I MPLEMENTATION OF B ASE I SOLATION

3.3.1 CONCEPTUAL / PRELIMINARY DESIGN

If your project passes the check list for the need for isolation, site suitability and structure suitability then the isolation system is implemented by stepping through a conceptual/preliminary design process:

• Define the seismic isolation objectives. Does the project suit full isolation or is it more suited to an energy dissipation solution?

• Decide on the seismic isolation plane and the location of isolation devices. This is often obvious, for example, a frame building will be isolated below the ground floor with isolators supporting each column. Other structures may not be so apparent and more than one option may be carried forward to the next stages.

• Select appropriate devices. This will depend on seismicity (some devices are better suited to low displacements, some have high damping etc.) and any restrictions on size – some devices require less vertical headroom. Generally, at this stage select several potential types.

• Assess the isolation system performance for each device type. At concept stage, a single mass approximation will provide displacements and base shear coefficients. If floor accelerations are critical you may do some analyses at this stage.

• Select one or more preferred devices.

This is usually the best point to evaluate the costs and benefits of seismic isolation for the project. You should have sufficient information to approach manufacturers of the preferred devices for hardware costs. You can estimate the cost of structural changes required to install the system. The maximum displacements can be used to cost the provision of clearances and specification of flexible utility connections.

The benefits will arise from the reduction in the base shear coefficient and the floor accelerations as they affect non-structural fixings. There may be direct, first cost savings associated with these. More likely, there will be indirect savings from increased seismic safety and reduced earthquake damage. Whether these can be included into the accounting depends very much on the building owner.

3.3.2 PROCUREMENT STRATEGIES

If seismic isolation proves effective and economical then the design process continues to the detailed design phase. The process at this stage depends on the approach taken to seismic isolation system procurement, whether prescriptive or performance based or a mix of the two.

The prescriptive approach is where you provide detailed device requirements, which usually include the maximum vertical loads for each combination, the design level and the maximum earthquake displacements and the effective stiffness and damping at each of these displacement levels. To use this method of procurement, you need a good knowledge of device characteristics. You do not want to specify requirements which are very difficult, or even impossible, to achieve.

The performance based approach is where you specify the performance you want to achieve and leave the detailed device properties up to the vendor. For example, you might specify an effective period, maximum displacements and maximum base shear coefficients at the design level and maximum earthquake. In this variation, you would usually require the vendor to submit analysis results to demonstrate compliance and you should also perform some analyses yourself to verify this. Sometimes these two approaches are mixed – the specification details the device characteristics that will achieve the required system performance. Vendors are then permitted to submit alternative devices that will at least match this performance.

Which approach you take depends on your confidence in designing isolation systems, how much control you want to retain over the process and your capability of evaluating a range of isolation systems.

The two approaches represent a current ambivalence in seismic isolation system supply, as to whether the product is a commodity (where the prescriptive approach is typical) or an engineered system (where the performance approach is typical). In the early 1980’s isolation was clearly an engineered system and design almost always involved vendors at the early stages of a project. As

patents expire, more manufacturers enter the market and the product is moving more to a commodity. However, this process is not proving simple as we do not have the equivalent of steel beam safe load tables.

Eventually there will no doubt be tables of standard isolation devices and progress in this has been made in Japan. In the U.S. the wide variations in seismic requirements, particularly the near fault effects, preclude this approach.

TABLE 3-2 PROCUREMENT STRATEGIES

Approach Description Advantages Disadvantages

Prescriptive Specify detailed device characteristics, including stiffness and damping. May specify sizes.

Structural engineer retains control.

Simple to evaluate bids.

Requires a structural engineer expert in isolation design.

Limits potential bidders. May not be optimal system. Performance Specify performance

requirements of the isolation system (period, displacement, damping). Vendors design devices.

Does not require expertise in device design.

Wider range of bidders. Less engineering effort at design stage.

Difficult to evaluate bids. May need to check analysis of a large number of systems.

Combined Specify a complying system as for prescriptive

approach.

List performance of this system and allow other devices that can match this.

Widest range of bidders. Most likely to attract optimal design.

Requires design expertise. Difficult to evaluate bids. May need to check analysis of a large number of systems.

3.3.3 DETAILED DESIGN

Once the concept is accepted and a procurement strategy established, the detailed design follows much the same process as for any other structural design:

• Analyze the structure and assess the detailed structural performance for the selected system. • Develop either device characteristics or performance criteria for the project specifications.

• Design and detail the connections of the isolators to the structure above and below. • Document as for any other design – contract drawings and specifications.

3.3.4 CONSTRUCTION

As for detailed design, the construction phase proceeds as for any other structure although there are some additional requirements for a seismic isolation system:

• Codes typically require prototype and quality control tests. You will need to supervise these tests and evaluate the results for compliance with the specifications. These may also affect the construction schedule as typically 2 to 3 months are required to manufacture, test and evaluate prototype bearings.

• The installation may have special requirements, particularly for seismic retrofit. • A program is required for maintenance, servicing and post-earthquake inspection.