An important consideration in the planning and budgeting of a wood-framed, multi-story housing project is which type of structural system to employ, specifically for the floor and roof structures. Using a simple lean tool called "Choose By Advantages" supports us in helping clients evaluate these systems for the best fit on a project. This decision-making methodology uses multi-criteria decision-analysis to compare the advantages of a set of alternatives.
Two structural systems recently explored by our team, wood trusses and wood joists, help to demonstrate the evaluation process and illustrate the importance of identifying the system that is most advantageous for a particular project.
- 18” deep floor trusses at 2’-0” on center spanning from party wall to party wall
- Bottom of trusses provides flat surface to apply gypsum layers for fire rating of floor
- Mechanical, plumbing, and electrical systems run in between diagonals when perpendicular to the structure and can be "tucked in" to the depth of the structure when running parallel
- MPE systems hosted inside the structural depth require fire-rated assemblies when penetrating the gypsum ceiling membrane
- One consistent plane of fire-rated construction, with no redundant framing and drywall for bulkhead
- Concealed MPE systems inside the system
- Wood joists spaced 16” deep joist at 18” spacing spanning from the party walls to party wall
- Bottom of joists provides flat surface to apply gypsum layers for fire rating of floor
- Mechanical, plumbing, and electrical systems run underneath the joists when running perpendicular to the structure
- MPE systems hosted outside the structure depth require a drop ceiling or bulkhead to conceal the systems.
- Higher ceiling heights where MPE systems are not required. In this design, MPE systems are typically carried in a drop-ceiling in the service area of the unit (bathrooms, closets, etc.) and the balance of the unit can have higher ceilings
- MPE penetrations do not require rated penetrations. MPE systems to do not interrupt the fire-rated floor or ceiling assembly. Less maintenance and Contractor can establish structural rating integrity prior to MPE system installation
- If exposed MPE systems are compatible with the aesthetic of the design, extra bulkheads and framing can be eliminated
Factors such as prevalence of material supply (joists vs trusses), familiarity of the system by general contractors, regularity of the unit geometries, and the client's desired aesthetics for the space, will impact the determination of which system is more advantageous. In a recent project, the client desired a consistent ceiling height and concealed MPE systems, and the general contractor demonstrated considerable savings by omitting extra bulkheads and framing. Therefore, the wood truss system was chosen.
Our goal in this endeavor is to clearly communicate the advantages of multiple systems to our clients, allowing them to make a decision that aligns with their goals for a particular project.