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Chapter 3 ISOLATOR DEVICES AND SYSTEMS
3.6 LEAD RUBBER BEARINGS
3.6.1 Introduction
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It is normal practice to design bridge bearing installations so that negative pressures do not occur in the rubber under the combined action of non-seismic loads and motions. It is also appropriate to design isolated structures so that non-seismic actions do not cause negative pressures.
However, when seismic actions cause negative pressures in isolator mounts, their duration and frequency are so low that considerable negative pressures might be tolerated (Tyler, 1991). In general, an isolator design should be adopted which avoids very high negative pressures during seismic action. In the particular case of high uplift forces under the corner columns of two-way frame structures, high negative pressures in corner rubber bearings may be avoided by attaching the bearing tops to the bottom beams of the frames designed to allow corner uplift, for example as described by Huckelbridge (1977).
3.5.6 Other Factors in Rubber Bearing Design
In practice the application of laminated-rubber bearings to seismic isolation calls for sophisticated design and specialised manufacturing technology. The rubber must be formulated for long-term stability and resistance to environmental factors, particularly deterioration due to ozone and ultraviolet light. The bonds (vulcanising) between the rubber and the interleaved metal plates must resist the large and varying operating stresses. Bearings must be provided with end and side rubber cover to inhibit corrosion of the metal plates and to remove rubber-surface deterioration from regions of high operating strains. The rubber cover and additional surface materials may be used to increase fire resistance. Interleaved steel plates must have adequate strength to resist rubber shear forces. However, some plate bending may reduce the build-up of rubber tension when large displacements give high end moments. Bearing end-plates must provide for dowels or for other means of preventing end slip under high shear forces. Such shear connections must operate despite end moments and in some cases when uplift occurs.
The effect of a fire on the performance of rubber elastomeric bearings and lead rubber bearings has been checked by Miyazaki (1991) in Japan, by heating the outside of bearings to greater than 800oC for more than 100 minutes while carrying a vertical load.
After this heating the rubber elastomeric bearings and the lead rubber bearings performed in a satisfactory way without any appreciable change in their force-displacement loops or load bearing capacities.
3.5.7 Summary of Laminated Rubber Bearings
Laminated rubber bearings are already in use in bridges, in order to accommodate thermal expansion. Their modification for the seismic isolation of buildings and bridges is a fairly simple engineering concept, but in practice it requires sophisticated design and specialised manufacturing technology.
The lead rubber bearing was invented in April 1977 by W.H. Robinson when he saw a rubber elastomeric bearing while trying, with little success, to get a cylindrical lead shear damper to operate at large strains. The steel plates in the elastomeric bearing were immediately seen to present a solution to the problem of how to control the shape of the lead during large plastic deformation. A glued elastomeric bearing was drilled out to take a lead plug, as shown in Figure 3.16, and was tested immediately, and the results forwarded to the New Zealand Ministry of Works and Development (MWD). In the next few weeks, the MWD redesigned the isolators for the William Clayton Building (see Chapter 8), replacing the planned design (elastomeric bearings plus steel dampers) with lead rubber bearings, which were substantially less costly to install, and they provided a 650 mm diameter elastomeric bearing for testing with a range of lead plugs. At the same time the Bridge Section of the MWD designed the Toe Toe and Waiotukupuna bridges to take lead rubber bearings. Thus, during a very short and exciting time, lead rubber bearings were invented, tested and usedin practical applications.
Before describing the lead rubber bearing in detail, it is worthwhile considering the reasons for choosing lead as the material for the insert in the isolators.
The major reason is that the lead yields in shear at the relatively low stress of ~10 MPa, and behaves approximately as an elastic-plastic solid. Thus a reasonably sized insert of ~100 mm in diameter is required to produce the necessary plastic damping forces of ~100 kN for a typical 2 MN rubber bearing. Lead is also chosen because, as noted for the lead-extrusion damper, it is 'hot-worked' when plastically deformed at ambient temperature, and the mechanical properties of the lead are being continuously restored by the simultaneous interrelated processes of recovery, recrystallisation and grain growth (Wulff et al (1956); Birchenall (1959) and Van Vlack (1985)). In fact, deforming lead plastically at 20oC is equivalent to deforming iron or steel plastically at a temperature greater than 400oC. Therefore, lead has good fatigue properties during cycling at plastic strains (Robinson & Greenbank ( 1976). Another advantage of lead is that it is used in batteries, and so it is readily available at the high purity of 99.9 per cent required for its mechanical properties to be predictable.
An elastomeric bearing, as described in Section 3.5, is readily converted into a lead rubber bearing by placing a lead plug down its centre, Figure 3.16. The hole for the lead plug can be machined through the bearing after manufacture or, if numbers permit, the hole can be made in the steel plates and rubber sheets before they are joined together. The lead is then cast directly into the hole or machined into a plug before being pressed into the hole. For both methods of placing the lead, it is imperative that the lead plug is a tight fit in the hole and that it locks with the steel plates and extrudes a little into the layers of rubber. To ensure that this occurs, it is recommended that the lead plug volume be 1 per cent greater than the hole volume, enabling the lead plug to be firmly pressed into the hole. Thus, when the elastomeric bearing is deformed horizontally, the lead insert is forced by the interlocking steel plates to deform in shear throughout its whole volume.
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Figure 3.16: (a) Lead rubber bearing which consists of a lead plug inserted into a vulcanised laminated rubber bearing. The form shown here is suitable for applications where there is no applied tension.
(b) Lead rubber bearing for William Clayton Building (see Chapter 6). Note the 300 mm rule placed on the bearing. Load capacity 3 MN, stroke + 100 mm. (Robinson, 1982.) (c) Lead rubber bearing under static test. (Robinson, 1982.)
(d) Lead rubber bearing for William Clayton Building under dynamic test (1979). The motive force was supplied from the drive of a converted caterpillar tractor: vertical load up to 4 MN, frequency 0.9 Hz, maximum power 100 kW, maximum shear force 400 kN, stroke + 90 mm. (Robinson, 1982.)
(e) Lead rubber bearing with top and bottom plates vulcanised to the rubber, suitable for applications requiring applied vertical tension. (Robinson, 1982.)
(a) (b)
(d) (c)
(e)