Which railing features a wave pattern intended to improve rigidity?

• A. Box beam guardrails
• B. W-beam bridge railings
• C. Tubular rail system
• D. Thrie beam railings

Answer: (D)

Rigidity in guardrail design comes from how the rail shape distributes impact forces. Thrie-beam railings use a three-rail assembly with a corrugated, wave-like profile. That wave pattern increases stiffness by boosting the moment of inertia and creating multiple contact points along the impact path, helping the rail resist bending and deflection and directing a vehicle in a controlled way. Other rail types rely on different cross-sections and do not employ that wave configuration to the same extent, so they don’t gain the same rigidity advantage.