Question: I noticed that with the updates of codes it seems that with the passage of time the seismic parameters (response spectral accelerations) are increasing. For example if we compare earlier versions of AASHTO (if we talk about bridges) and the 2013 version, the earthquake changed from 475 years to 1000 years return period earthquake, while the R factor and load factor for earthquake remained same. Wasn't this change in early 2000's a drastic? The earlier structures (bridges for Aashto) required retrofit? Should have been a huge retrofitting activity!For Pakistan we have only PGAs for 475 years in PBC. Practicing [...]