chapter  3
28 Pages

Seismic response, seismic design

Recent research on the inelastic earthquake response of non-symmetric, multistory buildings designed in accordance to modern codes, has indicated that ductility demands under the action of design-level earthquakes are not distributed evenly throughout the structure, as it would be desirable for a well balanced design. In fact, it has been found that frames at the so called “flexible” edges of the buildings exhibit higher ductility demands, compared to the frames at the opposite sides, the so called “stiff” edges of the building. This has been observed in reinforced concrete buildings designed according to the European and US earthquake resistant design codes, EC8 and IBC, respectively. In some cases the differences in the computed ductility factors between elements at the two opposite edges exceeded 100%. This is undesirable because it may lead to premature failure of members at the “flexible” edges. In the present paper this phenomenon is verified for a number of eccentric steel braced frame buildings, also designed in accordance with the EC8 code. One, three and five story buildings have been used, with original mass eccentricities εm = 0 (symmetric), εm = 0.10 and εm = 0.20, but due to space limitations results are presented only for the 3 and 5 story buildings. The verification is made through non linear, time history, inelastic dynamic analyses of detailed building models of the plastic hinge type. Groups of ten, two component, artificial motion pairs, generated from historical records to closely match the design spectrum, are used as input. Mean values of ductility factors, based on axial deformations for brace members and on plastic hinge rotations for beams, are the indices compared for the stiff and flexible edges of each building along the x and y directions. Subsequently, a design modification is introduced to alleviate the problem. This modification was to increase the bracing sections at the flexible edges and decrease them at the stiff edges by factors based on the top floor edge design displacements. The results from the modified design are quite satisfactory and meet the objective of more uniform distribution of ductility demands. The improvement can be seen by comparing ductility demands in Figures 1 and 2 (original vs. modified designs) for braces and beams of the 5-story eccentric buildings with εm = 0.10 and εm = 0.20

Advances and Trends in Structural Engineering, Mechanics and Computation – Zingoni (Ed.) © 2010 Taylor & Francis Group, London, ISBN 978-0-415-58472-2

A.S. Elnashai & A. Lewis Mid-America Earthquake Center, Civil and Environmental Engineering Department, University of Illinois at Urbana-Champaign, Illinois, USA

ABSTRACT

The MAE Center team traveled to Haiti February 27th through March 6th as part of an Earthquake Engineering Research Institute (EERI) field reconnaissance team. The paper reports early observations from the Mw 7.0 Haiti earthquake of January 12th, 2010. Considering the economic impact of this event, the Haiti earthquake may be the most destructive modern natural disaster ever.