The 1/3 rule for wall stability

Structure · Columns · printed page 47 (PDF page 48)

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Book index entries

Structural

Text in the illustration

middle third of the wall
force
center of gravity
procedure
measure the amount that the wall is out of plumb
make a scale diagram - drawing a line representing the force of gravity through the center of the structure
if this line does not lie within the center 1/3 of the wall, the wall is unstable
the same concept applies to bowing or leaning walls
If there are joists resting on the walls, even less lean is allowed before the wall is unstable
stable
borderline
unstable
(the center of gravity for the wall extends outside the middle third)

Generated explanation

The illustration compares an upright wall, two progressively leaning walls, and a bowed wall using the middle-third rule for stability. Each wall has its middle third shaded, a center-of-gravity marker, and a vertical downward arrow showing the gravity force line relative to its base. The upright wall is labeled stable, the slightly leaning wall borderline, and the more tilted and bowed walls unstable because the gravity line falls outside the middle third at the base. Horizontal arrows indicate lateral forces on the walls. The printed procedure calls for measuring the out-of-plumb displacement and making a scale diagram, and notes that walls supporting joists tolerate even less lean.

Components

walls wall bases

Defects shown

out-of-plumb walls leaning walls bowed walls unstable walls with gravity force lines outside the middle third of the base

Related illustrations

0263

The 1/3 rule for wall stability

· p. 43

Four wall profiles illustrate the one-third rule for wall stability, comparing a plumb wall, two increasingly leaning walls, and a bowed wall. Each profile marks the wall's middle third, its center of gravity, and a vert