the tributary area dynamic.  Additional bracing in the form of blocks or cross bracing is used

to keep the joists from buckling or twisting under load.  Joists are typically solid wood

planks.  They may also be composite structures such as truss joists, and Ι-shaped beams

comprised of wooden top and bottom chords glued to a narrow strip of wood paneling.

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The wood framed walls of the structure are then erected on top of this platform.  They are

usually comprised of many evenly spaced vertical 2 x 4 inch wooden studs. Wooden panels,

called shear panels, are nailed to the outside face of the stud wall framing to brace the wall

against lateral loads and to keep the studs from buckling under load.  Compressive forces

are induced by the load of the above ground structures such as the roof rafters or floor joists

that rest on the top plate of the load bearing walls.  The walls transfer these compressive

forces to the foundation.  Shear forces are induced in the walls by loads applied parallel to

their plane such as wind and seismic shaking.

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For example, strong winds try to overturn the wall which is fastened to the foundation.  This

causes the top plate of the wall to try to shear past the bottom plate just as we saw before

for beams and trusses.  Diagonal bracing is used between the studs in addition to the shear

.

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panels, which resist the shear forces by means of plate action.  That is, the entire wall acts

as one integral plate.  And all the walls are joined together edge to edge at their commonly

shared edges to make a plate-like box structure.  Where openings are cut into the stud wall

framing for windows or doors, wide wooden header boards are used to span the top of the

opening so as to carry the downward compressive load of the missing stud columns.  The

ends of the header rest on supports, called cripple studs that transfer the load to the ground.

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