In this method of modelling, the gap elements were placed between the colliding structures to measure contact forces.

Progressively, these force-based models were implemented in commercial software as gap elements to model structural pounding. These models have been widely used and modified numerous times however, they suffer from a lot of uncertainties associated with their contact parameters (i.e contact stiffness, coefficient of restitution, and damping ratio) as proper methods/formulas to calculate the parameters for contact between flat surfaces do not exist. The research gradually evolved to analytical force-based models called contact element models (other names are impact element or gap friction element models) that simulate pounding between structures idealised as lumped masses. Simulating pounding between two structures and the extent of pounding damage started from a simple model that could only estimate the elasticity and plasticity of a damage using the pre- and post-impact velocities. Over the past decades, simulating structural pounding and assessing the resultant pounding forces have attracted the attention of many researchers. The extent of seismic pounding damage ranges from minor crushing to catastrophic collapse of the colliding structure, even though recent design codes provide guidance on mandatory separation gaps to prevent pounding still, in dense metropolitan areas with growing populations and constructors willing to make the maximum use of available lands, structural pounding is becoming a significant matter that needs attention in the design of new buildings.

Seismic pounding damage has been observed in several earthquakes such as Mexico City earthquake in 1985 (Rosenblueth and Meli, 1986), Loma Prieta in 1989 (Kasai and Maison, 1997), and more recently Gorkha earthquake in 2015 (Shrestha and Hao, 2018). It occurs as a result of out-of-phase responses of structures due to differences in their dynamic properties. Structural pounding occurs when two or more structures with insufficient separation distances in between collide under earthquake loading.

Advance steel 2015