Homes are increasingly being designed with more open spaces, larger rooms and atypical framing conditions. Not surprisingly, these changes have resulted in an increased demand for Engineered Wood Products (EWP), notably beams that can carry heavier loads across greater spans. Parallam® PSL provides superior strength and stiffness to address the more demanding conditions of many modern home designs.
When a beam is directly attached to repeated framing members (joists or trusses) or fastened to sheathing in a floor system, the beam is continually braced for design purposes. Design properties and allowable load tables are established assuming beams are braced.
Some framing conditions, however, do not provide continuous lateral support to a beam. Common examples include dropped garage headers; beams that are “upset” into a wall; and, dropped beams installed to support concentrated loads from above. When a beam is not provided with sufficient bracing along its compression edge, the load carrying capability and stability may be affected.
Designing for Lateral Stability
A designer should consider the field conditions and whether the framing provides continuous lateral support to a beam. In cases where a beam is fully braced (1st set of pictures above) additional steps are not needed beyond the design checks for sufficient strength, acceptable deflection and bearing. However, if the beam will not be fully braced, the designer must check the beam’s maximum allowable unbraced length (Lu). Weyerhaeuser’s Forte® software provides the Lu when a beam is designed.
Verifying the ability of a beam to support a load based on the maximum unbraced length is detailed in the National Design Specification® for Wood Construction (NDS) and requires calculating the beam’s stability factor (CL). This factor reduces the member’s allowable bending stress based on the actual unbraced length. Accounting for this reduction can result in the need to increase the depth and/or width of a beam. Solid section beams like Parallam® PSL (which are inherently more laterally stable than multiple ply beams of the same width) often work without a size increase despite the lack of bracing. Because wide beams have a larger Lu, it is simpler and more cost effective to select a solid section Parallam® PSL for unbraced field conditions.
To understand the impact a lack of bracing can have on the required member size, consider a 3-ply 1.75” x 16” Microllam® LVL beam spanning 18’-3” and installed with only lateral support at the ends—a common situation for dropped garage headers. If the beam were braced along its length it could support 20’ of roof tributary load (approximately 47,000 ft-lb applied moment). However, since it is unbraced even a much larger 4-ply 24” Microllam® LVL could not support the load. The table below summarizes the calculated CL factors and resulting moment capacities due to the lack of continuous bracing.
This demonstrates the tremendous design advantages that wide beams offer in cases where the beam has little or no bracing in the span. As shown in the table above, a single 5.25” x 16” Parallam® PSL had an Lu equal to the span of the beam, meaning the beam stability factor did not need to be a calculated and the beam would not need to be braced within the span. In order for a multiple ply Microllam® LVL of the same cross section to work, it would need to be braced every 5’-1” along the length of the beam. It’s also worth noting that a multi-ply beam with twice the wood volume and more than double the moment capacity (the 4-ply 1.75” x 24” Microllam® LVL) still has an unbraced length less than the beam span.
Multi-ply members also must be properly connected to adequately transfer loads to adjacent plies. Connections become further complicated in cases where large point loads are supported on the side of a multi-ply beam. Unfortunately, many designers omit connection details from their design drawings which results in code official red-tags. Weyerhaeuser provides tables to help determine the proper connection details needed to complete construction documents.
Whether a multi-ply beam is properly fastened also impacts the maximum unbraced length of the beam. For example, if the 3-ply 1¾” x 16” mentioned earlier was poorly fastened the beam plies would work independently, reducing the 5’-1” Lu down to only 2’-6”.
Wide, solid section Parallam® PSL beams are a convenient option since there are no plies to fasten. In addition, because connections do not have to be calculated and detailed, Parallam® PSL beams save time both for the designer and the framer in the field.
Specifying a solid section beam is often an optimal solution. Multi-ply beams can require additional design time to check unbraced length and determine the appropriate fastener pattern; and can result in a larger beam. Specifying solid Parallam® PSL avoids the need to address ply-to-ply connections and provides a more laterally stable beam.