Performance Expectations in Current Codes

Uniform Building Code

The Uniform Building Code (UBC) follows a prescriptive, traditional approach, as was stated in the introductory section of notes. The performance goals, which are shown below, are traditional as expected.

Earthquake Intensity
Frequency of Occurence
Desired Performance
Several times during service life
No damage to structure or nonstructural compontents
One or more times during service life
Limited dampage to nonstructural components and no significant damage to structure
Major (Catastrophic)
(10% exceedance in 50 years)
Rare and unusual event as large as any experienced in vicinity of site
No collapse of structure or other damage that would create a life safety hazard

The definitions of the frequency of an earthquake's occurence and the corresponding desired performance are vague and subjective. Terms such as "limited damage" and "one or more times" are good examples of this. Three levels of performance under different earthquakes are desired, but only one design earthquake is given - the major event. Since no design earthquakes are given for the minor and moderate events, an indirect approach is employed to attempt to achieve the desired performance. Various checks are employed, but no design checks are specifically tied to a limit state goal, such as the drift limit. The uncertainty as to which limit state a particular provision is targeting and the ambiguity of in the definitions discussed above lead to wide variations in interpretation of the code. Thus, the performance of structures designed according to the UBC can vary widely as well.

Other Current Codes

Other current codes also contain ambiguous performance goals and are plagued by the same variability in interpretation and performance as the UBC.