United States
Hong KongWood framing and load paths
A load path carries force continuously from roof to foundation. Any interruption - an opening, a removed stud, a cantilever - must be carried by a member sized for it, and spans come from code tables.
One idea explains most framing questions: load has to get to the ground, and it takes a path.
The path
Roof load to rafters. Rafters to walls, through studs, to the plate. Then the floor structure, the foundation, and finally the soil.
Every element in that chain carries what is above it. Remove one and the load does not disappear; it redistributes.
Openings
A window or door interrupts the studs. A header spans the opening and carries their load to trimmers at each side, which carry it down to the plate.
Header size depends on span and load, and it comes from a table. Undersized headers sag, and the sag shows in doors that stop closing.
Allowable span depends on species, grade, spacing and load. Nobody memorizes it. Tab the span tables and these become quick marks rather than three-minute searches.
Lateral load
Wind and seismic force act sideways. The vertical load path does not resist them, so shear walls do, transferring lateral force to the foundation through sheathing, nailing and hold-downs.
Change the nailing pattern and you change the capacity. That is why shear wall nailing is inspected.
Connections
The path is only as good as its joints. Hurricane ties, hold-downs and straps exist because nails in withdrawal are weak and uplift pulls the roof off.
Moisture
Nineteen percent moisture content is the usual threshold above which lumber is considered green. Green lumber shrinks as it dries, which loosens connections and cracks finishes applied over it.
Common questions
What is a load path?
The continuous route by which load travels from roof to foundation. Interrupting it means something must carry the difference.
What carries an opening in a wall?
A header spanning the opening, transferring load to trimmers at each side.
Do I memorize span tables?
No. They are lookups, which makes tabbing them worthwhile.
What resists lateral load?
Shear walls, through sheathing, nailing pattern and hold-downs.
What moisture content counts as green?
19 percent is the usual threshold, above which shrinkage on drying loosens connections.