Moving Toward Performance-based Engineering
Vision 2000


Vision 2000 is an attempt by practicing engineers to improve current codes in a significant manner. The definitions of performance states being developed are being incorporated in the appendices of the SEAOC Recommended Lateral Force Requirements and Commentary. Vision 2000 focuses on defining what constitutes a frequent, rare or very rare earthquake, and on describing in detail what the performance states are that one wants for different types of events and structures. This is not yet a design method, but that is coming. All figures and tables shown below are after Vision 2000, SEAOC.

Basic Approach

A relationship, which is shown below graphically, is developed between the performance objective, type of facility, probability of earthquake occurence, which is then tied to response parameters related to each performance objective. These parameters are identified and some initial estimates are quantified.

As shown, the performance objective increases (i.e. there should be less damage) for a high probability earthquake (one that may occur several times during the life of the structure) or for an important structure or dangerous occupancy (i.e. a hospital or dynamite plant). Conversely, more damage is acceptable for a rare, severe earthquake or for less critical or temporary facilities. Thus, a building would be expected to suffer more damage if it were subjected to a more severe, less likely earthquake. Also, a more critical building would be exepected to have less damage for the same earthquake probability.

A basic structure would be expected to have essentially no damage if subjected to an even with a 10% probability of occurence in 30 years whereas it would be near collapse if subjected to an event with a 10% probability within 100 years. One can substitute more appropriate numbers for a particular project, or upgrade the characterization of the structure (to an essential facility, for instance, where the structure would be designed to remain life safe during the very rare event).

This method removes some of the ambiguity from the current SEAOC recommendations. The method still needs to indicate what performance parameters to consider (drift, stress, plastic hinge rotation, acceleration, etc. and what limits are to be imposed to achieve a particular performance objective. Some information on performance parameters was provided in Vision 2000 for basically the first time, but it was for the most part based on consensus rather than on test data or quantitative field observation.

Performance States

Vision 2000 has four performance (i.e. limit) states:

Fully operational
Continuous service. Negligible structural and nonstructural damage.
Operational
Most operations and functions can resume immediately. Structure safe for occupancy. Essential operations protected, non-essential operations disrupted. Repair required to restore some non-essential services. Damage is light.
Life Safe
Damage is moderate, but structure remains stable. Selected building systems, features, or contents may be protected from damage. Life safety is generally protected. Building may be evacuated following earthquake. Repair possible, but may be economically impractical.
Near Collapse
Damage severe, but structural collapse prevented. Nonstructural elements may fall. Repair generally not possible.


Occupancy Classifications

Three occupancy types are considered in Vision 2000:

  • Safety Critical Facilities
    - Large quantities of hazardous materials such as toxins, radioactive materials, or explosives with significant external effects of damage to building
  • Essential/Hazardous Facilities
    - Critical post-earthquake facilities such as hospitals, communications centers, police, fire stations, etc.
    - Hazardous materials with limited impact outside of immediate vicinity of building such as refineries, etc.
  • Basic Facilities
    - All other structures

Earthquake Classifications

The earthquake intensity is described quantitatively in probabilistic terms as follows:

Earthquake Classification
Recurrence Interval
Probability of Occurence
Frequent
43 years
50% in 30 years
Occasional
72 years
50% in 50 years
Rare
475 years
10% in 50 years
Very Rare
970 years*
10% in 100 years

* need not exceed mean + 1 standard deviation for the maximum deterministic event


Drift Limits

Drift limits have recently been added to Vision 2000, and are shown below.

Limit State
Permissable Maximum Drift (%)
Permissable Permanent Drift (%)
Fully operational
0.2
negligible
Operational
0.5
negligible
Life Safe
1.5
0.5
Near Collapse
2.5
2.5

These limits raise several questions. How do we calculate the maximum drift (or maximum permanent drift) and prove we satisfy these criteria? Why are these criteria selected? Will a building at 2.6% drift collapse?


Damage to Steel Moment Frames

Since there is a large jump in damage from the Operational limit state to the Life Safe limit state, another limit state related to repairability may need to be added. In this new state, damage would be limited to make repair quick and/or economically feasible. However, since damage is difficult to quantify and economics issues are owner sensitive, intermediate states are difficult to incorporate in a code.

Limit State
Damage Description
Fully operational
Negligible
Operational
Minor local yielding at a few places. No observable fractures. Minor buckling or observable permanent distortion of members.
Life Safe
Hinges form. Local buckling of some beam elements. Severe joint distortion. Isolated connection fractures. A few elements may experience fracture.
Near Collapse
Extensive distortion of beams and column panels. Many fractures in connections.

 


Limitations

The Vision 2000 approach has several limitations. First, it does not suggest analytical approaches nor methods to assure the reliability of the structure. Also, intermediate limit states are difficult to quantify. In addition, note that Vision 2000 is an uncoupled approach - that is, we end up with a deterministic procedure based on a probabilistically determined spectrum. Load and resistance factors still remain to be determined to provide desired reliability. Lastly, identification of limit states by a subjective name (i.e. fully operational) may lead to legal problems. The Japanese have avoided this potential problem by using a letter system (category A, B, C, etc. performance objective). Probabilistic specification fo response parameters may be better, however.