Performance Assessment of Reinforced Concrete Planar Frame Using Nonlinear Analysis
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Date
2015-10
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Addis Ababa University
Abstract
Ground excitation is the major cause of failure in reinforced concrete structures constructed
in seismically active zone. This can be due to either the unpredictable nature of the seismic
excitation or due to lack of proper structural design, detailing and construction. During
seismic excitation the response of structures should be estimated appropriately. For response
determination of structures, codes fail to predict the real time response of structures. Hence
existing performance assessment should be done using a reliable and easily applicable
analysis procedure.
The revised Ethiopian building code (EBCS-EN-2014) shows a major change on the expected
peak ground acceleration (from 0.05g to 0.1g) for Addis Ababa. For the past several years
large numbers of buildings are designed and constructed using the previous building code.
The performance of these structures is highly affected by this change. There for it is
mandatory to assess the performance of the existing structures using provisions of revised
code. This study focuses on condominium buildings as they are constructed in large scale all
over the country and as they are areas where people are congregated. The study is done under
the assumption that premature failures are not expected to happen.
A planar frame was taken from the representative 3D case study building. The capacity
demand comparison was done for each member of the frame using EURO CODE and ACI
provision for stiffness reduction factor. The frame fails to satisfy minimum code
requirements even after the consideration of moment redistribution.
In the aim of saving the structure load reduction was performed as engineering solution. The
solution improve the structural performance but still it is unsafe.
A mechanistic solution was also proposed to utilize the reserve capacity of the structure. The
solution being pushover analysis with fiber based modeling which capture the nonlinear
properties. Seismostruct and SAP2000 software were used for analysis.
Element level verification was done using both software followed by frame level verification
which will address effect of redundancy. At relatively low loads prediction obtained from
both software give consistent result with the experimentally obtained data At a relatively high
load the result obtained from Seismostruct gives a better prediction of the experimental result.
With the aim of investigating the stiffness reduction factor recommended by EURO CODE
and ACI extent and progress of crack checked. For this particular frame it is found out that the columns reduction factor is closer to ACI provision while the beam reduction factor is
closer to EURO CODE provision.
Finally the case study building was assessed for its performance level, for soil class B the
result shows that the structure‟s performance level do not satisfies drift requirements for
operational and immediate occupancy but it is adequate for life safety and collapse
prevention.
From the study it is found that the use of pushover analysis results in 25.6% increment in
shear capacity from Seismostruct and 16% increment in shear capacity from SAP2000 for
this particular frame. The higher increment from Seismostruct is expected due to the plastic
zone consideration which is a plastic hinge in SAP2000. The advantage obtained by using the
pushover analysis may increase or decrease depending on the rate of progressive hinge
formation, length of the plastic hinge and other parameters.
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Keywords
seismic excitation, base shear, pushover, performance level, plastic hinge, plastic zone, fiber based modeling