What is Nonlinear Analysis? A nonlinear analysis is an in-depth investigation of how a building will move in an earthquake. The stiffness of the building is analyzed using computer software. There are two main procedures. A push over analysis is a two-dimensional analysis in which the loads are increased on the building until it is pushed over. The other option is a time history analysis, where the building will be analyzed in three dimensions for specific earthquake ground motions over time. In both cases, the building is forced to move a code prescribed distance and member forces and stresses are then checked against code limits. I liken the building’s design as Muhammad Ali in his fight against George Foreman. Muhammad Ali’s “Rope a Dope” strategy was to lean against the rope and let the rope absorb George Foreman’s punches. Huge punches will be thrown, but the building is designed to move and dissipate the energy rather than stand there and get knocked out. Best Applications for Capacity Based Design and Nonlinear Analysis In seismic areas, BHB believes capacity based design should be used regardless of the design method chosen. It is required by code except in certain building types and usage. BHB uses it in all buildings. Building owners and architects should ask for it on all buildings. Architects should ask where the fuses occur and work with the structural engineer to limit non-structural damage in these areas. Nonlinear analysis is not for every job; however, it is a great tool on high-value buildings. Large multi-story buildings will perform better because of the advanced analysis. The analysis can also be used in historical buildings to minimize impacts of a seismic upgrade. In any building, occupants will be safer. It is easier to control earthquake damage because it is better understood where it will occur. Benefits of Capacity Design and Nonlinear Design Strengthening buildings into expensive, tank-like structures does not lead to better performance in earthquakes. Engineers instead should use capacity based design to dissipate energy and allow buildings to move properly in an earthquake. Advanced analytical procedures, such as nonlinear analysis, can be used to ensure the building performs to code and the owner’s objectives while saving money. Through advanced seismic design procedures, such as capacity based design and nonlinear analysis, structural engineers can design for the unknown. Rather than guessing at the forces, we can design our buildings to dissipate energy and be safe havens, regardless of the strength of the earthquake. b Brett Goodman is a principal at BHB Structural. He is a registered structural engineer in Idaho, and he has 17 years of structural engineering experience. He can be reached at brett.goodman@bhbengineers.com. Architects & Engineers aecriskpro.com | (208)440-7075 Insurance and Risk Management for 100% Employee-Owned 37
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