Report - PEER - University of California, Berkeley

large rotational response can be explained by noting that both the CR and CV arelocated on the same left hand side of CM. On the other hand, the two models withstrength distributions that satisfied the balanced CV-CR location criteria show similarlow rotational response.4. CONCLUSIONSDespite of all the codified torsional provisions available, damage caused by excessiverotations has been observed in all recent major earthquakes. One reason may be thatsome of the damaged buildings were built prior to the adoption of the provisions. It isalso possible that some of the damaged buildings were built without attention beingpaid to the codified provisions. A third possible reason may be that the currentprovisions are not as effective as we think. Many parametric studies have been carriedout for the last two decades to validate the provisions. Most of them were based onmodels having lateral force resisting elements with independent strength and stiffnessproperties. However, recent studies suggest that code procedures may not be effectivewhen the interdependence of strength and stiffness in the resisting elements isconsidered. This issue deserves further investigation.Extending ideas developed on single storey models to multistorey structures canbe problematic. Properties such as strength and stiffness are no longer scalarquantities. It may no longer be possible to use a single value for the plastic centroid orthe rigidity center in the most general case of horizontally irregular multistoreystructures. There is hope, however, that those concepts can be applied to specialclasses of multi-storey structure that are vertically regular. To classify multi-storeystructures and to extend lessons learned from single mass model studies to specialclasses of multi-storey structures remains a challenge. At present, simple shortcuts donot appear to be available for predicting the response, particularly the shear demand,of horizontally irregular multistory structures where higher modal contributionsbecome significant.In this age of performance-based seismic design, it is only fair to ask thefollowing questions. Are we wiser now than in the days when Rosenblueth andHousner warned the profession about the dangers of asymmetry? Do we now havebetter design guidelines to mitigate torsional damage due to horizontal irregularity?The answer to the first question is a tentative “may be”. Regarding the answer to thesecond question, we believe that the jury is still out.ACKNOWLEDGEMENTThe support of the Natural Science and Engineering Research Council (NSERC) ofCanada to the second author is gratefully acknowledged.380