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Three types of joint detailing are suggested in Chinese specifications to reinforce CFT column-steel beam connections, including internal diaphragms (ID), external diaphragms (ED) and through diaphragms (TD). Previous research has mainly focused on a single type of connection detail; very little comparative research has been conducted on the differences of these details in influencing the behavior of the connections. Full-scale experimental research and finite element analyses (Abaqus) are conducted to study the effect of joint details and axial force ratios on the failure mechanisms of CFT column-steel beam connections. Force transfer mechanisms and shear deformations of the joint are further investigated using advanced computational models. The results show that the connections with reinforcement details suggested by Chinese specifications provide satisfactory seismic performance. ED connections exhibit the highest shear strength and lowest ductility. ID connections provide the highest ductility and lower shear strength. TD connections exhibit similar performances as ID connections. The comparison of these connections shows that connection type, diaphragm thickness and tube thickness have a significant influence on the joint confinement and may lead to different failure mechanisms of the connections. A high axial force ratio may also cause an undesirable column yielding mechanism if the reinforcement of the joint is inadequate. Keywords: CFT connection; failure mechanism; diaphragm; axial force ratio; FE analysis 1. Introduction The principle of a "strong joint-weak element and Concrete filled steel tube (CFT) columns have become s trong column-w eak beam" is recommended for commonplace in high-rise building design throughout structural design in China because beam yielding the world in recent years because of their excellent results in better ductility, higher safety margins seismic performance (Morino S. et al., 2003, Ricles and easier repair after damage compared to column J.M. et al., 2004, Rong B., 2007). In CFT columns the yielding or joint yielding. Structural designers simply concrete increases the local buckling strength of the need to verify if the relevant codified equations steel tube, and the steel tube provides confinement to the are satisfied in their element design. The equations concrete. This synergistic behavior results in structures consider the geometries of the elements (beam, column with high strength and stiffness, capable of resisting large and joint), material properties and axial force ratios. lateral cyclic deformations. However, this synergistic Previous research has mainly focused on the seismic composite action between the steel tube and the concrete performances of a particular connection detail on small cannot be fully achieved without proper reinforcement scale specimens (Morino S. et al., 1993, Lu X. et al., of the connections. Three major reinforcing details for 2000, Elremaily A. et al., 2001, Nishiyama I. et al., CFT column-steel beam connections are suggested in 2004, Han L.-H. et al., 2007, LEE S.-H. et al., 2010). Chinese specifications (CECS, 2004), including internal Very little research on CFT column systems have diaphragms (ID), external diaphragms (ED) and through considered the relation between the codified equations diaphragms (TD) connections. and connection behavior, the effects of reinforcing details on failure mechanisms, the key factors governing the structural failure, and the coupled effects *Contact author: Liping Kang, Ph.D. Candidate, of these factors. Therefore a comparison of these Tongji University, B107, Civil Engineering Bldg., connection details is needed to clarify their differences 1239 Siping Rd., Shanghai, 200092, China in reinforcing the joint and possibly leading to different Tel: +86-137-7422-3384 failure mechanisms of the connections. In addition, E-mail: email@example.com CFT column sections are becoming larger and larger ( Received April 6, 2014 ; accepted October 28, 2014 ) Journal of Asian Architecture and Building Engineering/January 2015/204 197 in real practice (Fan H. et al., 2009), so full-scale is 4200mm. Diaphragms and beams are connected to experiments are necessary to better predict the seismic the steel tube column with full penetration welding behavior of these connections. Experiments and finite at the ED and ID connections. Columns are divided element analyses using Abaqus were conducted for into three sections and welded to the beams at the TD full-scale CFT column-steel beam connections with connections. Parameters studied include connection various diaphragms to investigate the effect of different reinforcement types (ED/ID/TD connection), thickness connection details and axial force ratios on failure of the diaphragms (10mm/14mm), thickness of the mechanisms of the connections. steel tube at the joint area (14mm/18mm), beam details (regular/dog bone beam), and axial force 2. Experimental Research ratios (0.11/0.34/0.68) as shown in Table 1. All of 2.1 Test Setup these specimens are designed to meet the principle of Nine full-scale CFT column-steel beam connections a "strong joint-weak element; strong column-weak are tested (Fig.1.) under constant axial force and cyclic beam" by referring to the equations for RC structures lateral displacement at the top of the column. The and steel structures. Capacity design concept is used bottom of the column is pinned to the ground, and both to induce intended failure mode. Material properties beam-ends are supported vertically (Z direction) so that of the steel and the concrete are tested and the results the beam can move freely in the horizontal direction are shown in Tables 2. and 3. Displacement control is (X direction). The connection specimen consists of a adopted in the test, so that the story drift angle of the CFST column (□-400X400X14) and an I-shaped built- connection is equal to 1/1000, 1/800, 1/500, 1/400, up steel beam (I-500X250X8X10). The slenderness 1/300, 1/200, 1/100, 1/50, 1/30 and 1/20 in each of the beam flange and web is close to the slenderness loading level respectively. Two cycles are conducted limit for plastic design in the Chinese code. The for the first two loading levels, and three cycles are column height is 3000mm and the total beam length performed for all other loading levels. Fig.1. Connection Details (unit: mm) Table 1. Connection Specimen Details Connection Steel tube at the Axial force Axial force Reinforcement type Diaphragm details specimens joint ratio (kN) JD11 400x14 0.11 1482 Width 150mm; thickness External diaphragm JD12 400x14 0.34 4445 10mm JD13 400x14 0.68 8890 JD22 400x14 0.34 4445 Thickness: 10mm; hole JD22-B (dog bone 400x14 diameter: 250mm 0.34 4445 beam) Thickness: 14mm; hole Internal diaphragm JD22-D 400x14 0.34 4445 diameter: 250mm JD22-T1 400x18 Thickness: 10mm; hole 0.34 4445 JD-23 400x14 diameter: 250mm 0.68 8890 Width 500mm; hole Through diaphragm JD-32 400x14 0.34 4445 diameter: 250mm 198 JAABE vol.14 no.1 January 2015 Xilin Lu 2.2 Test Results away from the joint (where the beam flange meets the 2.2.1 Failure Modes external diaphragm); ID connection exhibited brittle Typical failure modes of the connections are shown failure in the beam close to the joint; TD connection i n Fi g . 2 . E D c o n n e c t i o n s JD11 , JD1 2 , a n d JD1 3 exhibited brittle failure at the beam diaphragm initially yielded in the beam flanges close to the transition. These observations show that the diaphragm external diaphragms, and the yielding then extended to type has a significant influence on the failure the external diaphragms. Finally, the beam webs began mechanism (ductile/brittle) of the connection and the to buckle locally, leading to large local buckling in location of the yielding. the flanges at the transition between the beam and the The axial force ratio of JD23 is the same as JD13 diaphragm. The web buckling decreased the capacity but their failure modes vary. This is because the of the overall specimen to carry loads. Therefore, the external diaphragms provide a strong reinforcement resistance of the specimen decreased rapidly after the to the steel tube and confinement to the concrete in story drift angle reached 1/50. The loading was stopped the joint. The column retains its strength even under a at the story drift angle of 1/20, when the strength very high axial force ratio. The confinement provided decreased to less than 85% of the maximum value. by internal diaphragms to the joint is weaker, so the Table 2. Material Properties of the Steel Steel thickness fy ft Es Elongation (mm) (MPa) (MPa) (GPa) (%) 8 333.88 525.57 206 29.0 10 354.62 553.83 211 29.5 14 324.41 498.49 206 29.3 18 336.80 512.68 218 33.6 Table 3. Material Properties of the Concrete Concrete fc (MPa) fc' (MPa) ft (MPa) Ec (GPa) 47.8 45.0 2.35 32.6 C40 ID connections showed various failure modes depending on the connection details. Specimens Beam yields: JD11,JD12,JD13,JD22-B,JD22-T1 JD22, JD22-B, JD22-D, and JD22-T1 yielded initially in the beams, while JD23 yielded in the column. Specimens JD22 and JD22-D initially yielded in the beam flanges, followed by severe local buckling of the beam flanges at the transition zone, local buckling of the beam webs and finally by fracture of the beam flange. The failure modes of JD22-B and JD22-T1 are similar to that of JD22. However, the beam flanges of JD22-B and JD22-T1 did not crack even though the overall specimen shear strength decreased to less than 70% of its maximum value. This shows that both thicker steel tubes at the joint and dog bone beam details improve the brittle failure mode of internal Beam fractures: JD22,JD22-D,JD32 diaphragm connections. In addition, JD22-B's yielding and buckling occurred within the dog bone section. The column of the JD23 connection yielded under the high axial force (0.68 P/P ), and this was followed by local buckling of the tube forming an "elephant foot" around the column, and then by weld fractures of the steel tube at the "elephant foot" and crushing of the infilled concrete. The fracture of TD connection JD32 occurred at a story drift angle of 1/50, at the location where the beam flange and the through diaphragm meet. This crack grew rapidly with the increase of the displacements, resulting in a serious decrease in shear strength. Comparison of JD12, JD22 and JD32 shows the Column yields: JD23 influence of the connection type on failure modes: ED connections exhibited ductile behavior at locations Fig.2. Typical Failure Modes of the Connections JAABE vol.14 no.1 January 2015 Xilin Lu 199 steel tube buckles and the inner concrete crushes. displacement at the top of the column to the height This observation shows that the level of confinement of the column. All the hysteresis curves show a provided by the diaphragms has a significant effect on pinching effect close to the origin of the coordinate; the failure mode of the connections. this is probably caused by initial gaps in the unloaded A comparison of JD22 and JD23 shows that the configuration between the bolts and nuts in the testing failure mode of the connection changes from beam system which is difficult to eliminate in the test. The failure (JD22) to column failure (JD23) with the Vc-Rt curves for JD11, JD12, JD22, JD22-B, and increase of the axial force ratio. JD22-D are stable up to a story drift angle of 1/20. 2.2.2 Load-deformation Hysteresis Curves However, the strength of the JD13 and JD23 decreases Story shear force (Vc) versus story drift angle (Rt) rapidly after the 1/50 deformation level (approximately relations for all specimens are shown in Fig.3. The the maximum strength point) because of the high axial story drift angle is defined as the ratio between the force ratio. JD32 also shows a serious strength decrease (a) JD11 (b) JD12 (c) JD13 (d) JD22 (e) JD22-B (f) JD22-D (g) JD22-T1 (h) JD23 (i) JD32 Fig.3. Story Shear Force-Story Drift Angle Curves of the Connections 200 JAABE vol.14 no.1 January 2015 Xilin Lu (a) Axial force ratio (b) Connection type (c) Connection geometry Fig.4. Influence Factors of Story Shear Force-Story Drift Angle Envelop Curves of the Connections in the second and third cycle at the 1/30 loading level; point. this is caused by fast crack growth initiated in the beam 2.2.4 Shear Strength and Ductility flange and through diaphragm transition. JD22-T1 The shear strength and ductility of the connections was the first specimen tested, and the test was stopped are listed in Table 4. before the story drift rotation reached 1/20 because The story drift angle ductility factor is used to of considerations concerning damage to the loading characterize the ductility of the connections. The system. The available data for JD22-T1 shows stable yield strength of the specimen is calculated with the strength up to the 1/30 loading level. method proposed by Han (Han L., 2004), in which 2.2.3 Load-deformation Envelope Curves the yield deformation is defined as the deformation Fig.4. shows the envelope curves used to characterize at the intersection of the initial stiffness line and the the effect of axial force ratio and joint details on the tangent line at the maximum strength point of the load- story shear force-story drift angle relations. High deformation relationship curve. axial force decreases shear strength of the connection The ED connection (JD12) provides the highest and intensifies the strength deterioration after the capacity and the lowest ductility, while the ID maximum strength. The ED connection (JD12) shows connection (JD22) provides a lower shear capacity higher strength and more severe strength deterioration but higher ductility than the ED connection. The TD than ID (JD22) and TD (JD32) connections. The dog connection (JD32) provides similar shear strength bone specimen (JD22-B) decreases the global strength but lower ductility compared to the ID connection. of the connection but shows little initial stiffness Increasing the axial force ratio causes earlier yielding, reduction. Increasing both the thickness of the steel and lower shear strength and ductility (compare JD11, tube (JD22-T1) and diaphragm (JD22-D) in the joint JD12 and JD13). Increasing both the diaphragms area increases the global strength while intensifying thickness (JD22-D) and steel tube thickness (JD22-T1) the strength deterioration after the maximum strength in the joint increases the shear capacity while Table 4. Main Test Results of Connection Specimens Yielding Maximum Ultimate Connection Ductility Direction Strength Story drift Strength Story drift Strength Story drift specimens factor (kN) angle (kN) angle (kN) angle + 429.8 -0.016 505.6 -0.019 429.8 -0.034 JD11 2.123 - -439.5 0.017 -517.0 0.020 -439.5 0.035 + 402.0 -0.013 494.7 -0.018 420.5 -0.027 JD12 1.860 - -428.0 0.014 -490.3 0.017 -416.8 0.026 + 450.0 -0.013 535.5 -0.019 455.2 -0.027 JD13 1.839 - -337.0 0.012 -397.0 0.014 -337.5 0.019 + 283.0 -0.011 361.9 -0.020 307.6 -0.040 JD22 3.433 - -330.0 0.012 -405.7 0.031 -344.8 0.040 + 293.0 -0.014 379.9 -0.031 322.9 -0.038 JD22-B 2.586 - -308.0 0.013 -377.6 0.017 -321.0 0.032 + 313.0 -0.012 401.1 -0.020 340.9 -0.034 JD22-D 2.668 - -312.0 0.012 -425.2 0.019 -361.4 0.031 + 313.0 -0.012 406.2 -0.024 345.3 -0.035 JD22-T1 2.455 - -355.0 0.013 -422.2 0.018 -358.9 0.028 + 311.0 -0.010 483.7 -0.017 411.1 -0.032 JD23 2.775 - -316.0 0.010 -437.1 0.012 -371.5 0.024 + 316.0 -0.013 405.1 -0.020 344.3 -0.033 JD32 2.668 - -305.0 0.012 -365.1 0.013 -310.3 0.033 JAABE vol.14 no.1 January 2015 Xilin Lu 201 decreasing the ductility. The dog bone beam detail 3.2.1 Shear Deformations of the Joint (JD22-B) tends to decrease both the ultimate strength The LVDT gauges are placed on the surface of the and ductility. steel tube webs in the test, so the shear deformations The story drift angle at yield for the connections at the steel tube webs from the Abaqus simulations ( ) tested is between 0.01~0.0163 rad, which is 5.5~9.0 are compared to the tested results ( ) in Table 5. The times that of the elastic limit of the story drift angle experimental shear deformations of JD12 and JD22D for RC structures (1/550), and 3.0~5.4 times that for are not deemed reliable because of instrumentation steel structures (1/300). The ultimate story drift angle issues found during testing. The simulated shear of the connections is between 0.0228~0.04rad, which deformations are 0.95-2.15 times the tested results with is 1.14~2 times that of the plastic limit of the story an average of 1.48 times; smaller shear deformations drift angle for RC and steel structures (1/50). All the from the test may be due to the high strain hardening connections tested exhibited excellent ductility. effect of the steel tube which provides higher confinement to the inner concrete than the model. 3. Finite Element Analysis It is found from the Abaqus analysis that the steel 3.1 FE Modeling tube flanges in the joint experience out-of-plane Computational models were created in Abaqus (Horizontal, X-direction) deformations and the largest to further study the differences between these deformation occurs at the middle of the steel tube connections. A half model with symmetrical boundary flange. Shear deformations at the middle of the steel conditions was used in order to save computational tube flanges ( ') of all specimens are listed in Table 5. memory and time. Three-dimensional eight node brick The value of ' for ED connection is the largest and elements with full integration and incompatible modes that of ID connections is the smallest. Increasing the (C3D8I) were adopted for all the elements. A finer thickness of the diaphragms or steel tube in the joint mesh was used in the joint area in order to capture the decreases the shear deformations. Higher axial force complex stress distribution at this location. A bilinear ratio tends to increase the shear deformations at this stress-strain relationship with a strain hardening of 1% location. was adopted for the steel. The constitutive model of the The value of the shear deformation ratio η = '/ γ γ concrete suggested by Han (Han L., 2004) was adopted is further calculated in Table 5. This parameter was for the inner concrete, considering the confinement used to study the confinement difference of various from the steel tube to the concrete. A "tie constraint" connection types. It is obvious that the stronger was used between the concrete and the steel tube confinement the connection provides, the smaller η because there was very limited slippage observed in will be. The analysis shows that external diaphragms the failed specimens. provide more confinement to the connections than 3.2 FEA Results internal diaphragms, and thicker diaphragms or steel Monotonic loading was used in finite element tubes in the joint provide higher confinement to the analysis to avoid convergence problems. The Abaqus joint. model agrees well with the test results insofar as initial and unloading stiffnesses are concerned. The estimated 4. Force Transfer Mechanisms of the Joint shear strength is 60% and 80~95% of the test results The force transfer mechanism of the joint is further for JD23 and all other connections, respectively. studied to clarify the differences of connections Higher shear strength from the test may be caused by with various diaphragm details. The tension (Ft) a high strain hardening of the steel in the test, whereas and compression force (Fc) in an ED connection strain hardening of 1% is adopted for steel in the is transferred from the beam flanges to the column model. through two paths: (1) one part is transferred to the Table 5. Shear Deformations of the Joint Simulated shear Simulated shear deformation Shear deformation Specimens deformation at the at the middle of the steel tube η =γ '/γ from the experiment η =γ /γ / 2 steel tube web ( ) flange ( ') ( ) γ γ γ JD11 0.0012 0.0043 3.7 0.0024 0.96 JD12 0.0009 0.0042 4.5 0.0117 ---- JD13 0.0012 0.0045 3.8 0.0014 1.72 JD22 0.0021 0.0256 12.4 0.0044 0.95 JD22B 0.0016 0.0155 9.5 0.0015 2.15 JD22D 0.0025 0.0191 7.6 0.0004 ---- JD22T1 0.0012 0.0147 12.3 0.0019 1.29 JD23 0.0040 0.0283 7.0 0.0041 1.97 JD32 0.0017 0.0244 14.0 0.0026 1.33 202 JAABE vol.14 no.1 January 2015 Xilin Lu steel tube flange and the inner concrete; (2) the other diaphragm deformations of JD22T1 are lower than part is transferred to the external diaphragm, and those of JD22. This is probably because the thicker then to the steel tube web and the inner concrete. The steel tube has a stronger stiffening effect on the inner forces from the beam flanges in an ID connection are concrete in the joint, resulting in a higher stiffness in transferred to the steel tube flanges first, and then are transferring the forces in the concrete. Increasing the transferred to the inner concrete through: (1) friction thickness of the diaphragm (JD22D) also increases between the diaphragm and concrete, and (2) the the confinement of the joint, and this leads to smaller compression force at the inner hole. Connections with diaphragm deformations. TD transfer the forces in some combinations of the paths specified for ID and ED connections described 5. Conclusions above. Seismic performance of full-scale CFT column- Fig.5. shows the forces transferred through external steel beam connections with various diaphragms was diaphragms of JD12 versus story drift angle relations. investigated experimentally and computationally to Tension force transferred through Diaph-1 is smaller study the effect of joint details and axial force ratios on than that transferred through Diaph-2. However, the failure mechanisms of the connections. compression force transferred through Diaph-1 is more The experimental research shows that: than 4 times that transferred to Diaph-2. This analysis (1) All of the specimens exhibit excellent seismic shows that concrete provides an efficient transferring performance, including high shear strength and path for compression while the external diaphragm ductility when compared to steel and RC structures. provides an efficient transferring path for tension. ED connections show the highest shear strength but The effect of connection type on the total force lowest ductility among the three types of connections transferred to the diaphragm versus diaphragm tested. ID connections have higher ductility and lower deformation relation is shown in Fig.6. The total shear strength than ED connections. TD connections force transferred to the joint is in the following order: have similar shear strength but lower ductility JD12>JD32>JD22. The deformation of JD12 is around compared to ID connections. Increasing the thickness 80% of that in JD22 and JD32. This comparison of diaphragms and steel tube increases shear strength shows that the external diaphragms provide a stronger but decreases ductility. Increasing the axial force ratio confinement to the joint than internal diaphragms, and tends to decrease both the shear strength and ductility this leads to a smaller diaphragm deformation. Both of the connections. the total forces transferred through the diaphragm and (2) Axial force ratio has a significant effect on yielding mechanism. Very high axial force ratio may lead to column failure even though the equations specified in the Chinese codes (CECS, 2004) are satisfied. (3) Different failure mechanisms of ED and ID connections under high axial force ratio indicates that the reinforcement details affect the failure mechanism of the connections; strong reinforcement of the connection is necessary to ensure the beam yielding mode, especially with a high axial force ratio. Fi n i t e e l e m e n t a n a l y se s p r o v i d e sa t i sf a c t o ry (a) Tension (b) Compression simulations to the test results in light of the maximum story shear forces and joint shear deformations. The Fig.5. Forces Transferred in the External Diaphragm of JD12 ratio of the shear deformation at the middle of the steel tube flanges (maximum joint shear deformation) to that at the steel tube web surface (minimum joint shear deformation) shows that external diaphragms provide stronger confinement to the connections than internal connections, and thicker diaphragms/steel tube in the joint tends to increase the confinement of the connections. The analysis on force transferring mechanism shows that different connection types transfer the forces from the beam flange in different ways, and each connection type shows different force transfer mechanisms for (a) Connection type (b) Connection geometry tension and compression. The concrete in the joint is efficient in transferring compression forces, while Fig.6. The Influencing Factors on the Total Force Transferred diaphragms provide an efficient path for transferring to the Diaphragm JAABE vol.14 no.1 January 2015 Xilin Lu 203 tension forces. Thicker steel tubes in the joint provide more confinement to the concrete, and this leads to smaller diaphragm deformations. In a word, connection details including diaphragm types, diaphragm thickness and steel tube thickness in the joint provide different confinement to the joint which may cause different failure mechanisms of the connections. High axial force ratios may also cause undesirable column yielding mechanisms of the connections if the reinforcement of the joint is inadequate. Acknowledgement This research was sponsored by the National Natural Science Foundation of China under the Integration Project of the Major Research Plan of NSFC (Award Number 91315301-4). The authors greatly appreciate this support. References 1) CECS (2004). Technical Specifications for Structures with Concrete-filled Rectangular Steel Tube Members. Beijing, China: China Engineering Construction Standardization Association. (in Chinese) 2) Elremaily, A. and Azizinamini, A. (2001). 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Building Structure, 37(9), pp.1-6. (in Chinese) 204 JAABE vol.14 no.1 January 2015 Xilin Lu
Journal of Asian Architecture and Building Engineering – Taylor & Francis
Published: Jan 1, 2015
Keywords: CFT connection; failure mechanism; diaphragm; axial force ratio; FE analysis
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