Experimental evaluation of damage effect on dynamic characteristics of concrete encased composite column-beam connections
Introduction
The application of steel and concrete in the same section as structural elements which are also known as composite structural members goes to decades ago. Composite structural elements provide the combined behavior of two or more structural materials such as steel and concrete. Due to sufficient structural performances such as high stiffness, enough strength, good ductility and large energy absorption capacity, composite steel-concrete structural members are used in vertical and horizontal structural elements widely in recent years [19].
Concrete encased composite columns constructed from the encasing of I, H, pipe or tube steel profile in reinforced concrete columns are one of the best composite structural elements in vertical bearing systems. When the encased steel composite section is tube or circular hollow, it is named as concrete encased concrete filled steel tube section. The main advantages of these types of columns are, having high stiffness and enough strength with small cross section in comparison to other types. In addition, the fire resistance of these types of columns is higher than conventional steel and concrete columns, because the encased profile is fully covered with a concrete layer [36]. Another remarkable advantage which should not be ignored is the corrosion resistance of these columns. Using such types of columns is preferred in ashore areas. On the other hand, the big disadvantage of the concrete encased composite column is the difficulty associated with its construction.
Determination of structural response for concrete encased composite column using finite element analysis is very difficult because of many uncertainties and assumptions in the modeling process. Therefore, non-destructive experimental measurements should be performed to validate the numerical results and/or to obtain the real structural behavior. Ambient vibration based operational modal analyses method is one of the most efficient and useful tool which can be easily applied for these sort of structures.
Many numerical and experimental researches have been conducted in order to evaluate the structural performance of concrete encased composite columns. An and Han [5] studied the behavior of concrete-encased CFST columns under combined compression and bending. It is inferred that the column-beam connection zones show best performance in lateral dynamic loads such as earthquake, wave or wind. This phenomenon is studied by many researchers with respect to cyclic loading considering different types of connection details [8,10,15,16,24,[28], [29], [30],34]. Ren et al. [26] conducted an experimental study to determine the performance of concrete encased concrete filled steel tube columns under compression and torsion at the same time. Chen et al. [9] presented a detailed study related to depth ratio of encased profile which is directly affecting the flexural ductility of the composite columns. A series of experimental tests were carried out for post fire behavior of composite structural elements to investigate the efficiency of exterior concrete cover [32,33]. Although a great deal of studies related to concrete encased composite columns, beams and connection types are available, there are no enough researches regarding evaluation of the dynamic characteristic of composite structures based on ambient vibration test results.
The rest of the paper is laid out as follows: Section 2 summarizes the basic formulation of ambient vibration test using Enhanced Frequency Domain Decomposition (EFDD) and Stochastic Subspace Identification methods (SSI) with Modal Assurance Criterion (MAC) value. Section 3 presents a brief test setup, column-beam connection type and loading protocol with its geometric configuration. Section 4 presents the modal characterization of the composite columns by means of experimental measurements. This section is also devoted to discussion and comparison of results. Finally, Section 5 draws the main conclusions of this study.
Section snippets
Enhanced frequency domain decomposition (EFDD) method
EFDD method is an extension of FDD technique. In this method, modes are simply picked locating the peaks in singular value decomposition plots calculated from the spectral density spectra of the responses. In EFDD, the single degree of freedom (SDOF) Power Spectral Density (PSD) function that is identified around a peak of resonance, is taken back to the time domain using the Inverse Discrete Fourier Transform. The natural frequency is obtained by determining the number of zero-crossing as a
Test setup
In this paper, it is aimed to determine the damage effect on dynamic vibration characteristics of concrete encased composite columns considering different column-beam connection types. For experimental investigation, four half scale composite column-steel beam specimens were constructed in laboratory. Fig. 1 shows three dimensional view of the test setup including geometrical dimensions. Each structural part was designed according to the AISC 360-10 [1]. The steel used in the study was
Ambient vibration test
Ambient vibration tests were carried out to extract the dynamic characteristics such as natural frequencies, mode shapes and damping ratios. In the ambient vibration tests, B&K 3560 type data acquisition system with 17 channels, B&K 4506-B003 type tri-axial accelerometers which have 500 mV/g sensitivity and 0.3-2000 Hz frequency range, signal cables, PULSE [23] and OMA [20] software were used as the test equipment. Ambient vibration tests were carried out during 15 min considering 0-100 Hz
Conclusions
This paper is the first one that presents vibration based modal parameter identification of concrete encased composite columns considering different column-beam connection types for undamaged and damaged conditions using ambient vibration tests. Our work has led us to conclude the following noteworthy points:
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The natural frequencies obtained as a result of the ambient vibration tests applied to the undamaged state of the concrete encased composite columns using EFDD and SSI methods were very
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