What is mechanical stress?
In building engineering, stress usually means mechanical stress: the force acting on a material per unit of area. The formula is σ = F / A, where F is the force in newtons and A the area in mm². The unit is N/mm², which equals MPa (megapascal). Stress is what a structural engineer compares with the strength of a material: as long as the stress that occurs stays below the permissible value, the component holds.
The Dutch word spanning has two other meanings in construction: electrical voltage (volts) and the span of a beam or arch. This article is about the mechanical meaning.
Types of stress
- Compressive stress. The material is squeezed, for example a column or foundation.
- Tensile stress. The material is stretched, for example a tie rod or the underside of a loaded beam.
- Shear stress. Two parts tend to slide past each other, for example at the bearing of a beam or in a bolt.
- Bending stress. In bending, compression occurs on one side and tension on the other.
Worked example
A 100 × 100 mm column carries 10 kN (10,000 N). The area is 10,000 mm². The compressive stress is then 10,000 / 10,000 = 1 N/mm².
For comparison, characteristic strengths from the standards:
- Structural steel S235: yield strength 235 N/mm².
- Concrete C20/25: characteristic cylinder compressive strength 20 N/mm².
- Softwood C24: characteristic bending strength 24 N/mm².
A structural engineer does not calculate with these values directly, but with design values that include safety factors.
Application
- Materials behave differently. Concrete and masonry are strong in compression but weak in tension; that is why concrete is reinforced. Steel is strong in both tension and compression.
- Stress concentrations. At corners, holes and openings the stress rises locally. That is often where the first cracks appear.
- Smaller cross-section = higher stress. A chase or hole in a beam or column reduces the cross-section. Always have this assessed.
Common mistake: confusing stress with strength. Stress is what occurs in the material as a result of the load; strength is the maximum the material can withstand.
Related terms
- Compressive strength
- Modulus of elasticity
- Effective stress
- Span
- Stress crack
- Horizontal thrust
- Reinforcing steel
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