Thesis Open Access

Seismic Performance Evaluation of Stiffness Irregular Rc Buildings Due To Story Height Difference

Abdulhamid Yassin

Thesis supervisor(s)

Tesfaye Alemu ( PhD )

In high rise buildings or multi story buildings, soft-story construction is common because of story height
difference due to architectural consideration. These provide stiffness irregularities on the building
structures, which reduce the stiffness of the lateral load resisting system and sudden reduction in stiffness
causes higher stress to be concentrated at the stiffness reduced story columns, which lead the columns to
be unable to provide adequate resistance during the earth quake.
Taking into consideration the above points, the structural building frames must be evaluated and designed
by considering the effect of soft story on seismic performance of the building. The main focus of this
research is to study the seismic performance of stiffness irregular building due to significant story height
difference on RC buildings by implementing numerical models on the basis of finite element principles.
For these two regular buildings G+10 and G+20 is developed to be considered as base case structure, and
cases that represent irregular structures are defined by modifying vertical distribution stiffness of the base
case. The stiffness irregularity is created by increasing the height of ground floor and second floor of
base case 200%. Accordingly, different model cases of RC building are analyzed and designed on ETABS
2016.2.1 (CSI ETABS 2016, Integrated Building Design Software, Computers and Structures Inc.
Berkeley). Analysis and design of the proposed building model cases followed the conventional design
approach as prescribed on the new Ethiopian Buildings Code Standards (ES EN: 2015). The design
outputs of main structural elements are then used for the numerical model on SeismoStruct [SeismoSoft,
2016]. Pushover and nonlinear dynamic analyses are carried out for obtaining the response in terms
fundamental periods, base shear-top displacement, inter-story drift, lateral displacements, and fragility
curve at different performance levels.
The result of this research shows the following: the effect of stiffness irregularity on fundamental period
have more significant contributions as the soft-story number and number of story or building height
increase, for all cases the Story drifts demands increase in the soft story and decrease in most of the other
stories and drift of most floors are greater than the allowable drift according to ES EN limit (drν≤0,005h),
the seismic base shear for stiffness regular building are greater than the stiffness irregular one and finally
the probability of failure for each limit state capacities of stiffness irregular building model is greater than
stiffness regular buildings models in all cases.

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