Abstract: Conservation laws of momentum and energy are introduced to model pounding between adjacent bridge girders based on collinear compact between rod theory. An equivalent rigid-body compact model is proposed. The coefficient of restitution depends only on the length ratio l₂/l₁ and impact spring stiffness ratio α. A simplified analysis of seismic pounding in a concrete bridge indicates that the equivalent rigid-body compact model can acquire almost the same structural response as that elastic rod compact model does.