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