A crane boom can lift loads that weigh many tons because its steel members share the forces instead of letting one solid beam take everything. When a load hangs from the boom tip, gravity pulls downward and creates tension, compression, bending, and shear inside the structure. Engineers design crane booms as lattice trusses so the steel is arranged where it is most useful.
Understanding these stresses helps explain why a long, lightweight boom can be strong enough for construction work.
In a loaded boom, the top members often carry compression while the lower members carry tension, depending on boom angle and support points. Diagonal lattice members redirect forces through triangles, which are hard to distort and reduce bending in individual pieces. The boom must also resist shear near supports and connections, where forces transfer between pins, welds, and steel members.
Safe crane operation depends on both the boom design and the load chart, which limits how much weight can be lifted at each radius and boom angle.
Key Facts
- Stress = force per area, σ = F/A
- Tension pulls a member longer, while compression pushes a member shorter.
- Bending moment increases with load and distance, M = Fd
- Shear force acts parallel to a cross section and can cut or slide material layers.
- A triangular truss carries loads mostly as tension and compression in straight members.
- Increasing boom radius increases tipping risk and bending moment on the crane.
Vocabulary
- Stress
- Stress is the internal force per unit area in a material when it is loaded.
- Tension
- Tension is a pulling force that stretches a structural member.
- Compression
- Compression is a pushing force that squeezes a structural member and can cause buckling if the member is long and slender.
- Bending moment
- A bending moment is the turning effect of a force that causes a beam or boom to curve.
- Truss
- A truss is a structure made of connected straight members, usually arranged in triangles, to carry loads efficiently.
Common Mistakes to Avoid
- Treating the boom as a single solid bar is wrong because a lattice boom works by sending forces through many connected members, not by using one uniform piece of steel.
- Ignoring the load radius is wrong because the same load creates a larger bending moment and tipping risk when it is farther from the crane base.
- Assuming all boom members carry the same force is wrong because some members are in tension, some are in compression, and some may carry little force depending on the load path.
- Forgetting buckling in compression members is wrong because a slender steel member can fail by bending sideways even if the compressive stress is below the crushing strength of steel.
Practice Questions
- 1 A 12,000 N load hangs from a boom tip 8.0 m from the pivot. What bending moment does the load create about the pivot? Use M = Fd.
- 2 A steel tie member in tension carries 45,000 N and has a cross sectional area of 0.0030 m^2. What is the tensile stress in the member? Use σ = F/A.
- 3 A crane boom is redesigned from a solid beam into a triangular lattice truss with diagonal members. Explain how this change helps the boom resist tension, compression, bending, and shear without greatly increasing its weight.