Safety Assessment Program Evaluator Student Manual

CA Emergency Mgmt. Agency Safety Assessment Program Evaluator Manual April 2011frequencies are shorter, the strongest effects are felt by shorter, stiffer structures whose periodsof vibration are closer to matching the period of the ground waves. Taller, more flexiblestructures will be affected most severely by the long frequencies that will continue out beyondthe near field.In the near field, the strong shaking that is felt by structures will have significant vertical andlateral components. Since the vertical load system of buildings is designed for more than deadload, or gravity load by itself, the additional vertical forces are normally not a threat to thestructure. However, lateral shaking from earthquakes subjects structures to both shear andoverturning forces. These threats require structures to have a complete lateral force resistancesystem, which may either be part of the vertical load system, or separate from it.In the far field, longer seismic waves that are undamped by local geology cause unusual effectsthat can severely damage taller, longer period structures. When the fundamental period of a sitematches that of the structure built on it, seismic shaking can cause resonance that amplifies thestructure‟s response. The collapse of buildings between 10 and 20 stories in height in Caracas in1967, and of 8 to 12 story buildings in Mexico City in 1985, are unfortunate examples of thiseffect.Photo courtesy FEMAFigure 2-2 – Shear cracking in masonry infill wallsThe above photo illustrates classic X-shaped shear cracking that occurs in response to lateralseismic forces.33