Part 3 — Types of Beams You Are Likely to Encounter If a surgeon doesn't possess a foundation of human anatomy and physiology knowledge, would you let that surgeon cut you open and probe around inside your body? Would you let a fire officer send you inside a burning structure if that...
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Part 3 — Types of Beams You Are Likely to Encounter
If a surgeon doesn't possess a foundation of human anatomy and physiology knowledge, would you let that surgeon cut you open and probe around inside your body? Would you let a fire officer send you inside a burning structure if that fire officer doesn't possess a foundation of fireground anatomy and physiology: building construction?
Perhaps the most basic of structural elements used by man is the beam. As you recall from parts one and two of "Behold the Beam" (Firehouse®, July and August 2010), a beam is a structural member that deflects when loaded. "Deflection" is a fancy word for bending; because a beam can bend, it can support a load. In short, a beam performs three important structural functions within a structure: it spans, it deflects and it transfers. However, if a beam is overloaded and deflects too much, it will fail. If a compressive support fails, thus doubling the beam's span, it can also fail. Every beam has a sort of bending "sweet spot." As long as there is a neutral axis, with compression on one side and tension on the other side, a beam will support and transfer its load to the next member in the structural hierarchy.
Part one reviewed how a beam works and part two described the various configurations of beams. Part three begins to define 13 types of beams you are likely to encounter during pre-incident planning (or while browsing your local lumber retailer). The capabilities and performance of the beam types are similar, but often there are significant differences; for example, sawn wood versus engineered wood versus steel.
Why is this information relevant to firefighters and fire officers? It is very simple: The more you know about and understand various building construction methods, techniques and materials, the more likely you will be to make informed strategic decisions when confronted with a building being assaulted by fire. I hearken to the wisdom of my friend and colleague Stewart Rose: It is more difficult to be a good strategist than to be a great tactician. Building construction knowledge is an essential component of good fireground strategy.
Here are 13 types of beams you are likely to encounter:
- Sawn wood
- Glue laminated lumber (Glulam)
- Laminated strand lumber (LSL)
- Parallel strand lumber (PSL)
- Laminated veneer lumber (LVL)
- Wide-flange steel
- LiteSteelbeam (LSB)
- Pre-cast concrete
- Faux (fake)
1. Sawn Wood
Sawn (or milled) wood beams — boards, lumber, and timber — are available in a variety of species and grades. Among these are two major classes of wood: hardwood and softwood. In general, hardwoods are used for furniture, wall paneling and flooring and softwoods are used for general building construction. Since natural sawn lumber is "engineered" by nature, the quality of the wood varies from tree to tree and even within each tree. Of the tree species used for sawn timber, Douglas fir is the strongest of the softwoods.
The primary factor used to determine the structural application of sawn wood is grain direction. Ideally, compressive and tensile loads are applied parallel to the grain. With the load applied parallel to the grain, a given piece of wood can withstand one-third more force in compression than in tension. In fact, the allowable compressive force perpendicular to the grain is about one-fifth to half the allowable compressive force parallel to the grain.
Because the strength of wood is greater across the grain than parallel to the grain, tensile forces perpendicular to the grain will cause wood to split. (For a gnarly workout, grab an ax and try splitting a cord of wood perpendicular to the grain.) Because wood has a more favorable strength-to-weight ratio, wood is — pound for pound — stronger than steel.
General construction softwoods are classified by the industry as "yard lumber." Yard lumber is further classified as follows: