APA Literature




Common Glulam Layups

Glulam beams are manufactured by gluing together a series of boards (laminations). Glulam beams are generally much stronger than solid sawn beams of the same size because significant defects are rejected as the laminations are selected and the laminations can be arranged so the strongest laminations are in areas of high stress. It is much easier and less expensive to reject a portion of a board or even an entire lamination than the entire beam. Arranging the laminations allows the placement of the stronger laminations to match beam stresses. Obviously, rearrangement of the wood fibers is not an option with a solid sawn beam.

The key (shown on the right) deciphers color coding used to indicate lamination grades in the glulam combinations 24F-V4, 24F-V8, Comb. 3, and Fire Rated combinations shown below. Different layups and strengths are associated with each combination.
Layup Key

24F-V4 Layup

A simply supported beam (one support at each end) with normal loading will tend to bend in the shape of a smile. This places the highest tension stress at the bottom of the beam and the highest compression stress at the top of the beam. This is called normal or positive bending and the Douglas Fir 24F-V4 layup shown on the right works very well with these stresses. Notice the high grade laminations at the bottom of the beam. This is the most common layup used in our plants. If this beam is installed upside down, however, its capacity is reduced considerably. APA-EWS certified 24F-V4 beams are rated at 77% of their normal bending capacity when inverted.
Combination 24F-V4

24F-V8 Layup

When a beam has more than two supports or cantilevers over a support, it will tend to bend in the shape of a frown over the support. This is called reverse or negative bending and places the highest tension stress at the top of the beam and the highest compression stress at the bottom of the beam. Between supports, however, the beam is frequently subjected to positive bending. The Douglas Fir 24F-V8 layup shown on the right is the best layup for this condition. This is a balanced layup and has equal strengths for positive and negative bending. Notice the tension laminations at both the top and the bottom of the beam.
Combination 24F-V8

Comb. 3 Layup

Square or rectangular columns are frequently subjected to bending stresses in two directions. When glulams are laid flat for bridge decking they will be subjected to weak axis bending (the load is applied parallel to the gluelines). In trusses the various members may be subjected to heavier axial stresses (tension & compression) than bending stresses. Under any of these conditions it might not make sense to place the best wood at the "top" and "bottom" of the beam and a lesser wood in between. The Douglas Fir Comb. 3 layup shown on the right works well with these stresses. This is the most common column layup used in our plants. Other column combinations frequently used are Comb. 5 (stronger and more expensive) and Comb. 2.
Combination 3

Fire Rated Glulam Beams

When wood beams are subjected to fire the top is usually protected from direct flames by a floor, roof, or whatever the beam is supporting. This means that the beam is only exposed to fire on three of its four faces (two sides and the bottom). Charing is a normal part of the burning process. As the beam chars it produces an insulating layer that actually protects the inner core. A fire rated beam is able to retain a significant portion of its capacity for a specified period of fire exposure (usually one hour or longer). APA has a technical note (Y245) that is available for free download. When a beam specification requires a fire rating the layup is changed by adding a tension lamination to the bottom of the beam and removing a core lamination (usually L3) so that the beam depth stays the same. See the fire rated 24F-V4 (Above Right) and the fire rated 24F-V8 (Below Right).

Please note that specifying a fire rated layup does not guarantee that the beam will retain its full capacity for one hour of fire exposure. An engineering analysis is required, particularly on smaller beams.
Fire Rated 24F-V4
Fire Rated 24F-V8

Alaskan Cedar

Every combination specifies a layup of specific lamination types including wood species and grade. Different combinations are used for Douglas Fir beams and for Alaskan Cedar. 20F-V12, 20F-V13, and Comb. 70 are common AC combinations that are similar to DF 24F-V4, 24F-V8, and Comb. 3, although the AC beams are not as strong. AC costs are higher due to its scarcity, but it is frequently used because of its natural resistance to decay.

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