Base Isolation with Lead Rubber Bearings
A base-isolated building is not stronger: it is slower. The bearings lengthen the natural period of the structure until the earthquake no longer finds it in resonance, and the price of that choice is that the building has to be able to slide. Everything else on site follows from this: anything crossing the isolation plane — a pipe, a cable, a stair, a pavement — has to be designed to move, otherwise it becomes a rigid bridge that cancels the system.
Overview
TAV. 00The process covers building a base isolation plane with lead rubber bearings, from defining the movement gap through to the long-term inspection plan. The logic differs from any other structural work: here you are not building something that resists, you are building a plane along which the building can move, and the quality of the result depends far more on what must not touch than on what is built. The phases follow that order: first establish how far the building has to be able to slide and defend that space, then build the two substructures to steelwork tolerances, then install and test the bearings, and finally resolve every crossing. The last phase does not close the site: it opens the service life, because an isolation system has to be looked at periodically and must stay reachable.
The site score
TAV. HThe detail of the craftThe gap is design, not a finishing joint
Define the gap and defend it
The first decision is how far the building must be able to move relative to the ground, and that displacement becomes a continuous seismic gap around the whole perimeter. The gap has to be drawn in plan as a design element and not improvised on site: it crosses pavements, ramps, external stairs, thresholds, fences and every incoming service. In the same step the position of the isolation plane is chosen — under the first floor structure, under a basement, on top of existing columns — and the resulting void is checked to be walkable by a person.
Definition in plan and section of the perimeter seismic gap according to the design displacement, identifying every element that crosses it and the level of the isolation plane.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
PLATE 01
Not stronger: slower
Traditional strengthening increases capacity and leaves the building where the earthquake finds it. Isolation moves the building out of the range where the shaking is most effective, by lengthening its natural period: it is a choice about frequency, not about strength.
The diagram is qualitative and illustrates the principle. Spectra, periods, design displacements and verification criteria are to be taken from the design and the applicable standards.
PLATE 02
The rigid bridge that cancels the system
The bearings are deliberately soft. Any rigid element connecting ground and building is far stiffer than they are, so it takes the action: it does not reduce the isolation, it bypasses it at that point.
The diagram illustrates the mechanism. Joint movement capacities, construction details and verifications are to be taken from the design and the applicable standards.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QHow far does an isolated building actually move?
Far enough to make the perimeter gap a visible element and to require genuine seismic joint covers at every crossing. The design displacement depends on the site, the structure and the devices chosen, and must be read from the calculation report: it is that figure, not a general rule, that sizes the gap, the flexible service joints and the sliding bearings for stairs and ramps.
Why lead inside the bearing?
Because it yields at low stress and does so stably, repeatably and without degrading cycle after cycle: it is an almost ideal damper at ambient temperature. The laminated rubber around it provides vertical capacity and re-centring, the core turns energy into heat. The lead is fully encapsulated within the device and has no contact with the environment.
Can an existing building be isolated?
Yes, and it is one of the most effective interventions on existing buildings, but the site work is different in nature: the structure is cut on a horizontal plane, the load is transferred to temporary jacks, the devices are inserted and the load is returned. The difficulty is not strictly technical, it lies in the number of already existing crossings, all of which have to be remade flexible.
How often should the bearings be inspected?
At the intervals set by the inspection plan, and in any case after a felt seismic event. Routine inspection, though, is almost never about the devices: it is about the space around them. Checking that the gap is clear, that the void has not been occupied and that no new service crosses the plane rigidly is worth, in practice, far more than a close examination of the rubber.