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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:
- Boards — Less than two inches deep and at least two inches wide. Boards are graded for appearance rather than strength. Boards are used for siding, subflooring and interior finish work.
- Dimension lumber — Generally, two to four inches deep and at least two inches wide. Used extensively for general construction, dimension lumber is graded for strength rather than appearance. Dimension lumber includes rafters, joists and light-framing elements such as wall studs. "Light framing" lumber is two to four inches deep and two to four inches wide.
- Structural lumber — At least five inches deep and two inches wide and graded for strength and structural application. Beams and stringers must be two inches wider than the depth. Posts and timbers are at least five inches deep by five inches wide. (Width cannot be more than two inches greater than the depth.)
- Timbers — Five inches or more in the smallest dimension.
- Heavy timbers — Columns must be no less than eight by eight inches when supporting floor loads and not less than six inches in the smallest dimension when supporting a roof or ceiling. Beams and girders must not be less than (nominal) six inches wide and 10 inches deep. The smallest nominal dimension for sawn-wood timber-truss components that will support a floor load cannot be less than eight inches.
Lumber is measured in "board feet," with one board foot equal to the volume of a piece of wood with a nominal dimension of 12 inches square and one inch deep. (A 2,100-square-foot home would require roughly 13,125 board feet of lumber.)
The term "nominal" refers to the dimensions of a piece of lumber before drying and surface planning; the term "dressed" refers to the actual dimensions of lumber after drying and surface planning. Dressed sizes are always shown with inch marks (") and nominal sizes are always shown without inch marks. Sawn lumber is generally available in lengths from six to 24 feet.
2. Glue Laminated (Glulam) Lumber
In 1933, the first glue laminated beam was used in the United States at the Peshtigo School in Madison, WI. When you hear "Glulam," you can be certain it is a substantial beam. Rather than a composite of thin wood veneers or tiny chunks of wood, Glulams are a layered composite of full-dimensional lumber and high-strength glue. It is not difficult to find a contemporary Type III commercial building featuring a panelized roof supported by huge Glulam girders. (Example: a concrete tilt-up warehouse with a panelized wood roof supported by steel columns.)
A typical Glulam girder is basically a stack of glued together two-by-four-inch dimensional lumber and is very strong, thus the complete name: glue laminated lumber beam. When nature and human engineering collaborate, incredible strength and reliability result. Pound for pound, an engineered Glulam girder will always be stronger than the most perfect, defect-free, sawn heavy timber that nature could grow. A unique feature of a Glulam beam is that it can be cambered; cambering is the upward pre-bending of a beam into a frown-shape so that the beam will flatten when loaded, rather than bending downward into a smile-shape when loaded. For additional resistance to tension, a Glulam hybrid is available that incorporates laminated veneer lumber (LVL) along the bottom of the beam.
Although glued together and thus classified "EWB" (engineered wood beam), Glulam girders are not vulnerable to failure when the glue heats. The fire performance of a Glulam girder can be expected to be at least that of a perfect heavy-timber girder of the same dimension. Be more concerned with the fire performance of unprotected steel columns that are often used to support Glulam girders and, as always, pay attention to the weakest part of any structural system: the connections.
Glulam beams can be factory curved and are often used as ground-to-ground arches in plank-and-beam structures. It is often desirable to leave finished Glulams exposed rather than hidden above ceilings. Glulams are also used for contemporary timber truss components.
As strong as they are, Glulam beams can and do fail. The primary cause of Glulam failure is improper design and overloading. Overloading can cause shear failure at the supported ends and mid-span tensile failure along the bottom of the Glulam. Manufacturing defects can also make Glulams vulnerable to overload failure. The good news is that contemporary building codes make overload failure of glue laminated lumber beam unlikely.
During pre-planning visits, look for Glulam beams that have been reinforced with post-tensioning rods along the beam bottom. These rods should be considered a red-flag indicator of a Glulam in distress. The steel tension rod will fail when heated to around 800 degrees Fahrenheit.
3. Laminated-Strand Lumber (LSL)
A laminated-strand lumber beam is an engineered, composite, factory-made beam. LSLs are used routinely as headers above windows and doors within wood frame walls. LSL is manufactured using small strands (up to 12 inches long) of hardwoods that normally would not be considered suitable for structural applications. The face-side appearance of LSL is similar to that of oriented strand board (OSB).
As with other engineered composite wood products, LSL is an alternative to flitch beams and steel lintels. LSL is also used as rim boards, intermediate span beams and purlins. LSL offers the added benefit of good resistance to a lateral force such as the wind (winds up to 120 mph!) Although not common, LSL can also be used as columns and wall studs. LSL wall studs can reach 30 feet.
Rather than run plumbing around an LSL beam, plumbers can run pipes through an LSL beam. A hole up to 4 5/8 inches in diameter can be drilled through a 14-inch LSL beam without compromising its structural integrity. The maximum length of an LSL beam is around 64 feet.
4. Parallel-Strand Lumber (PSL)
You're probably familiar with Weyerhaeuser's proprietary name for its PSL products: Parallam (correctly pronounced paral-lam, not para-lam). Introduced in 1988, Parallam is often made using waste material left over from the plywood manufacturing process. Structural applications include floors, walls and roofs.
Glulam, LSL, PSL and LVL all belong to a general category of engineered wood products known as structural composite lumber (SCL). SCL products are reliably straight and true, free of knots, resist twisting, shrinking, and bowing, and are split resistant and very strong.
Made from long, thin strands of wood bonded together in a microwave process, PSL is consistently straight and strong and resists shrinking, warping and splitting. The strength of PSL is similar to that of LVL. The PSL manufacturing process provides for the removal of natural wood growth defects such as knots. PSL beams are manufactured in widths up to seven inches and with depths up to 18 inches. Manufacturers claim that a PSL beam can span nearly 70 feet. To achieve additional strength and rigidity without increasing depth, PSL beams can be built up at the job site.
PSL is not designed to be appearance grade, so it is frequently left rough and unfinished, concealed within voids or behind sheetrock. However, "architectural-grade" PSL is offered that, after sanding and finishing, will provide "a unique architectural look and feel."
A significant environmental benefit of PSL is that the manufacturing process uses almost the entire log, making PSL an efficient use of a renewable resource: trees. PSL is used as beams and columns for contemporary post-and-beam structures, as well as for light framing beams and headers; PSL is often used for masonry wall lintels. You will find the use of PSL increasing throughout residential construction and as intermediate and large structural members in commercial building construction.
The strength properties of PSL are similar to those of a solid-sawn wooden beam of comparable dimension. PSL is more expensive than other LSL, LVL or a Glulam. PSL and LSL belong to a sub-category of SCL referred to as "long strand lumber."
5. Laminated-Veneer Lumber (LVL)
LVL is a composite consisting of several layers of wood veneer and adhesive. LVL is not new a technology; the LVL process was used during World War II to make airplane propellers. LVL has been used for beams and headers in building construction since the 1970s. You may hear LVL referred to by its proprietary name, MicroLam. LVL is also referred to as parallel-laminated veneer (PVL).
Although the face-side appearance of LVL resembles plywood, LVL is not plywood. LVL is engineered so that the grain of each veneer layer runs in the same direction (long) for uniform edge-load strength (on-edge, as a beam) or face-load strength (flat, as a plank). This "parallel lamination" provides increased uniformity and predictability compared with a material of the same dimension that is "cross laminated" (cross-banded) such as plywood.
Parallel lamination makes LVL an excellent choice for long-span load-bearing applications up to 80 feet. As with other structural composite lumber products, LVL is engineered to resist warping, shrinking, slitting, bowing, twisting and crowning. In other words, LVL — and its SCL kin — offer reliable dimensional stability. Pound-for-pound, a given LVL beam delivers greater load-bearing capacity than the most perfect solid-sawn lumber of the same dimension.
LVL is manufactured using dried and graded wood veneers that are coated with waterproof adhesive, assembled into a pattern, formed into billets and cured in a heating press. After curing, the billets are cut into stock for rimboards, headers, beams and the flanges for wood I-joists. (I have not yet seen dimensional LVL used as floor joists.) Like PSL, LVL can be built up (fastened side by side) to create a stronger beam rather than having to increase the depth of a single LVL. Because of the parallel lamination, LVL is not recommended for use as columns and posts (compressive members).
To Be Continued...
Next time, there will be more beams to behold when we describe the final eight of the 13 beam types:
10. Wide-flange steel
11. LiteSteelbeam (LSB)
12. Pre-cast concrete
13. Faux (fake)
NOTE: John Wooden, my inspiration for the "Fire Station Pyramid of Success," died on June 4, 2010, at the age of 99, a few months shy of his 100th birthday. Firehouse® published the five-part series in 2008. Although the world lost a great man, the influence of his character and what I call his respectful leadership will endure. Look for an updated homage to Coach Wooden in the coming months.
MARK EMERY, EFO, is a shift battalion chief with the Woodinville, WA, Fire & Life Safety District. He is a graduate of the National Fire Academy's Executive Fire Officer program and an NFA instructor specialist. Emery received a bachelor of arts degree from California State University at Long Beach and is a partner with Fire Command Seattle LLC in King County, WA. He is in no way affiliated with or an advocate for the truss manufacturing or building construction industries. He may be contacted at firstname.lastname@example.org or access his website www.competentcommand.com.