Patent Description:
Elastomeric isolators are commonly used in the structure of buildings or other construction works in seismic areas, to separate the superstructure from the foundation or other supporting structure. The isolator, or bearing, must have a high vertical but low horizontal stiffness, resulting in a significant reduction of the seismic accelerations that are transmitted to the superstructure. Such bearings are specified in section <NUM> of the <NPL>.

A known design of an elastomeric isolator is referred to as a lead rubber bearing (LRB). Such an isolator has one or more holes formed in an elastomeric body to receive a lead core or plug. <CIT>, <CIT> and <CIT> disclose known elastomeric isolator designs.

<FIG> is a view of a typical LRB. The elastomeric body <NUM> is usually reinforced by plate-shaped steel members <NUM> embedded in the rubber material <NUM>. The reinforcement members <NUM> are arranged perpendicular to the direction of the main effort to which the bearing is subjected, usually horizontally. Thus, they allow absorption of some shear strain by the elastomeric body <NUM> in case of dynamic efforts caused, for example, by earthquakes. In the example shown, there is a central hole formed vertically through the elastomeric body and a lead plug <NUM> inserted in the hole. Frames plates <NUM>, <NUM> are arranged on both sides of the elastomeric body <NUM> for connecting the bearing to the foundation and the superstructure.

To describe the behavior of a LRB in case of earthquake, a bilinear model as illustrated in <FIG> can be schematically referred to. In <FIG>, the horizontal axis represents displacement while the vertical axis represents horizontal force. At first, the behavior is predominantly governed by the shear stiffness K<NUM> of lead. When the lead yields at the plastic limit at point (SY, FY), the shear stiffness K<NUM> of rubber is involved until the maximum displacement SD is reached.

At the end of this displacement at point (SD, FD), when there is no more horizontal speed, the lead material can recrystallize and recover its shear stiffness. The lead plug can then react to the horizontal acceleration in the opposite direction, until the plastic limit is again reached, at which point, the resisting effort depends only on the stiffness of rubber.

This results in a cyclic behavior when the structure is subjected to periodic accelerations, as illustrated in <FIG>. The cycle in the displacement/force relation has a surface area proportional to the energy dissipated by the isolation bearing, while the effective stiffness Keff determines the natural frequency of the bearing. These two parameters determine the performance of the seismic isolation, and thus the behavior of the structure in case of earthquake.

In practice, the behavior in the first cycle, as shown in <FIG>, is not maintained during subsequent cycles. This known phenomenon is taken into account in the design of LRBs. For example, the AASHTO standard (see "<NPL>) requires that the minimum effective stiffness measured during the three first cycles is not be less than <NUM>% of the maximum effective stiffness. It also requires that the minimum energy dissipated per cycle (EDC) measured during the specified number of cycles is not less than <NUM>% of the maximum EDC.

When cycles are repeated, the plastic yield limit of the lead material decreases, which causes a reduction of the effective stiffness Keff and a reduction of the dissipated energy. This behavior is taken into account when computing the response of the structure to the seismic event (forces), either by considering a mean or minimum value for the plastic yield limit of the lead, or by performing iterative calculation with a variable value at each cycle.

An object of the present invention is to propose an enhanced LRB design, having a more stable dynamic behavior and thus better efficiency.

According to a first aspect, there is disclosed a lead rubber bearing according to claim <NUM>. Said lead rubber bearing comprises a deformable body comprising rubber layers laminated with metal reinforcement layers, and at least one lead plug received in a hole formed through the laminated layers of the deformable body. The deformable body has a thermally conductive interface in contact with the lead plug to provide thermal conduction between the lead plug and at least some of the metal reinforcement layers.

In conventional LRBs, the reason for the change in the mechanical behavior of the lead plug lies in the increase of its temperature. The absorbed seismic energy is transformed into heat, first in the lead and then by conduction in the adjacent materials which mainly consist of rubber having a low thermal conductivity. The characteristics of the lead material, in particular its yield limit, are impacted by the temperature increase. For example, the temperature of a lead cylinder with a diameter of <NUM> and a length of <NUM>, used as an LRB plug, increases by <NUM> if the LRB is subjected to a lateral deformation cycle of ±<NUM> (<NUM>% shear strain), assuming a shear yield limit of <NUM> MPa. If no heat diffusion take place, such heating of the lead plug gives rise to a ~<NUM>% drop of the yield limit for the second cycle. In conventional LRBs, the lead plug is in contact with flexible rubber material that is not thermally conductive.

The LRB proposed herein allows quicker diffusion of the heat generated in the lead plug, through the thermally conductive interface and the metal reinforcement layers.

Given that the heated generated in the lead plug can be evacuated more efficiently, its performance will not be degraded as much as in conventional LRBs. The metallic reinforcement layers can act like radiator fins to evacuate the heat promptly (due to their high thermal conductivity) and massively (due to the high specific heat of the surrounding rubber) away from the lead plug.

The thermally conductive interface includes at least one thermally conductive deformable portion in contact with at least one of the metal reinforcement layers, and exposed at an inner surface of the hole to be in contact with the lead plug.

The thermally conductive deformable portion may consist of rubber material loaded with thermally conductive particles, for example graphite particles.

The thermally conductive deformable portion have an inner part forming at least part of a wall of the hole receiving the lead plug, and radial extensions belonging to the rubber layers of the deformable body laminated with the metal reinforcement layers. The rubber layers of the deformable body may include thermally non-conductive portions surrounding the radial extensions of the thermally conductive deformable portion.

Typically, the thermally conductive deformable portion has a thermal conductivity of more than <NUM> W. Preferably, that thermal conductivity is more than <NUM> W.

In an embodiment, the thermally conductive interface includes edges of the metal reinforcement layers that are exposed at an inner surface of the hole to be in contact with the lead plug.

In another aspect, there is proposed a method of manufacturing a lead rubber bearing according to claim <NUM>. The method comprises stacking rubber layers alternating with metal reinforcement layers, the stacked layers having at least one hole therethrough, forming a deformable body with the rubber layers, the metal reinforcement layers and the hole therethrough, wherein forming a deformable body comprises curing the rubber layers, and inserting a lead plug in the hole of the deformable body. The deformable body is formed to have a thermally conductive interface in contact with the lead plug to provide thermal conduction between the lead plug and at least some of the metal reinforcement layers.

At least some of the stacked rubber layers include a first ring of rubber material loaded with thermally conductive particles, disposed adjacent to the hole, and a second ring of rubber material not loaded with thermally conductive particles, disposed around the first ring. Curing the rubber layers may be performed by allowing part of the rubber material of the first ring to flow and cover edges of the metal reinforcement layers, so as to form a wall of the hole receiving the lead plug. After curing, the rubber material loaded with thermally conductive particles may have a thermal conductivity of more than <NUM> W. K-<NUM>, preferably more than <NUM> W.

When the thermally conductive interface includes edges of the metal reinforcement layers exposed at an inner surface of the hole to be in contact with the lead plug, forming the deformable body may comprise, after curing the rubber layers, removing rubber material at the inner surface of the hole to expose the edges of the metal reinforcement layers. Alternatively, forming the deformable body comprises, before curing reinforcement layers. Alternatively, forming the deformable body comprises, before curing the rubber layers, disposing a removable molding plug in the hole, in contact with the edges of the metal reinforcement layers.

Other features and advantages of the Lead Rubber Bearing or LRB disclosed herein will become apparent from the following description of non-limiting embodiments, with reference to the appended drawings, in which:.

The description which follows is illustrated with schematic drawings where the same reference numerals are used to designate similar parts. <FIG> show LRBs having a general configuration similar to that of <FIG>, with a cylindrical lead plug <NUM> inserted into a hole formed centrally in a generally cylindrical deformable body <NUM>. It will be appreciated that the teachings of the present invention are applicable to LRBs having a variety of other geometries, with any number of lead plugs.

<FIG> is a schematic cross-sectional view of the deformable body of the conventional LRB of <FIG>, where the arrangement of the rubber layers <NUM> laminated with the metal reinforcement layers <NUM> of the deformable body <NUM> is better seen. The metal reinforcement layers <NUM> are fully embedded in the rubber material. The thermal conduction between them and the lead plug <NUM> is poor because the hole <NUM> that receives the lead plug <NUM> is lined with a thickness of rubber material <NUM>.

The situation is improved when the wall of the hole <NUM> that receives the lead plug <NUM> is processed to expose the inner edges <NUM> of the metal reinforcement layers <NUM> as shown in <FIG>.

The lead plug <NUM> is then in direct contact with the metal reinforcement layers <NUM>. The thermal conduction to take away the heat generated in the lead plug <NUM> is then governed by the thermal conductivity of lead and stainless steel, namely <NUM> W. K-<NUM> and <NUM> W. K-<NUM>, respectively. In contrast, the thermal conductivity of rubber (such as the rubber layer <NUM> present at the wall on the hole <NUM> in <FIG>) is <NUM> W. K-<NUM>, while its specific heat is <NUM> J. Therefore, an embodiment as shown in <FIG> avoids excessive heating of the lead plug <NUM>. This results in a more stable behavior on the LRB, in terms of displacement/force cycle, and thus in better isolation performance under seismic forces.

To manufacture an LRB as shown in <FIG>, alternating rubber and metal layers having a central hole therethrough are conventionally stacked, and then the rubber layers <NUM> are cured to form the deformable body <NUM>. Afterwards, the lead plug <NUM> is forcibly inserted into the hole <NUM>. The hole in each metal reinforcement layers <NUM> has substantially the same diameter as the lead plug <NUM>, so that an intimate contact between the lead and the stainless steel is obtained when the plug is inserted.

In a possible manufacturing method, a molding core having a diameter slightly smaller than that of the lead plug <NUM> is inserted in the central hole <NUM> of the stack of rubber and metal layers <NUM>, <NUM> when the rubber material is cured, so that a deformable body <NUM> as shown in <FIG> is obtained, with a layer <NUM> of rubber material at the inner surface of the hole <NUM>. After curing, that layer <NUM> is removed, e.g. by abrasion, to expose the inner edges <NUM> of the metal reinforcement layers <NUM>.

Alternatively, a molding core having the same diameter as the lead plug <NUM> can be used. When the molding core is removed after curing of rubber material, the inner edges <NUM> the metal reinforcement layers <NUM> are exposed at the inner surface of the hole <NUM>.

<FIG> illustrate another embodiment of a LRB in accordance with the invention. In that embodiment, part of the usual rubber used in the layers <NUM> is replaced by a ring of rubber material modified to have a much higher thermal conduction and located adjacent to the lead plug <NUM> and in contact with the metal reinforcement layers <NUM>.

Thermally conductive rubber materials can be obtained, for example, by replacing, in the mixture, part of the carbon black by expanded graphite that has a thermal conductivity <NUM> times larger.

Alternatively, other kinds of thermally conductive particles are added to the rubber composition, such as small fibers or flakes of highly conductive materials, e.g. copper. The present invention is not limited to any particular way of making a rubber material thermally conductive.

<FIG> show the layers of metal <NUM> and of rubber <NUM> that are stacked around a molding core <NUM> that has a diameter equal to or slightly smaller than that of the lead plug <NUM>. The diameter of the molding core <NUM> is smaller than that of the central hole formed in the metal reinforcement layers <NUM>, for example <NUM> to <NUM> smaller. Each rubber layer <NUM> includes a ring <NUM> of thermally conductive rubber disposed around the molding core <NUM> and surrounded by another ring <NUM> of conventional thermally non-conductive rubber.

<FIG> illustrates the configuration of the deformable body <NUM> after curing of the rubber material under pressure, which provides strong adherence between the rubber and the stainless steel of the reinforcement layers <NUM>.

The rubber material of the inner rings <NUM> is transformed into a thermally conductive deformable portion that includes an inner part <NUM> forming the wall of the hole <NUM> that will receive the lead plug <NUM> and radial extensions <NUM> belonging to the rubber layers <NUM> of the deformable body <NUM>. The inner part <NUM> is formed by the rubber material that flows from the inner rings <NUM> during the heat treatment and migrates to the surface of the molding core <NUM>. Its outer surface is in contact with the inner edges of the metal reinforcement layers <NUM>. The radial extensions <NUM> of the thermally conductive deformable portion are also in contact with the metal reinforcement layers <NUM> on their upper and lower sides. Around the radial extensions <NUM>, the rubber material from the rings <NUM> remains as thermally non-conductive portions to provide the required deformability and the shear stiffness K<NUM>.

The embodiment of <FIG> does not alter the flexibility and elasticity of the rubber layers <NUM>. It also ensures a durable contact of the metal reinforcement layers <NUM> with the conductive material that conveys the heat generated in the lead plug <NUM>.

For comparison purposes, we consider the example of a lead rubber bearing having a generally cylindrical deformable body <NUM> with an outer diameter of <NUM>, made of fourteen rubber layers <NUM> having a thickness of <NUM> with <NUM>-thick stainless steel reinforcement plates <NUM> between them. The deformable body <NUM> has a central hole where a cylindrical lead plug with a diameter of <NUM> is inserted.

<FIG> shows such an LRB in a conventional configuration where there is a <NUM>-thick rubber layer <NUM> lining the inner wall of the hole <NUM> that receives the lead plug <NUM>.

If <NUM> kJ are dissipated in the lead plug <NUM> of that LRB, the temperature increase of the lead material after <NUM> is <NUM>. The energy of <NUM> kJ corresponds to that of two cycles of horizontal deformation of ±<NUM>% (±<NUM>) of the bearing with the usual value of <NUM> MPa as the yield limit of lead under shear stress.

<FIG> shows an illustrative LRB according to an embodiment of the invention, which has geometry similar to that of <FIG>, except that the inner edges <NUM> of the metal reinforcement layers <NUM> are in contact with the lead plug <NUM>.

Here, if the same amount of seismic energy (<NUM> kJ) is dissipated in the lead plug <NUM>, the temperature increase of the lead material after <NUM> is only <NUM>.

<FIG> shows an illustrative LRB according to another embodiment of the invention, which again has geometry similar to that of <FIG>, except that there is a thermally conductive deformable portion with an inner part <NUM> and radial extensions <NUM> as discussed with reference to <FIG>. The inner part <NUM> has a thickness of <NUM> along the radial direction, while the radial extensions <NUM> have a length of <NUM>.

In example <NUM>, the thermal conductivity of the rubber material of the thermally conductive deformable portion <NUM>, <NUM> is six times larger than that (<NUM> W. K-<NUM>) of the rubber of the outer ring <NUM>, i.e. <NUM> W.

Here, if the same amount of seismic energy (<NUM> kJ) is dissipated in the lead plug <NUM>, the temperature increase of the lead material after <NUM> is <NUM>.

In example <NUM>, the configuration of the LRB is the same as in example <NUM>, but the thermal conductivity of the rubber material of the thermally conductive deformable portion <NUM>, <NUM> is sixty times larger than that of the rubber of the outer ring <NUM>, i.e. <NUM> W.

The examples show that constructive measures as proposed by the present invention significantly reduce the temperature increase of the lead plug <NUM> of the LRB in case of earthquake. This results in an improved dynamic behavior of the LRB.

If variations of the shear yield limit of lead as a function of temperature according to the literature are taken into account (about <NUM> MPa/°C), the gain afforded by the improved thermal contact with the lead plug <NUM> is as indicated in Table <NUM> below.

The comparison shows that a thermally conductive deformable portion <NUM>, <NUM> of <NUM> W. K-<NUM> to form the thermally conductive interface between the lead plug <NUM> and the metal reinforcement layers <NUM> (example <NUM>) achieves performances almost as good as direct lead-steel contact (example <NUM>), while it provides a more durable contact with the lead plug.

The thermal conductivity at the thermally conductive interface is preferably more than <NUM> W. Still a significant improvement over conventional LRBs is obtained in example <NUM>, where the thermally conductive deformable portion <NUM>, <NUM> is only <NUM> W. Generally, the behavior of the LRB is improved when the thermal conductivity of the thermally conductive deformable portion <NUM>, <NUM> is more than <NUM> W.

Claim 1:
A lead rubber bearing, comprising:
a deformable body (<NUM>) comprising rubber layers (<NUM>) laminated with metal reinforcement layers (<NUM>); and
at least one lead plug (<NUM>) received in a hole (<NUM>) formed through the laminated layers of the deformable body,
wherein the deformable body (<NUM>) has a thermally conductive interface in contact with the lead plug (<NUM>) to provide thermal conduction between the lead plug and at least some of the metal reinforcement layers (<NUM>),
characterized in that
the thermally conductive interface includes at least one thermally conductive deformable portion (<NUM>, <NUM>) in contact with at least one of the metal reinforcement layers (<NUM>), and exposed at an inner surface of the hole (<NUM>) to be in contact with the lead plug (<NUM>), and wherein the thermally conductive deformable portion has:
an inner part (<NUM>) forming at least part of a wall of the hole (<NUM>) receiving the lead plug (<NUM>); and
radial extensions (<NUM>) belonging to the rubber layers (<NUM>) of the deformable body (<NUM>) laminated with the metal reinforcement layers (<NUM>).