Loading a beam

• When a beam is loaded it will bend
• Bending movement creates stress
• The top of the beam is compressed and the underside is stretched
• This is because there are compressive forces at the top and stretching/tensile forces at the underside
• If the tensile force is greater than the tensile strength of the material it cracks on its underside
• tensile strength-the resistance of a material to breaking under tension
• If a beam is only supported on the centre tension forces occur at the top of the beam and compression on the underside

Neutral axis

• The central axis of the beam is called the neutral axis
• Neutral axis-is that part of the beam that remains unstressed when the beam is bent/carrying a load
• The section above the neutral axis shortens (due to compression)
• The section below the neutral axis lengthens (due to tension)
• The length of the neutral axis does not change.
• Please note that on a centrally supported beam the section above the neutral axis lengthens and the one below shortens
• On a loaded beam the tension are equal to the compression forces bu they act in the opposite direction
• The greater the distance is from neutral axis the greater the tensile and compression forces

To access more topics go to the Combined Science Notes page.