There is a buzz around the construction industry about the benefits of framing multifamily projects up to five stories with wood rather than steel. Yet, while wood framing is adaptable, cost effective and easy to work with, there are construction challenges to multi-story wood construction. Below, we discuss some of those challenges and identify several technical measures to help reduce the impact on wood frame construction.
One of the key issues with wood frame construction is Shrinkage. The IBC recognizes Shrinkage in code 2304.3.3. An excerpt from the code reads, “Wood walls and bearing partitions shall not support more than two floors and a roof unless an analysis satisfactory to the building official shows that shrinkage of the wood framing will not have adverse effects on the structure or any plumbing, electrical or mechanical systems, or other equipment installed therein due to excessive shrinkage or differential movements caused by shrinkage.”
Multi-story wood construction requires a number of considerations for builders. As Structural Consultant Alan Birbeck noted, “The construction of a 5-story, type-3 wood frame requires an understanding of framing tolerances, shrinkage, interaction with dissimilar materials, and the contractor should develop a proactive quality control procedure and review with the Architect and Engineer”. Mr. Birbeck also noted, “the approximate shrinkage in conventional 2x wood frame is as follows: 4-story – 5/8”, 5-story – 7/8”.” Other sources, however, warn of even substantially higher amounts of vertical shrinkage in a 5-story wood framed structure. Nevertheless, all members involved with the construction of a multi-story wood frame project must research, understand and plan for the impact of shrinkage for each specific project.
Mitigating the Effects of Shrinkage
In order to lessen the effects of shrinkage, a structural engineer may specify a hybrid type of balloon framing, incorporate sheathing gaps for movement, and increase rough opening sizes. And when these changes occur, they should be noted in the Architectural plans, Structural Engineering and Submittals. Likewise, the plans should address movement in windows and doors, exterior claddings (especially problematic with brick because it grows), pipe or conduit penetrations in the envelope, plumbing DWV and supply lines, and dissimilar materials.
The Western Wood Products Association (WWPA) Tech Notes Report #10 calls for the implementation of a variety of technical measures with these tall stacked wood structures, including the following:
Site storage – storing unprotected lumber directly on the ground must be avoided. Lumber that has increased moisture content from rain should be allowed to dry before attaching sheathing elements.
Expansion joints or slip type architectural details are often employed in these areas at each floor elevation. When detailing panel sheathing, the panels should not extend across the floor joists if the joists are sitting on top of the wall.
Metal hold downs and ties should be retightened before finishing materials are installed.
Window and doors should be installed into prepared openings in exterior walls when lumber is as close to equilibrium moisture content as possible. Windows should be independent from brick veneer or detailed to allow for up to ½” movement per floor. Further a clear gap should be provided to allow for differential movement at the bottom window rail and the sill below.
Doors should be installed with a minimum 1/8” gap between the jamb and top of door. For all rough openings a gap should be left at the top of the door.
MEPs – Using flexible joints for electrical, mechanical, and plumbing work between floors may eliminate potential problems resulting from wood shrinkage.
Making It Work
While the items listed above are some of the ways to help lessen the impact of shrinkage, these only work if 1) the specifications and adaptations are clearly marked within the project plans and 2) the construction team follows the plans. Two cost effective ways to help ensure both of these occur is by having 1) a third-party peer plan review and 2) onsite QA/QC inspections. The peer plan review examines the project plans to identify and fix errors before construction begins. The QA/QC inspections during construction help identify any construction errors and defects while there is still time to fix them.
No matter what quality assurance process is used, it is imperative that one exists. As more and more owners and builders elect to pursue wood frame construction for multi-story projects, we can expect the effects of shrinkage to play an even bigger role in construction. Still, it’s possible to take advantage of the cost savings and adaptability of wood construction without degrading the end product or taking on substantial liabilities by having a strong partnership with Architects, Engineers, Builders and the Quality Control Third-Party Team. In the end, maintaining the quality and integrity of construction is all of our responsibilities; so, let’s do all we can to ensure safe and quality construction.
For more information, contact Burgess Construction Consultants, Inc. at 888-644-6489.