Åu=
CHAPTER 5: ISOLATION SYSTEM DESIGN
5.1 INTRODUCTION
This Chapter describes a design procedure for seismic isolation systems. Many isolation systems use a combination of elastomeric bearings types (lead-rubber, high damping rubber and plain rubber) which are designed specifically for the applied loads and displacements.
Other isolation solutions, such as sliding systems and the friction pendulum system, are based on devices which are designed by the supplier for the particular application.
The design procedures here are used to design elastomeric bearing types and perform a preliminary assessment of the performance of an isolation system which incorporates one or more types of device. The procedure is suitable for design office use to select the types and properties of devices which will achieve the desired performance. The design characteristics developed by the procedure are used as input to a detailed analysis and evaluation of the isolated structure.
Because of the complexity of hardware design, and empirical aspects of design for most types of isolators, it is usual to obtain assistance from manufacturers. As base isolation technology has evolved, manufacturers have realized that structural engineers do not have the skills to design hardware and so will provide this assistance.
There are codes available (U.S, British and Australian) which provide design rules for devices used in isolation, such as elastomeric bearings and Teflon sliding bearings. However, most of these codes are for non-seismic bridge applications and need to be adapted to use for seismic isolation applications.
5.1.1 Assessing Suitability
Not all structures are suited for seismic isolation and the first stage in the design procedure is to check suitability. The checks should examine the need for isolation, the suitability of the site and the suitability of the particular structure. Table 5.1 lists some of the items which should be assessed prior to commencing any detailed design.
Some structures are more suited than others to isolation, as listed in Table 5.2. Most examples of isolated buildings fall into one or more of these categories. This does not exclude other building types, but most projects will have one or more of (1) requirements for continuing operation (2) low ductility (3) historic merit or (4) valuable contents.
114
Item Checks 1. Need for Isolation
Level of Earthquake Risk
Seismic Design Requirements
Is Earthquake Design Required?. Seismic isolation is best suited for moderate and high seismic areas.
If seismic design adds to costs significantly, then isolation is likely to be more effective.
2. Site Suitability Geologic Conditions Site Subsoil Conditions
Distance to Fault
Potential for resonance effects may rule out isolation (e.g. Mexico City).
Stiff Soil is best for isolation. As site conditions become softer, isolation becomes less effective and more expensive.
Near fault motions may add to the response at the isolated periods. If the distance to the nearest active fault is small isolation
displacements may be excessive.
3. Structure Suitability Weight of the Structure Period of the Structure
Structural Configuration
Heavy structures tend to be the most cost effective to isolate.
Generally, the period of the non-isolated structure should be less than 2 seconds, although there are exceptions.
Isolators are usually placed in a crawl space or basement. If a slag-on-grade is planned, this will need to be replaced with a
suspended floor.
Large aspect ratio of the structural system (height to width ratios) may cause overturning problems.
For bridges, tall piers may make the period too long.
For retrofit, assess how difficult to separate the structure from the ground.
Table 5.1: A Suitability Check List
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
Table 5.2: Most Suitable Buildings for Isolation
115 5.1.2 Design Development for an Isolation Project
If a project appears to be a suitable candidate for isolation, the level of design of the isolation system depends on the procurement strategy to be adopted for the isolation system. Specifications will be either prescriptive or performance based on some combination of the two, as listed in Table 5.3.
A prescriptive specifications provides details of the devices to be supplied (materials, dimensions etc.) as for other structural components such as steel frames, concrete walls etc.
A performance specification states the performance to be achieved and requires the suppliers to design devices to meet this performance (such as for a design-and-build contract). Each of these approaches has advantages and disadvantages and in most cases a combined specification is most effective. In this case, the engineer would supply properties of a complying system (e.g. effective stiffness and damping for HDR devices) but also supply the expected performance of this system to allow vendors of other systems to design and bid systems with at least equal performance.
Description Advantages Disadvantages
Prescriptive Specification.
Specify detailed device characteristics, including stiffness and damping.
May specify sizes.
Structural engineer retains control.
Simple to evaluate bids.
Requires the structural engineer to be expert in isolation design.
Limits potential bidders.
May not be optimal system.
Performance Based Specification Specify performance
requirements of the isolation system (period, displacement, and 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 Prescriptive / Performance Specification 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.
Table 5.3: Procurement Strategies
116