Comparison of steel-concrete composite column and steel column

Accepted Manuscript Comparison of steel-concrete composite column and steel column Piotr Lacki, Anna Derlatka, Przemysław Kasza PII: DOI: Reference:

S0263-8223(17)33195-1 https://doi.org/10.1016/j.compstruct.2017.11.055 COST 9121

To appear in:

Composite Structures

Received Date: Revised Date: Accepted Date:

27 September 2017 13 November 2017 20 November 2017

Please cite this article as: Lacki, P., Derlatka, A., Kasza, P., Comparison of steel-concrete composite column and steel column, Composite Structures (2017), doi: https://doi.org/10.1016/j.compstruct.2017.11.055

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Prof. Piotr Lacki, Ph.D. Anna Derlatka*, Ph.D. Przemysław Kasza Czestochowa University of Technology Dąbrowskiego 69, 42-201 Częstochowa *[email protected]

Comparison of steel-concrete composite column and steel column Keywords: steel-concrete composite column; steel reinforced concrete column; steel; FEM

Abstract The aim of the work was numerical analyses of a steel-concrete composite column and a steel column. An internal column 3.60 m in length was considered. The column was on the second storey of a six-storey building designed for retail and services. The column was subjected to compression and uniaxial bending. The existing steel column was made from a welded Hprofile. In the first stage of the work, the composite column was designed as an alternative to the existing steel column using the analytical method. A steel reinforced concrete column with a steel H-profile was selected. The second part of the work consisted in modelling the steel and composite columns. The geometries, loads and boundary conditions used in simulations of the columns were the same as in the analytical calculations. Numerical analysis was carried out using the ADINA System based on the finite element method. In the steel column, the stresses and displacements were considered. In the composite column, the stresses in the steel and concrete elements, the stresses distributions in the reinforcement bars and displacements of the whole column were evaluated. 1. Introduction Steel-concrete composite columns are new composite members. They are widely used due to their high load-bearing capacity, full usage of materials, high stiffness and ductility and large energy absorption capacity as pointed out by the authors of [1,2]. The steel reinforced concrete (SRC) member (Fig. 1a), also known as the concrete encased composite member, is the result of filling the empty space in a steel H-profile with concrete [3]. Combining reinforced concrete (RC) and structural steel sections provides several advantages over traditional reinforced concrete and steel members. The concrete provides fire resistance to the steel section and restrains the steel member from buckling [4,5]. Applying steelconcrete composite columns has a beneficial impact on the course and values of concrete strains in relation to reinforced concrete columns. However, SRC columns require longitudinal and transverse reinforcement to prevent the concrete from spalling while being subjected to axial load, fire, or an earthquake [6]. A well-confined concrete core is vital for the column to develop a satisfactory plastic hinge rotation capacity. On the other hand, the reinforcement cage in SRC columns creates difficulty in concrete casting of the beamcolumn connections [7,8].

As presented in pracach [9–15], concrete-filled steel tube (CFST) columns (Fig. 1b) are also a primary type of composite columns. They can reduce the time of construction by eliminating formwork and reinforcement. CFST column-walls can be quite flexible in shapes, so that they can be easily fit into all kinds of designs [16,17]. However, the local buckling is always a major concern in the design of concrete-filled thin-walled steel tube columns. As presented in papers [18,19], in order to increase the confinement of concrete and make concrete casting easy in SRC columns, replacing the longitudinal and transverse reinforcement by a thin-walled steel tube (tubed SRC) is proposed. The tube is placed at the perimeter of the cross section and it terminate near the beam-column joint. In paper [20], two types of tubed SRC column (c,d) are distinguished: circular tubed SRC (CTSRC) and square tubed SRC (STSRC).

Fig. 1. Examples of composite columns sections: a) steel reinforced concrete (SRC), b) Concrete Filled Steel Tube (CFST), c) square tubed SRC (STSRC), d) circular tubed SRC (CTSRC) To date only a few 3-D finite element models have been found in the literature highlighting the behaviour of concrete encased steel composite columns. The authors of [21] investigated the behaviour of pin-ended axially loaded concrete encased steel composite columns. A nonlinear 3-D finite element model was developed to analyse the inelastic behaviour of steel, concrete, longitudinal and transverse reinforcement bars as well as the effect of concrete confinement. The main objective of study [22] was to present an efficient nonlinear 3-D finite element model to analyse concrete encased steel composite columns at elevated temperatures. Paper [23] presents the carrying capacity of the column in the form of a steel round tube filled with concrete depending on the ways of loading. A small number of publications of composite columns FEM models provide the basis for an analysing of such columns. In [24] a three dimensional elasto-plastic finite element formulation was employed to investigate the strength degradation of reinforced concrete piers wrapped with steel plates which corrode at the pier base. A challenge in the numerical calculations is to take into account material cracking. In this paper, the fracture model described in [25] was applied. Literature gives different ways to take into account the material's fracture. One of the noteworthy comments of crack analysis is presented in [26], in which a homogeneous elastic, orthotropic solid containing three equal collinear cracks, loaded in tension by symmetrically distributed normal stresses were

considered. Authors of [27] determined the fundamental solutions for an unbounded, homogeneous, orthotropic elastic body containing an elliptical hole subjected to uniform remote loads. The aim of the study [28] is elaboration of a new testing method and estimation of the fracture toughness in Mode III of concretes with F fly-ash (FA) additive. 2. Goal and scope of work The aim of the work was to perform numerical analysis of a steel column and a steel reinforced concrete column. The numerical analysis was carried out using the ADINA System based on the finite element method. In the steel column, the stresses and displacements were considered. In the composite column, the stresses in the steel and concrete elements, the forces distributions in the reinforcement bars and displacements of the whole column were evaluated. In the first stage of the work, the composite column was designed as an alternative to the existing steel column using an analytical method. The existing steel column in the building was made from a welded, steel S235 H-profile (Fig. 3a). The internal column of the length 3.60 m was considered. The column was on the second storey of a six-storey building designed for retail and services (Fig. 2). The column was subjected to compression and uniaxial bending. Based on the load combination, the axial force was NEd = 2045.1 kN, while the bending moment was MEd = 180.27 kNm.

Fig. 2. Internal forces in analysed column The choice of composite column construction was contributed to the fact that several years ago a tram line was built near the renovated building. Tramway traffic causes additional vibration in the construction of buildings. Composite columns have a higher energy absorption capacity than steel columns.

Based on the analytical calculations according to [29], a steel reinforced concrete column with the following cross-section was obtained (Fig. 3): − HEB 260 steel profile made from steel S235 − 4 reinforcement bars 12 mm in diameter made from A-IIIN RB500W steel − C30/37 concrete material

Fig. 3. Cross-section of: a) steel column, b) steel reinforced concrete column The resistance in compression calculated analytically amounted 90%, whereas the resistance in combined compression and uniaxial bending was 50%. The analytical calculations according to standard PN-EN 1994-1-1 [29] do not take into account stretching of the concrete or stress distribution in the reinforcement. Additionally, standard [29] does not explicitly define any method of calculating the displacement of the composite column. Column displacement can only be determined by the numerical model using the finite element method. 3. Numerical model The second part of the work consists in modelling the steel and composite columns. The geometries, loads and boundary conditions of the columns were used as in the analytical calculations. 3D-solid, 27-node finite elements were used for the steel profiles in both columns and concrete. Truss, 3-node finite elements were used for the reinforcement bars. The model was loaded with axial force NEd = 2045.1 kN and a pressure of 0.464 MPa (Fig. 4) acting on the lateral surface of the column. The pressure was intended to reflect bending moment MEd = 180.27 kNm acting on the supports.

The steel column model has 4104 3-D solid elements. The composite column model has 23292 3-D solid elements and 2112 truss elements. The calculations are performed in 10 time steps.

Fig. 4. Column models with load and boundary conditions: a) steel column, b) steel reinforced concrete column A plastic isotropic material model with the following parameters: – – – – – –

Density ρ = 7850 kg/m3 Poisson’s ratio ν = 0,3 Modulus of elasticity Ea = 210 GPa Yield strength fy = 235 MPa Ultimate tensile strength fu = 360 MPa Elongation A5 = 25%

was used for the steel profiles made from S235 structural steel. A plastic isotropic material model with the following parameters: – – – – – –

Density ρ = 7850 kg/m3 Poisson’s ratio ν =0.3 Modulus of elasticity Es = 205 GPa Yield strength fsk = 500 MPa Ultimate tensile strength fu = 550 MPa Elongation A5 = 10%

was used for the reinforcement bars made from A-IIIN RB500W steel.

A model with the following parameters: – – – – – – –

Density ρ = 2500 kg/m3 Tangent modulus at zero strain Ecm = 32 GPa Uniaxial cut-off tensile stress fctm = 2.9 MPa Uniaxial maximum compressive stress (SIGMAC) fcm = 38 MPa Uniaxial compressive strain at SIGMAC εc1 = 2.2‰ Uniaxial ulitimate compressive stress fck = 30 MPa Uniaxial ulitimate compressive strain εcu2 = 3.5‰

was used for the C30/37 concrete material. The basic concrete material model characteristics are [25]: − Tensile cracking failure at a maximum, relatively small principal tensile stress − Compression crushing failure at high compression − Strain softening from compression crushing failure to an ultimate strain, at which the material totally fails 4. Results Results for the steel and composite columns are presented in Table 1.

Time step

Column

X-displacement, mm

Y-displacement, mm

Z-displacement, mm

Stress XX, MPa

Stress YY, MPa

Stress ZZ, MPa

Principal stress, MPa

Table 1. Results for steel and composite columns

1

Steel

-0.3

-0.00

-0.2

-33.9

Composite, steel part

-0.1

-0.00

-0.1

Steel

-0.6

-0.01

-0.4

-179.1; 54.7 -15.34; 0.7 -67.0

Composite, steel part

-0.2

-0.00

-0.2

Steel

-0.8

-0.02

-0.6

Composite, steel part

-0.4

-0.01

-0.3

-11.1; 8.8 -7.6; 5.2 -5.0; 0.7 -21.9; 17.4 -16.7; 12.2 -10.3; 1.4 -32.7; 26.1 -26.4; 19.3

-10.2; 2.4 -23.2; 11.2 -3.6; 2.4 -20.1; 4;8 -52.3; 27.8 -7.4; 2.6 -30.0; 7.1 -82.3; 44.2

-37.4; 10.5 -33.4; 24.0 -5.2; 3.0 -68.9; 18.0 -72.1; 54.0 -10.1; 2.7 -102.9; 27.0 -111.8; 75.0

Composite, concrete part 2

Composite, concrete part 3

-34.9; 8.4 -31.6; 1.6 -100.1 -53.9; 12.9

Composite, concrete part 4

Steel

-1.1

-0.03

-0.8

Composite, steel part

-0.5

-0.01

-0.5

Steel

-1.4

-0.03

-1.0

Composite, steel part

-0.6

-0.01

-0.6

Steel

-1.7

-0.04

-1.2

Composite, steel part

-0.7

-0.02

-0.7

Steel

-1.9

-0.05

-1.4

Composite, steel part

-0.9

-0.02

-0.8

Steel

-2.2

-0.06

-1.6

Composite, steel part

-1.0

-0.02

-1.0

Steel

-2.5

-0.06

-1.8

Composite, steel part

-1.1

-0.02

-1.1

Steel

-2.8

-0.07

-2.0

Composite, steel part

-1.2

-0.03

-1.2

Composite, concrete part 5

Composite, concrete part 6

Composite, concrete part 7

Composite, concrete part 8

Composite, concrete part 9

Composite, concrete part 10

Composite, concrete part

-48.1; 2.4 -133.3 -72.6; 17.4 -64.1; 3.0 -166.4 -91.3; 21.7 -82.5; 3.7 -199.6 -109.5; 26.0 -100.4; 4.3 -232.8 -127.0; 30.0 -118.6; 5.3 -266.0 -144.9; 34.1 -137.3; 6.3 -299.4 -161.7; 43.3 -156.4; 7.4 -325.0 -179.1; 54.7 -175.9; 8.6

-15.7; 2.2 -43.5; 34.7 -36.2; 25.8 -21.3; 2.9 -54.4; 43.4 -46.1; 32.1 -26.9; 2.8 -65.2; 52.1 -55.6; 38.3 -32.7; 2.8 -76.0; 60.8 -64.3; 44.4 -38.6; 2.7 -86.9; 69.5 -74.7; 50.7 -44.7; 3.2 -97.6; 78.2 -90.4; 58.0 -50.9; 3.6 -106.0; 86.9 -107.3; 65.0 -57.2; 4.5

-11.1; 2.6 -39.9; 9.5 -111.6; 59.5 -14.4; 2.6 -49.8; 11.8 -140.0; 74.4 -17.4; 2.5 -59.7; 14.2 -166.8; 89.4 -19.8; 2.3 -69.7; 16.5 -190.3; 104.4 -21.8; 3.6 -79.6; 18.9 -216.9; 120.0 -23.8; 2.9 -89.6; 21.3 -233.0; 136.9 -26.3; 3.8 -103.4; 24.6 -248.1; 152.0 -28.3; 3.3

-14.74; 2.5 -137.0; 37.5 -151.2; 105.0 -18.63; 3.0 -171.1; 45.0 -190.2; 150.0 -22.4; 3.5 -205.1; 52.5 -228.2; 180.0 -25.7; 4.0 -239.2; 60.0 -263.9; 180.0 -28.5; 5.0 -273.3; 75.0 -297.2; 210.0 -32.1; 5.0 -307.7; 90.0 -313.8; 280.0 -35.0; 6.0 -367.5; 105.0 -331.4; 240.0 -37.0; 6.0

The principal stresses distribution (Fig. 5) shows that compression stresses prevail in the steel column. The maximum compression stress is 321 MPa. It is located at the head of the column, on the outer surface of the flange, which is not loaded by pressure. The tensile stresses are on

the outer surface of the flange on which the transverse load of the column is acting. The maximum tensile stress of 90 MPa is on this flange.

Fig. 5. Distribution of principal stresses in steel column, MPa As shown in Fig. 6, the largest displacements with respect the X and Z axes of the steel column are ∆lx = 2.1 mm and ∆lz = 2.8 mm. The locations of the largest displacements are presented in Fig. 6.

Fig. 6. Steel column displacements with respect X and Z axes, mm The principal stresses distribution in the concrete material (Fig. 7) shows that the principal compressive stresses are on most of the column surface. The maximum compression stress is 24 MPa. The maximum value is located at the base of the column, on the opposite side of the applied pressure. The tensile stresses in the concrete element are at the ends of the columns, on the applied pressure side. The maximum tensile stress is 4 MPa.

Fig. 7. Distribution of principal stresses in concrete of composite column, MPa Compressive stresses prevail in the HEB profile (Fig. 8). The maximum compressive stress (201 MPa) is at the column head, on the outer surface of the flange, which is not loaded by pressure. The maximum tensile stress (201 MPa) is in the corner between the flange and the web.

Fig. 8. Distribution of principal stresses in HEB profile of composite column, MPa The distribution of axial stresses in the reinforcement (Fig. 9) shows that the reinforcement bars are compressed, which is due to the nature of the work of the column. In contrast, the binders are stretched because during compression of the column, the column strives to increase the cross-section, which in consequence leads to stretching of the extreme fibres.

Fig. 9. Distribution of axial stresses in reinforcement of composite column, MPa

The displacement of the composite column can only be determined by the numerical model, since the PN 1994-1-1 standard [29] does not define the method of calculation. The maximum displacements with respect to the X and Z axes of the analysed column are ∆lx = 1.2 mm i ∆lz = 1.3 mm.

Fig. 10. Composite column displacements with respect X and Z axes, mm 5. Discussion After loading the existing steel column, the compressive stress of 321 MPa exceeds the steel yield strength of 235 MPa. Therefore, changing the structure of the column is suggested. In the designed composite column, the maximum compressive stress of 25 MPa in the concrete material is less than the compressive strength of the concrete. In analytical calculations, the stretching of concrete is omitted. By contrast, the numerical model provides real stress. The maximum stretching value of 4 MPa exceeds the tensile strength of 2.9 MPa. Exceeding the tensile strength will result in scratches when the maximum load is reached. The maximum compressive and tensile stresses in the steel section of the composite column are 201 MPa, which is 86% of the yield strength. This proves compatibility with the analytical calculations of the load carrying capacity. One of the advantages of steel-concrete composite columns is the small displacement compared to a steel column. The displacement of the steel column relative to the Z axis was 2.1 mm, while the displacement of the column to the Z axis was 1.3 mm. The difference is

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