D AMAGE P ROBABILITY

Major earthquakes have a low probability but high consequences. For benefit cost studies, they have a low annual probability and so the earthquake damage cost may be very low on a net present value calculation. However, the low annual probability may not be reassuring to an owner who wants to know “What happens if the earthquake occurs next year?” For evaluating this, cost benefit analysis can be based on conditional probability, assuming that the event occurs within the design life of the building. This approach tends to markedly increase the B/C ratio.

3.4.8 SOME RULES OF THUMB ON COST

The additional engineering and documentation costs compared to a non-isolated design will probably be at least 20% and may be much more for your first project. The total range of costs will be about that shown in Table 3-5. Excluding reduced damage costs, the added costs may range from a minimum of –3.5% to +12% of the total building cost.

TABLE 3-5 ISOLATION COSTS AS RATIO TO TOTAL BUILDING COST

Item Lower

Bound BoundUpper

Engineering and Documentation Isolators

Structural Changes

Architectural & Services Changes Savings in Structural System Reduced Damage Costs

0.1% 0.5% 0% 1% -5% -25% 0.5% 5% 5% 5% 0% -50%

3.5 STRUCTURAL DESIGN TOOLS

3.5.1 PRELIMINARY DESIGN

Isolator design is based on material and section properties as for any other type of structural section. Similar tools are used as for example for reinforced concrete sections, such as spreadsheets. The solution for the response of an isolated system based on a single mass is a straightforward procedure although for a non-linear system the solution will be iterative. Again, spreadsheets can be set up to solve this type of problem. In my experience, the complete isolator design and performance evaluation can be performed using a single spreadsheet.

3.5.2 STRUCTURAL ANALYSIS

The analysis of an isolated building uses the same procedures as for a non-isolated building, that is, in increasing order of complexity, equivalent static analysis, response spectrum analysis or time history analysis. The criteria for an isolated structure to be to be designed using the equivalent static load method are so restrictive that this method is almost never used. The most common methods are dynamic, as would be expected given the characteristics of an isolated building.

Most isolated structures completed to date have used a time history analysis as part of the design verifications. Codes often permit a response spectrum analysis, which requires a lot less analytical effort than time history. Recent codes (e.g. UBC) do not require time history analysis unless the site is especially soft or the isolation system selected has special characteristics (lack of restoring force or dependence on such factors as rate of loading, vertical load or bilateral load). Although a response spectrum analysis may be used for most structures, the procedure is usually more complex than for non-isolated structures as a linear analysis procedure is used to represent a non-linear system. For most isolation systems, both the stiffness and the damping are displacement dependent. However, for a given earthquake the displacement is itself a factor of both stiffness and damping. This leads to an iterative analysis procedure – a displacement is assumed, stiffness and damping calculated and the model analyzed. The properties are then adjusted based on the displacement from the analysis.

Because the response of the isolated structure is dominated by the first mode the performance evaluation based on a single mass approximation will generally give a good estimate of the maximum displacement and so the number of iterations is usually not more than one or two. The response spectrum analysis can be performed using any computer program with these capabilities (e.g. ETABS, SAP2000, and LUSAS). Most of these programs can also be used for a time history analysis if required.

As discussed later, the studies we have performed suggest that the response spectrum analysis seriously under-estimates overturning moments and floor accelerations for most isolation systems. Until this issue is resolved, we should not use this method for final design. Note that our most common linear elastic analysis tools, ETABS and SAP2000, can be used to perform the time history analysis and so this is not an undue impediment to use.

3.6 SO, IS IT ALL TOO HARD?

To most engineers, seismic isolation is a new technology and the sheer scopes of things to consider may make it just seem too hard. Current codes do not help as, for example, the UBC has a complete section on seismic isolation which will be entirely new territory to an engineer starting out in isolation design.

The key is to realistically evaluate your structure, and not to have too high an expectation of cost savings from the outset. If the project that you select is a good candidate for isolation then the procedure will follow in a straightforward manner. If it has characteristics which make it a marginal or bad candidate than eventually problems will arise with the isolation system design and evaluation. These may be such that you will abandon the concept and never want to try it again. So, pick your target carefully!

As discussed earlier, the reduction in earthquake forces achieved with isolation does not translate into a similar reduction in design forces. The reason for this is ductility, as the force reductions permitted for ductility in non-isolated buildings are similar to those achieved by the isolation system. However, the isolated building will have a higher degree of protection against earthquake damage for the same, or lower, level of design force.

These features of isolation lead to building types which are more suitable for isolation for other because of particular characteristics. Table 3-6 list categories which are most suited. If you

examine the HCG project list at the start of these guidelines, you will see that most completed projects fall into one of these categories.

Items which are positive indications of suitability are:

• Buildings for which continued functionality during and after the earthquake are essential. These buildings generally have a high importance factor, I.

• Buildings which have low inherent ductility, such as historic buildings of unreinforced masonry. These buildings will have a low ductility factor, R.

• Buildings which have valuable contents.

If any of these conditions apply to your project then it will generally by easier to justify the decision to isolate than is none apply.

TABLE 3-6 SUITABLE BUILDINGS FOR ISOLATION

Type of Building Reasons for Isolating

Essential Facilities Functionality

High Importance Factor, I Health Care Facilities Functionality

High Importance Factor, I Old Buildings Preservation

Low R

Museums Valuable Contents

Manufacturing Facilities Continued Function High Value Contents