Accidental torsion in buildings due to base rotational excitation

Abstract

This investigation is concerned with accidental torsion in buildings resulting from rotational excitation (about a vertical axis) of the building foundations as a result of spatially non‐uniform ground motions. Because of this accidental torsion, the displacements and deformations in the structural elements of the building are likely to increase. This increase in response is evaluated using actual base rotational excitations derived from ground motions recorded at the base of 30 buildings during recent California earthquakes. Accidental torsion has the effect of increasing the building displacements, in the mean, by less than 5 per cent for systems that are torsionally stiff or have lateral vibration periods longer than half a second. On the other hand, short period (less than half a second) and torsionally flexible systems may experience significant increases in response due to accidental torsion. Since the dependence between this increase in response and the system parameters is complex, two simplified methods are developed for conveniently estimating this effect of accidental torsion. They are the ‘accidental eccentricity’ and the ‘response spectrum’ method. The computed accidental eccentricities are much smaller than the typical code values, 0.05bb or 0.1b, except for buildings with very long plan dimensions (b ≥ 50 m). Alternatively, by using the response spectrum method the increase in response can be estimated by computing the peak response to each base motion independently and combining the peak values using the SRSS rule.

Effect of Material Variability and Mechanical Eccentricity on the Seismic Vulnerability Assessment of Reinforced Concrete Buildings

“…Also, non-uniform distribution of the columns material in r.c. buildings can produce torsional effects [24][25][26][27]: hence the definition of "material eccentricity" e m . Conventional analyses on existing buildings generally consider uniform materials by assuming an average value obtained from an experimental test.…”

“…It is a consequence of their dependence only on the average mechanical properties of the columns. In other terms, material eccentricity involves torsional modes [27,30] as well as a geometric eccentricity but this aspect is not discussed in the current Standards [20]. The multimodal and adaptive pushover appear the most suitable methods to consider torsional effects here induced [31,32].…”

Torsional Response Induced by Lateral Displacement and Inertial Force

“…As causes of the torsional response of buildings, researchers have studied eccentric distribution of stiffness (Georgoussis, 2009;Stathi et al, 2015), damping (Goel, 2000;Lin and Tsai, 2007), and the mass of a structure (Chandler and Hutchinson, 1986), in addition to spatially non-uniform or torsional ground motion input (De la Llera and Chopra, 1994;Heredia-Zavoni and Barranco, 1996;Basu and Giri, 2015;Falamarz-Sheikhabadi and Ghafory-Ashtiany, 2015;Gičev et al, 2015). A comprehensive review article has also been written by Anagnostopoulos et al (2015).…”

Effect of stiffness variability on the response of isolated structures

“…The structure was assumed to have multiple lateral load resisting planes located symmetrically about the centreline of the building, each with a sti!ness that varied randomly from the nominal design sti!ness. Numerical response spectrum analyses were conducted to study the e!ects of the sti!ness variability [5] and base rotation [6]; results were also compared to those of Monte Carlo simulations [5]. While their studies focused on the response of buildings, many of their "ndings are applicable to the response of isolated structures.…”

“…Among the factors considered were rotational motions at the building foundation, uncertainty in the sti!ness of structural elements, uncertainty in the location of the structure center of mass, and uncertainty in sti!ness and mass distributions of the stories other than the one analyzed. Their model consisted of a rectangular single-storey building that was free to translate and rotate (a two-degree-of-freedom system) [5,6]. The structure was assumed to have multiple lateral load resisting planes located symmetrically about the centreline of the building, each with a sti!ness that varied randomly from the nominal design sti!ness.…”