Systems for Absorbing Seismic Energy

1. Seismic dampers
Our Office deals particularly with the application of seismic dampers, having applied them in several projects in Greece. Dampers are very sophisticated devices belonging to “cutting edge” technologies, which are included in the structural system of new or existing buildings, usually through steel diagonal braces. During an earthquake they are pressed and stretched sequentially and these movements activate the absorption of seismic energy, which is eventually converted to heat.

This way, a percentage up to 40% of seismic energy can be absorbed, relieving at the same time the structural system of the building in a proportional manner. As the Eurocodes do not cover this subject, we use the American ones (FEMA 450 for new buildings and FEMA 356 for existing ones).

Dampers are usually used in existing buildings, achieving a great improvement of their behavior against earthquakes, due to significantly reduced seismic forces, which are finally applied to the structures.

They can also be used in new buildings, thus ensuring a more economical and much lighter structural system, with reduced sections of the structural elements, or even avoiding the use of shear walls.

“Fluid viscous” dampers are placed on the upper two floors of an existing building (one damper at each side).

Deflected shape of the building before the installation of the dampers.

Deflected shape of the building after the installation of the dampers (analysis models).

“Fluid viscous” dampers in a new building.

“Friction type” dampers in a new building.

There are two main categories of dampers:

• “Deflection dependent” dampers, like “metallic yielding” or “friction” type devices.
• “Velocity dependent” dampers, like “fluid viscous” and “viscoelastic” type devices.

A type of “metallic yielding” damper, very modern and efficient and very much used lately especially in Japan, is the “Buckling Restrained Unbonded Braces” (BRUB) system, see photo on the right.

In a hollow steel diagonal bracing member, a steel plate is inserted, and the gap is filled with concrete mortar. The plate is appropriately coated in order to have no, or very little, friction to the mortar, so that it will be able to slide freely, when stressed in tension or compression.

The plate is designed in such a way so that to yield in axial force, tension or compression, during the design earthquake, creating that way an enlarged hysteresis loop, which leads to significant absorption of seismic energy. J.M has designed such dampers, which e which have been implemented by specialized workshops.

BRUB dampers are applied to existing buildings either through the PARSANT method, or directly between the concrete beams. They can also be used in new buildings for improving their seismic behavior.

2. Magnifier of Horizontal Displacement (MHD)
It is a device, developed by Mr. John Marneris, which multiplies the relevant horizontal seismic deformations (drift) between two consecutive floors. The increased deformation is transferred to the dampers (deflection dependent ones, such as ΒRUB). This increase can be 2 to 5 times greater and this results in greater deformation of the dampers (contraction/elongation), resulting in the production of a larger hysteretic loop, and ultimately a greater absorption of seismic energy.