Seismic Systems
that Stand Up to Nature

Answers:

1. The goal of each seismic device is to dissipate or absorb the force of the earthquake, decreasing the demand on the building structure and helping to keep it intact. What type of device is selected depends on the location of the project, design parameters, the owner's requirements, building codes, and costs. Basic code requirements say that a building must be intact enough to allow safe egress immediately following a quake. Among the systems represented, base isolators, paired with braces, shear walls, or dampers, are generally the most effective for preventing seismic damage. Shear walls alone, however, are most cost effective.

2. Shear walls may be added to an existing building or designed into a new one to add strength and stiffness. Normally made of reinforced concrete, they may be constructed from other materials as well. Shear walls are usually heaviest at the base, where seismic forces are strongest, and become thinner as they go up through the building. They are also one of the most cost-effective devices, though their thickness-often 12 to 18 inches or more-limits building design and eats up square footage.

3. Braced framing stretches diagonally from the beam and column joints to create a triangulated vertical frame. Bracing limits building design because it requires consistent floor design so the braces can go through from top to bottom without interrupting the usage of space. If each floor is programmatically different, positioning the bracing is an architectural challenge. Bracing may be shaped in a variety of ways-x, v, and chevron shapes are among the most common.

4. There are four varieties of dampers: visco-elastic, friction, metallic, and viscous. Dampers soak up the energy of earthquake-induced motion, thereby limiting structural damage. They are also effective at stopping the swinging motion a building is prone to with earthquake vibrations. Directing the earthquake energy to the dampers stablizes the structure; the piston component, common to all types of dampers, pumps back and forth, releasing the energy-often in the form of friction or heat

5. Base isolators come in three varieties, high-damping rubber, lead-core rubber, and friction pendulum. All three have basically the same effect, allowing the building to move independently of the shifting ground. They perform differently, however. Rubber or elastomeric isolators are available in varying degrees of softness which allow different levels of movement. These allow the building to move, but pull it back into place. Lead core inserts in a rubber isolator absorb some of the seismic energy and force the isolator to snap back into place more quickly, bringing the building back to level. Friction pendulums consist of sliders which slip around on a concave, nonstick steel plate. They rely on the weight of the structure to recenter the building after an event