The Shear Panel
Standardizing the shear panel construction was the key to creating the structural frame. These panels are built to carry the gravity loads of the 400 sq ft. A structural engineer told me that these panels will perform best when loaded. This was accomplished by placing these at each corner and transferring the rest of the loads through a beam to the shear panels.
Shear panel studs are 16" or 24" on center to match the wall panel framing. The outside studs are doubled although I recommend using double studs for the entire panel if your layout is 24" OC. Tie-down brackets are placed at each side of the shear panel. 2 per panel, 4 per corner of the structural frame.
The x-strapping is not optional regardless of the sheeting design for the building for 2 reasons. First, shear panels interior to the building will not get a plywood sheeting and would not provide shear resistance. Second, if someone uses that so-called cardboard "structural" sheeting, we can count of the shear capabilities of the panel itself. The strapping is the best way to achieve corner to corner shear resistance and positively ties the opposite top corner to the anchor bolt at the foundation or the structural frame below.
Beams
The beam between the shear panels is the next, most important structural element. This beam is required to support the floor loads and the wall panel above it while keeping a dimension of 3-1/2"w x 12"d x 12' long. Added support is provided by a 2x12 rim joist installed inside of the structural frame. The rim joist ties the shear panels and beam together, the floor trusses are connected to the rim joist.
The primary reason for using a beam rather than exterior wall framing is to allow the openings below it to be up to 12' wide and floor to ceiling. With the beam carrying the building loads we remove design restrictions on the wall panel. Headers above windows and doors will not be required or will be much lighter in design.
Floor Trusses
The floor truss is the simplest part of the structural frame. The only requirement is they do not exceed 12" deep while supporting an open span of 20 feet. At 12" deep we are creating a 9' ceiling height in a 10' tall frame and 8' ceiling in the 9' frame.
Floor trusses are easily installed on standard joist hangers. Floor truss panels can also be prefabricated in 2 sections. The rim joists are cut to 10' long with the floor trusses end nailed or installed with joist hangers. These sections are then lifted into place between the frames on-site.
Metal or wood trusses are acceptable no matter what material is being used for the primary framing. I would expect to see wood I-Joist floor trusses in a metal framed home to help reduce costs yet maintain the structural qualities of steel for the primary framing.
Structural Sheeting
The horizontal sheeting across the top of the structural frame is the only required sheeting. This applies at the top level frame before the roof trusses are installed. Because the structural frame assembly carries the loads, the roof trusses can be engineered much lighter than traditionally done.
When building multi-story buildings, the sheeting between floors should be 1-1/8" T&G plywood. At the top level, below the roof trusses, use 3/4" T&G plywood.
Horizontal sheeting is not required but I design 1/2" plywood as the first layer of the exterior. This not only adds to the seismic and wind resistance but helps reduce exterior noise. In a metal frame home I use it as a thermal break where the plywood is fastened to the studs and the siding is fastened to the plywood within 2" of each stud but not into the stud itself.
I only recommend using plywood sheeting, no OSB and certainly not the cardboard "structural" sheeting where that is allowed. OSB is too susceptible to moisture damage weaking the structural value. This example shows hidden damage which can occur behind a stucco finish. I know, no sheeting should get wet but when it does, plywood responds and recovers much better than OSB.
