ASCE 7 Chapter 17 Seismic Isolation Design Requirements: An Owner's Guide

Most owners meet ASCE 7 Chapter 17 the same way: the structural engineer says the building will be isolated, and a few weeks later the schedule grows a testing line and a review line, and the site plan gains a moat nobody had drawn. This article walks through what "Seismic Design Requirements for Seismically Isolated Structures" asks for, section by section, then places it next to the two documents an international owner hears about in the same conversation: the AASHTO guide specifications for bridges and Chapter 14 of Turkey's TBDY 2018. It ends with the questions worth putting to your engineer before the design freezes.

For the MCE_R design level and the moat in more depth, see seismic isolation design under ASCE 7-22. This one covers the whole chapter.

What Chapter 17 covers

Chapter 17 of ASCE/SEI 7-22 governs seismically isolated buildings wherever the International Building Code is adopted. Section 17.1 states the scope in one sentence: every seismically isolated structure, and every portion of it, is designed and built to this chapter plus the applicable requirements of the rest of the standard. Isolation does not replace the ordinary seismic provisions; it sits on top of them.

Three definitions from Section 17.1.1 carry the rest of the chapter:

• Isolation interface: the boundary between the upper part of the structure, which is isolated, and the lower part, which moves with the ground.
• Isolator unit: a horizontally flexible and vertically stiff element that permits large lateral deformation under the design earthquake.
• Isolation system: more than the bearings. It includes every element that transfers force between them, all connections to the rest of the structure, and the wind restraint, energy dissipation and displacement restraint devices if the design relies on them.

That last definition matters commercially: "the isolation system" is wider than the catalog item you are buying.

Isolation system requirements

Section 17.2.4 lists what the isolation system must satisfy regardless of the analysis method:

• Environmental conditions, wind and fire (17.2.4.1 to 17.2.4.3): aging, creep, fatigue and temperature are design inputs; a wind restraint system at the interface limits movement under wind; fire protection matches the columns and walls around it.
• Lateral restoring force (17.2.4.4): the lateral force at the maximum displacement must be at least 0.025 W greater than the force at half that displacement, checked for both upper bound and lower bound properties. This applies to every isolator type, sliding and elastomeric alike.
• Displacement restraint, vertical load stability and overturning (17.2.4.5 to 17.2.4.7): the bearings stay stable under the total maximum displacement with the vertical loads that go with it, and the building does not tip on them.
• Inspection and replacement (17.2.4.8): access for inspecting and replacing units is designed in, and the building carries a monitoring, inspection and maintenance program.
• Quality control (17.2.4.9): a test program for the isolator units is part of the structural design and incorporates the production testing of Section 17.8.5.

Section 17.2.8 handles a fact every buyer should know: bearing properties are not single numbers. They shift with temperature, loading rate, aging, contamination and, for rubber, scragging. The chapter applies property modification factors, the lambda factors, to turn nominal tested properties into an upper bound and a lower bound set, and both sets are analyzed. The soft set usually governs displacement, the stiff set usually governs the forces going into the superstructure.

The displacement vocabulary

Maximum displacement, D_M, is the lateral displacement of the isolation system under the risk targeted maximum considered earthquake, excluding the extra movement from actual and accidental torsion. It is the number the isolation system is designed for, computed separately at each property bound.

Total maximum displacement, D_TM, adds the torsional component back in. Section 17.1.1 names three uses for it: verifying the stability of the isolation system, designing structure separations, and the vertical load testing of prototype isolators. The moat width is set by D_TM, not D_M, and so is the displacement at which prototype bearings are stability tested. When a value engineering round proposes narrowing the gap around the building, this is the requirement it trades against.

Two roads to an analysis

Section 17.4 sets the default: isolated structures use the dynamic procedures of Section 17.6 unless they qualify for the equivalent lateral force procedure of Section 17.5. Where supplementary viscous dampers are used, response history analysis is mandatory.

Equivalent lateral force procedure

The ELF procedure is a static calculation built on the effective period and effective damping at D_M. Section 17.4.1 permits it only if every condition on its list holds, each checked at both property bounds with the stricter result governing. The conditions include: Site Class A, B, C or D; an effective period at D_M of no more than 5.0 s; a superstructure of no more than four stories or 65 ft (19.8 m), unless there is no tension or uplift on the isolators; effective damping at D_M of no more than 30 percent; an isolated period more than three times the fixed base period; no structural irregularity; and an isolation system that meets the stiffness, restoring force and displacement capacity criteria. Even in dynamic analysis, the chapter uses ELF results as a floor for several design quantities, so the calculation is done either way.

Dynamic procedures

Response spectrum analysis is allowed only when the structure, site and isolation system satisfy a subset of the ELF conditions (Section 17.4.2.1). Response history analysis, in which the model is driven by a suite of scaled ground motions with the bearings modeled nonlinearly, is permitted for any isolated structure and required for every one that does not qualify for response spectrum analysis (Section 17.4.2.2). This is where most real projects land, from hospitals on soft sites to tall or irregular superstructures. Anything with dampers lands here by rule.

Testing requirements for isolator units

Section 17.8 is the chapter's proof requirement: the properties used in analysis come from tests of a selected sample of components before construction, and the wind restraint system is tested too if the design uses one (17.8.1). Three layers follow:

1. Qualification tests (17.8.1.1): the manufacturer submits, for the registered design professional's approval, data quantifying heating under cyclic motion, loading rate, scragging, production variability, temperature, aging, exposure and contamination. The data must cover the same component types, materials and sizes, from the same manufacturer supplying the job.
2. Prototype tests (17.8.2): project specimens go through a prescribed sequence under average dead load plus half the live load effect. Section 17.8.2.2 spells it out: twenty fully reversed cycles at the wind design force; three cycles at each of 0.25, 0.5, 0.67 and 1.0 times D_M, or an equivalent dynamic sequence at the effective period; three cycles at 1.0 D_M; and a durability set at 0.75 D_M whose cycle count depends on the site spectrum but is never fewer than ten. Load bearing units repeat the D_M cycles under two additional vertical load cases. Prototype tests may be waived for similar units already tested, but only with the design review's acceptance (17.8.2.7).
3. Production tests (17.8.5): a program set by the registered design professional that tests 100 percent of the isolators going into the building in combined compression and shear, at not less than two thirds of D_M from lower bound properties. Mean results must fall within the specification range set by the lambda factors of Section 17.2.8.4.

Hold onto the production requirement. Prototype specimens are not the bearings that ship; the production program verifies those. If your RFQ is silent on it, read what to require in an isolator RFQ before it goes out.

The role of the design review

Section 17.7 requires an independent design review of the isolation system and its test programs by one or more individuals with knowledge of the subject, at least one a registered design professional. The standard fixes the scope: the design criteria including site specific spectra and ground motion histories; the preliminary design with device selection and the values of D_M, D_TM and the lateral force level; the qualification data and property modification factors for the chosen device; the prototype testing program; the final design of the entire structural system with all supporting analyses; and the production testing program. The review starts before the ground motions are chosen and ends after the bearings are built. Timing and reviewer qualifications are covered in our peer review article.

Bridges: the AASHTO guide specifications

A search for "guide specifications for seismic isolation design" lands on a different document: the AASHTO Guide Specifications for Seismic Isolation Design, published by the American Association of State Highway and Transportation Officials for highway bridges. The base text is the 4th edition of 2014, rewritten to follow the way seismic hazard is defined in the AASHTO LRFD Bridge Design Specifications and expanded with an Appendix B of fourteen design examples built on two benchmark bridges. Interim revisions in 2023 aligned it with the 9th edition of the LRFD Bridge Design Specifications and the 2nd edition of the Guide Specifications for LRFD Seismic Bridge Design, and adopted a risk targeted design spectrum based on the 2018 USGS National Seismic Hazard Model.

The two documents share a philosophy: bounded properties verified by bearing tests. They are not interchangeable. Chapter 17 is written for buildings; the AASHTO text is written for bridges. A building specification that cites AASHTO, or a bridge specification that cites Chapter 17, signals a template nobody checked.

Turkey: TBDY 2018 Chapter 14

Turkey's building earthquake code, TBDY 2018, gives isolated buildings their own Chapter 14. Owners with projects in both countries will recognize the structure at once.

The vocabulary matches; the numbers do not. A specification written for one code cannot be translated into the other by swapping section numbers.

What an owner should ask the engineer

1. Which analysis procedure, and why? If ELF, which Section 17.4.1 conditions were checked and at which bound. If response history, who selects and scales the ground motions.
2. What are D_M and D_TM at both bounds? These size the bearings, the moat, the utility crossings and the test rig. They belong in the basis of design, not the shop drawings.
3. Which bearing type did the design assume? A switch from elastomeric to sliding bearings, or the reverse, reopens the property bounds and possibly the analysis. Our LRB and FPS comparison explains why the two families do not behave alike.
4. Is the production test program written yet? Section 17.8.5 makes the engineer responsible for it. Without it at tender, bidders are pricing different things.
5. When does the design reviewer start? The first item on the Section 17.7 list is the design criteria. A reviewer hired after the analysis reviews rework.
6. Who owns the moat after handover? Section 17.2.4.8 asks for an inspection and maintenance program. The facilities team should receive it and know that the gap around the building is a structural element.

If you would like these questions worked through on a live project, book a 30 or 60 minute online consultation. For the wider picture, start with the complete guide to seismic isolation.

Sources

• ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 17, sections as cited. asce.org
• AASHTO, Guide Specifications for Seismic Isolation Design, 4th Edition (2014) with 2023 Interim Revisions. AASHTO Journal · TRID record
• Türkiye Bina Deprem Yönetmeliği (TBDY 2018), Bölüm 14, Resmî Gazete, 18 March 2018. resmigazete.gov.tr
• Design supervision and control scope under TBDY 2018, including Chapter 14 buildings. insapedia.com

Frequently Asked Questions

What does ASCE 7 Chapter 17 cover?

Chapter 17 of ASCE/SEI 7-22 sets the seismic design requirements for seismically isolated structures: general design and isolation system requirements, seismic hazard inputs, selection between the equivalent lateral force and dynamic analysis procedures, minimum displacements and forces, independent design review, and prototype and production testing of isolator units.

What is the difference between maximum displacement and total maximum displacement?

Maximum displacement, D_M, excludes the extra movement caused by actual and accidental torsion and is the design displacement of the isolation system. Total maximum displacement, D_TM, includes torsion and is used to verify isolator stability, to size structure separations such as the moat, and for the vertical load testing of prototype isolators.

When can the equivalent lateral force procedure be used for an isolated building?

Only when every condition in Section 17.4.1 is met at both property bounds, including Site Class A to D, an effective period of no more than 5.0 s, a superstructure of no more than four stories or 65 ft unless there is no uplift on the isolators, effective damping of no more than 30 percent, and no structural irregularity. Otherwise the dynamic procedures of Section 17.6 apply, and response history analysis is required for any isolated structure with supplementary viscous dampers.

Are the AASHTO Guide Specifications for Seismic Isolation Design the same as Chapter 17?

No. The AASHTO guide specifications cover seismically isolated highway bridges; ASCE 7 Chapter 17 covers buildings. They share the same isolation philosophy and both rely on bounded properties and bearing tests, but the analysis limits and testing provisions are written for different structures and cannot be swapped.

How does TBDY 2018 Chapter 14 differ from ASCE 7 Chapter 17?

TBDY 2018 uses two design levels, computing the isolation system displacement at DD-1 with lower bound properties and superstructure forces at DD-2 with upper bound properties, while ASCE 7-22 uses the single MCE_R level. The Turkish chapter also requires CE marked isolators, prototype tests in an independent laboratory accredited to TS ISO/IEC 17025, and production control tests on a random 30 percent of each unit type, expanding to all units if any fails.