Study of composite action and bonding capacity in CFT-columns

CONTENT
1 Introduction
2 Composite column
	2.1 History of composite columns
		Figure 1. Schematic cross-section of an Emperger-column
	2.2 Concrete-filled tubular columns
3 Load introduction
	3.1 General
	3.2 Concrete only
	3.3 Steel tube only
		Figure 2. Load introduction with the console.
	3.4 The whole section is loaded
4 Material properties
	4.1 General
	4.2 Steel under pressure
		Table 1. Material properties of steel according to Eurocode 1993
		Figure 3. Tensile behavior of steel during pull test.
	4.1 Concrete under pressure
		Figure 5. A Development of microcracks in concrete when load stage increases.
		Figure 6. Generation of the gap between the concrete core and steel tube.
		Figure 7. Concrete core and steel tube, when equilibrium with lateral deformations is achieved.
		Figure 8. Steel tube and concrete core when confining is activated.
	4.2 Triaxial stress state in the steel tube
		Figure 9. Strain-diagram and coordinate system.
		Figure 10. Load stage in steel tube when confining is active.
		Figure 11. Loading stage in concrete core then confining is active.
		Figure 12. Strain diagram on steel tube.
	4.3 The confining effect
		Table 2. Material properties of concrete in various grades.
		Figure 13. Effect of confining to single aggregate in microscale.
5 Mechanical behavior
	5.1 Friction
	5.2 Bonding mechanism
		5.2.1 Adhesion
			Figure 14. A Schematic view of assumed load-slip curve during the push-out test.
		5.2.2 Terminology
			Figure 15. Microlocking.
			Figure 16. Macrolocking.
		5.2.3 Microclocking
			Figure 17. An amplifying/diluting effect to microlocking.
				Table 3. Material properties of different stones Geoscience, Texas The University Houston.
			Figure 18. Keying effect of aggregate teeth.
		5.2.4 Macrolocking
			Table 4. Allowed tolerance in steel tubes from EN 10219-2:2006.
			Figure 19. Tolerance in steel tube.
			Figure 20. Generation of normal force in macrolocking stage.
		5.2.5 Mechanical shear connector
			Figure 21. Schematic cross-sectional view of weld bead and Hilti nail.
			Figure 22. “Rising” of weld caused by mechanical keying and slipping.
	5.3 Surface roughness
		5.3.1 General
		5.3.2 Ra-Value
			Figure 23. Simplified Ra -approximation.
			Figure 24. Surface with peaks and high frequency.
			Figure 25. Surface with round peaks and low frequency.
		5.3.3 Rz-value
			Figure 26. Idea of Rz-value.
6 Material behavior
	6.1 Push-out test
	6.2 Measurement and instrumentation
	6.3 Pressure and lateral stiffness
		Figure 28. Deformations caused by concrete pressure in square profile.
			R is the rate of placement [m/h]
			Table 5 a. ????,-????.-values
			Table 5 b. ????,-????.-values
		Figure 29. Distribution of concrete pressure.
	6.4 Deformations during push-out test
		Figure 30. retells findings of Tao et al. in schematic picture of bonding pattern.
		Figure 31. Retells idea of Tao et al. (Tao et al. 2011, 490). Pinching in the push-out test.
	6.5 Effect of the shrinkage
		Figure 32. Comparison of autogenous shrinkage to overall shrinkage. Values are calculated with formula 3.8 from Eurocode 1992 and with initial values, Do=323.9 mm, td=28 d, ts=3 mm, Rh=60% and fck(x)=20, 40, 60, 80.
		where * marks values for total shrinkage and dot mark value for autogenous shrinkage. At X-axis is concrete grade and Y-axis is shrinkage.
		Figure 33. Percentage of autogenous from overall shrinkage. Values are calculated with formula 3.8 from Eurocode 1992 and with initial values, Do=323.9 mm, td=28 d, ts=3 mm, Rh=60%, fck(x)=20, 40, 60, 80 and assumed cement type is N. Where x marks pe...
	6.6 Effect of surface roughness
		Table 6. common values for roughness range in different steel surfaces
	6.7 Wetting of material
		Figure 34. Contact angle that determines the wetting category.
	6.8 Oxidation
		Figure 35. Oxidation of steel surface.
	6.9 Effect of pitting mechanism
	6.10 Effect of corrosion
	6.11 Mill Scale
7 Shear transfer in structure
	7.1 Transfer length
		Figure 36. Development of shear transfer trough length.
		Figure 37. Schematic graph of location - strain curves.
	7.2 Beam-column load introduction
		Table 7. Eurocode based bonding capacity values
	7.3 Calculation method
		7.3.1 Simplified calculation method
			Figure 38. Contact area and parameters.
			Figure 39. The calculation method of the moment stage in column joint.
		7.3.2 FEM
			Figure 40. General mesh.
			Figure 41. The Mesh in cross-section and top of consoles.
			Figure 42. The close up of mesh in the joint area.
			Figure 43. The stress stage in the console joint.
			Figure 44. Deformations in the console joint.
			Figure 45. Stress stage in the concrete core when a load is introduced via the console.
			Figure 46. Fem analyses of CFT structure with consoles that are similar than Shakir-Khalil used.
			Figure 47. The dimensions of square pipe console.
			Figure 48. Stress stage distribution at infill concrete when Shakir-Khalil (knife plate) console is used.
			Figure 49. Stress stage distribution at infill concrete when tube console is used.
			Figure 50. Stress distribution in steel tube.
			Figure 51. Stress distribution in concrete core.
			Figure 52. The stress distribution in steel tube with knife plate console.
			Figure 53. The stress distribution in steel tube with tube console.
	7.4 Construction order
		Figure 54. Schematic picture of sliding mechanism if consoles are loaded before casting.
8 Test preparation
	8.1 Analyzing Roik´s test setup
		Table 8. Shear bonding values from DIN 18806
	8.2 Diameter and concrete mixture
	8.3 Performing the push-out test
	8.4 What happen during the test.
	8.5 Test setup
		Figure 55. Curing of specimen.
		Figure 56. Positioning of LVDT- gauges.
		Figure 57. Assumed development of bonding.
		Figure 58. Positioning of strain gauges.
	8.6 The test specimens
		Figure 59. Example of step up weld in specimen from the group R.
			Table 9. Parameters of the test specimens.
			Table 10. Characteristics of steel tubes
9 Inspection of the specimens
	9.1 Material properties
		9.1.1 Steel
			Table 11. Tested properties of steel tubes.
		9.1.2 Concrete
			Table 12. Measured concrete properties.
			Table 13. the development of shrinkage in air curing.
			Table 14. Shrinkage (mm) after 28 days. Comparing of the result in different storage conditions.
	9.2 Inspection of the test tubes
		Figure 60. Longitudinal location of measurement points (M(n) for surface roughness measurement.
		Figure 61. Cross-sectional location of measurement points (M(n)) for roughness measurement.
10 Test results
	10.1 Group R
		Figure 62. Load-slip curve for the specimen’s form the group R.
	10.2 Group 3
		Figure 63. Example of external stud that was welded on outer surface of steel tube for facilitate the handling of specimens.
		Figure 64. Load-slip curves for all of the specimens from the group 3.
	10.3 Group 4
		Figure 65. load-slip curves for the specimens from the group 4.
	10.4 Group 5
		Figure 66. Load-slip curves for the specimens from the group 5.
	10.5 Group 6
		Figure 67. Load-slip curves for specimens from the group 6.
	10.6 Group 7
		Figure 68. Load-slip curves for specimens from the group 7.
	10.7 Group 8
		Figure 69. Load-slip curves for the specimens from the group 8.
	10.8 Strain
		10.8.1 Confinement
			Figure 70. Gauge positions of the specimen 4 from the group 5.
			Figure 71. Gauge positions of the specimen 3 from the group 5.
			Figure 72. Gauge positions of the specimen 3 and 4 from the group 6.
			Figure 73. Development of axial strain value in the specimen 3 from the group 5.
			Figure 74. Load-strain diagrams; results from axial gauges at top part of the specimen 3 and 4 from the group 5 and specimen 3 and 4 from the group 6 are presented.
			Figure 75. Load-axial strain diagram for the specimen 4 for the group 5.
			Figure 76. Positioning of horizontal gauges in the specimens 3 and 4 in the group 8
			Figure 77. Evaluation points at slip-load curve.
			A78. axial and lateral strain of the specimens 3 and 4 from the group 8 at slippage of 1.85mm.
			Figure 79. Axial and lateral strain of the specimens 3 and 4 from the group 8 at slippage of 4mm.
			Figure 80. Axial and lateral strain of the specimens 3 and 4 from the group 8 at slippage of 10mm.
			Figure 81. Axial and lateral strain of the specimens 3 and 4 from the group 8 at slippage of 20mm.
			Figure 82. Development of strain in gauges at position 6. from the specimens 3 and 4 from the group 8. All data have been smoothened with exponential smoothing with factory 0.98.
			Figure 83. Development of strain in gauges at position 10. from the specimens 3 and 4 from the group 8. All data have been smoothened with exponential smoothing with factory 0.98.
			Figure 84.  Development of confinement in the specimen 3 from the group 8.
			Figure 85. Development of confinement in the specimen 4 from the group 8.
			Figure 86. Development of confinement in the specimens 3 and 4 from the group 8, at position of gauge 1.
			Figure 87. Development of confinement in the specimens 3 and 4 from the group 8, at position of gauge 2.
			Figure 88. Development of confinement in the specimens 3 and 4 from the group 8, at position of gauge 3.
			Figure 89. Development of confinement in the specimens 3 and 4 from the group 8, at position of gauge 4.
			Figure 90. Development of confinement in the specimens 3 and 4 from the group 8, at position of gauge 5.
		10.8.2 Weld
			Figure 92. Example of mark that is caused by weld during the test.
			Figure 93. The stress-slip curves from all of the specimens from the groups 3,5 and 6.
			Figure 94. The stress-slip curves from all of the specimens from the groups 4,7 and 8.
		10.8.3 Transfer length
			Figure 95. Strain values from gauges attached in the specimen 4 from the group 5.
			Figure 96. Strain values from the both specimens at the group 5.
			From figure 97. Both of the specimens from the group 6.
			Figure 98. Strain values from gauges attached to the specimen 3 the group 5 and average of specimens from the group 6 are presented.
		10.8.4 Affect of the weld
		10.8.5 Shrinkage
			Table 16. Effect of shrinkage to ideal radius after 28 days
			Figure 99. Concrete test cube with Finnish aggregate.
			Figure 100. Concrete test cube with partly Lithuanian aggregate.
11 Conclusions
Bibliography