Developing targeted and economical retrofit solutions for Pre-Northridge buildings
Steel moment-resisting frames are one of the most common lateral systems found in low rise and high rise steel-framed buildings. Many buildings built throughout the 1960’s to 1990’s in California were built with steel moment-resisting frame systems.
The 1994 Northridge Earthquake in the Los Angeles area and the 1995 Kobe Earthquake in Japan dramatically illustrated several weaknesses in traditional steel moment frame performance and design. Column fracture occurred in several buildings at the beam-column joint, and weld metal was found to have inadequate toughness and ductility to handle the cyclical loading associated with earthquakes. Further research and testing of “pre-Northridge” moment frame connections after those earthquakes revealed several other potential problems with steel moment frames including: weak column-strong beam configurations, “soft” story behavior, excessive building drift, partial joint column splice weld fracture, and ineffective welding inspection.
Forell/Elsesser has evaluated and retrofitted several “Pre-Northridge” steel moment frame buildings. The use of non-linear dynamic analysis enables us to simulate the expected behavior of the existing structure under seismic loading, maximize the utility of the existing structural system, and identify and prioritize potential weaknesses. We then develop unique, viable, and cost-effective options for each structure. In some cases, the building has proven to be generally adequate, needing only selective retrofitting for weaker elements. In other cases, the moment frame system has been shown to be inadequate and a new supplementary lateral system, such as ductile braced frames or passive dampers, is introduced to supplement the existing lateral system.
With our on-going participation in the Pacific Earthquake Engineering Research Center’s (PEER) Seismic Evaluation and Retrofit of Older Tall Buildings Group, Forell/Elsesser continues to stay involved in code development, research, and investigations of this important issue facing many building owners. Our deep understanding of the issues surrounding the older steel moment frame structures enables us to develop both targeted and economical retrofit solutions.