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Concrete filled steel tubes
- 2. CONTENTS INTRODUCTION HISTORY OF CFT COMPONENT BEHAVIOUR COMPARISON MATERIALS PRESTRESSED CFT APPLICATIONS ADVANTAGES AND LIMITATIONS CONCLUSION BIBLIOGRAPHY
- 3. What is CFT
- 4. INTRODUCTION Concrete filled steel tubes (CFT) uses the advantage of both steel and concrete Infill concrete in steel tube delays local buckling Steel tube reinforces the concrete to resist tensile forces Improves its compression stress and ductility
- 5. Typical CFT cross sections
- 6. HISTORY CFT has been used in China for almost 50 years Used as main column in subway stations at Beijing from 1966 CFT used as to avoid having large size of columns in buiding’s in 1980s Since 1990s many buildings has CFT columns in Beijing and other parts in China
- 7. COMPONENT BEHAVIOUR The circular cross section provides the strongest confinement to the core concrete. Local buckling is more likely to occur in square and rectangular cross sections square and rectangular hollow sections are increasingly used in construction since being easier in beam-to-column connection and high cross sectional bending stiffness In CFT confinement of concrete is provided by steel tube and local buckling of steel tube is improved due to concrete core
- 8. Schematic failure mode of hollow steel tube, concrete and CFT columns
- 9. COMPARISON Tests was conducted for comparison between Steel tube, RC, Steel Tube plus RC, and CFT The geometric dimension is taken as a constant for all the materials considered. Test results gives that ultimate strength of CFT is even larger than the steel tube and RC column combined. Ductility of CFT is significantly enhanced when compared with steel tube and concrete alone
- 10. Axial compressive behaviour of CFT column
- 11. MATERIALS CFT comprises of : Steel Concrete
- 12. STEEL Mild steel, High strength steel, Fire resistant steel etc. Outward profile of steel should not be less than 100mm Wall thickness of steel tube should not be less than 3mm Circular hollow steel section, D/t less than or equal to 100(235/fy)
- 13. For Rectangular steel section, D/t less than or equal to 40(235/fy) where, D- Outer diameter or depth of circular or rectangular section t- wall thickness of the tube Fy - yield strength of steel
- 14. Concrete Normal weight concrete and High strength concrete can be used Water/Cement ratio should not exceed 0.4 Self- Consolidation Concrete is preferred Strength of steel and concrete should be suitably matched Combination of High Strength Concrete with High strength steel is preferred
- 15. For fy of steel tube from 235N/mm2 to 345N/mm2 , compressive strength of concrete should be 40 MPa to 60 Mpa For fy of steel tube from 235N/mm2 to 345N/mm2 , compressive strength of concrete should be 40 MPa to 60 Mpa
- 16. PRESTRESSED CFT Advanced technique in the field of CFT Increases the load carrying capacity of CFT considerably. Prestressed CFT truss girders usually consist of CFT chords, hollow steel tube braces, and high-strength prestressing strands.
- 17. (a) Xiangjiaba Bridge (b) Zidong Bridge Typical applications of CFT truss girders in bridges.
- 18. (a) Details of the transition block (b) Details of the anchorage block Cross-section view of the prestressing strand (Unit: mm)
- 19. Elevation view of the prestressed truss girder (unit: mm).
- 20. ADVANTAGES The occurrence of the local buckling of the steel tube is delayed, and the strength deterioration after the local buckling is moderated The strength of concrete is increased due to the confining effect provided from the steel tube, and the strength deterioration is not very severe, since the concrete spalling is prevented by the tube. Drying shrinkage and creep of concrete are much smaller than ordinary reinforced concrete. Forms and reinforcing bars are omitted and concrete easting is done by pump-up method, which lead to savings of manpower and constructional cost and time
- 21. Concrete improves the fire resistance performance, and the amount of fireproof material can be reduced or its use can be omitted Better cost performance is obtained by replacing a steel structure by CFT structure The size of column is smaller, increases the usable floor area CFT columns used concrete 62% less and steel 5%~10% less than that of RC columns. It is about 55% lighter than that of RC. Hence, the foundation cost can be reduced. The force resulting from earthquake is smaller. CFT columns are safer and more reliable in seismic region, The high-strength concrete can be used and the brittle failure can be prevented.
- 22. LIMITATIONS Limited knowledge regarding the behaviour of CFT The interaction of the two materials poses a difficult problem in the determination of combined properties such as moment of inertia and modulus of elasticity the majority of the tests to date have been conducted on relatively small specimens, often 6 inches in diameter or smaller
- 23. APPLICATION OF CFT In tall buildings, only partial columns of building adopted in early days, then greater part of columns adopted, then all of the columns were adopted CFT has higher compression capacity and ductility. It is good for the application of arch bridge.
- 24. First CFT Arch Bridge in China: Wangchang East River Bridge (Span 115 m)
- 25. CANTON TOWER Canton Tower is the fourth-tallest freestanding structure in the world by pinnacle height (604 m) The structure consists of a space lattice composite frame and an RC core 24 inclined CFT members are utilised Maximum tube dia and wall thickness are 2000mm and 50mm respectively The cross section of CFT columns used in High rise buildings are large.
- 26. CANTON TOWER, GUANZHONG, CHINA CFT column Mega CFT Column cross section
- 27. The mega column cross section is divided into many chambers The longitudinal stiffener, reinforcing tie bars and internal diaphragms are used to increase internal stability Several vent holes and man holes are provided for concrete placement and installation
- 28. GANHAIZI BRIDGE, CHINA World’s longest CFT truss bridge with the world's highest bridge piers of CFT lattice Uses CFT for nearly the entire structure. Total length of bridge is 1811m and have 36 spans of 62.5 m Pier height is 110m Super structure is composed of CFT truss girders and precast concrete deck slabs.
- 29. • Compared to traditional bridges, this kind of bridge can reduce more than 50 per cent of its own weight and has a very good seismic performance
- 34. CONCLUSION The scope of CFT has been extended greatly by researchers and engineers with rapid development of research and application of CFT structures in China and all over the world Characteristic of CFT is that the structural properties can be improved due to the composite action between steel tube and filled concrete. CFT can be treated as an alternative system to the steel or RC system Studies on the feasibility of the CFT system should be fully evaluated for its widely expanded applications in the future.
- 35. REFERENCE 1. ASTM. 1991 „„ASTM standards in building codes: specifications, test methods, practices, classifications, terminology,‟‟ Philadelphia. 2. British Standards Institution ~BST1994. Design of composite steel and concrete structures. Euro code 4, ENV 1994-1-1, London. 3.Elwi, A. A., and Murray, D. W. ~1979!. „„A 3D hypo elastic concrete constitutive relationship.‟‟ J. Eng. Mech. Div., Am. Soc. Civ. Eng., 4.Cheng Hongtao, Dissertation of the doctoral degree in engineering(D), Harbin Institute of Technology, Harbin 2001. 5. Zhong CFT, Concrete Filled Steel Tubular Structures (M), Heilongjiang Science- Technical Publishing House, Harbin, 1995. 6. Design Regulation of Composite Structures(S), DL/T 5085-1999. 7. Design Regulation of Composite Structures---- Square CFT Members(S), GJB4142- 2000. 8.Bazant Z.P. and Kim S.S. Plastic-Fracturing Theory for Concrete.(J), Journal of Engineering Mechanics Division. 1979, 105(EM3).