FLUID VISCOUS DAMPERS USED FOR SEISMIC ENERGY DISSIPATION IN STRUCTURES

Transcription

1 FLUID VISCOUS DAMPERS USED FOR SEISMIC ENERGY DISSIPATION IN STRUCTURES Douglas Taylor 1, Philippe Duflot 2 1 Taylor Devices Inc, 90 Taylor drive, North Tonawanda, NY , USA 2 Taylor Devices Europe, 18 rue J-B Vannypen, Brussels 1160, Belgium ABSTRACT The new political climate at the end of the Cold War in 1990 caused a restructuring period for the American defense industry. One of the results was that previously restricted military technologies became available to the general public. Highly advanced products and services suddenly appeared in the marketplace, seemingly out of nowhere. Perhaps the best known of these is the now ubiquitous Internet, which in reality came from 1970's defense technology. In the civil engineering field, high capacity fluid viscous dampers, used extensively in military applications, have found commercial applications on buildings and bridges subjected to seismic and/or wind storm inputs. Because fluid damping technology was proven thoroughly reliable and robust through decades of Cold War usage, implementation on commercial structures has taken place very quickly. This presentation provides a brief overview of fluid damping technology with specific case studies being provided from four of more than eighty major buildings and bridges now equipped with fluid viscous dampers. Keywords: Fluid viscous damper; damping technology; energy dissipation INTRODUCTION TO FLUID VISCOUS DAMPER TECHNOLOGY Damping is one of many different methods that have been proposed for allowing a structure to achieve optimal performance when it is subjected to seismic, wind storm, blast or other types of transient shock and vibration disturbances. Conventional approach would dictate that the structure must inherently attenuate or dissipate the effects of transient inputs through a combination of strength, flexibility, and deformability. The level of damping in a conventional elastic structure is very low, and hence the amount of energy dissipated during transient disturbances is also very low. During strong motions, such as earthquakes, conventional structures usually deform well beyond their elastic limits, and eventually fail or collapse. Therefore, most of the energy dissipated is absorbed by the structure itself through localized damage as it fails. The concept of supplemental dampers added to a structure assumes that much of the energy

2 input to the structure from a transient will be absorbed, not by the structure itself, but rather by supplemental damping elements. An idealized damper would be of a form such that the force being produced by the damper is of such a magnitude and function that the damper forces do not increase overall stress in the structure. Properly implemented, an ideal damper should be able to simultaneously reduce both stress and deflection in the structure. Although the concept of large added-on fluid viscous dampers seemed viable, the normal development cycle time to thoroughly test and implement the technology could be expected to take many years. This is especially true when taking into consideration the traditionally long approval cycles for incorporation of new technology into the building and bridge codes. When the Cold War ended in 1990, it was revealed that several branches of the U.S. military had been using large fluid viscous dampers for more than 35 years as part of their strategic defense programs. More importantly, the manufacturers of these formerly classified components were interested in transitioning the technology for civilian use. The question was however : Can a fluid viscous damper, designed to protect missile silos and ships against nuclear explosions, be used cost-effectively for earthquake protection? If so, then the technology could be quickly implemented, since the products were already in production. For example, Figure 1 is a declassified photo, circa 1961, of a test on a 200 tonnes force fluid viscous damper built for the U.S. Air Force. The use of the damper was on the base isolation system for the Atlas Ballistic Missile, based within an underground launch silo. Dampers of this type were produced in many different forces and deflections for use by the U.S. Army, Navy and Air Force. In this context, demonstration of the benefits of damping technology on civilian structures could take place immediately, using existing dampers and the seismic test facilities already available at U.S. university research centers. Figure 1: Test for 200 tonnes damper for US Air Force (1961)

3 The application of fluid viscous dampers as part of seismic energy dissipation systems was experimentally and analytically studied. Fluid damping technology was validated by extensive testing on one-sixth to one-half scale building and bridge models in the period Testing took place at the Multidisciplinary Center for Earthquake Engineering Research (MCEER), located on the campus of the State University of New York at Buffalo in USA. The damper design subjected to test had seen extensive military use on diversified military projects, including the Atlas, Peacekeeper (MX) and Tomahawk Missiles, and the B-2 Stealth Bomber. The effects of added supplemental fluid damping in a structure subjected to earthquake transients is depicted in the test results provided in Figures 2 and 3. The test results used the 1940 El Centro earthquake transient as a test input. The structure was a single story, steel building frame, using steel moment frame connections. Figure 2 shows the response of the test structure under a scaled input of 33% of the 1940 El Centro earthquake. Note the small hysteresis loop revealing that the test structure was at the onset of yield. Structural damping in the frame was in the 1½ 2% range. In comparison, Figure 3 is the same structure with 20% added damping, obtained by the addition of two small linear fluid viscous dampers installed as diagonal brace elements. The large energy dissipation of added damping is readily apparent in the oval damping curve superimposed over the structural spring rate curve. Note also that the input in Figure 3 is the full 100% El Centro earthquake and yet base shear and deflection of the frame are virtually unchanged from the undamped case of Figure 2. Thus, the addition, in this case, of 20% added linear damping to the structure increased its earthquake resistance by a factor of 3, compared to that of the same structure without added damping. What it is however most important to note is that this threefold performance improvement was obtained without increasing the stress or deflection in the structure. In fact, it is this tremendous performance improvement that has caused much of the interest in fluid viscous dampers for structural engineering use. Figure 2 : One-story structure, no dampers El Centro 33,3%

4 Figure 3 : One-story structure, two dampers El Centro 100% When test reports were first published by the U.S. National Earthquake Research Center at Buffalo, they included tests showing similar performance gains with all other notable earthquakes for which transient records were available. Full scale implementation began shortly after this first report was published. Additional testing revealed that fluid viscous dampers were also well suited to improve the performance of structures subjected to high velocity wind storms. Of course, since these products have been used previously by the U.S. military to attenuate weapons detonations, the ability of fluid viscous dampers to improve structural resistance to blast was already well understood. DESCRIPTION OF THE FLUID VISCOUS DAMPERS Fluid viscous dampers operate on the principle of fluid flow through orifices. A stainless steel piston travels through chambers that are filled with silicone oil. The silicone oil is inert, non flammable, non toxic and stable for extremely long periods of time. The pressure difference between the two chambers cause silicone oil to flow through an orifice in the piston head and seismic energy is transformed into heat, which dissipates into the atmosphere. The force/velocity relationship for this kind of damper can be characterized as F = C.V α where F is the output force, V the relative velocity across the damper, C is the damping coefficient and α is a constant exponent which is usually a value between 0,3 and 1,0. Fluid viscous dampers can operate over temperature fluctuations ranging from 40 C to +70 C. The orifice construction utilized is similar to that in the classified application for the U.S. Air Force B-2 Stealth Bomber and is considered state of the art [1]. Fluid viscous dampers have the unique ability to simultaneously reduce both stress and deflection within a structure subjected to a transient. This is because a fluid viscous damper varies its force only with velocity, which provides a response that is inherently out-of-phase with stresses due to flexing of the structure.

5 PROJECT EXAMPLES The tested performance of structures with fluid viscous dampers showed that tremendous gains in performance could be realized at relatively low cost. Thus, implementation of fluid damping technology began relatively swiftly, with wind protection usage beginning in 1993, and seismic protection usage beginning in Today, more than eighty major structures are using fluid viscous dampers to obtain enhanced performance during seismic or wind excitation. Four of these projects are described here, selected for their diversity. The Arrowhead Regional Medical Center at Colton, California This project was the first application for fluid viscous dampers in the seismic protection field. The five buildings of this complex use a total of 186 dampers, each being rated at 145 tonnes force. The dampers are used to dissipate seismic energy, and are installed in systems parallel with rubber base isolation bearings. The square meter medical center is located in San Bernardino County, between the cities of Ontario and San Bernardino. The location is within 8 km of the San Andreas Fault, and 10 km of an intersecting fault. The location of the medical center was determined by available Federal Government funding, provided with a requirement that the hospital complex be located between the two cities noted, and with a very easy access. The original design concept from the project s engineering team utilized only base isolation bearings, manufactured from so-called high damping rubber. However, the near fault location generated site transients in the 1,4 m/s range, requiring that the bearings provide deflection in excess of plus or minus 1,5 meters! No rubber base isolation bearing of this size had ever been fabricated. Of course, the large deflection also required that new technology be created with respect to accommodating seismic displacements at utility connections, parking ramps, access walkways, etc. In an attempt to reduce deflections, the project team evaluated the improvements from adding dampers in parallel with the base isolation bearings The design group investigated adding 37% fluid damping to the 8% damping expected from the rubber bearings, for a total damping level of 45% critical. Higher damping levels were found to cause decreased deflections, but higher stress, so the 45% damping level was considered as optimal for both stress and deflection. The use of the fluid viscous dampers allowed bearing displacement to be reduced to only plus or minus 0,6 meters, a reduction so substantial that the project goals could be accomplished economically. One of these goals was to produce the most seismic resistant civilian structure in the world, and this was achieved easily with fluid viscous dampers.. A fluid viscous damper with nonlinear damping of F = CV 0,4 was selected for use in the project. An additional advantage of the fluid viscous dampers was that the long period rubber bearings and fluid viscous dampers would be able to fully reset the building after a seismic event with no permanent offset. A photograph of a typical damper at this facility is provided as Figure 4. Each damper is 4,3 meters in length and weighs nearly 1500 kg. The San Francisco Civic Center Office Building Those who are familiar with seismic designs in San Francisco will agree that this historic city is literally the home of the braced steel seismic resistant frame. When the 1994 Northridge, California earthquake revealed problems with steel moment frames, one would normally anticipate the desire for braced frames to become even stronger. One can then imagine the surprise within the structural engineering community during 1996, when erection began on the 14-story San Francisco Civic Center office building. This square meter structure

6 combined 292 fluid viscous dampers with the so-called post-northridge moment frame to optimize performance while maintaining a cost-effective project budget. Two different damper force levels were used by the engineer, of 100 tonnes and 55 tonnes force. All dampers were plus or minus 100 mm stroke. The dampers were used in diagonal brace elements, with a bolted flange connection to attach dampers to their brace extenders. The piston rod end of the damper incorporated a clevis with spherical bearing, as did the opposite end of the brace extenders. The dampers were supplied with building attachment clevises, which consisted of simple tang plates that were bolted to a gusset plate at the building s beam to column connections. Low exponent damping in the V 0,4 range was selected, combined with a building frame that can provide extensive inelastic deformation. The use of low exponent damping tended to limit damper forces if the frame was loaded into the inelastic range by an earthquake greater than magnitude 8. A photograph of the building during construction is provided as Figure 5, the dampers being in the diagonal brace elements. The Hotel Woodland Woodland, California This historic 4-story vintage structure is in the U.S. Federal Registry of National Historic Landmarks. The building is constructed from non-ductile reinforced concrete (RC), consisting of an RC frame at the ground level, and RC shear walls at levels 1, 2 and 3. This type of structure is generally identified as having a soft first story. The owner wished to convert the building into office space, and elected to upgrade the seismic resistance. A simplistic approach would be to add external shear walls, but of course this would essentially eliminate the historic exterior of the building. An extensive structural analysis was performed, looking at potential improvement areas. The most promising solution was to add fluid viscous dampers to the first story of the structure. To accomplish this, steel moment frames were added to the first story to provide stiffness and strength, which the existing lightly reinforced concrete columns did not have. Chevron bracing was added to install the dampers, allowing damper loads to be limited to the horizontal plane only, the dampers being mounted parallel to the floors. A total of sixteen dampers was used, with two dampers in each chevron brace. Each damper was rated at 45 tonnes output force, with available displacement of plus or minus 50 mm. Figure 6 shows the damper installation. The Pacific Northwest Baseball Stadium-Seattle, Washington This major league baseball park opened during the 1999 season and features a three-section retractable roof of steel truss construction. When fully extended, the roof measures 192m x 200m, is 64m in height and weighs tonnes. Potential inputs to the roof include Zone 4 seismic transients and high winds. Added fluid damping was selected at an early stage of the design process to reduce lateral seismic and wind loads to the roof. The reduced loading from the damped structure reportedly provided a net savings of $4,2 million on the project. Additional dampers were added between roof sections to eliminate the potential for longitudinal pounding damage. The latter application proved mundane, using virtually off-shelf dampers in the 900 kn-1800 kn range. The use of dampers in the lateral direction proved much more difficult, since the only available mounting point was to use large dampers in diagonal braces between the column and roof trusses. The dampers were located relatively close to the intersection of these two structural elements and available mounting regions dictated that a total of only eight dampers could be used. These eight were required to reduce both stress and deflection in the tonnes roof, requiring that each damper be capable of kn output under maximum credible

7 earthquake conditions with plus or minus 380 mm deflection. The design was made much more difficult due to the 7 m required pin to pin length for the dampers, coupled with a restriction from the architect that would not permit a conventional flange connection between damper and the extender. The damper design was further restricted by the architect with a requirement that there be no more than a 25% diameter change at any point along the entire length of the device. CONCLUSIONS The use of fluid viscous dampers for seismic and wind protection of commercial and public structures began in the 1990's. Implementation has occurred rapidly, compared with other technologies. This is due largely to the widespread use of these products on Cold War era defense and military programs. When the Cold War ended, much of the fluid viscous damper technology was declassified and transitioned to the public for commercial use. These were proven through extensive testing and widespread use throughout the military and defense sector. When fluid viscous dampers are used for seismic or wind protection, the end result is a predictable reduction of both stress and deflection in the structure. Indeed, this simultaneous stress and deflection reduction is unique to fluid viscous dampers. Optimal performance is dependent on the type of structure and the level of performance required. Today, more than one hundred major buildings and bridges are using fluid viscous dampers as a primary seismic or windstorm design element. Damper sizes being used range from as little as 5 tonnes force to more than 900 tonnes force, with deflections as low as 5 mm and as high as 1,5 meters. Indeed, it can be said that the use of supplemental fluid viscous dampers will be one of the primary solutions for seismic and wind protection in the structures of the 21 st century, or in the more succinct form quoted by one California structural engineer : When in doubt damp it out. REFERENCES 1. Constantinou, M.C., Symans, M.D. Experimental and analytical investigation of seismic response of structures with supplemental fluid viscous dampers. In: Technical Report NCEER , National Center for Earthquake Engineering Research, Buffalo, New York, 1992.