The impact interaction between adjacent structures or structural components during earthquake excitation when relative displacements exceed the separation gap and cause collision, producing impulsive forces, high local accelerations and stress transients that are not captured by standard linear response assumptions.

Principle

When relative seismic displacements of neighbouring masses exceed available clearance, contact occurs and dynamic impact loads form that are dominated by local stiffness, contact duration and relative velocity; these impulsive forces contain high‑frequency content and can produce damage disproportionate to linear inertial force estimates.

Demonstration

Illustrative scenario → Situation: Two adjacent buildings with a designed separation of 100 mm experience out‑of‑phase motion in a severe ground shaking record. Recognition: Relative displacement at an intermediate story reaches 130 mm. Action: Collision (pounding) occurs; contact force spike is applied over a short duration. Consequence: Localized damage to cladding and infill, concentration of shear and moment demands at impacted floors, possible progressive failure if connections were not designed for impulsive loads.

Misapplication

Assuming a nominal gap prevents pounding without checking mode shapes, higher‑mode contributions or differential foundation motion. The semantic error is treating geometric clearance as a scalar safety margin independent of dynamic phasing and relative velocities.

Consequence

If accounted for, designers can specify larger separations, energy‑absorbing devices, or inter‑building links to control impacts; if ignored, pounding can cause localized collapse, loss of nonstructural elements, transfer of unexpected loads to diaphragms and vertical systems, and increased repair costs and downtime.

Reversal

If buildings are intentionally tied with flexible connectors or share a continuous diaphragm, pounding is prevented but the structures then act as a single dynamic system and may demand a different global seismic design; alternatively, provision of sacrificial fenders or dampers at separation points converts impacts into controlled energy dissipation.

Boundary

Clearly within: adjacent structures with independent dynamic characteristics and finite gap such that relative displacement can exceed clearance. Boundary case: separation where soil‑structure interaction causes differential movement but contact is mitigated by yielding infill. Clearly outside: single monolithic structures, or cases of purely radiated wave interaction where no physical contact occurs.

Semantic Tension

Urban density and minimal footprint (small gaps for efficiency) versus required seismic separation and resilience: tight urban planning maximizes land use but increases pounding risk unless compensated by specialized detailing or isolation strategies.

Synthesis

Pounding is a collision‑driven phenomenon controlled by relative displacements and velocities rather than by static clearances alone; preventing it requires dynamic consideration of mode shapes, phasing and local contact detailing or else accepting and designing for the impulsive loads it generates.