Concrete-Filed Steel Tubular (CFST) Members

Advantages of CFST

Higher confinement in the concrete

?The steel column section can add confinement to the concrete core, which causes an increment both in strength and ductility for the concrete. Circular CFST cross-sections provide a higher confinement than rectangular CFST, due to the shape of circular section provides the higher hoop stresses. This confinement is also influenced by the diameter-to-thickness ratio (D/t) of the tubes. In Fig. 3, it clearly shows that the ultimate strength for a concrete-filled steel tube is even larger than the summation of the strength of the steel tube and the RC column, which is described as “1(steel tube) + 1(concrete core) greater than 2 (simple summation of the two materials)” (Han et al., 2014).

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Delay of the steel local buckling

Local buckling of the chord, especially around the joint, can be prevented or delayed due to the support from the core concrete to the steel tube and thus can increase the strength and the ductility of the structures (Xu et al., 2014). Even if the concrete cracks, the delay of local buckling in CFST sections will still occur since the concrete expands and bears against the steel tube, maintaining the concretesteel contact. Due to the concrete core forces all local buckling modes outward, thinner steel sections may be used that still ensure the yield strength will be reached in the tube before buckling occurs.

Higher compressive and flexural strength

?The concrete filling gives a higher load compressive capacity without increasing the outer dimensions. With concrete CFST built-up columns is very beneficial in producing a great moment inertia through the position of the chords being far from the centroid of the column. When the spacing is larger in the pier, the lacing connections can even using truss hollow sections.

?Earthquake and Fire Resistance

?CFST columns have become the preferred form for many seismic-resistant structures. Subjected to severe earthquakes, concrete encasement cracks resulting in reduction of stiffness but the steel core provides shear capacity and ductile resistance to subsequent cycles of overload (Shanmugam & Lakshmi, 2001). In contrast to reinforced concrete columns with transverse reinforcement, the steel tube also prevents spalling of the concrete and minimizes congestion of reinforcement in the connection region, particularly for seismic design. Therefore, CFST columns have been used for earthquake-resistant structures. Compared to hollow steel tube structure, the fire resistance can be considerably increased by concrete filling. The concrete can work as a fireproofing to the steel section.

?Rapid construction

The tedious process of framework preparation and steel fixing in the RC construction is absent in CFST structures since the steel tube acts as the framework, which decreases workload (Abed et al., 2013). In moderate- to high-rise construction, CFST column can ascend more quickly than a comparable reinforced concrete structure since the steelwork can precede the concrete by several stories (Webb, 1993). The hollow steel tubes are also conveniently hoisted and stitched. Once the concrete has hardened and the composite action has been developed, the system can achieve its final strength and stiffness, to support the designated gravity and lateral loads.

Savings in the construction costs

?Due to the fast erection and an optimal design, the constructions costs may be reduced. Because of its higher strength, a composite column is lighter than a typical RC column with a similar strength, which reduces the loads on and cost of the foundation, and the cost and amount of reinforcement bars. The steel section can word as formwork and is stiffened by the concrete in CFST columns, is much lighter than a conventional steel column, which also reduces substantially the steel costs.

The other advantages of CFST column is listed below:-

1. The size of column is smaller, increases the usable floor area by 3.3% (5500m2).

?2. CFST columns used concrete 62% less and steel 5%~10% less than that of RC columns.

?3. Compared with steel column, CFST ones used steel is 50% less and decreases cost 45%.

?4. It is about 55% lighter than that of RC. Hence, the foundation cost can be reduced. The

force resulting from earthquake is smaller.

?5. The cost on transportation and assembly of columns can be reduced because they are built by hoisting the empty steel tube first, then pour concrete into it.

?6. CFST columns are safer and more reliable in seismic region, The high-strength concrete can be used and the brittle failure can be prevented.

?7. Steel tube of CFST columns are generally less than 40mm thick. It is easily available, cheap and can be conveniently fabricated and assembled.

?Disadvantages

Limited Applications

From the many structure has been built, it can be seen that the use of CFST are trusses also limited to columns, piers, arch ribs, etc. Currently, very few precedents of CFST trusses using in beams. Because beams are generally made of rectangular shape, while rectangular CFST has a more complex mechanical performance, tedious requirements and poor economic returns.

Complex load transferring mechanism

?For transport or erection it may be that bolted joints are preferred or required, whereas for space structures prefabricated connectors are generally used. However, the simplest solution is to profile the ends of the members which have to be connected to the through member (chord) and weld the members directly to each other. Nowadays, end profiling does not give any problem and the end profiling can be combined with the required bevelling for the welds. Although the directly welded joint is the simplest and cleanest solution, the load transfer is rather complex due to the non-linear stiffness distribution along the perimeter of the connected braces. The design rules have been based on simplified analytical models in combination with experimental evidence, resulting in semi-empirical design formula (Wardenier et al., 2010).

Significant second-order effects

Collapse prevention is a fundamental objective of earthquake resisting design. Collapse may occur if an individual story displaces sufficiently so that the secondorder effects (termed P-Δ or P-δ effects) fully offset the first-order story shear resistance and instability occurs. It is one of the major concerns in seismic design to avoid excessive P-Δ effects (Gupta & Krawinkler, 2000). Similarly, CFST built-up columns can be used in high-rise structure, while slender columns with small diameter of the tubes will inevitable increase the P-Δ effects.

Fire resistance

Concrete filled steel tubular (CFST) columns have good load bearing performance and fire resistance because the core concrete can help to bear load and absorb heat, However, fire protection is still necessary to ensure structural fire safety of CFST columns particularly when the columns have big slenderness or subjected to high load level.

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Fire safety of building structures is becoming more and more important in recent years all around the world. Structural members should be designed satisfying specific fire resistance rating multi-storey buildings often require structural fire resistance rating of 120 min or more. Fire proof coating, including non-reactive type (eg. sprays, cement mortar and concrete encasement) and reactive type (eg. thin film intumescent fire coating), are normally used to protect steel and steel-concrete composite structures and they can form an effective fire insulation layer. The nonreactive fire proof coating materials have relatively constant physical and thermal properties and no chemical reaction would occur when heated. However, the non-reactive fire proof coatings have several defects such as high thickness, heavy weight, poor durability and difficult maintenance. Moreover, in order to achieve aesthetic appearance, additional decoration is also inevitable. The reactive fire proof coating, such as the intumescent fire coating (IFC), is composed of constituents including resin, dehydrating catalyser, carbonizing agent, vesicant, fillers and additives. Intumescent coating has advantages such as light weight, high performance, simple construction, aesthetic appearance, easy to maintenance and so on. Chemical reaction would occur when IFC is heated, and large amount of heat is absorbed by smoke and fume produced in the chemical reaction.

??Determination of IFC for steel structures is normally dependent on three parameters

in practice:

?1- required fire resistance rating

2- section factor of a specific structural member

3- a limiting steel temperature

?Intumescent fire coatings are usually used to protect steel structures with fire resistance rating of less than 90 min ,and some products can provide fire protection up to 120~150 min

?IFC are usually affected by several factors such as the heating rate of the protected steel, the coating thickness and the steel section factor, fire protection for the CFST columns classified as fire resistance class I [1] in an airport terminal building in China (Fig. 1).

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References:-

Liew, J. Y., & Xiong, M. (2015). Design Guide for Concrete Filled Tubular Members with High Strength Materials to Eurocode 4. Singapore: Research Publishing Blk 12 Lorong Bakar Batu, #2-11, 349568 SINGAPORE.

?song ,Q.zhou,K.han,L.feng,Y.(2018) Fire resistance of circular concrete-filled steel tubular (CFST) column protected by intumescent coating. China Southwest Architectural Design and Research Institute. Article in Journal of Constructional Steel Research.(DOI: 10.1016/j.jcsr.2018.03.038)

?maca,J.pecce,M.zonta,D.clemente,P.(2015) SEISMIC BEHAVIOR OF CONCRETE FILLED STEEL TUBULAR BUILT-UP COLUMNS. UNIVERSITY OF TRENTO Engineering of Civil and Mechanical Structural Systems

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