Keywords: seismic, earthquake, reinforced concrete buildings, modelling.

The primary objective of this study is to mitigate earthquake hazards on existing reinforced concrete residential buildings; and to provide an educated answer to the inquisitive question "what will happen to the numerous building structures that were designed with a considerable safety factor to resist gravity loads and maybe wind loads, without consideration of seismic loads?" A case study building that reflects the design and construction practice in the region was considered in this study. The earthquake static forces, gravity loads and design requirements have been developed in compliance with the Uniform Building Code, 1997 (UBC-97) [1]. Two-dimensional analysis of a representative mathematical model was analyzed using the ETABS-software [2]. A simplified analytical two-dimensional model was developed constituting a simple dual model. A structural non-linear analysis program SNAP-2DX [3] was used for the inelastic static pushover (ISP) analysis as well as for the inelastic dynamic time history (IDTH) analysis. The IDTH response of the study building is considered unsatisfactory; the building has failed no later than 6 seconds of the selected ground acceleration record, which consists of 45 seconds. The story shear resulting from the IDTH analysis exceeded at lower stories 170% that of UBC-provision as well as that of the ISP analysis. Underestimation of lateral story shear at lower stories is very precarious since it may trigger a soft-story mechanism if not accounted for.

The inelastic static procedure, colloquially known as "pushover analysis", has become a standard method [4,5] for estimating seismic deformation demands in building structures as well as their local and global capacities. However, it may lead to gross underestimation of story drifts, story shears, and may eventually fail to identify correctly the location of formed plastic hinges.

The building's overall lateral response was found inadequate from the survivability standpoint. It has been verified that by designing buildings with abundant safety margins for gravity load resistance (more than 110% for beams in bending and more than 200% for columns under axial loads) does not guarantee the survivability of the structure and consequently safety of its inhabitants against developed earthquake forces. The study-building's lateral resistance was below the seismic codes' requirements, 78.8% of the designated UBC lateral loads, and response parameters such as inter-story drift index and rotational ductility ratio have well exceeded the allowed practical limits set out by the design codes. It has been concluded also that the use of an inelastic static procedure is recommended for structures in which "higher mode effects" are not significant, that is, in relatively low-rise buildings. If these higher mode effects are significant, then the procedure needs to be supplemented with a non-linear dynamic analysis.

1

Uniform Building Code, Seismic Design Manuals, 3 volumes, Intl. Conf. of Building Officials, 1997.

2

Computers and Structures Inc., CSI, "ETABS: Integrated Analysis, Design, and Drafting of Building Systems", Berkeley, CA, USA, 2008.

3

D. Rai, S.C. Goel, J. Firmansjah, "SNAP2DX: Structural Nonlinear Analysis Program", UMCEE96-21, University of Michigan, Ann Arbor, 1996.

4

L.F. Ibarra, R.A. Medina, H. Krawinkler, "Hysteretic Models that Incorporate Strength and Stiffness Deterioration", Earthquake Engineering Structural Dynamics, 34(12), 1489-1511, 2005. doi:10.1002/eqe.495

5

D. Vamvatsikos, C.A. Cornell, "Incremental Dynamic Analysis", Earthquake Engineering Structural Dynamics, 31(3), 491-514, 2002. doi:10.1002/eqe.141

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