Influence of Gross vs Transformed Cross Section in Computation of Lateral Displacements.

Abstract eng:
Lateral displacements due to earthquake loading are a limit state in design of building structures. After Loma Prieta (1989), Mexico (1985), Northridge (1994), Kobe (1995) and Nazca, Peru (1997) extended damage was attributed to excessive displacements. This meant large economic losses. Afterwards proposals were made to design based on displacement control. It became an important concern (SEAOC Vision 2000). After 1997 Peruvian Seismic Standard became very demanding lateral displacements and which effective stiffness should be considered in seismic analysis became an important issue. World standards and researchers have proposed different criteria as to what moment of inertia, or effective stiffness, should be considered in seismic analysis. This paper summarizes a study to evaluate how computed lateral displacements in buildings change due to various stiffness reduction criteria for reinforced concrete members (ACI 318, FEMA, Paulay & Priestley, E.030 Peru). A set of reinforced concrete buildings were design and spectral dynamic analyses performed. Variables included: a) structural configuration (regular or irregular in plan), b) earthquake resistant system (framed or dual) and c) height (4, 8, 12 and 20 stories). Amounting to a total of 16 building prototypes. Each of these was then analyzed with gross cross section (uncracked) and again for each of the three effective stiffness reduction criteria (cracked sections) given a total of 64 buildings. Lateral displacements for each of these buildings in both directions were tabulated for each of the stiffness reduction criteria to evaluate increment in displacements as compared to buildings with elements with uncracked sections. As expected an important increment on displacements was found with significant differences depending upon the reduction criteria and structural system. Differences were also found with building height. Largest increments were due to ACI criteria, smallest with FEMA,s and Paulay and Priestley criteria produced intermediate results. As an average, displacements using cracked sections increased around 60% as compare to those with uncracked sections. This information was taken into account in the 2016 version of Peruvian seismic standards that will now require seismic analysis be performed using uncracked sections.

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