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Forces in Structures
Tension, Compression, Shear, and Bending
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Structures stay standing because forces are directed, balanced, and transferred safely into the ground. Engineers study how loads move through beams, columns, arches, and trusses so that bridges and buildings can support weight without failing. Understanding these force paths helps explain why some shapes are strong even when they use less material. It also shows why design choices affect safety, cost, and efficiency.
Different parts of a structure often carry different kinds of internal force, especially tension, compression, shear, and bending. A truss spreads loads through triangular members, an arch pushes forces outward and downward, and a cantilever resists rotation at its fixed end. In buildings, floors send loads into beams, beams into columns, and columns into foundations. Engineers calculate these effects so materials stay below their stress limits and the structure remains stable under changing loads.
Key Facts
- Static equilibrium requires sum of forces = 0 and sum of torques = 0.
- Stress = force/area, or sigma = F/A.
- Strain = change in length/original length, or epsilon = delta L/L.
- Hooke's law for elastic materials: stress = E x strain.
- For a beam, bending moment and shear force both change along the span depending on the load.
- Triangles are widely used in trusses because their shape stays fixed unless member lengths change.
Vocabulary
- Tension
- Tension is a pulling force that stretches a structural member.
- Compression
- Compression is a pushing force that shortens or squeezes a structural member.
- Shear
- Shear is a force that causes one part of a material to slide past another part.
- Bending moment
- Bending moment is the turning effect of loads that makes a beam curve or bend.
- Load path
- Load path is the route by which forces travel through a structure to the supports and foundation.
Common Mistakes to Avoid
- Confusing mass with force, because mass is measured in kilograms while force is measured in newtons and depends on gravity. Always convert weight using W = mg when needed.
- Assuming every member in a structure carries the same type of force, which is wrong because some members are in tension while others are in compression. Check the geometry and support conditions before labeling forces.
- Ignoring support reactions, which is wrong because supports provide the forces and torques needed for equilibrium. Always include reaction forces in a free body diagram.
- Thinking a stronger structure always needs more material, which is wrong because shape and force distribution matter greatly. Triangles, arches, and deep beams can increase strength without simply adding mass.
Practice Questions
- 1 A 1200 N load rests on a horizontal beam supported equally at both ends. If the load is placed at the center, what upward reaction force acts at each support?
- 2 A steel column carries a compressive force of 50000 N and has a cross sectional area of 0.01 m^2. What is the compressive stress in the column?
- 3 A truss bridge uses many triangular sections instead of rectangular sections. Explain why triangles make the structure more stable and how they help control deformation.