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Experimental study on hysteretic behavior of composite frames with concrete-encased CFST columns
Journal of Constructional Steel Research 123 (2016) 110–120
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Journal of Constructional Steel Research
Experimental study on hysteretic behavior of composite frames with concrete-encased CFST columns Kun Wang a,⁎, Shen-Feng Yuan a, Zai-Xian Chen b, Hai-Xiang Zhi a, Gao-Lin Shi a, Da-Fu Cao a a b
College of Civil Science and Engineering, Yangzhou University, Jiangsu, Yangzhou 225127, China School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China
a r t i c l e
i n f o
Article history: Received 16 July 2015 Received in revised form 17 April 2016 Accepted 23 April 2016 Available online xxxx Keywords: Concrete-encased steel beam Concrete-encased CFST column Lateral cyclic loading Seismic performance Composite frame
a b s t r a c t This paper proposed an innovative frame structure composed of concrete-encased steel beams and concreteencased concrete filled steel tube (CFST) columns for long span building, and two one-bay and one-storey specimens were tested under lateral low cyclic loading. The testing process, such as cracks developments and failure patterns were observed, and the seismic performance such as hysteretic behavior, skeleton curves, rigidity degradation, energy dissipation, and residential deformation were investigated. Additionally, the strain variations for longitudinal bars, flanges of H-shaped steel and steel tube in potential zone of the plastic hinges of beams and columns were analyzed. The test results indicated that two specimens both behaved perfect energy dissipation and ductility even prestressed; and the initial cracks of prestressed specimen was restrained and appeared later than that of unprestressed one due to the prestressing effects; otherwise, during the whole loading the circular strains of steel tubes in the bottom of columns varies unevenly, and steel tubes in compressive zone could supply effective constraint on corresponding concrete. According to the strain variations, the sequence of plastic hinges of the two specimens presented that they occurred on the beam ends firstly and then on the bottoms of columns and mechanism of energy dissipation for beam plastic hinges and a delayed occurring of plastic hinges on column bottom were expected. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Concrete-encased steel composite (CESC) frame is a type of structure where the steel, such as H-shaped steel, steel tube, steel-angles and steel trusses, were embedded in reinforced concrete (RC) frame to minimize the cross-section dimensions and raise the rigidities and capacities [1–6]. So far, the concrete-encased steel composite frame has been utilized widely in practice for long-span, heaver loading and superhigh buildings. Prestressed concrete-encased steel composite (PCESC) frame has both advantages of CESC frame and RC frame, where the PCESC frame not only has favorable ductility and energy dissipation due to the embedded steel, but also has higher cross-section capacities and minimized cross-section size, deflection and crack width owing to the prestressing effects [7–8]. Thus, the prestressed concrete-encased steel composite frame is also expected to satisfy the demands for large space and anti-seismic performance. In general, the welded cross-shaped steels are adopted in columns, and welded or rolled H-shaped steel are utilized in beams for PCESC frame. However, it is found that the prestressing tendons and longitudinal rebars will pass through the flanges of welded cross-shaped steel ⁎ Corresponding author at: College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China. E-mail address: [email protected] (K. Wang).
http://dx.doi.org/10.1016/j.jcsr.2016.04.024 0143-974X/© 2016 Elsevier Ltd. All rights reserved.
and the capacities of panel zone could be decreased in engineering application, additionally the beam-column connection construction is also difficult to be achieved. However, if the concrete-encased steel columns are replaced by the angle-steel concrete columns [9–11], where lattice angle-steel skeletons are embedded without steel bars, to simplify the connection construction, there are still some issues, such as high cost of producing of angle-steel skeletons as well as the incomplete design approaches for anchoring the embedded H-shaped steel in panel zone. Therefore, an innovative type of composite frame structure with concrete-encased steel beams and concrete-encased concrete filled steel tube (CFST) columns, in which concrete filled steel tube stud is embedded, is proposed to solve the above problems and to get better seismic behavior, on the basis of the previous investigations. Fig. 1 depicts a schematic view of the innovative composite frame and beamcolumn connection where external diaphragms are utilized. Several advantages of the innovative composite frame over the conventional PCESC frames could be found as follows: (1) the prestressing tendons and longitudinal rebars of beam could pass through the panel zone from the two sides of the embedded steel tube, and the steel tube could be kept continuous without weakening; (2) the concrete encased CFST column could economize the construction cost by saving the labor of welding process with purchasing the steel tubes, compared to the cross-shaped steels in general concrete-encased steel columns
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Fig. 1. Schematic view of composite frame and beam-column joint.
and angle-steel skeletons in angle-steel concrete columns; (3) the embedded steel tube could enhance the shear capacity and ductility of the panel zone to optimize the mechanical performance. At present, a number of studies have been reported on the behavior of concrete-encased steel beams with prestressing and concreteencased CFST columns. Lee [12] and Kim [13] had test and analysis the prestressed composite beams with corrugated web; Fu [14] and Wang [15] experimentally investigated the bending capacity of prestressed concrete-encased steel beams; Choy [16] and Wang [17] had experimentally investigated the shear capacity of prestressed concrete encased steel beams; Yao [18] tested the shear capacity of prestressed ultra high strength concrete encased steel beams; Xue [19] carried out tests on the hysteretic behavior of prestressed concreteencased steel beams, and the results indicated that the prestressed specimens have perfect ductility and energy dissipation; Nie [20] built an axial compressive bearing capacity design formulas of the concrete-encased CFST columns; the tests separately conducted by Li [21], Han [22], Zhao [23] and Ji [24] indicated that the concrete-encased CFST columns has good seismic performance according to the hysteretic behavior test results, and the bearing capacity formula were proposed; Ji [25] investigated the effect of cumulative damage on the seismic behavior of the novel columns through experimental testing. According to the prior investigations, some design approaches have been adopted in the Chinese standard: Technical specification for steel tube-reinforced concrete column structure (CESC188:2005). Additionally, Nie [26] and Liao [27] conducted experimental testing on the connections of concrete-encased CFST column to RC beam and steel beam. Although many researchers had conducted seismic performance tests and analysis on composite frame with various section
configurations [28–30], so far the proposed innovative composite frame in this paper has not been reported. The aim of this paper is to investigate the hysteretic behavior of composite frame structure with concrete-encased CFST columns. Thus, two composite frame specimens of one-bay and one-storey were manufactured, in which each specimen is composed of two concrete-encased CFST columns and one concrete-encased steel beam to represent a typical frame element in a building. Amongst the two frame specimens, one is prestressed with bonded posttensioned technique and the other is not. Then the two frame specimens are tested under low cyclic lateral loading, and test data pertaining to the behavior of the composite frame specimens are present, and the energy dissipation, rigidity degradation, residential deformation, ductility and strain variations are investigated, and the hinge mechanism is obtained. The results of this work will be helpful for popularizing the new type of frame structure in seismic zone. 2. Experimental program 2.1. Design of testing specimens Two test specimens of one-bay and one-storey composite frames designated as SRCF and PSRCF were designed, and the dimensions and reinforcement details were given in Fig. 2, where the testing specimen PSRCF was prestressed by using bonded post-tensioned technique and the SRCF was not. For the two specimens, the same cross-sectional dimensions of beams with 150 mm × 200 mm and columns with 240 mm × 240 mm were adopted. The seamless circular steel tubes of 108 mm × 8 mm and welded H-shaped steels of
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Fig. 2. Reinforcements of two testing specimens.
120 mm × 50 mm × 3 mm × 5 mm were arranged inside the columns and beams of the testing specimens respectively and the connection of H-shaped steel and steel tube was designed by using the external diaphragm with thickness of 5 mm referencing to specification [15]. The main differences between the two specimens lay in that the beam of specimen PSRCF was designed as prestressed member, where rebars with diameter of 8 mm of grade HPB300 were arranged, while rebars with diameter of 10 mm of the same grade were adopted in beam without prestressing for specimens SRCF. Otherwise, all the specimens were designed according to the criterions of “strong column-weak beam” and “strong joint - weak member”. For the specimen PSRCF, a prestressing tendon with grade of 1860 and nominal diameter of 12.7 mm was distributed in the form of three-segment parabola in the beam, and the duct was grouted after the tendon was tensioned. Due to the dimensional limitations for the test specimen PSRCF, the web of the welded H-shaped steel embedded in beam was placed to one side, thus the prestressing tendon could be arranged to pass through the steel tube in the panel zone. The
prestressing tendon was tensioned at one end of the beam with the tension control stress of 1395 N/mm2. The wedge-type anchorages for single prestressing tendon were fixed at the both ends of the beam, and the effective prestress of the tendon was about 1020 N/mm2 by calculating.
Table 1 Mechanical properties of steel plates, steel tube, rebars and stirrups. Material type
Size
Yielding strength fy/N/mm2
Tensile strength fu/N/mm2
Flange of H-shaped steel Web of H-shaped steel Circular steel tube Longitudinal rebars in beam
−5 −3
230.3 305.2 267.4 390.3 303.7 342.0 504.1
314.2 413.0 448.9 576.8 451.8 524.3 672.1
Longitudinal rebars in column Stirrups
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Fig. 3. Loading apparatus and loading methods.
2.2. Material properties The two testing specimens of one-bay and one-storey composite frame were casted by using the fine stone concrete products with the grade of C40 and carefully cured; whilst several cubic concrete specimens of 100 mm × 100 mm × 100 mm were made for assess the compressive
strength. According to the tests, the average cubic concrete compressive strength fcu was 48.7 N/mm2. The grade of seamless circular steel tube and plates for manufacturing the H-shaped steel was Q235, the grade of longitudinal rebars in beam, the longitudinal rebars in column and stirrups was HPB300, HRB335 and HRB400, respectively. The relevant coupons were made from the steel plates, steel tube, rebars and stirrups to
Fig. 4. Strain gauge distributions of PSRCF.
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Fig. 5. Cracks distribution for two specimens.
Fig. 6. Failure modes for two specimens.
be under tension tests for determining the material properties. The mechanical properties of steel tube, steel plate, rebars and stirrups were given in Table 1. 2.3. Loading apparatus and loading history Fig. 3 gives a view of loading apparatus and loading methods for the specimens. The foundation of the testing specimen was anchored on the floor by using strong bolts, and the lateral load was applied at one beam end along the beam axial direction by hydraulic servo loading system. Two vertical concentrated loads of 40 kN were imposed on the top of the beam and kept constant in the whole loading process by the hydraulic jacks, which are supported on the upper reaction steel beam. In order to make the free lateral sway of the specimens under lateral load and the vertical loads, two sliding devices were put into use between the hydraulic jacks and reaction beam. Additionally, the lateral braces were utilized to prevent the specimens form lateral away of out-plane. At the beginning of the tests, the vertical loads were firstly reached to 30% of the predetermined values and then unloaded to zero. After that, the vertical load reached to the predetermined loads and kept constant in the subsequent loading process. For the lateral cyclic loading, the
load-displacement hybrid control loading rule was employed in the tests, and the yielding of the test specimens was determined by the obvious flexion point formed in the hysteretic curves. Before the yielding, the load control rule was adopted, with one cycle at every increasing load level; after yielding, the displacement control was employed, the displacement increased on a multiple of yielding displacement, with three cycles at every displacement level, until the lateral load dropped to about 85% of the peak load. In the process of cyclic loading, the rightward loading was regarded as the forward direction. 2.4. Measurements In order to investigate the characteristics of plastic hinge formed on beams and columns, strain gauges were mounted on flanges of Hshaped steel and longitudinal rebars at two ends of beam about 50 mm away from the column edge, whilst the longitudinal rebars in bottom of columns about 50 mm away from the foundation top are also fixed with strain gauges. Additionally, four pairs of strain gauges in perpendicular direction were fixed on the steel tube at the same cross-section position of longitudinal rebars strain gauges for each column bottom to observe the longitudinal and circular strain of steel
Fig. 7. Hysteretic curves of two specimens.
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Fig. 8. Skeleton curves for two frames.
Fig. 10. Rigidity degradation K vs. displacement.
tube. The distribution of strain gauges for the two test specimens were the same, and Fig. 4 gives the serial numbers and distributions of strain gauges for specimen PSRCF. The main measurements include four parts as follows: (1) the loaddisplacement (P-Δ) curves under lateral cyclic loading; (2) the strains of flanges of H-shaped steel, steel bars as well as steel tubes in plastic hinges; (3) the development and distribution for the cracks and the failure modes for the two testing specimens.
crack width on the top was approximately 2–3 mm, whilst the concrete at the right side on the bottom of the right column was crushed; when the displacement reached to + 7Δy, the concrete at the beam end crushed, and the main crack run through the beam section, at this moment the concrete in the plastic hinges zone on the bottom of the two columns spelled seriously. The observation acquired from the leftward loading was similar to these of the rightward. Similarly, the specimen PSRCF was at the elastic stage without any cracks under the vertical load. When the lateral load reached to + 100 kN in the rightward direction, two bending cracks with the width about 0.01 mm formed on the bottom of the beam at left end and on the top of the beam at right end, respectively. At this moment, there was one horizontal crack found at the left side on the bottom of right column. With the increasing of loading, new bending cracks were observed at the beam and column ends, and the existing cracks kept developing; when the lateral load reached to +160 kN, the horizontal displacement increased distinctly compared to these of previous loading, and there was inflection point found in the hysteretic curves. By this time the displacement was 7 mm, and the cracks closed after unloading. The displacement control was applied after specimen PSRCF yielding, and the control displacement increased by the same displacement Δy = 7 mm. During the three cycles of displacement Δy = 7 mm, the cracks on the bottom of the two columns developed in the diagonal downward direction with a 45°; when the horizontal displacement reached to +2Δy, the slightly concrete crushing was found on the top of the beam at the left end as well as on the bottom at the right end with crack width of about 1–2 mm; as the horizontal displacement increased to + 4Δy, the concrete crushed at the right side on the bottom of right column; when the horizontal displacement reached to + 7Δy, the concrete for beam ends crushed with major cracks running through the cross-section, and the concrete peel off severely in the plastic hinges zone of the right and left column bottoms. The observations obtained from the leftward loading are analogous to these of the rightward.
3. Testing procedure 3.1. Experimental observations The testing specimen SRCF was at elastic stage and no cracks were observed when only the predetermined vertical load was imposed on the top of beam. With the lateral load increased to + 50 kN in the right direction, the first vertical bending crack with the width of 0.02 mm was found on the top of the beam at the left end. Accompanied by the increasing lateral load, some new bending cracks were observed at the two beam ends, and the existing crack kept propagating. When the lateral load increased to +100 kN, there was one horizontal bending crack formed at the left side on the bottom of left and right columns, respectively. When the lateral load increased to +180 kN, the horizontal displacement increased obviously compared to these under the preceding cyclic loading, and there was an evident inflection point in the loaddisplacement curves, and by this time the displacement was about 8 mm. After that, the displacement control mode was accepted, and the control displacement increased by the same displacement Δy = 8 mm, where Δy was yielding displacement. During the three cycles of displacement Δy, the cracks on the bottom of the two columns began to propagate in the diagonal downward direction of about 45°; when the horizontal displacement reached to +2Δy, sign of crushing for concrete was observed on the bottom of the beam at the right end; as the horizontal displacement increased to +3Δy, the concrete crushing on the bottom of beam at right end was getting worse, and the maximum
Fig. 9. Energy dissipation coefficient E vs. displacement.
Fig. 11. Residential deformation ratio vs. displacement.
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3.2. Cracks distribution and failure mode Fig. 5 illustrated the cracks distribution of the two test specimens when reached damage. It can been seen that the initial crack occurring of specimen PSRCF was obviously posterior to that of specimen SRCF at the same lateral loading level from the previous observations, and the average crack spacing of beam for specimen PSRCF was a little less than that for specimen SRCF. After the specimens yielding, the cracks on the columns in the range of steel tube diameter developed in the inclined downward direction. This could be due to the fact that relevant slip between steel tube and outer concrete were formed without effective shear connection. It was found from Fig. 6 that, the beam ends were damaged severely compared to the column bottoms, where the concrete peeled off seriously and the steel bars and H-shaped steel were exposed. Finally, the concrete on the bottom of the columns was crushed and corresponding longitudinal rebars buckled. Additionally, a few cracks were observed on the top of the columns, and it indicated that the failure mechanism of strong column-weak beam could be formed around the beamcolumn joints. On the other hand, one or two wispy inclined cracks
were found in the panel zone without shear failure, and the objective for strong-joint and weak-member was achieved. According to the experimental results, the beam ends were damaged seriously compared to the columns, and the perfect mechanism of beam hinges for energy dissipation were formed, whilst the seismic design concept of strong-column and weak-beam was realized. 4. Analysis of tests results 4.1. Load-displacement hysteretic curves and skeleton curves Fig.7 gives the load-displacement hysteretic curves for the two specimens under cyclic loading. It illustrated that the hysteretic curves presented the straight line shape without distinct loops under the forward and backward loading at the beginning of the loading; accompanied with the increase of lateral load, the hysteretic curves gradually deviated from the straight line with accelerating deformation, and residual deformation generated when unloading. After yielding, the capacities of the specimens improved in a certain degree with the increasing of the horizontal displacement; when the capacities of specimens passed the
Fig. 12. Strains of longitudinal bars and steel flanges for beam and column ends under right loading.
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Fig. 13. Bottom Steel tube strains of columns under forward loading.
peak loads, the capacities descended gradually. From an overall perspective, the hysteretic curves of the two specimens presented the plump spindle shape without distinct pinch, and behaved perfect seismic performance. Fig. 8 shows the skeleton curves for the two specimens, which illustrated the major characteristics in the loading process. At the elastic stage, the lateral sways for the two specimens were closed to each other at the same lateral load level; after yielding, the yield rigidity reduced with the increasing of the displacements; when the load exceeded the peak load, the skeleton decreased gently, showing a favorable ductility. Additionally, due to the prestressing effect, the yielding rigidity, capacity and slope of the declined segment of skeleton curve for specimen PSRCF were larger than those for specimen SRCF.
4.2. Energy dissipation capacity The earthquake-resistant properties for structure depended on the energy dissipation capacity when it comes into the plastic stage. The magnitude of the energy dissipating could be reflected by the area of the hysteretic loops according to Chinese standard JGJ101-96 [31]. Fig. 9 shows the energy dissipating capacity comparison of the two specimens. It can be seen from the Fig. 9 that, the energy dissipation capacity of specimen SRCF was slightly larger and then smaller than that of specimen PSRCF at the initial and subsequent stage, respectively. This indicated that the energy dissipation capacities of the two test specimens were almost the same, and the composite frame imposed with prestressing still has favorable energy dissipation capacity.
Fig. 14. Distribution of plastic hinges in two frames.
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4.3. Rigidity degradation Rigidity degradations of the two specimens were observed in the tests according to Chinese standard JGJ101-96 [31]. Fig.10 shows the relation of loop rigidity coefficient (K) and horizontal displacement (Δ) for the two specimens in the forward and backward direction. It was found that the hysteretic loop rigidities of the two specimens decreased with the increasing displacement, and the two rigidity degradation curves were closed to each other without significant distinction. 4.4. Deformation restoring capacity The relationship between residential deformation ratio and horizontal displacement was given in Fig.11, where the expression for residential deformation ratio was Δ0/Δu, and the Δ0 and Δu was the experienced maximum horizontal displacement and residual deformation after unloading at every cycle, respectively. It was illustrated that the residential deformation ratio for the two specimens were closed to each other in the forward direction, however the residential deformation ratio of specimen PSRCF was a little smaller than that of SRCF, and this could be due to the sufficient plastic development in specimen SRCF in the forward direction. 4.5. Ductility The yielding points for the skeleton curves were determined by the energy equivalent methods []. The test results for yielding load, yielding displacement, maximum load and displacement ductility coefficients were listed in Table 2. It illustrated that the yielding displacements for the two specimens in forward and backward direction were basically the same; the displacement ductility coefficients of the specimen PSRCF were smaller than these of specimen SRCF, whilst the displacement ductility coefficients in forward direction were greater than these in backward direction. In addition, the ductility for the prestressed specimen PSRCF was not get worse evidently, and this may be due to the fact that amount of energy dissipation formed by the steel web in beam and steel tube of column in plastic hinges zone, and this resulted in larger plastic deformation. 5. Strain variations and plastic hinges occurring sequence The values of the strain gages affixed on longitudinal bars, H-shaped steel flanges and steel tubes in the plastic hinges of beam and column ends were recorded, and the rule of variation of strain could be obtained by the relation between the strain values and displacement corresponding to the peak point of every hysteretic loop in the whole test loading. 5.1. Strain of longitudinal bars and steel flanges of beam ends and longitudinal bars of column bottoms Under the forward loading, the strain variations for the longitudinal bars and H-shaped steel flanges of beam ends and longitudinal bars of column bottoms were given in Fig.12, and the monitoring point arrangements were shown in Fig 4. It was illustrated that: (1) When the horizontal load was less than 120 kN (here Δ = 4.2 mm), the strains of the longitudinal bars and H-shaped steel flanges
at beam ends and longitudinal bars of column bottoms increased with the increasing of horizontal displacement, and all the measured strains were less than yielding strain, and the specimen was at elastic stage. (2) When the horizontal load reached to 140 kN (Δ = 5.1 mm), the upper flange of H-shaped steel at right end of beam was tensile to yield firstly (s13 and s14 in Fig. 12d), meanwhile the bottom longitudinal bars at the beam right end approached to compressive yield(s15 and s16 in Fig. 12d). (3) With the increasing load reached to 160 kN (Δy = 7 mm), the upper tensile longitudinal bars yield at the right end of beam (s9 and s10 in Fig. 12b). (4) When the horizontal displacement reached to 2Δy, the bottom tensile longitudinal bars at the left side of two columns (s18 in Fig. 12e, s29 in Fig. 12f) and the lower tensile steel flange at left end of the beam (s7 and s8 in Fig. 12c) had been yield, whilst the lower tensile longitudinal bars of beam at the left end approached to yield (s1 and s2 in Figure12a), and the lower compressive steel flange yield at the right end of beam (s15 and s16 of Fig.12d). (5) When the horizontal displacement reached to 3Δy, the tensile longitudinal bars (s3, s4, s9, s10) and steel flanges (s7, s8, s13 and s14) at two ends of beam and tensile longitudinal bars (s17, s18 and s29) at the bottom of two columns had got yield, meanwhile the bottom longitudinal bars at right side of left column (s19, s20 in Fig.12e) and the upper steel flange at the left end of beam (s6 in Fig. 12c) were compressed to yield. (6) When the horizontal displacement reached to 4Δy, the bottom longitudinal bars at right side of the right column approached to yield (s31 and s32 in Fig. 12f), and after that the values of strain overflowed. The phenomenon in backward loading was basically similar to that in forward loading. It was observed that, the tensile H-shaped steel flanges yield a little earlier than the longitudinal bars at the same cross-section, and the reason was that the yielding strain of the steel flanges was relatively smaller than that of the longitudinal bars due to the smaller yielding strength; the yielding time of the bottom longitudinal bars of the two columns were approximate at the displacement of 2Δy, however at the moment the strain of the lower H-shaped steel flange at the left end of beam exceeded the yielding strain a lot, indicating that the lower tensile steel flange at the left end of beam began yielding earlier than the tensile bottom longitudinal bars of the columns. In addition, the compressive H-shaped steel flanges and longitudinal bars all reached yielding eventually, which was one of the reasons for the perfect ductility of the structure. The strain variations of the longitudinal bars and steel flanges at two ends of beam as well as bottom longitudinal bars of two columns for specimen SRCF were analogous to that for specimen PSRCF. 5.2. The steel tube strain variations on the column bottom Fig. 13 gives the strain variations of the bottom steel tube of two columns for specimen PSRCF under the horizontal in the forward loading. From Fig. 13, some phenomenon was observed as follows: (1) When the horizontal displacement reached to Δy, the strains for the steel tube were not exceed the yielding values, indicating that the steel tube were at the plastic stage.
Table 2 Displacement ductility factor of specimens.
SRCF in forward direction SRCF in backward direction PSRCF in forward direction PSRCF in forward direction
Yielding load Py/kN
Yielding displacement Δy/mm
Peak load Pu/kN
Ultimate displacement Δu/mm
Ductility coefficient μ = Δu/Δy
199.0 194.0 199.0 200.0
9.0 10.0 9.0 10.0
281.7 256.7 284.3 275.5
55.3 54.6 48.9 48.8
6.1 5.5 5.4 4.9
Note: Here Δy is the yielding displacement determined by the energy equivalent methods from skeleton curves.
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(2) When the horizontal displacement reached to 2Δy, the longitudinal strain s33 of the bottom steel tube at the left side of right column firstly reached to the tensile yielding shown in Fig. 13b, and the remainder strain gages at other locations do not exceed the yielding strain. (3) When the horizontal displacement reached to 3Δy, the longitudinal strain (s21 and s33) of the bottom steel tube at the left side of right and left column had been damaged, but the tendency of the strains indicated that the strain gages had exceeded tensile yielding, meanwhile the longitudinal strain, such as s25, s37 and s39, also reached to yielding, meanwhile the longitudinal compressive strains of the bottom steel tube on the right side of the two columns had reached to the yielding values. (4) When the horizontal displacement reached to 4Δy, bottom steel tube circular strains, such as s26, s28, s38, and s40 of the right and left columns reached to compressive yielding strain; when the horizontal displacement reached to 5Δy, the circular strain s36 at the right side of the right column reached to tensile yielding; when the displacement reached to 6Δy, the circular strain s24 at the right side of the left column reached to tensile yielding.
The strain variations of backward loading were basically the same to the forward loading. From Fig. 13, the circular strains of steel tube distributed unevenly at the same cross-section position. The circular strains of steel tube in the compressive zone had the tendency of tension, so the steel tube could offer the effective constraint to the inner concrete; on the contrary, the circular strain of steel tube in the tensile zone has the tendency of compression, and the steel tube could not offer the effective constraint. The strains of steel tube at the bottom of the column for specimen SRCF were the same. 5.3. Sequence of the plastic hinges occurring The sequence of the plastic hinges occurring could be defined by the values of the strain gages tied on the steel flanges and longitudinal bars of the beam and column, according to the assumption that yielding of cross-sectional I-shaped steel flange and tensile steel bars is recognized as yielding of beam and column respectively. It reveals that the sequence of the plastic hinges occurring for the two specimens are the same as shown in Fig. 14, presenting the “beam end-beam end-column bottomcolumn bottom” sequence. It illustrated that the beam hinges dissipation mechanism could be achieved and the occurring time for column bottom plastic hinges could be postponed. 6. Discussions In this paper, there is no vertical load imposed on the top of the two columns and only two concentrated forces of 40 kN applied on the beam, thus the column axial compressive ratio is very small, and this may result in perfect energy dissipation and good ductility for the two frame specimens. However, the further research such as numerical simulation should be carried out under the situation of higher axial compressive ratio. 7. Conclusions (1) The hysteretic curves for the two specimens under relatively small axial compressive ratio without vertical load on the top of the columns are comparatively plump with spindle shape and have perfect seismic performance; otherwise the specimens also have favorable energy-dissipating capacity even though they are prestressed. (2) The circular strain for the steel tube presents the uneven variation tendency. The effective constraint for the concrete in the steel tube in the compressive zone could be achieved by the steel tube under circular tension.
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(3) The occurring time of the cracks of the concrete-encased steel beam could be put off when prestressed, and the performance of the structure at the service stage could be improved. (4) The beam plastic hinges dissipation mechanism could be achieved for the innovative type of structure and the occurring time for the column plastic hinges on the bottom of column could be postponed.
Acknowledgments The authors appreciate the support of the Natural Science Foundation of Jiangsu Province (BK20140489), the Science and Technology projects of Ministry of Housing and Urban-rural Development (2014K2-035) and the Natural Science Foundation for Colleges and Universities in Jiangsu Province (11KJB560006).
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Contents lists available at ScienceDirect
Journal of Constructional Steel Research
Experimental study on hysteretic behavior of composite frames with concrete-encased CFST columns Kun Wang a,⁎, Shen-Feng Yuan a, Zai-Xian Chen b, Hai-Xiang Zhi a, Gao-Lin Shi a, Da-Fu Cao a a b
College of Civil Science and Engineering, Yangzhou University, Jiangsu, Yangzhou 225127, China School of Civil Engineering, Harbin Institute of Technology, Heilongjiang, Harbin 150090, China
a r t i c l e
i n f o
Article history: Received 16 July 2015 Received in revised form 17 April 2016 Accepted 23 April 2016 Available online xxxx Keywords: Concrete-encased steel beam Concrete-encased CFST column Lateral cyclic loading Seismic performance Composite frame
a b s t r a c t This paper proposed an innovative frame structure composed of concrete-encased steel beams and concreteencased concrete filled steel tube (CFST) columns for long span building, and two one-bay and one-storey specimens were tested under lateral low cyclic loading. The testing process, such as cracks developments and failure patterns were observed, and the seismic performance such as hysteretic behavior, skeleton curves, rigidity degradation, energy dissipation, and residential deformation were investigated. Additionally, the strain variations for longitudinal bars, flanges of H-shaped steel and steel tube in potential zone of the plastic hinges of beams and columns were analyzed. The test results indicated that two specimens both behaved perfect energy dissipation and ductility even prestressed; and the initial cracks of prestressed specimen was restrained and appeared later than that of unprestressed one due to the prestressing effects; otherwise, during the whole loading the circular strains of steel tubes in the bottom of columns varies unevenly, and steel tubes in compressive zone could supply effective constraint on corresponding concrete. According to the strain variations, the sequence of plastic hinges of the two specimens presented that they occurred on the beam ends firstly and then on the bottoms of columns and mechanism of energy dissipation for beam plastic hinges and a delayed occurring of plastic hinges on column bottom were expected. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Concrete-encased steel composite (CESC) frame is a type of structure where the steel, such as H-shaped steel, steel tube, steel-angles and steel trusses, were embedded in reinforced concrete (RC) frame to minimize the cross-section dimensions and raise the rigidities and capacities [1–6]. So far, the concrete-encased steel composite frame has been utilized widely in practice for long-span, heaver loading and superhigh buildings. Prestressed concrete-encased steel composite (PCESC) frame has both advantages of CESC frame and RC frame, where the PCESC frame not only has favorable ductility and energy dissipation due to the embedded steel, but also has higher cross-section capacities and minimized cross-section size, deflection and crack width owing to the prestressing effects [7–8]. Thus, the prestressed concrete-encased steel composite frame is also expected to satisfy the demands for large space and anti-seismic performance. In general, the welded cross-shaped steels are adopted in columns, and welded or rolled H-shaped steel are utilized in beams for PCESC frame. However, it is found that the prestressing tendons and longitudinal rebars will pass through the flanges of welded cross-shaped steel ⁎ Corresponding author at: College of Civil Science and Engineering, Yangzhou University, Yangzhou 225127, China. E-mail address: [email protected] (K. Wang).
http://dx.doi.org/10.1016/j.jcsr.2016.04.024 0143-974X/© 2016 Elsevier Ltd. All rights reserved.
and the capacities of panel zone could be decreased in engineering application, additionally the beam-column connection construction is also difficult to be achieved. However, if the concrete-encased steel columns are replaced by the angle-steel concrete columns [9–11], where lattice angle-steel skeletons are embedded without steel bars, to simplify the connection construction, there are still some issues, such as high cost of producing of angle-steel skeletons as well as the incomplete design approaches for anchoring the embedded H-shaped steel in panel zone. Therefore, an innovative type of composite frame structure with concrete-encased steel beams and concrete-encased concrete filled steel tube (CFST) columns, in which concrete filled steel tube stud is embedded, is proposed to solve the above problems and to get better seismic behavior, on the basis of the previous investigations. Fig. 1 depicts a schematic view of the innovative composite frame and beamcolumn connection where external diaphragms are utilized. Several advantages of the innovative composite frame over the conventional PCESC frames could be found as follows: (1) the prestressing tendons and longitudinal rebars of beam could pass through the panel zone from the two sides of the embedded steel tube, and the steel tube could be kept continuous without weakening; (2) the concrete encased CFST column could economize the construction cost by saving the labor of welding process with purchasing the steel tubes, compared to the cross-shaped steels in general concrete-encased steel columns
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Fig. 1. Schematic view of composite frame and beam-column joint.
and angle-steel skeletons in angle-steel concrete columns; (3) the embedded steel tube could enhance the shear capacity and ductility of the panel zone to optimize the mechanical performance. At present, a number of studies have been reported on the behavior of concrete-encased steel beams with prestressing and concreteencased CFST columns. Lee [12] and Kim [13] had test and analysis the prestressed composite beams with corrugated web; Fu [14] and Wang [15] experimentally investigated the bending capacity of prestressed concrete-encased steel beams; Choy [16] and Wang [17] had experimentally investigated the shear capacity of prestressed concrete encased steel beams; Yao [18] tested the shear capacity of prestressed ultra high strength concrete encased steel beams; Xue [19] carried out tests on the hysteretic behavior of prestressed concreteencased steel beams, and the results indicated that the prestressed specimens have perfect ductility and energy dissipation; Nie [20] built an axial compressive bearing capacity design formulas of the concrete-encased CFST columns; the tests separately conducted by Li [21], Han [22], Zhao [23] and Ji [24] indicated that the concrete-encased CFST columns has good seismic performance according to the hysteretic behavior test results, and the bearing capacity formula were proposed; Ji [25] investigated the effect of cumulative damage on the seismic behavior of the novel columns through experimental testing. According to the prior investigations, some design approaches have been adopted in the Chinese standard: Technical specification for steel tube-reinforced concrete column structure (CESC188:2005). Additionally, Nie [26] and Liao [27] conducted experimental testing on the connections of concrete-encased CFST column to RC beam and steel beam. Although many researchers had conducted seismic performance tests and analysis on composite frame with various section
configurations [28–30], so far the proposed innovative composite frame in this paper has not been reported. The aim of this paper is to investigate the hysteretic behavior of composite frame structure with concrete-encased CFST columns. Thus, two composite frame specimens of one-bay and one-storey were manufactured, in which each specimen is composed of two concrete-encased CFST columns and one concrete-encased steel beam to represent a typical frame element in a building. Amongst the two frame specimens, one is prestressed with bonded posttensioned technique and the other is not. Then the two frame specimens are tested under low cyclic lateral loading, and test data pertaining to the behavior of the composite frame specimens are present, and the energy dissipation, rigidity degradation, residential deformation, ductility and strain variations are investigated, and the hinge mechanism is obtained. The results of this work will be helpful for popularizing the new type of frame structure in seismic zone. 2. Experimental program 2.1. Design of testing specimens Two test specimens of one-bay and one-storey composite frames designated as SRCF and PSRCF were designed, and the dimensions and reinforcement details were given in Fig. 2, where the testing specimen PSRCF was prestressed by using bonded post-tensioned technique and the SRCF was not. For the two specimens, the same cross-sectional dimensions of beams with 150 mm × 200 mm and columns with 240 mm × 240 mm were adopted. The seamless circular steel tubes of 108 mm × 8 mm and welded H-shaped steels of
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Fig. 2. Reinforcements of two testing specimens.
120 mm × 50 mm × 3 mm × 5 mm were arranged inside the columns and beams of the testing specimens respectively and the connection of H-shaped steel and steel tube was designed by using the external diaphragm with thickness of 5 mm referencing to specification [15]. The main differences between the two specimens lay in that the beam of specimen PSRCF was designed as prestressed member, where rebars with diameter of 8 mm of grade HPB300 were arranged, while rebars with diameter of 10 mm of the same grade were adopted in beam without prestressing for specimens SRCF. Otherwise, all the specimens were designed according to the criterions of “strong column-weak beam” and “strong joint - weak member”. For the specimen PSRCF, a prestressing tendon with grade of 1860 and nominal diameter of 12.7 mm was distributed in the form of three-segment parabola in the beam, and the duct was grouted after the tendon was tensioned. Due to the dimensional limitations for the test specimen PSRCF, the web of the welded H-shaped steel embedded in beam was placed to one side, thus the prestressing tendon could be arranged to pass through the steel tube in the panel zone. The
prestressing tendon was tensioned at one end of the beam with the tension control stress of 1395 N/mm2. The wedge-type anchorages for single prestressing tendon were fixed at the both ends of the beam, and the effective prestress of the tendon was about 1020 N/mm2 by calculating.
Table 1 Mechanical properties of steel plates, steel tube, rebars and stirrups. Material type
Size
Yielding strength fy/N/mm2
Tensile strength fu/N/mm2
Flange of H-shaped steel Web of H-shaped steel Circular steel tube Longitudinal rebars in beam
−5 −3
230.3 305.2 267.4 390.3 303.7 342.0 504.1
314.2 413.0 448.9 576.8 451.8 524.3 672.1
Longitudinal rebars in column Stirrups
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Fig. 3. Loading apparatus and loading methods.
2.2. Material properties The two testing specimens of one-bay and one-storey composite frame were casted by using the fine stone concrete products with the grade of C40 and carefully cured; whilst several cubic concrete specimens of 100 mm × 100 mm × 100 mm were made for assess the compressive
strength. According to the tests, the average cubic concrete compressive strength fcu was 48.7 N/mm2. The grade of seamless circular steel tube and plates for manufacturing the H-shaped steel was Q235, the grade of longitudinal rebars in beam, the longitudinal rebars in column and stirrups was HPB300, HRB335 and HRB400, respectively. The relevant coupons were made from the steel plates, steel tube, rebars and stirrups to
Fig. 4. Strain gauge distributions of PSRCF.
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Fig. 5. Cracks distribution for two specimens.
Fig. 6. Failure modes for two specimens.
be under tension tests for determining the material properties. The mechanical properties of steel tube, steel plate, rebars and stirrups were given in Table 1. 2.3. Loading apparatus and loading history Fig. 3 gives a view of loading apparatus and loading methods for the specimens. The foundation of the testing specimen was anchored on the floor by using strong bolts, and the lateral load was applied at one beam end along the beam axial direction by hydraulic servo loading system. Two vertical concentrated loads of 40 kN were imposed on the top of the beam and kept constant in the whole loading process by the hydraulic jacks, which are supported on the upper reaction steel beam. In order to make the free lateral sway of the specimens under lateral load and the vertical loads, two sliding devices were put into use between the hydraulic jacks and reaction beam. Additionally, the lateral braces were utilized to prevent the specimens form lateral away of out-plane. At the beginning of the tests, the vertical loads were firstly reached to 30% of the predetermined values and then unloaded to zero. After that, the vertical load reached to the predetermined loads and kept constant in the subsequent loading process. For the lateral cyclic loading, the
load-displacement hybrid control loading rule was employed in the tests, and the yielding of the test specimens was determined by the obvious flexion point formed in the hysteretic curves. Before the yielding, the load control rule was adopted, with one cycle at every increasing load level; after yielding, the displacement control was employed, the displacement increased on a multiple of yielding displacement, with three cycles at every displacement level, until the lateral load dropped to about 85% of the peak load. In the process of cyclic loading, the rightward loading was regarded as the forward direction. 2.4. Measurements In order to investigate the characteristics of plastic hinge formed on beams and columns, strain gauges were mounted on flanges of Hshaped steel and longitudinal rebars at two ends of beam about 50 mm away from the column edge, whilst the longitudinal rebars in bottom of columns about 50 mm away from the foundation top are also fixed with strain gauges. Additionally, four pairs of strain gauges in perpendicular direction were fixed on the steel tube at the same cross-section position of longitudinal rebars strain gauges for each column bottom to observe the longitudinal and circular strain of steel
Fig. 7. Hysteretic curves of two specimens.
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Fig. 8. Skeleton curves for two frames.
Fig. 10. Rigidity degradation K vs. displacement.
tube. The distribution of strain gauges for the two test specimens were the same, and Fig. 4 gives the serial numbers and distributions of strain gauges for specimen PSRCF. The main measurements include four parts as follows: (1) the loaddisplacement (P-Δ) curves under lateral cyclic loading; (2) the strains of flanges of H-shaped steel, steel bars as well as steel tubes in plastic hinges; (3) the development and distribution for the cracks and the failure modes for the two testing specimens.
crack width on the top was approximately 2–3 mm, whilst the concrete at the right side on the bottom of the right column was crushed; when the displacement reached to + 7Δy, the concrete at the beam end crushed, and the main crack run through the beam section, at this moment the concrete in the plastic hinges zone on the bottom of the two columns spelled seriously. The observation acquired from the leftward loading was similar to these of the rightward. Similarly, the specimen PSRCF was at the elastic stage without any cracks under the vertical load. When the lateral load reached to + 100 kN in the rightward direction, two bending cracks with the width about 0.01 mm formed on the bottom of the beam at left end and on the top of the beam at right end, respectively. At this moment, there was one horizontal crack found at the left side on the bottom of right column. With the increasing of loading, new bending cracks were observed at the beam and column ends, and the existing cracks kept developing; when the lateral load reached to +160 kN, the horizontal displacement increased distinctly compared to these of previous loading, and there was inflection point found in the hysteretic curves. By this time the displacement was 7 mm, and the cracks closed after unloading. The displacement control was applied after specimen PSRCF yielding, and the control displacement increased by the same displacement Δy = 7 mm. During the three cycles of displacement Δy = 7 mm, the cracks on the bottom of the two columns developed in the diagonal downward direction with a 45°; when the horizontal displacement reached to +2Δy, the slightly concrete crushing was found on the top of the beam at the left end as well as on the bottom at the right end with crack width of about 1–2 mm; as the horizontal displacement increased to + 4Δy, the concrete crushed at the right side on the bottom of right column; when the horizontal displacement reached to + 7Δy, the concrete for beam ends crushed with major cracks running through the cross-section, and the concrete peel off severely in the plastic hinges zone of the right and left column bottoms. The observations obtained from the leftward loading are analogous to these of the rightward.
3. Testing procedure 3.1. Experimental observations The testing specimen SRCF was at elastic stage and no cracks were observed when only the predetermined vertical load was imposed on the top of beam. With the lateral load increased to + 50 kN in the right direction, the first vertical bending crack with the width of 0.02 mm was found on the top of the beam at the left end. Accompanied by the increasing lateral load, some new bending cracks were observed at the two beam ends, and the existing crack kept propagating. When the lateral load increased to +100 kN, there was one horizontal bending crack formed at the left side on the bottom of left and right columns, respectively. When the lateral load increased to +180 kN, the horizontal displacement increased obviously compared to these under the preceding cyclic loading, and there was an evident inflection point in the loaddisplacement curves, and by this time the displacement was about 8 mm. After that, the displacement control mode was accepted, and the control displacement increased by the same displacement Δy = 8 mm, where Δy was yielding displacement. During the three cycles of displacement Δy, the cracks on the bottom of the two columns began to propagate in the diagonal downward direction of about 45°; when the horizontal displacement reached to +2Δy, sign of crushing for concrete was observed on the bottom of the beam at the right end; as the horizontal displacement increased to +3Δy, the concrete crushing on the bottom of beam at right end was getting worse, and the maximum
Fig. 9. Energy dissipation coefficient E vs. displacement.
Fig. 11. Residential deformation ratio vs. displacement.
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3.2. Cracks distribution and failure mode Fig. 5 illustrated the cracks distribution of the two test specimens when reached damage. It can been seen that the initial crack occurring of specimen PSRCF was obviously posterior to that of specimen SRCF at the same lateral loading level from the previous observations, and the average crack spacing of beam for specimen PSRCF was a little less than that for specimen SRCF. After the specimens yielding, the cracks on the columns in the range of steel tube diameter developed in the inclined downward direction. This could be due to the fact that relevant slip between steel tube and outer concrete were formed without effective shear connection. It was found from Fig. 6 that, the beam ends were damaged severely compared to the column bottoms, where the concrete peeled off seriously and the steel bars and H-shaped steel were exposed. Finally, the concrete on the bottom of the columns was crushed and corresponding longitudinal rebars buckled. Additionally, a few cracks were observed on the top of the columns, and it indicated that the failure mechanism of strong column-weak beam could be formed around the beamcolumn joints. On the other hand, one or two wispy inclined cracks
were found in the panel zone without shear failure, and the objective for strong-joint and weak-member was achieved. According to the experimental results, the beam ends were damaged seriously compared to the columns, and the perfect mechanism of beam hinges for energy dissipation were formed, whilst the seismic design concept of strong-column and weak-beam was realized. 4. Analysis of tests results 4.1. Load-displacement hysteretic curves and skeleton curves Fig.7 gives the load-displacement hysteretic curves for the two specimens under cyclic loading. It illustrated that the hysteretic curves presented the straight line shape without distinct loops under the forward and backward loading at the beginning of the loading; accompanied with the increase of lateral load, the hysteretic curves gradually deviated from the straight line with accelerating deformation, and residual deformation generated when unloading. After yielding, the capacities of the specimens improved in a certain degree with the increasing of the horizontal displacement; when the capacities of specimens passed the
Fig. 12. Strains of longitudinal bars and steel flanges for beam and column ends under right loading.
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Fig. 13. Bottom Steel tube strains of columns under forward loading.
peak loads, the capacities descended gradually. From an overall perspective, the hysteretic curves of the two specimens presented the plump spindle shape without distinct pinch, and behaved perfect seismic performance. Fig. 8 shows the skeleton curves for the two specimens, which illustrated the major characteristics in the loading process. At the elastic stage, the lateral sways for the two specimens were closed to each other at the same lateral load level; after yielding, the yield rigidity reduced with the increasing of the displacements; when the load exceeded the peak load, the skeleton decreased gently, showing a favorable ductility. Additionally, due to the prestressing effect, the yielding rigidity, capacity and slope of the declined segment of skeleton curve for specimen PSRCF were larger than those for specimen SRCF.
4.2. Energy dissipation capacity The earthquake-resistant properties for structure depended on the energy dissipation capacity when it comes into the plastic stage. The magnitude of the energy dissipating could be reflected by the area of the hysteretic loops according to Chinese standard JGJ101-96 [31]. Fig. 9 shows the energy dissipating capacity comparison of the two specimens. It can be seen from the Fig. 9 that, the energy dissipation capacity of specimen SRCF was slightly larger and then smaller than that of specimen PSRCF at the initial and subsequent stage, respectively. This indicated that the energy dissipation capacities of the two test specimens were almost the same, and the composite frame imposed with prestressing still has favorable energy dissipation capacity.
Fig. 14. Distribution of plastic hinges in two frames.
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4.3. Rigidity degradation Rigidity degradations of the two specimens were observed in the tests according to Chinese standard JGJ101-96 [31]. Fig.10 shows the relation of loop rigidity coefficient (K) and horizontal displacement (Δ) for the two specimens in the forward and backward direction. It was found that the hysteretic loop rigidities of the two specimens decreased with the increasing displacement, and the two rigidity degradation curves were closed to each other without significant distinction. 4.4. Deformation restoring capacity The relationship between residential deformation ratio and horizontal displacement was given in Fig.11, where the expression for residential deformation ratio was Δ0/Δu, and the Δ0 and Δu was the experienced maximum horizontal displacement and residual deformation after unloading at every cycle, respectively. It was illustrated that the residential deformation ratio for the two specimens were closed to each other in the forward direction, however the residential deformation ratio of specimen PSRCF was a little smaller than that of SRCF, and this could be due to the sufficient plastic development in specimen SRCF in the forward direction. 4.5. Ductility The yielding points for the skeleton curves were determined by the energy equivalent methods []. The test results for yielding load, yielding displacement, maximum load and displacement ductility coefficients were listed in Table 2. It illustrated that the yielding displacements for the two specimens in forward and backward direction were basically the same; the displacement ductility coefficients of the specimen PSRCF were smaller than these of specimen SRCF, whilst the displacement ductility coefficients in forward direction were greater than these in backward direction. In addition, the ductility for the prestressed specimen PSRCF was not get worse evidently, and this may be due to the fact that amount of energy dissipation formed by the steel web in beam and steel tube of column in plastic hinges zone, and this resulted in larger plastic deformation. 5. Strain variations and plastic hinges occurring sequence The values of the strain gages affixed on longitudinal bars, H-shaped steel flanges and steel tubes in the plastic hinges of beam and column ends were recorded, and the rule of variation of strain could be obtained by the relation between the strain values and displacement corresponding to the peak point of every hysteretic loop in the whole test loading. 5.1. Strain of longitudinal bars and steel flanges of beam ends and longitudinal bars of column bottoms Under the forward loading, the strain variations for the longitudinal bars and H-shaped steel flanges of beam ends and longitudinal bars of column bottoms were given in Fig.12, and the monitoring point arrangements were shown in Fig 4. It was illustrated that: (1) When the horizontal load was less than 120 kN (here Δ = 4.2 mm), the strains of the longitudinal bars and H-shaped steel flanges
at beam ends and longitudinal bars of column bottoms increased with the increasing of horizontal displacement, and all the measured strains were less than yielding strain, and the specimen was at elastic stage. (2) When the horizontal load reached to 140 kN (Δ = 5.1 mm), the upper flange of H-shaped steel at right end of beam was tensile to yield firstly (s13 and s14 in Fig. 12d), meanwhile the bottom longitudinal bars at the beam right end approached to compressive yield(s15 and s16 in Fig. 12d). (3) With the increasing load reached to 160 kN (Δy = 7 mm), the upper tensile longitudinal bars yield at the right end of beam (s9 and s10 in Fig. 12b). (4) When the horizontal displacement reached to 2Δy, the bottom tensile longitudinal bars at the left side of two columns (s18 in Fig. 12e, s29 in Fig. 12f) and the lower tensile steel flange at left end of the beam (s7 and s8 in Fig. 12c) had been yield, whilst the lower tensile longitudinal bars of beam at the left end approached to yield (s1 and s2 in Figure12a), and the lower compressive steel flange yield at the right end of beam (s15 and s16 of Fig.12d). (5) When the horizontal displacement reached to 3Δy, the tensile longitudinal bars (s3, s4, s9, s10) and steel flanges (s7, s8, s13 and s14) at two ends of beam and tensile longitudinal bars (s17, s18 and s29) at the bottom of two columns had got yield, meanwhile the bottom longitudinal bars at right side of left column (s19, s20 in Fig.12e) and the upper steel flange at the left end of beam (s6 in Fig. 12c) were compressed to yield. (6) When the horizontal displacement reached to 4Δy, the bottom longitudinal bars at right side of the right column approached to yield (s31 and s32 in Fig. 12f), and after that the values of strain overflowed. The phenomenon in backward loading was basically similar to that in forward loading. It was observed that, the tensile H-shaped steel flanges yield a little earlier than the longitudinal bars at the same cross-section, and the reason was that the yielding strain of the steel flanges was relatively smaller than that of the longitudinal bars due to the smaller yielding strength; the yielding time of the bottom longitudinal bars of the two columns were approximate at the displacement of 2Δy, however at the moment the strain of the lower H-shaped steel flange at the left end of beam exceeded the yielding strain a lot, indicating that the lower tensile steel flange at the left end of beam began yielding earlier than the tensile bottom longitudinal bars of the columns. In addition, the compressive H-shaped steel flanges and longitudinal bars all reached yielding eventually, which was one of the reasons for the perfect ductility of the structure. The strain variations of the longitudinal bars and steel flanges at two ends of beam as well as bottom longitudinal bars of two columns for specimen SRCF were analogous to that for specimen PSRCF. 5.2. The steel tube strain variations on the column bottom Fig. 13 gives the strain variations of the bottom steel tube of two columns for specimen PSRCF under the horizontal in the forward loading. From Fig. 13, some phenomenon was observed as follows: (1) When the horizontal displacement reached to Δy, the strains for the steel tube were not exceed the yielding values, indicating that the steel tube were at the plastic stage.
Table 2 Displacement ductility factor of specimens.
SRCF in forward direction SRCF in backward direction PSRCF in forward direction PSRCF in forward direction
Yielding load Py/kN
Yielding displacement Δy/mm
Peak load Pu/kN
Ultimate displacement Δu/mm
Ductility coefficient μ = Δu/Δy
199.0 194.0 199.0 200.0
9.0 10.0 9.0 10.0
281.7 256.7 284.3 275.5
55.3 54.6 48.9 48.8
6.1 5.5 5.4 4.9
Note: Here Δy is the yielding displacement determined by the energy equivalent methods from skeleton curves.
K. Wang et al. / Journal of Constructional Steel Research 123 (2016) 110–120
(2) When the horizontal displacement reached to 2Δy, the longitudinal strain s33 of the bottom steel tube at the left side of right column firstly reached to the tensile yielding shown in Fig. 13b, and the remainder strain gages at other locations do not exceed the yielding strain. (3) When the horizontal displacement reached to 3Δy, the longitudinal strain (s21 and s33) of the bottom steel tube at the left side of right and left column had been damaged, but the tendency of the strains indicated that the strain gages had exceeded tensile yielding, meanwhile the longitudinal strain, such as s25, s37 and s39, also reached to yielding, meanwhile the longitudinal compressive strains of the bottom steel tube on the right side of the two columns had reached to the yielding values. (4) When the horizontal displacement reached to 4Δy, bottom steel tube circular strains, such as s26, s28, s38, and s40 of the right and left columns reached to compressive yielding strain; when the horizontal displacement reached to 5Δy, the circular strain s36 at the right side of the right column reached to tensile yielding; when the displacement reached to 6Δy, the circular strain s24 at the right side of the left column reached to tensile yielding.
The strain variations of backward loading were basically the same to the forward loading. From Fig. 13, the circular strains of steel tube distributed unevenly at the same cross-section position. The circular strains of steel tube in the compressive zone had the tendency of tension, so the steel tube could offer the effective constraint to the inner concrete; on the contrary, the circular strain of steel tube in the tensile zone has the tendency of compression, and the steel tube could not offer the effective constraint. The strains of steel tube at the bottom of the column for specimen SRCF were the same. 5.3. Sequence of the plastic hinges occurring The sequence of the plastic hinges occurring could be defined by the values of the strain gages tied on the steel flanges and longitudinal bars of the beam and column, according to the assumption that yielding of cross-sectional I-shaped steel flange and tensile steel bars is recognized as yielding of beam and column respectively. It reveals that the sequence of the plastic hinges occurring for the two specimens are the same as shown in Fig. 14, presenting the “beam end-beam end-column bottomcolumn bottom” sequence. It illustrated that the beam hinges dissipation mechanism could be achieved and the occurring time for column bottom plastic hinges could be postponed. 6. Discussions In this paper, there is no vertical load imposed on the top of the two columns and only two concentrated forces of 40 kN applied on the beam, thus the column axial compressive ratio is very small, and this may result in perfect energy dissipation and good ductility for the two frame specimens. However, the further research such as numerical simulation should be carried out under the situation of higher axial compressive ratio. 7. Conclusions (1) The hysteretic curves for the two specimens under relatively small axial compressive ratio without vertical load on the top of the columns are comparatively plump with spindle shape and have perfect seismic performance; otherwise the specimens also have favorable energy-dissipating capacity even though they are prestressed. (2) The circular strain for the steel tube presents the uneven variation tendency. The effective constraint for the concrete in the steel tube in the compressive zone could be achieved by the steel tube under circular tension.
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(3) The occurring time of the cracks of the concrete-encased steel beam could be put off when prestressed, and the performance of the structure at the service stage could be improved. (4) The beam plastic hinges dissipation mechanism could be achieved for the innovative type of structure and the occurring time for the column plastic hinges on the bottom of column could be postponed.
Acknowledgments The authors appreciate the support of the Natural Science Foundation of Jiangsu Province (BK20140489), the Science and Technology projects of Ministry of Housing and Urban-rural Development (2014K2-035) and the Natural Science Foundation for Colleges and Universities in Jiangsu Province (11KJB560006).
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