WOOD
bonded to the desired depth. High structability. Most wood I-beams are increased stiffness and deflection limits
strengths are achieved in a similar way made with “knockouts” or precut hole to reduce floor vibrations.
as SCL by selecting high-quality mate- locations to simplify the installation of
rial and dispersing defects throughout mechanical and electrical system pipe/ Comparison
the members. Additional strength is conduit. These members are useful for Natural and engineered wood prod-
achieved by placing the highest-quality beam and girder applications where the ucts are available in a variety of grades,
laminations in specific zones within the higher strength of SCL is not needed, sizes, and geometries that offer the
members. Glulams can be manufac- but sawn lumber is not efficient. Since designer many options for framing.
tured as unbalanced bending members wood I-beams are slender, more care Figure 1 summarizes strength and size
with designated compression and ten- has to be taken during erection to limitations of wood products, as well as
sion zones to achieve higher efficiency. maintain stability. uses for each type of product.
The members are also manufactured When designing with SCL it is Figure 2 includes a comparison of
with a designated top and bottom for important to consider environmental engineered wood systems in typical
simple span beam applications. Glu- conditions that may be encountered floor framing applications. The design
lams are most often selected for their both during construction and in service. assumes a spacing of 16-inches on-cen-
visual appeal where the structural wood When SCL gets wet during construc- ter, repetitive members, floor dead loads
framing will remain exposed. tion and then dries under load, the of 15 pounds per square foot (psf ), floor
Prefabricated wood I-Joists are effi- long-term dead load deflection can be live loads of 40 psf, and deflection cri-
ciently shaped members — similar to more than anticipated from standard teria of L/360 for total load and L/480
W shapes in structural steel. They are calculations. We have encountered for live load. Relative costs (based in
formed by gluing either sawn lumber or several instances where SCL members, the Northeastern United States) are for
LVL flanges to a plywood or oriented even when continuously dry, have more comparison. It is clear that for shorter
strandboard web. The composite nature than twice the calculated long-term dead spans, dimension lumber will be the most
takes advantage of the high strength of load deflection. In addition, wood I-joist efficient system. Wood I-joists may be a
quality sawn lumber or LVLs in con- floor systems can experience undesirable viable option if issues with dimension
junction with the high shear strength of vibration characteristics — even when lumber submittals, as previously men-
plywood. The “I” shape also results in designed for code-stipulated deflection tioned, are an anticipated problem or if
a lightweight member for ease in con- limits. Some manufacturers recommend greater consistency between members
is desired. For moderate spans, wood
I-beams are both cost effective and
comparable in depth to the dimension
Figure 2: A cost comparison of engineered wood systems in typical
lumber systems. At long spans, wood-I
floor framing applications
joists are still cost effective, but other
Span Type Size Approximate $/ft SCL options become reasonable.
Due to the uniqueness of natural
10 feet Dimension Lumber (1) 2x8 1.00
building materials, the architect and
Wood-I (1) TJI 9-1/2” 1.60 engineer need to be aware of the ben-
LVL (1) 1-3/4” x 7-1/4” 2.40 efits and limitations of each option.
Parallam (1) 3-1/2” x 9-1/2” 9.20 Consideration must be made not only
Glulam (Arch. Grade) (1) 3-1/2” x 7-1/4” 12.00 to the final constructed building, but
also to the methods of construction
20 feet Dimension Lumber (3) 2x10 3.00
that may affect both the strength and
Wood-I (1) TJI 11-7/8” 2.20 stiffness of various wood products.
LVL (1) 1-3/4” x 11-7/8” 3.40
Parallam (1) 3-1/2” x 9-1/2” 9.20
Glulam (Arch. Grade) (1) 3-1/2” x 9-1/4” 12.00 Matthew Johnson, P.E., is an asso-
30 feet Dimension Lumber (6) 2x12 10.00 ciate principal, and Ariane Fund is a
Wood-I (1) TJI 18” 4.10 staff II engineer. Both are with Simpson
Gumpertz & Heger Inc. (www.sgh.com),
LVL (1)1-3/4” x18” 7.25
in Waltham, Mass., and can be reached
Parallam (1) 3-1/4” 14” 14.00 at mhjohnson@sgh.com and aifund@sgh.
Glulam (Arch. Grade) (1) 3-1/2” x 14” 24.00 com, respectively.
38 Structural Engineering & Design April 2010 www.gostructural.com
