Creating Innovative Technology
Here is a look at some of the innovative technology that Structural Engineers develop and use to help structures resist earthquakes.
Base Isolation is a state-of-the-art design strategy that substantially decouples (isolates) a building from the damaging effects of earthquake ground motion. It can be used to dramatically reduce earthquake forces by factors of 5 to 10 on mid-rise buildings (2-15 stories tall), and make a Magnitude 8.0 earthquake seem like a much less damaging Magnitude 5.5 event.
A base-isolated structure is one supported by isolation elements--generally bearings or sliders--that are typically placed between the building and its foundation. An appropriate analogy would be the relationship between automobiles and their suspension system of springs and shock absorbers, which cushion the occupants from a bumpy ride.
The following illustrations show the effects of an earthquake acting on both a base-isolated building and a conventional, fixed-base building. During an earthquake, the ground beneath each building begins to move. In figure 1, isolators placed between the bottom of each column and its foundation, prevent most of the horizontal movement of the ground from being transmitted to the structure, and dissipate the energy imparted to the building by earthquake. The building feels less force and moves uniformly (like a rigid box) in a slow controlled fashion, resulting in substantially reduced damage to the structure and its contents.
Figure 2 is a "snapshot" of a conventional fixed-base building at one particular point of its earthquake response. This building is shown to be changing its shape-from a rectangle to a parallelogram. We say that the building is deforming. The primary cause of earthquake damage to buildings is the deformation that the building undergoes as a result of the inertial forces acting upon it. Conventional fixed-base construction can cause high floor accelerations in stiff buildings and large deformations in flexible structures. These two factors create challenges to ensuring the safety of building occupants and providing protection to building contents.
Figure 1: Earthquake Response of Base-isolated Building
Figure 2: Earthquake Response of Conventional Fixed-Base Building
The conventional fixed-base alternatives rely on the ability of a structure to flex and endure permanent deformation without failure. This prevents collapse in a major earthquake, but it could mean extensive damage to the structure and its contents. By reducing the forces imposed on a building, seismic isolation reduces the damage to the structure and its contents.
In some cases, the contents of a building are more valuable than the structure itself. Isolation offers an effective option for protecting an investment or minimizing downtime, and is a viable alternative for construction of new buildings or retrofit of historic buildings.
Lead-rubber bearings, made from layers of rubber sandwiched together with layers of steel, are among the frequently used types of base isolation bearings. (See figure 3. Source: MCEER) In the middle of the bearing is a solid lead "plug." On top and bottom, the bearing is fitted with steel plates, which are used to attach the bearing to the building's superstructure and foundation.
The bearing is stiff and strong in the vertical direction, but flexible in the horizontal direction.
Figure 3 : Lead-Rubber Bearing
Sliding Isolation Systems
Spherical sliding isolation systems represent one type of sliding isolation system. The building is supported by bearing pads that have a curved surface and low friction. During an earthquake, the building is free to slide on the bearings. Since the bearings have a curved surface, the building slides both horizontally and vertically (see figure 4). The force needed to move the building upward limits the horizontal or lateral forces that would otherwise cause building deformations.
Figure 4: Spherical Sliding Isolation Bearing
Friction Pendulum Sliding (FPS) bearings are another type of sliding isolation system. Devices are placed between a building's superstructure and its foundation, where they alter the force-response characteristics of the building. The FPS system makes use of spherically shaped, articulated sliding bearings. The unique feature of the FPS system is that movement of one part of the bearing with respect to others resembles pendulum motion in the presence of friction.
The U.S. Court of Appeals building in San Francisco , constructed in 1905, is an early example of American Renaissance Style the only such example in the western United States that has been retrofitted with FPS isolators.
US Court of Appeals - San Francisco, CA
Energy Dissipation Devices
Another major new technology for improving the earthquake resistance of buildings involves energy dissipation. During an earthquake, a certain amount of energy is transferred to the building. While buildings can dissipate, or damp, this energy, the capacity to do so before becoming deformed or damaged is quite limited.
A building dissipates energy either by undergoing large-scale movement or by sustaining increased strains in key building elements such as columns and beams. Both of these processes eventually result in some degree of damage. Structural engineers can greatly decrease the seismic energy entering the building, and thus decrease building damage, by equipping a building with additional devices that have high damping capacity.
A number of damping devices have been developed. What follows are some representative examples:
- Friction Dampers utilize frictional forces to dissipate energy
- Metallic Dampers utilize the deformation of metal elements within the damper
- Viscoelastic Dampers utilize the controlled shearing of solids
- Viscous Dampers utilize the forced movement (orificing) of fluids within the damper
Damping devices are often installed as part of bracing systems. Figure 5 shows one type of damper-brace arrangement, with one end attached to a column and one end attached to a floor beam.
Figure 5: Damping Device Installed with Brace