Τhe Behavior of Hybrid Reinforced Concrete-Steel Buildings under Sequential Ground Excitations

Abstract

In common construction practice, various examples can be found involving a building type consisting of a lower, older, reinforced concrete structure and a more recent upper steel part, forming a so-called “hybrid” building. Conventional seismic design rules give full guidelines for the earthquake design of buildings constructed with the same material throughout. The current seismic codes neglect to provide specific design and detailing guidelines for vertical hybrid buildings and limited existing research is available in the literature, thus leaving a scientific gap that needs to be investigated. In the present work, an effort is made to fill this gap in the knowledge about the behavior of this hybrid building type in sequential earthquakes, which are found in the literature to burden the seismic structural response. Three-dimensional models of hybrid reinforced concrete–steel frames are exposed to sequential ground excitations in horizontal and vertical directions while considering the elastoplastic behavior of these structural elements in the time domain. The lower reinforced concrete parts of the hybrid buildings are detailed here as corresponding to a former structure by a simple approximation. In addition, two boundary connections of the structural steel part upon the r/c part are distinguished for examination in the elastoplastic analyses. Comparisons of the arithmetical analysis results of the hybrid frames for the examined connections are carried out. The seismic response plots of the current non-linear dynamic time-domain analyses of the 3D hybrid frames subjected to sequential ground excitations yield useful conclusions to provide guidelines for a safer seismic design of the hybrid building type, which is not covered by the current codes despite being a common practice.

The Influence of Soil Deformability on the Seismic Response of 3D Mixed R/C–Steel Buildings

“…The foundation mat is dimensioned considering concrete grade C20/25 reinforced by steel bars Φ18/20 of steel type B500c in both horizontal directions, top and bottom, for all mixed models simplistically, according to current codes [2,3]. Concerning the interconnection of the steel and concrete parts, the subsequent situations are considered, following [6][7][8][9]: (i) a fixed support of the steel columns, i.e., transferring moments in both horizontal axes, named "uniform" in the current work; (ii) a stable interconnection of the steel vertical elements only along a single direction, such as the "minor internal axis" [1,2] of each column section, and an essentially "pinned connection" [6] on the other direction, such as the "major axis" [6] of the steel column section, suggesting that minimal-to-zero moment transfer is performed, named "release" here. The described support distinction is essential considering that in practical real construction, the "drilled-in epoxy type anchor rods" [6] used in the base plate connections of the steel vertical elements are unlikely to be fully anchored into a prior r/c one, meaning that the moment capacity of the base-plate connections may be limited.…”

“…Similarly, selected mixed 3D frames have been designed as new constructions [1][2][3] and examined by nonlinear time history (NLTH) analysis, considering the typical rigid soil assumption under strong earthquakes, as presented by Askouni and Papagiannopoulos [6][7][8]. For the select mixed 3D models analyzed by Askouni [9], which rested on rigid soil, the r/c part was considered as an older construction, while the steel part was considered as a custom construction, with sequential earthquake excitations showing a possible disadvantageous effect of the sequential type of excitation on the model response. The case of a mixed frame was analyzed by Pnevmatikos et al [10] using wavelet analysis by focusing on damage detection due to earthquake excitation.…”

“…The orientation angle of the ground excitation was found to affect the response of common buildings [37,38,[40][41][42]45,46] and special structures [47,48], so this is considered in the current analysis procedure, as mentioned in the next section. In addition, two boundary conditions of the connection of the higher part by steel upon the lower part by reinforced concrete are examined here similarly to [6][7][8][9]. This current work aims to investigate the behavior of low-to medium-rise mixed models under strong ground excitations, in 3D space in view of the effect of deformable supporting soil in comparison to the rigid soil assumption, which has not been clarified by the available worldwide literature so far, despite evidence that the SSI may strongly affect the seismic behavior of common structures [37][38][39][40][41][42][43][44].…”