Types of Modifications

Modifications fall into several categories:

Hybrid Solution

PRESSS (Precast Structural Seismic Structural Systems)is a U.S./Japan joint program for the development of structural methods suitable for ductile reinforcement and joining of rigid concrete plate and beam structures. As these systems comprise both post tensioning and external energy dissipation they are called hybrid solutions. Fiber reinforced concrete is an essential element in these structures as they allow the creation of structural regions capable of "plastic hinging", a feature that enables progressive flexible joint failure without catastrophic dismantling.

Isolation

Generally required for large masonry buildings, excavations are made around the foundations of the building and the building (in piecemeal fashion) is separated from the foundations. Steel or reinforced concrete beams replace the connections to the foundations, while under these, layered rubber and metal isolating pads replace the material removed, these in turn are attached below to new or existing foundations. These allow the ground to move while the building, restrained by its inertial mass, remains relatively static. The pads absorb energy, transforming the relative motion between the ground and the structure into heat. While the pads tend to transmit some of the ground motion to the building they also keep the building positioned properly over the foundation. Careful attention to detail is required where the building interfaces with the ground, as at stairways and ramps, to ensure sufficient free motion without damage from compression or dismantling or falling from extension.

Dampers

Dampers absorb the energy of motion and convert it to heat, thus "damping" resonant effects in structures that are rigidly attached to the ground. In these cases, the threat of damage does not come from the initial shock itself, but rather from the periodic resonant motion of the structure that repeated ground motion induces.

Reinforcement

The most common form of seismic retrofit is adding strength to the existing structure to resist seismic forces. The strengthening may be limited to connections between existing building elements or it may involve adding primary resisting elements such as walls or frames.