Why can an impact load produce stresses several times those of the same static load?

It is applied suddenly with kinetic energy, not slowly

What is the key danger of a cyclic load?

Failure can occur below the yield stress after many repetitions

Failure needs stress above the ultimate strength

It only causes temperature stresses

It acts only on the web of a beam

How are stresses from combined loading (e.g. bending plus torsion) found?

Find each separately and superpose, valid for linear elastic and small deformation

Use only the largest single action

Average the stresses of all actions

Practice · 5 marks

Types of loads on structures and effects

Short answer

Loads are classified by duration (static, dynamic, impact), permanence (dead, live), and distribution (point, UDL). Axial load causes normal stress σ=N/A; transverse load causes shear and bending stress σ=My/I; torque causes torsional shear τ=Tr/J; restrained thermal expansion causes thermal stress.

Classify the loads that act on a structure. Explain the effect each kind of load produces in a member.

A load is any external action (force, moment, pressure, temperature change, support settlement) that makes a structure deform and develop internal stresses. Loads are classified in more than one way.

Classification of loads

Basis	Types	Examples
Duration / variation with time	Static, dynamic, impact, cyclic (fatigue)	Self weight; machine vibration; falling weight; rotating shaft
Permanence	Dead, live	Self weight of beam; people, vehicles
Environmental	Wind, snow, earthquake, thermal	Wind on a tower; temperature rise in a rail
Distribution	Point, line (UDL, UVL), surface	Column on a beam; slab on a beam; water pressure

Static load: applied slowly, so inertia forces are negligible.
Impact load: applied suddenly with kinetic energy, so stresses can be several times those of the same static load.
Cyclic load: repeated many times; failure can occur below the yield stress (fatigue).

Effect of load on a member

The effect depends on how the load acts relative to the axis of the member.

Load	Internal action	Resulting stress	Example
Axial (tension or compression)	Normal force $N$	$\sigma = N/A$	Tie rod, column
Transverse (shear)	Shear force $V$	$\tau = VQ/(Ib)$	Rivet, beam web
Transverse load or couple	Bending moment $M$	$\sigma = My/I$	Floor beam
Twisting couple	Torque $T$	$\tau = Tr/J$	Transmission shaft
Internal or external pressure	Hoop and axial stress	$\sigma_\theta = pr/t$	Boiler shell
Temperature change	Thermal strain	$\sigma = E\alpha\Delta T$ if restrained	Bridge deck

In practice members carry combined loading, for example a shaft in bending and torsion, or an eccentric column in compression and bending. The stresses are then found for each action separately and combined by superposition, valid for linear elastic response and small deformation. The combined stress state is then examined with the stress tensor to find principal stresses and check failure.

✓ Answer checked by Claude Sonnet 5.5

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