### STRUCTURES: FORCE VS. STRESS

Force and stress refer to the same phenomena, but with different meanings. Force is an external action, measured in absolute units: # (pound), k (kip); or SI units: N (Newton), kN (kilo Newton). Stress is an internal reaction in relative units (force/area ), measured in psi (pound per square inch), ksi (kip per square inch); or SI units: Pa (Pascal), kPa (kilo Pascal). Axial stress is computed as:

f = P / A

where

f = stress

P = force

A = cross section area

Note: stress can be compared to allowable stress of a given material.

Force is the load or action on a member

• Stress can be compared to allowable stress for any material, expressed as:

F ≥ f (Allowable stress must be equal or greater than actual stress)

where

F = allowable stress

f = actual stress

The type of stress is usually defined by subscript:

Fa, fa (axial stress, capital F = allowable stress)

Fb, fb (bending stress, capital F = allowable stress)

Fv, fv (shear stress, capital F = allowable stress)

The following examples of axial stress demonstrate force and stress relations:

Note: The heel would sink into the wood, yield it and mark an indentation

f = P / A

where

f = stress

P = force

A = cross section area

Note: stress can be compared to allowable stress of a given material.

Force is the load or action on a member

• Stress can be compared to allowable stress for any material, expressed as:

F ≥ f (Allowable stress must be equal or greater than actual stress)

where

F = allowable stress

f = actual stress

The type of stress is usually defined by subscript:

Fa, fa (axial stress, capital F = allowable stress)

Fb, fb (bending stress, capital F = allowable stress)

Fv, fv (shear stress, capital F = allowable stress)

The following examples of axial stress demonstrate force and stress relations:

Note: The heel would sink into the wood, yield it and mark an indentation