Dampers Do the Job At Davis
Written by Brad Friedrerichs
Civil Engineering Magazine
September 1997
"One of the most jarring engineering lessons learned in recent earthquakes is that braced steelframes do not perform that well when the earth shakes violently. Friction dampers could be one cost-effective way to dispel disaster"
In Japan's Kobe earthquake (7.2 on the Richter scale) over 5,000 people were killed, 30,000 were injured and 300,000 were left homeless. The total cost of the damage was pegged at more than $150 billion. In the U S " 63 - people died in the Loma Prieta earthquake (7.1 Richter scale), which shook San Francisco. The Northridge earthquake in Los Angeles, which had a magnitude of 6.7, left 57 dead and caused some $20 billion in damage. In the aftermath of these devastating earthquakes, policy makers gave retrofitting a higher priority on their agendas and engineers continued searching for new and better solutions with a new sense of urgency.
The generally accepted engineering response has been to use moment-resisting frames. Though relatively stable under lower seismic forces, these are much more Susceptible to damage than previously thought, as demonstrated by the Northridge quake. Yet emerging seismic-resistant technology could offer greater protection against the age-old threat of earthquakes at huge cost savings.
FRICTION DAMPER SOLUTION
0ne such innovation is the friction damper. The benefit of friction dampers lies in their ingenious simplicity -they absorb energy. As friction is generally considered the best protction against damage to steel structures ' relatively few friction dampers are needed for substantial protection.
Their performance is not affected by temperature, velocity or structural stiffness caused by aging. Unlike other seismic devices, they are maintenance free and do not need repair or replacement after earthquakes.
The friction damper was invented in the late 1970s by Avtar Pall, an expert in structural design. Pall, founder of Montreal based Pall Dynamics was inspired by the friction brake in automobiles and by his belief that the same forces were at work (bodies in motion) in both the stopping of car and the shaking of the earth.
Just as a vehicle's friction brake extracts kinetic energy from a moving vehicle, the motion of a vibrating building can be controlled by dissipating energy in friction. Basically, friction dampers consist of a series of steel plates that are specially treated to develop the greatest amount of friction. These plates are clamped together with high-strength steel bolts and are then allowed to slip under predetermined loading.
During earthquakes, friction dampers slip at a predetermined load before any members of the structural frame reach their breaking or damage points. This allows for the dissipation of a major portion of the earthquake's energy. In effect, the building remains elastic and is able to withstand the potentially catastrophic seismic force.
The key is gauging the optimum slip load. By selecting the proper slip load, it is possible to tune the response of the structure to the seismic forces. Parametric dynamic studies have shown that the optimum slip load is independent of the time history of the earthquake motion and is actually a structural property.
The modeling of friction dampers is rather simple. Since the hysteretic loop of the friction damper is similar to the rectangular loop of a perfectly elastoplastic material, the slip load may be considered an artificial yield force. An elastic brace with a friction damper can be regarded as a brace yielding at slip load. After years of research, Pall developed several types of friction dampers suitable for use in new construction or seismic upgrades to existing structures.
In 1987, a nine-story frame structure was proof tested at the Earthquake Engineering Research Center at the University of California, Berkeley. The seismic force, estimated at five times that of the devastating 1985 Mexico City earthquake, which measured 8.1 on the Richter scale, did not cause any damage to the test structure. All members of the frame remained elastic (intact) for the 0.84g acceleration-the maximum capacity of the "shake table," which corresponds to the force of a large earthquake. On the other hand, the moment-resisting frame would have failed at about 0.3g acceleration.
Another test, carried out in 1985, involved a three-story frame on the shake table at the University of British Columbia. Again, the performance of the friction dampers was superior to that of the moment-resisting frame. Even at an earthquake force of 0.9g, no damage was caused to the friction damper frames, while the other frames suffered enormous damage. In 1987, friction dampers were first used on a building in the seismic retrofitting of Friction dampers will reposition themselves without permanent displacement.
DAMPERS AT DAVIS
Friction dampers are currently being used as part of the seismic upgrade of three elevated water towers located on the campus of the University of California, Davis. The Davis project marks the first use of the friction damper in California, according to manufacturer Pall Dynamics. Since its introduction in 1987, the friction damper device has been used in 20 projects worldwide.
Our firm's friction damper project at Davis may chart a new course in seismic retrofitting, at least in earthquake-stressed California, where there is a large market for the friction damper system. Loma Prieta demonstrated dramatically that a prior investment of only a few percent of the $10 billion the quake cost could have saved lives, prevented countless injuries and reduced the staggering economic losses.
Historically, braced steel frames have been used in buildings and other aboveground structures. These are effective at protecting the structure from high winds or even moderate earthquakes, but they perform poorly during major earthquakes, when larger amounts of energy are fed into a structure. In these cases, braced frames are especially challenged, as they are stiffer and thereby invite higher lateral inertial forces that are harder to dissipate.
The structure is called on to dissipate energy while undergoing small, inelastic deformations.
Other seismic retrofit solutions have included concrete shear walls and concentric and eccentric bracing. In some cases, these retrofits require the complete tearing down and rebuilding of a structure. The resulting conventional retrofitting is often too costly and time consuming.
Officials at the University of California Davis campus had these concerns in mind when the water towers providing domestic water supply to the campus area were found unfit to resist the required seismic loading. Designed in the late 1950s and the 1960s, they range in height from 120 to 132 ft.
We conducted a preliminary study on the Davis project which indicated that friction dampers would reduce the lateral force of an earthquake by 60%. Also, by adding the dampers, none of the columns or foundations of the towers would have to be strengthened, amounting to a substantial cost savings.
DAVIS DAMPERS DEPLOYED
0fficials at the Davis campus approved the use of friction damper technology, and in March 1996 crews started installing 48 friction dampers on the three water towers there-one 200,000 gal. tank and two 100,000 gal. tanks. To date, one 100,000 gal. tank is completed, and the other two tanks are expected to be finished by the end of the year.
Each water tower project involves a fiveman crew and about two or three months' project time. The chronological series of steps includes lead abatement; installation of temporary bracing; removal of existing bracing; welding new bracing; connecting the friction dampers to the braces; removing temporary bracing; and, finally, tensioning all members of the frame (with a full tank of water above). The braces on the Davis towers are considered "long" at between 40 and 60 ft, requiring a special tension-brace friction damper developed by Pall Dynamics.
On each water tower, all of the steel braces receive a sufficient amount of pretensioning to keep the braces taut. Braces are connected to the gussets and friction dampers with 1-in.-diameter ASTM A325 friction-type bolts. All gusset plates are 1 in. thick. Contact surfaces and bolted connections are coated with an inorganic zinc paint.
We took the additional safeguard of designing the friction dampers to be removed and adjusted if required. In the worst-case scenario, if an earthquake were to displace the tower but the tower did not revert to its original position, then the friction dampers could actually be removed and taken to a machine shop for adjustment. The project engineer also designed a mechanism to assist the towers in reverting to their initial location. Both measures were extreme precautions. All evidence to (late shows that friction dampers effectively reposition themselves without any incidence of permanent displacement.
The friction dampers are modeled as nonlinear elements equivalent to tension braces with energy dissipation. The dampers occur in each diagonal brace of the water towers. As in any engineering project, each individual piece of technology-in this case, the friction damper must be solidly constructed and free of defects.
Before being shipped by the manufacturer the friction dampers are subjected to load testing, with 10% of the lot randomly selected and tested by an independent professional engineer.
We consider the Davis project an example of successful value engineering. The University of California saved approximately $245,000 on a total project cost of about $950,000 (about 26% of the total project cost) by using friction dampers instead of conventional bracing.
