Performance Based Seismic Retrofit Design
A short description of ASCE 31 and ASCE 41 (formerly 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. There are inherent assumptions about the resiliency of a new building built into building codes that may not apply to a retrofit. For example, the building code defines earthquake forces that a building needs to be designed to. These forces depend upon the use of the building, and the type of structural system, both of which are intertwined with other parts of the code. 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.
Building codes, also, have always focused upon public safety. Most building owners are unaware that the building code does not protect their building or its contents. The only goal of a building code is to protect the inhabitants of buildings; the building may be damaged beyond repair.
‘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) funded research which has resulted in two two documents that allow performance-based evaluation and design:
ASCE 31 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 a basic evaluation to be performed without extensive 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.
ASCE 41 is one of the first true performance based seismic upgrade building codes. Sophisticated clients have been using the FEMA 356 prestandard guidelines as an alternative to the arbitrary ‘percentage of current (or some other) code’ approach since before year 2000. In 2006, the ASCE adapted FEMA 356, with minor changes, as a standard. ASCE 41-06 has been adopted as part of the International Existing Building Code.
ASCE 41 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 standard 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 ASCE 41 and ASCE 31, should not be taken too literally at this point. The terms should rather be looked at as relative qualitative building performance levels.
Structural engineers, also, have a tendency to forget that all of the complex analysis in ASCE 41 is literally sitting upon statistical, approximate ground motion studies. Every few years, the US Geological Survey has been revising their ground motion maps based on new research or discoveries, sometimes quite dramatically. Hopefully the rate and magnitude of changes in ground motions will slow in the coming years.
The benefit of these new methodologies, though, to a building owner 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.
*I am intentionally separating 'evaluation' from 'retrofit'. There is ongoing discussion among structural engineers about the exact technical approach, but the general consensus is that evaluation methodologies need to be more forgiving of structural deficiencies than retrofit methodologies. This prevents a type of hair trigger in which small changes in ground motion studies or material properties, for example, could trigger a retrofit.