Remember: conventional thinking meant that you increase mass or add compressive members to support more load; lightweight thinking means that you replace mass with mathematics and geometry (triangles) and rely on segmentation by geometry rather than segmentation with compression. On the contemporary fireground, ventilating "over the seat of the fire" means being supported by geometry rather than the mass of a conventional structural system.
- Reliance on tension - The reason that you can add geometry without increasing the size of the individual truss members (webs and chords) is that trusses (and modern buildings) rely on tension at least as much as compression. Using tension to resist a load allows you to reduce dead load because you eliminate compressive members such as columns.
Consider a rescue rope: For its size and weight, a rescue rope is incredibly strong when resisting a tensile load. However, the rope is useless when resisting a compressive load. To resist the exact same tensile load supported by a rope, but delivered as compression, you would need to discard the flimsy rope; the rope would be replaced with something shorter and more rigid, and its diameter would need to be increased. In other words, to resist the load delivered as compression, the rope would need to be replaced with a column.
You do not need rigidity or a lot of mass when resisting a tensile load, however, when the exact same load changes to compression the diameter must increase, the length must decrease, flexibility must disappear, and more mass must be added. Bottom line: conventional construction emphasized compression, lightweight construction emphasizes tension. Developers love tension because it reduces dead load.
- Exponential connections - A conventional simple beam has just two connections, one at each supported end. Replace the solid, simple beam with a truss and you multiply the connections. In addition to a connection at each supported end, you have dozens of connections within the plane of the truss itself. Each web member connects at the top chord and at the bottom chord, thus each web member requires two connections. Often, these connections are shared with adjacent web members.