Listening Section 4: Note & Table Completion: Question 4

Transcript-based practice (no audio).

Lecturer: In this lecture I want to explain how modern engineers protect buildings from earthquakes using a technique called base isolation. The basic idea is refreshingly simple: instead of fixing a structure rigidly to the ground and relying on the frame's strength to survive shaking, engineers insert a layer of flexible devices, called isolators, between the foundation and the structure above. When the ground shakes violently, these isolators allow the building to move largely independently of the ground, so far less seismic force reaches the structure itself.

There are two common types of isolator. The first is the lead-rubber bearing, a stack of alternating rubber and steel plates with a solid lead core running through the centre. The rubber layers let the bearing flex sideways, absorbing horizontal motion, while the lead core deforms plastically during shaking, dissipating energy as heat. The second common type is the friction pendulum bearing, which works quite differently: it uses a curved sliding surface, so the building literally slides across a shallow concave dish during an earthquake, and gravity guides the structure back to its original position once the shaking stops.

What both approaches achieve, from an engineering standpoint, is a period shift. Every building has a natural vibration period, the rate at which it sways if disturbed, and ordinary earthquake motion shakes a conventional, fixed-base building most violently at that period. By adding flexible isolators, engineers effectively lengthen the building's natural period, shifting it away from the ground's strongest shaking frequencies, which dramatically reduces the forces the structure actually experiences.

Of course, allowing a building to move introduces practical complications. Because the base can shift sideways by a significant distance during a major quake, designers must leave a seismic gap around the entire perimeter of the building, a clear space preventing collisions with neighbouring buildings or utility lines. For the same reason, every pipe and cable crossing that isolation joint has to be flexible, able to stretch without rupturing as the building shifts.

One of the best-known examples of this approach is the retrofit of San Francisco City Hall, completed in the 1990s, where isolators were installed beneath the historic building without altering its exterior appearance. Base isolation isn't cheap, it adds noticeably to a project's upfront cost, but that additional expense is usually offset by dramatically reduced repair costs and downtime after a major earthquake, since isolated buildings suffer far less structural damage.

Complete the notes below. Write NO MORE THAN TWO WORDS AND/OR A NUMBER for each answer.

Earthquake-Resistant Design: Base Isolation. Lecture Notes

Basic principle

• Devices placed between the foundation and the structure: (1) ______________

Isolator types

• First isolator type: the (2) ______________ bearing
• Second isolator type: the (3) ______________ bearing

Engineering effect

• Effect both isolator types achieve on the building's vibration: a (4) ______________

Practical requirements

• Clear space required around the building's perimeter: a (5) ______________
• Pipes and cables crossing the isolation joint must be: (6) ______________

Example and cost

• City containing a well-known base-isolation retrofit: (7) ______________
• Base isolation increases a project's (8) ______________