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Chapter 3 ISOLATOR DEVICES AND SYSTEMS
3.7 FURTHER ISOLATOR COMPONENTS AND SYSTEMS
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The development of practical linear hydraulic dampers is complicated by a number of factors including the increase in silicone liquid volume with temperature, about 10% for a 100oC temperature rise, and also the tendency of the silicone liquid to cavitate under negative pressure.
3.7.2 PTFE Sliding Bearings Non Lubricated PTFE Bearings
The weight of a structure may be supported on horizontally moving bearings consisting of blocks of PTFE (polytetrafluoroethylene) sliding on plane horizontal stainless-steel plates. Starting about 1965, such bearings were used to provide low-friction supports for parts of many bridge superstructures. The coefficient of friction of a PTFE bridge bearing is typically of the order of 0.03, when operating at the very low rates arising from temperature cycling of the bridge superstructure. However, it is found that the coefficient of friction is very much higher, and is dependent on pressure and sliding velocity, when the operating velocity is typical of that which occurs in an isolator during a design-level earthquake, and when the operating pressure is typical of that adopted for PTFE bridge bearings (Tyler, 1977). For operating conditions typical of seismic isolator actions during design-level earthquakes, the frictional coefficients ranged from about 0.10 to 0.15 or more.
Consider a set of the above PTFE bearings used as a seismic isolator. The first isolator period Tb1
arises from foundation flexibility only, and is typically very short. The second isolator period Tb2
tends to infinity and therefore provides no centring force to resist displacement drift. The yield ratio Qy/W is given by the bearing coefficient of friction and is therefore rather large and variable. The approximately-rectangular force-displacement loop gives very high hysteretic damping. However, absence of a centring force may result in large displacement drift if seismic inertia forces are substantially greater than the bearing frictional forces. Also high initial stiffness leads energy into higher modes, providing strong floor spectra of high frequencies.
An isolator with a wider range of applications is obtained if part of the weight of the structure rests on PTFE bearings, while the remainder of the weight rests on rubber bearings. The reduced sliding weight reduces the yield ratio Qy/W, while the rubber bearings can be used to give an appropriate value for the centring force, as indicated by the second isolator period Tb2, which should usually be in the range between 2.0 and 4.0 seconds.
Problems arising from a very short first period Tb1 may be removed by mounting the PTFE bearings on rubber bearings, as described below.
Lubricated PTFE Bearings
Lubricated PTFE bearings have quite small coefficients of friction, usually less than 0.02 (Tyler, 1977), for the pressures and velocities which they would encounter as seismic isolator mounts.
When an isolator has low-friction load-support bearings, then components to provide centring and damping forces need not support weights. For example, approximately linear centring and damping forces could be provided by blocks of high-loss elastomer, for which creep is not a problem without sustained loads. If higher linear damping is required, hydraulic dampers could be added. However, since almost every isolator application is tolerant of at least a moderate degree of non-linearity, it should usually be possible to provide some of the centring and damping forces by non-linear components, such as weight-supporting lead rubber bearings.
For high reliability, lubricated PTFE bearings should be serviced regularly. However, for high-technology applications, for example nuclear power plant isolation, maintenance should not present a serious problem.
3.7.3 PTFE Bearings Mounted on Rubber Bearings
In Chapter 2 it was found that a bilinear isolator with a short first period Tb1 results in relatively large higher-mode seismic accelerations and floor spectra. In Chapter 4 it is shown that these higher-mode seismic responses may be substantially reduced by increasing the first bilinear period Tb1 to exceed the first period of the unisolated structure T1(U).
A compound isolator component developed in France (Plichon, et al, 1980) consisted of a sliding bearing mounted on top of a rubber bearing. Initially the bearings were made of lead-bronze blocks sliding on stainless steel, while later designs replaced the lead-bronze blocks by PTFE blocks.
The flexibility of the laminated rubber components of the compound bearing can be chosen to give a first bilinear period Tb1 which exceeds T1(U), the first structural period. As in the previous section, the second bilinear period Tb2 may be limited to a value which prevents excessive displacement drift by supporting part of the structural weight directly on rubber bearings. This also reduces the value of Qy/W for the isolator.
3.7.4 Tall Slender Structures Rocking with Uplift
The seismic design loads and deformations of tall slender structures are normally associated with high overturning moments at the base level. If the narrow base of such a structure is allowed to rock with uplift, then the base moment is limited to that required to produce uplift against the restraining forces due to gravity. This base moment limitation will usually reduce substantially the seismic loads and deformations throughout the structure.
The feet of a stepping structure are supported by pads which allow some rotation of the weight- supporting feet, while the overall structure rocks with uplift of other feet. Laminated rubber or lead slabs have been used to allow this rotation. These feet pads also accommodate small irregularities and slope mismatches between the feet and the supporting foundations. The stepping feet move in vertical guides which prevent 'walking', which would give horizontal or rotational displacements of the base of the structure.
Rocking with stepping is particularly effective in reducing the seismic loads and deformations of top-heavy slender structures such as tower-supported water tanks (where the tanks should be slender or contain baffles to prevent large long-period sloshing forces during major earthquakes). Another top-heavy structure is a bridge with tall slender piers. The piers may be permitted to rock in a direction transverse to the axis of the superstructure, providing the superstructure can accommodate the resulting deformations.
The seismic responses of a slender rocking structure are related in some ways to the responses of a structure with an approximately rigid-plastic horizontally-deforming isolator, but there are also major differences.
For mode-1 seismic responses a rigid rocking structure may be assumed, with forces and displacements expressed as horizontal actions at the height of the centre of gravity. The cyclic force-displacement curve is then almost vertical for all forces below the uplift force (which corresponds to Qy with bilinear hysteresis) and almost horizontal for all displacements during uplift. The force-displacement curve is essentially bilinear elastic. An effective period may be derived using the secant stiffness for maximum seismic displacement. The effective damping will arise from any energy losses during structural and foundation deformations together with the contribution of any added dampers. The effective period and damping may then be used to relate the maximum seismic displacement to the earthquake displacement spectra, as in the case of any other non-linear isolator.
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Since stepping isolation is a very non-linear constraint, and since the equivalent first isolator period Tb, is substantially less than the first period of the unisolated structure, the maximum seismic acceleration responses of the higher isolated modes are expected to be relatively large.
With stepping the higher mode periods and shapes may be derived by assuming a zero base moment, instead of the zero base shear force assumed when the isolator acts horizontally.
With rocking isolation there is always a substantial centring force, which is given by the uplift force. This centring force ensures that there is little drift displacement to add to the spectral displacement. The substantial centring force, and the high first stiffness, of the rocking isolator also ensure that there is very little residual displacement after an earthquake, even when substantial hysteretic dampers have been introduced.
An early application of rocking with uplift, to increase the seismic resistance of a tall slender structure, is contained in a design study by Savage (1939). The 105 meter piers of the proposed Pit River road-rail bridge were designed with their bases free to rock with uplift under severe along-stream seismic loads. A New Zealand railway bridge at Mangaweka, over the Rangitikei River, with 69 meter piers, was designed and built with the pier feet free to uplift during severe along-stream seismic loads (see Chapter 8). A tall rocking chimney structure, built at Christchurch New Zealand, is described by Sharpe & Skinner (1983).
3.7.5 Further Components for Isolator Flexibility Tall Columns and Free Piles
Horizontal flexibility can be provided by tall first-storey columns or by free-standing piles. Such flexible columns must have adequate length to avoid Euler instability under combined gravity earthquake loads, while providing adequate horizontal flexibility. With tall columns, the end moments may be severe despite relatively low horizontal shears.
With deep free-standing piles it is usually convenient to provide dampers and stops or buffers at the pile tops since it is usually practical to anchor them at this level. This approach has been used in Union House, Auckland, which uses steel cantilever dampers, and the Wellington Central Police Station, which uses lead-extrusion dampers (see Chapter 8). If tall columns are used to isolate a tower block it would be possible to anchor dampers to a surrounding high stiffness high- strength mezzanine structure.
In both the above cases where isolation was provided by tall free-standing piles, the tall piles were required to support the structure on a high-strength soil which underlay a low-strength soil layer. The tall piles were made free-standing by surrounding them with clearance tubes.
Basement boxes, supported on shorter piles and embedded in the surface layer, were used to provide anchors for the hysteretic dampers and the buffers.
Hanging Links and Cables
It is possible to provide horizontal flexibility by supporting a structure with hanging hinged links or with hanging flexible cables (Newmark & Rosenblueth, 1971). Effective pendulum lengths of 1.0 and 2.25 meters would give isolator periods of 2.0 and 3.0 seconds respectively. The necessary overlap of the supports and the structure can certainly be provided but in most cases this would be somewhat inconvenient and probably expensive, particularly for the longer links required for the longer isolator periods.
When isolation is required for a relatively small item within a structure it would sometimes be appropriate to suspend it from anchors at a higher structural level.
Rollers, Balls and Rockers
An object can be supported on rollers or balls, between hardened steel surfaces, to provide a very low resistance to horizontal displacement. Again the object may be supported on rockers with rolling contact on plane or curved upper and lower surfaces, with the curvatures of the 4 contacting surfaces chosen to give a gravity centring action.
While simple in principle, the use of hard rolling surfaces to provide horizontally flexible isolator supports presents practical problems. These may include load sharing between the rolling components and the low load capacity of rolling units, particularly when only parts of the contacting surfaces are worked during the intervals between substantial earthquakes. It is therefore likely that rolling supports will normally be restricted to the isolation of special components of low or moderate weight.
3.7.6 Buffers to Reduce the Maximum Isolator Displacement Isolator Maximum Displacement
Isolators are normally designed to accommodate a travel greater than that which would occur during design earthquakes. However, during extreme low-probability earthquakes there is a possibility that the base of the structure will arrive at the end of the isolator design displacement when the structure still has considerable kinetic energy. If a stiff structure encounters a rigid base stop with considerable kinetic energy the ductility demand on the structure may be high, and may even substantially exceed the structure's design deformation capacity. The use of a resilient or energy-absorbing buffer can considerably increase the acceptable base impact velocity.
There are two components of structural shear strain when its base impacts a stiff buffer. One is a transient shear pulse which travels up the structure, with attenuation, and is reflected successively at the top and base. This transient shear pulse can be attenuated substantially by having a buffer stiffness which is substantially less than the inter-storey stiffness. The other component is an overall shear deformation, which can be substantially reduced by having a buffer stiffness lower than the overall structural stiffness. This is not practical in all cases.
During a low-probability extreme earthquake it is acceptable to permit much greater damage than is accepted for design-level earthquakes. The principal requirement is to prevent casualties and particularly to avoid the extreme hazard of structural collapse.
Typically a seismic gap and buffer system should be designed to ensure that a structure does not collapse for a base displacement which would be from 50% to 100% greater (in the absence of a buffer), than that provided to accommodate design-level earthquakes.
Omni Directional Buffers using Rubber in Shear
Consider a structure mounted on laminated rubber bearings which have a maximum horizontal rubber shear strain of 100% under design earthquakes. Under earthquakes of twice this severity the bearings would deform to a strain of approximately 200%, and store 4 times the elastic energy. Suppose that the earthquake energy is not reduced by the presence of buffers (in fact it is likely to be reduced by 20% or 30%). The energy to be stored or absorbed in the buffers is three times that stored in the bearings on buffer impact. If stiff rubber shear buffers are used they will be required to store almost 3 times the energy in the bearings. For a shear strain of 3 in the rubber buffers the energy density is 9 times that of the bearings and hence the rubber volume required for the buffers is a third of that in the bearings.
The stiffness of the buffers may be based on the maximum base shear acceptable for the
88 Omni Directional Buffers using Tapered Steel Beams
Steel-beam buffers can be made omni directional in the same way as rubber buffers can. They may be designed to yield at a level which limits the base shear on the structure to an acceptable level. They may be of lower cost but more costly to install than equivalent- capacity rubber buffers. Operationally they are superior because of their yield-limited resistance force and because of the capacity to absorb most of the energy put into them.
Buffer Anchors
For many structures it will be difficult to provide buffer anchors of the desired strength. If the buffer anchors deform in a controlled way with an appropriate level of resistance, they may themselves function as buffers and greatly reduce the demands on a buffer device or even remove the need for added buffers.
The basement box which provides stops for base displacement of the New Zealand Central Police Station has a level of soil and of pile resistance which allows it to provide considerable buffer action. Because the basement box is comparable in mass to a building storey, it is necessary to have a base-to-basement deformable interaction which has lower stiffness than the interstorey members, to attenuate impact shear pulses. Such a deformable interaction is provided by lead collars, around the columns near their tops, which may impact basement stops during extreme earthquakes.
3.7.7 Active Isolation Systems Active Control of Isolator Parameters
When it is necessary to control the floor accelerations accurately during frequent moderate earthquakes, it should be possible to exercise a large measure of control over isolator parameters by including a set of double-acting hydraulic dampers with their coefficient of velocity damping force under the direct control of electrical signals, which are a function of the measured floor accelerations and base displacements. In the event of control system failure or for large earthquakes, the isolator should revert to an essentially passive system, effective for severe earthquakes.
It should be possible to check on the performance of this system by applying an artificial floor acceleration signal or by monitoring the response of the isolator by measuring its effect on ground micro tremors.
Active Forces on Isolated Structures
Where a very low level of vibration is important it would be possible, in principle, to use an active system to ensure a very low level of building horizontal vibrations. For example, the building could be supported on lubricated PTFE mounts and the active drive would only have to provide the low frictional losses in the mounts. In practice some additional power would be necessary to provide some centring action. The displacements required to accommodate such an isolator would be large during a major earthquake. The system would be most practical if it was only required to provide a high degree of isolation during frequent moderate earthquakes.
To resist wind loads it would be necessary to provide some clamping system whenever wind loads exceed the force capacity of the isolator actuators. Alternatively the structure could be enclosed by wind shields, in special cases.
A more practical system for many applications is likely to be a linear isolator which provides sufficient attenuation of the first mode(s) and an active system to further attenuate some of the small higher-mode responses, if necessary.