1854 Structural Elements that Prevent Damage and Improve Dynamic Response

Improved structural performance during large earthquakes depends on a balanced structural system. Elements that share the same displacement during large earthquakes must be designed to have about the same stiffness. Otherwise, the stiffer elements will be forced to resist most of the earthquake force. Elements that share the same force are often provided with a fuse to limit the force and protect adjacent members.

Providing great strength to resist earthquakes is usually self-defeating. As the elements are made stronger, they attract larger earthquake forces. If an element along the load path cannot resist these forces, it will break, sometimes with disastrous consequences. Elements like shear walls are often used to limit displacements in buildings, but they are nonductile and will often shatter when unexpectedly large earthquakes occur.

Diaphragm Wall Construction Sequence

FIGURE 18.76 Construction sequence for diaphragm foundations. (Drawing courtesy of Japan's Public Works Research Institute.)

FIGURE 18.76 Construction sequence for diaphragm foundations. (Drawing courtesy of Japan's Public Works Research Institute.)

The use of isolation and damping devices has gained popularity because they can protect a structure during several large earthquakes without suffering damage or requiring replacement. Moreover, these devices have proven to be effective for new construction as well as for retrofitting vulnerable existing structures.

The new San Francisco Airport International Terminal is a large steel frame building with a truss roof (Figure 18.77). It is the largest base-isolated building in the world. To ensure that it would remain in service after a very large (magnitude 8) earthquake, 267 friction pendulum seismic isolation bearings were placed between the steel columns and the foundations (Figure 18.78 and Figure 18.79). Each bearing can move 20 in. while supporting 6000 kip. The bearings increased the building's fundamental period to 3 s and reduced the earthquake force by 70%.

These interesting devices are surprisingly simple in principle. For service level loads, static friction restrains the system. During an earthquake, the system operates like a pendulum (Figure 18.80) with the amount of damping controlled by dynamic friction and with the period

FIGURE 18.78 Layout of foundations and friction pendulum devices for the new San Francisco Airport International Terminal. (Photo courtesy of Earthquake Protection Systems Inc.)
FIGURE 18.79 One of the 267 friction pendulum devices for the new San Francisco Airport International Terminal. (Photo courtesy of Earthquake Protection Systems Inc.)

and stiffness as shown below (where R is the radius, W is the weight, and g is the acceleration due to gravity).

The period of the structure is increased by flattening the bearing's concave surface and the force-displacement relationship is modified by changing the friction or the dead load. Similarly, the design of the pyramid-shaped "Money Store'' in West Sacramento (Figure 18.81) included fluid viscous dampers (FVD) to absorb energy and allow the steel moment-resisting frame to remain elastic during large

FIGURE 18.80 The structure's period is controlled by the radius of the curved bearing. (Drawing courtesy of Earthquake Protection Systems Inc. Zayas et al., The FPS earthquake resisting system, University of California, Berkeley, CA, 1987.)

FIGURE 18.81 The uniquely shaped ''Money Store'' was designed with fluid viscous dampers to improve its seismic behavior. (Photo courtesy of Marr-Shaffer & Miyamoto, Structural Engineers.)

earthquakes. The FVDs are cylinders filled with liquid silicon. When a load is applied, a piston pushes through the viscous fluid in the cylinder, absorbing energy (Figure 18.82). Concentrically braced frames were built with FVDs between the diagonal braces and the columns to increase the damping ratio of the building to 15% and reduce the story drift ratio to 0.005 (Figure 18.83).

Rod make-up

To prevent "boing" or bouncing accumulator effect, a control valve releases ^■ fluid into a third chamber, the accumulator housing

Rod make-up

To prevent "boing" or bouncing accumulator effect, a control valve releases ^■ fluid into a third chamber, the accumulator housing

FIGURE 18.82 Schematic drawing of a fluid viscous damper. (Drawing courtesy of Taylor Devices Inc.)

FIGURE 18.83 The steel frames included fluid viscous dampers to reduce the story drift. (Photo courtesy of Marr-Shaffer & Miyamoto, Structural Engineers.)

Isolation and damping devices are equally adept at providing protection for seismically vulnerable existing buildings. The Long Beach VA Hospital is a concrete shear-wall structure that was found to be vulnerable to large earthquakes (Figure 18.84). Many retrofit strategies were studied before isolation was finally accepted as the best way to keep the hospital functioning after a large earthquake on the nearby Pales Verdes and San Andreas Faults. Because the hospital also had to remain in service during construction, the sequence of the construction was crucial and the contractor had to be responsive to any problems that developed that could impact the daily operations of the hospital. Most challenging was completely isolating the structure. A moat had to be dug around the building and flexible connections had to be designed for all the utility lines as they entered the building.

Lead-rubber bearings were installed a few feet above the base of all 150 concrete columns that supported the building (Figure 18.85). Each bearing was 22 in. tall and 24 in. in diameter. First, friction gripping devices were used to transfer the load from the column onto hydraulic jacks. Then, a 20-ft section of the column was replaced with a lead-rubber isolation bearing. These bearings have a lead core that provides an initial high stiffness for service loads. During large earthquakes the lead core yields, providing damping for the structure. The bearings used on the Long Beach VA Hospital will allow the ground to move 16 in. without impacting the building.

Isolation and damping devices can also be used on other structures. The All American Canal Bridge in California is one of the many bridges that uses lead-rubber bearings to isolate the superstructure from earthquake ground motions (Figure 18.86).

The Vincent Thomas Suspension Bridge (a mile from the Long Beach VA Hospital) was retrofit with 80 FVDs to absorb energy and prevent the bridge deck from pounding against the towers and cable bents (Figure 18.87). The connection between the towers and the truss was modified to allow very large relative movements. The truss section on each side of the towers was replaced with a new unit that includes a deck section with 26-ft-long finger joints and large viscous dampers to absorb energy and prevent the truss from pounding against the towers (Figure 18.88).

In general, isolation and damping devices have performed very well during large earthquakes. However, isolated structures have usually been too far away to experience really large accelerations. The one occurrence where isolation and damping devices were very close to a fault rupture was on the Bolu

FIGURE 18.84 The existing Long Beach VA Hospital was seismically retrofit with lead-rubber bearings. (Photo courtesy of Dynamic Isolation Systems, Inc.)
FIGURE 18.85 The concrete columns were cut at midheight and lead-rubber isolation devices were installed. (Photo courtesy of Dynamic Isolation Systems Inc.)
FIGURE 18.86 Lead-rubber isolation bearings on the All American Canal Bridge in California.
FIGURE 18.87 The Vincent Thomas Bridge is a three-span suspension structure built over the port of Los Angeles. Copyright 2005 by CRC Press

Viaduct during the 1999 Duzce Earthquake. However, in this installation the bearings were eccentric to the dampers, resulting in out-of-plane motions that locked up the dampers and caused significant damage to the bridge. It will probably take several earthquakes to work out all the bugs and come up with installations that work most effectively. Because of the increased use of isolation and damping devices on structures in highly seismic areas, there should be many more opportunities to study their behavior.

When isolation and damping devices are not used, a sacrificial element will often limit the force and increase the damping in a structure. New RC beams and columns are provided with welded hoops and spirals that allow these members to form plastic hinges during large earthquakes. Existing concrete members are sometimes wrapped in steel casings, fiberglass, carbon fiber, and many other materials to increase their shear capacity and allow for the formation of a plastic hinge (Figure 18.89).

Steel structures have undergone similar improvements. Beginning in the late 1970s eccentrically braced frames (EBFs) were developed that provide greater stiffness and ductility during earthquakes. The EBF has a ductile link between the connections that is specially designed to act as an energy dissipater. This concept has been expanded to include a variety of different configurations (Figure 18.90).

The poor performance at the connections of moment-resisting frames during the Northridge and Kobe Earthquakes has resulted in a great deal of research and testing. There are now a variety of welded beam-column joints that ensure ductile behavior in the members rather than brittle fracture of the joint. One popular new connection is the "dog bone'' (Figure 18.91). Testing has shown that plastic hinging will occur at the reduced section, protecting the connection.

There are a number of other devices and structural elements that improve structural response during large earthquakes. Restrainers, shear keys, catchers, and seat extenders are used to prevent bridge superstructures from falling from their supports. A variety of materials are wrapped around weak columns to increase their ductility. However, isolation and damping devices show the greatest potential

FIGURE 18.89 Steel shell being wrapped around a concrete bridge column to increase its ductility and shear strength.
FIGURE 18.90 Two of the many configurations that have been developed for eccentrically braced connections.

to control the amount of damage on a structure, particularly when the goal is to keep the structure in service after a very large earthquake.

By ensuring that the supporting soil remains undamaged, by avoiding sites near active faults or other secondary hazards, and by the effective use of isolation and damping devices, most serious earthquake damage can be avoided. However, every structure should also be provided with abundant ductility and

Drilled constant Drilled tapered

FIGURE 18.91 Examples of beams in moment-resisting frames that use a reduced flange section to prevent fracturing of the connection.

Drilled constant Drilled tapered

FIGURE 18.91 Examples of beams in moment-resisting frames that use a reduced flange section to prevent fracturing of the connection.

large seats in case an expectedly large earthquake were to occur. Moreover, all structures must be carefully designed to be relatively uniform and without eccentric loads.

References

[1] EERI, Scenario for a magnitude 7.0 earthquake on the Hayward fault, Earthquake Engineering Research Institute, HF-96, The Institute, Oakland, CA, 1996, 109 pages.

[2] Mualchin, L. and Jones, A.L., Peak acceleration from maximum credible earthquakes in California (rock and stiff-soil sites), DMG Open-file Report, 92-1, California Division of Mines and Geology, Sacramento, CA, 1992, 53 pages.

[3] National Research Council (U.S.) Committee on the Alaska Earthquake, The great Alaska earthquake of 1964, Publication (National Research Council [U.S.]) no. 1603, National Academy of Sciences, Washington, DC, 1968-1973, 8 v. in 10.

[4] Housner, G., et al., Competing against time, Report to the Governor George Deukmejian from The Governors' Board of Inquiry on the 1989 Loma Prieta earthquake, State of California Office of General Services, May 1990.

[5] EERI, The Mexico earthquake of September 19, 1985 — on the seismic response of the Valley of Mexico, Earthquake Spectra, 4, 3, August 1988, pages 569-589.

[6] USGS, The Loma Prieta, California Earthquake of October 17, 1989, United States Government Printing Office, 1998.

[7] NIST, The January 17, 1995 Hyogoken-Nanbu (Kobe) earthquake: performance of structures, lifelines, and fire protection systems, NIST Special Publication 901 (ICSSC TR18), U.S. National Institute of Standards and Technology, Gaithersburg, MD, July 1996, 538 pages.

[8] Japan Society of Civil Engineers (JSCE), Preliminary report on the Great Hanshin earthquake, January 17, 1995, Japan Society of Civil Engineers, 1995.

[9] EQE, Steel's performance in the Northridge earthquake, EQE Int. Rev., Fall 1994, pages 1-6.

[10] Mitchell, J.K., Baxter, C.D.P., and Munson, T.C., Performance of improved ground during earthquakes, Soil Improvement for Earthquake Hazard Mitigation, American Society of Civil Engineers, New York, 1995, pages 1-36.

[11] Zayas, V.A. Low, S.S., and Mahin, S.A., The FPS earthquake resisting system: experimental report, UCB/EERC-87/01, Earthquake Engineering Research Center, University of California, Berkeley, CA, June 1987, 98 pages.

Ronald O. Hamburger

Simpson Gumpertz & Heger, Inc., San Francisco, CA

Charles Scawthorn

Department of Urban Management, Kyoto University, Kyoto, Japan

0 0

Post a comment