No abstract

The equivalent static eccentricities of seismic forces are usually defined by codes with simple expressions of the static eccentricity. This paper presents certain formulae for the exact calculation of these eccentricities on the basis of the dynamic response of a simplified model. From the parametric analysis of such formulae the determinative role of the torsional and lateral stiffness of the system becomes obvious for the correct evaluation of the equivalent static eccentricities. Finally, a proposal is made for the improvement of the static torsional provisions of the current codes.

No abstract

According to the model of Penzien and Watabe, the three translational ground motion components on a specific point of the ground are statistically noncorrelated along a well-defined orthogonal system of axes p, w, and v, whose orientation remains reasonably stable over time during the strong motion phase of an earthquake. This orthotropic ground motion is described by three generally independent response spectra Sa, Sb, and Sc, respectively. The paper presents an antiseismic design procedure for structures according to the above seismic motion model. This design includes a) determination of the critical orientation of the seismic input, i.e., the orientation that gives the largest response, b) calculation of the maximum and the minimum values of any response quantity, and c) application of either the Extreme Stress Method or the Extreme Force Method for determining the most unfavorable combinations of several stress resultants (or sectional forces) acting concurrently at a specified section of a structural member.

Isotropic multistory buildings are the ones characterized by the property: all load-resisting planar frames have proportional lateral stiffness matrices. In the present paper it is proved that the modal analysis of an N-story isotropic asymmetric, torsionally coupled, building (a problem of order 3N) can be separated into two independent sub-problems: (a) a sub-problem that corresponds to a single-story asymmetric, torsionally coupled, building (a problem of order 3); and (b) a sub-problem that corresponds to an N-story, torsionally uncoupled, planar frame (a problem of order N). It is also demonstrated that the orientation of peak modal seismic forces of the building is independent of the orientation of seismic excitation, which affects only their size. The separation provides a better insight into the structural behavior of asymmetric multistory buildings under earthquake ground motion.