| Summary: | 博士 === 國立臺灣科技大學 === 營建工程系 === 100 === Reinforced concrete (RC) columns are the most important elements in public and school buildings for bearing vertical loads. Generally, columns are slender and can be formed to meet ventilation and lighting requirements. Once columns fail, the loss in overall building safety and stability induces irreparable damage.
Flexural failure usually occurs in slender columns; a great deal of discussion in the literature is about ultimate displacement, as calculated with the statistical method. However, there has been little discussion on post-bar buckling displacement and residual strength for flexural failure modes because test-related information is not available for investigation; most tests aim to investigate the reasons for buckling and its prevention. Laboratory safety and test difficulty are the main concerns, so few studies have focused on the residual strength and bar buckling displacement after ultimate displacement.
In old Taiwanese school buildings, brick walls are widely used as partition structures for larger space requirements, but the strength of the brick walls due to the out-of-plane direction is ignored in most cases. The National Center for Research on Earthquake Engineering (NCREE) investigated the residual strength in-situ pushover test for the seismic capacity of school buildings out-of-plane. Rather than attempting to change existing doors and windows, it seems more convenient to retrofit the confined brick walls between classrooms with carbon fiber reinforced plastic to improve their out-of-plane seismic capacities. The residual strength is derived from the fact that the infilled brick wall takes over the axial load from the columns and delays their axial failure.
In this study, the University of Washington database of columns was used to propose a formula for the post-bar buckling displacement and residual strength as well as a trilinear curve model for the load displacement at four points: the maximum state point, the bar buckling state point, post-bar buckling state point, and residual state point. For the RC frames infilled with brick walls, a residual strength model for brick walls is proposed; tests were conducted on five full-scale specimens.
Analysis based on the proposed model yielded the following results: (1) Fail indicators for the cover spalled with bar buckling. (2) Reinforcement of the yield strength increased 1.25 times over that calculated when the column strength was at the plastic hinge, and satisfied the test results of strength for the BB and PB states. (3) The residual strength of frames infilled with brick walls, however, was clearly observed. (4) The retrofitted specimens exhibited improved structural performance compared to non-retrofitted specimens in terms of both the maximum strength and residual strength. (5) This residual strength can prevent frames from immediate collapse.
For comparison, the analysis results were assumed to show a similar trend to the test results; this assumption is proved reasonable. To further validate this proposed model as being able to efficiently predict other laboratory experiments and practical application in field tests, research on four single-column specimens constructed at the NCREE was adopted to analyze the results. The proposed model was found to effectively and reasonably predict the test results. For practical engineering applications, a single frame of a classroom test containing the second floor with three spans by the NCREE was performed by pushover analysis. The setting of plastic hinges employed the proposed model of this study to simulate flexural failure. The results showed that the analysis results had a similar trend to the results of the collapse test.. For the model of the RC frames infilled with brick walls, the proposed analytical model predicted the out-of-plane load–displacement relationship of the frames with flexural failure.
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