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 for a couple of reasons. First, defective laminations can be rejected as the laminations are selected. It is considerably easier and less expensive to reject a portion of a beam than the entire beam. Second, the laminations can be arranged so that the strongest laminations are in areas of high stress, and less expensive laminations can be placed in areas of low stress.
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. A Reinforced combination may be seen here. Different layups and strengths are associated with each combination. There are several grades of tension laminations. The term "high grade" refers to tension and L1 laminations, while "low grade" refers to L2D, L2 and L3 laminations.
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 left 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 strength is reduced considerably. APA-EWS certified 24F-V4 beams are rated at 77% of their normal bending capacity when inverted. Key
24F-V8 Layup
When a beam has more than two supports or extends beyond 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 normal or 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. Key
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 left 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. Key
When wood beams are subjected to fire the sides and bottom burn and char. The top is usually protected from direct flames by a floor, roof, or other material. The charing of the beam produces an insulating layer that protects the core. A fire rated beam is able to retain a significant portion of its capacity for a specified period of time (usually one hour).
An engineering analysis is usually required to determine how much capacity remains after a one hour fire. APA-EWS has a technical note (EWS-Y245) that can be downloaded in Adobe pdf format. 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 (to the right) and the fire rated 24F-V8 (to the left). Key
Please note that specifying a one hour fire rating 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.
Every combination specifies a layup of specific lamination types including wood species and grade. Different combinations are used for AC beams and for Douglas Fir. 20F-V12, 20F-V13, and Comb. 70 are AC beam combinations that are similar (respectively) to 24F-V4, 24F-V8, and Comb. 3 DF beams, 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.
