Performance Based Seismic Retrofit Design

A short description of FEMA 310 and FEMA 356 based seismic evaluation and design methodologies.

The state of the art in seismic evaluation and retrofit is constantly in flux, and after every major earthquake undergoes major revisions. Most recently, much research and effort has gone into so called 'performance based designs.'

The problem in the past has been that seismic retrofits and evaluations were somehow linked to current codes, which are usually designed for new buildings. It was up to the engineer working on that particular project to decide which parts of the code to use, what percentage of the code forces to design to, and which parts were prohibitively expensive. Some cities with large numbers of old buildings, such as San Francisco, created their own sub-chapters of the building code that attempted to address this issue and at least create a uniform standard.

Still, these code-based approaches have typically been fundamentally arbitrary. ‘Performance based design’ aims to provide an alternative, potentially more rational approach. For the design of seismic retrofits, performance based design can be understood as design based upon two distinct parameters: the magnitude of the earthquake, and the expected performance of a building in that earthquake.

The Federal Emergency Management Agency (FEMA) has published two documents that allow performance-based evaluation and design:

FEMA 310 provides a process for seismic evaluation of existing buildings. It makes some simplifying assumptions about the use of the building and size of earthquake that enable an evaluation to be performed without extensive geotechnical investigations. The evaluation is broken up into three 'tiers' of progressively more complex calculation and investigation. A building owner may have only one level of evaluation, or may decide that the building's importance merits going straight to the most sophisticated method of evaluation in order to determine if an upgrade is needed.

• Tier 1 is a fast screening evaluation that looks for fundamental flaws in structures. It relies heavily on engineering judgement - less on calculations - through a series of checklists of potential building flaws. Tier 1 tends to be quite conservative. Elements of a building that do not conform during the tier 1 evaluation should be evaluated according to the tier 2 evaluation, or the building owner may choose to stop at this point and perform a seismic upgrade on those components.
• Tier 2 is a more calculation-intensive evaluation, and may require material testing from the building. Depending upon the potential problems with the building, a tier 2 evaluation may require a three dimensional computer model to be made of the building. Again, the building components not conforming in the tier 2 evaluation may be upgraded, or analyzed further.
• Tier 3 is a highly calculation intensive evaluation. FEMA 310 does not specifically list an evaluation method for this, but recommends using FEMA 356. For projects that plan to use FEMA 356 for their upgrade anyway, it is often most efficient to go immediately to this tier of evaluation, since the same investigation and calculations needed for the evaluation will also cover what is needed for design.

FEMA 356 is currently a 'prestandard' and will soon be transformed into some of the first true existing building performance based seismic upgrade building codes. Sophisticated clients have been using the FEMA 356 guidelines as an alternative to the arbitrary ‘percentage of current (or some other) code’ approach since before year 2000.

FEMA 356 breaks buildings down into components and evaluates each of the components based upon their expected performance in a given seismic event. This approach dramatically increases the complexity of the analysis and design work, but also theoretically provides a rational, flexible way to aim for more efficient and better-understood building performance.

Typically, unless buildings have very special performance requirements, retrofits conform to what the prestandard calls the 'Basic Safety Objective.'

The BSO is intended to approximate the earthquake risk to life safety traditionally considered acceptable in the United Sates. Buildings meeting the BSO are expected to experience little damage from relatively frequent, moderate earthquakes, but significantly more damage and potential economic loss from the most severe and infrequent earthquakes that could affect them. The level of damage and potential economic loss experienced by buildings rehabilitated to the BSO may be greater than that expected in properly designed and constructed new buildings. - FEMA 356 commentary.

The BSO includes a performance objective of 'Life Safety' at a more frequently occurring 'BSE-1' (10% probability of exceedance in 50 years - mean return period 474 years) earthquake, and 'Collapse Prevention' at a much rarer 'BSE-2' (2% probabiliy of exceedance in 50 years - mean return period 2,475 years) earthquake. The concept is that in a very rare – but devastating – earthquake, the structure would be prevented from completely collapsing, but would have virtually no reserve strength left. In more frequent earthquakes – the BSE-1 earthquake is close to the code 'Design Basis Earthquake' – the structure would retain some reserve strength and also would provide protection for the inhabitants against some of the worst life safety hazards.

Performance based design is still in its infancy. Unfortunately, it is very difficult to properly test – prior to an actual earthquake – how well a design procedure is both going to perform, and more importantly be interpreted by engineers and actually be implemented by builders. Terms such as ‘Life Safety’ and ‘Immediate Occupancy’, which have technical meanings in FEMA 356 and FEMA 310, should not be taken too literally at this point. The terms should rather be looked at as relative qualitative building performance levels. The benefit of these new methodologies, though, to a client are that they take the rather hidden assumptions that are always present in our building codes up to this point – that a building is ‘safe’, for instance – and make them into clear, up front design decisions. The hope is that this will lead to a more sophisticated understanding and approach to building performance.