This book will appeal to specialist engineers with enquiring minds and, in a broader sense, to all who live in buildings, especially those in a seismically active region. While the continual development of building codes permits the design of new construction to resist earthquake loads more effectively, a major problem is that buildings which may once have conformed to past seismic codes become structurally unacceptable according to today's codes and need upgrading or 'retrofitting'. Poor construction methods also result in buildings that do not conform to any codes. Existing 'vulnerable' buildings that require strengthening in earthquake-prone countries immensely outnumber all recent construction.