The Variation in Elastic and Inelastic Seismic Demand - Essay Example

Steel being the choice of material, this geometric form was adopted with few changes by architects of a different era and became a distinctive feature of Modern Architecture (Arnold 1996). Besides the architectural factor, several steel building, rising up with architectural setback, is the outcome of the zoning and planning regulation mandated for the region. Geometric form with architectural setback mandated by urban planning and zoning regulations for high-rise buildings and skyscrapers located in medium and higher-density districts. The first zoning laws in New York first mandated this building form, in 1916 to reduce the shadow of the high-rise building on streets and sidewalks (Naeim 1989). According to New York zoning law, the buildings established between 1916 and 1960 were raised up to a certain height. The height of the lower part is defined with respect to the width of the street on which the building was established. Hence, in narrow streets, the setback started at the lower story. Starting from that level, they had to set back until the planned area of one-fourth of the whole site was reached. (New York Skyscrapers 1996)

 In the early years, the effect of setbacks on the seismic performance of steel frames was not the main concern of architects and engineers, especially, if they were constructed in medium or low-seismicity regions. Although few steel frames experienced severe damage during a past earthquake, no direct correlation between the damage and vertical irregularity was found (Youssef, Bonowitz & Gross 1995). In several regions, restrictions set for different buildings are believed unnecessary for buildings designed with steel frames. (Arnold 1996) and (Naeim 1989)

Earthquake-resistant design provisions and guidelines address vertical geometric irregularity as an issue related to the accuracy and reliability of analytical methods, used to define elastic and inelastic seismic demands. Once the structural configuration is defined, a primary evaluation procedure is employed to assess the presence of irregularity.