Finite element analysis and parametric study of EWECS composite columns with double H-shaped steel

The behavior of composite column that consists of an exterior wood panel with concrete encased steel (CES) core, hereafter referred to as Engineering Wood Encased Concrete-Steel (EWECS) composite columns, is investigated . Nonlinear analysis is done by using finite element software, ANSYS APDL, to study the seismic performance of the columns. Verification of the finite element modeling is done by comparing and corresponding experimental result that reported by one of the authors, then it is used as a reference for parametric study. The parameters in the parametric study are the use of fiber reinforced concrete (FRC), the use of Indonesian wood and the use of friction element. The results are presented in the form of hysteresis characteristics, failure mode, and principal stress distribution. It is demonstrated that the seismic performance of the EWECS composite columns can be accurately predicted by proposing finite element modeling. Obtained results from the parametric study show that various FRC, different wood, and the contact element influences the hysteresis loops and behavior of the columns. The flexural capacity of the columns is improved about 7-17% by adding steel fiber. In addition, the typical Indonesian wood (Matoa) enhances the flexural strength about 3.3%. Moreover, the use of a friction element affects the seismic behavior significantly.


Introduction
The innovation of the composite column structure has been widely researched and developed. In some countries, wood materials have been very popularly used as structural elements. Many houses and buildings are made of wood because of their environmentally friendly material and historical value. However, many developed countries such as Japan have regulation to strictly limit the story number of the wooden building [1]. A new composite structure has been developed in Japan as a solution for this limitation called Engineering Wood Encased Concrete-Steel (EWECS) composite structure. This composite structure consists of EWECS columns and EWES beams [2]. Fig. 1 shows the EWECS composite structure scheme for medium-rise buildings, such as apartments and offices.
The EWECS column is a new composite column, consisting of a steel encased concrete (CES) core covered by the wood panel, as shown in Fig. 2. Economic and structural advantages can be realized in this composite column. The concrete serves to withstand the local buckling of steel and increase the ductility of the structure [2].
The use of wood panel on this structure has several benefits. During construction, wood panel serves as a formwork of the column, which will reduce the construction costs. In addition, the woody panel can improve the behavior of structures on the column through its action of confining the CES cores and resisting to bending moments, shear, and buckling. This advantage makes EWECS columns applicable to the actual structure as an alternative to the SRC column, which has a weakness because it is difficult in the construction process [3].
Some experimental studies on the seismic performance of EWECS composite columns subjected to combined constant axial load and cyclic lateral load as a seismic simulation have been conducted by one of the authors in Japan [2][3]. Basically, an experimental study is ideal for studying behavior and structural failure. However, experimental studies take time and costly. In addition, the experimental study also requires adequate facilities, space, setting, and labour. If it is done correctly, finite element (FE) analysis is a powerful tool that can be an attractive alternative as a substitute and validate for experimental testing. This underlies the authors for developing finite element models using FEM-based software ANSYS APDL v14 [4]. The FE analysis software has been proved to enable the engineer perform multiple tasks, building computer models of the structural elements, applying loads and studying structural responses like the stress levels [5].
Models are developed by taking into account the nonlinear materials response. The FE analysis results are verified with the test results conducted by one of the authors [3]. Moreover, parametric studies were carried out with parameters such as the use of Fiber-reinforced Concrete (FRC use of friction element between wood and concrete influence of these parameters EWECS composite columns

The geometry of 3D
In this analysis considered based on described in Fig. 2 sections are 1600 mm. The Double-H-shaped cross s were used as the The dimensions test specimen model. In order with reasonable carefully determined elements, which range between obtain the computing time. 5095 elements considered as non  [4]. FRC), the use of Indonesian wood use of friction element between wood and concrete of these parameters EWECS composite columns EWECS composite structural system eometry of 3D analysis, square size based on the in Fig. 2. In a sections are 400 x 400 mm . The height to depth shaped cross steel were used as the encased steel of the column dimensions and geometrical specimen are used to construct the In order to get the reasonable computational determined. The aspect ratio of the , which is used in this analysis between 1 and 4. most accurate results in computing time. The total elements. In this as non-linear behavior material Detail of test specimen and 3D model view use of Indonesian wood use of friction element between wood and concrete of these parameters on the performance of EWECS composite columns will be discussed structural system eometry of 3D model , square size composite the experimental program In all models, the column cross mm 2 with the column height of depth ratio of the column teels of 300 x 150 steel of the column geometrical configuration of the are used to construct the the high accuracy computational time, the . The aspect ratio of the used in this analysis The model was discretized most accurate results in The total numbers of this analysis, the linear behavior material of test specimen and 3D model view use of Indonesian woods, and use of friction element between wood and concrete. The on the performance of will be discussed.
composite column experimental program, as , the column cross with the column height of of the column is 150 x 6.5 x 9 mm steel of the column.
configuration of the are used to construct the finite element accuracy of the results he mesh size . The aspect ratio of the solid used in this analysis, was kept in the The model was discretized most accurate results in with lowest of element used he material linear behavior material [6]. of (4):

Encased
Where E s (f y ) of encased steel MPa, respectively. yield criterion is

Wood
In this work obtained from the material test. The ultimate compressive strength modulus of 10500 MPa. The data from the experiment were used in the the force is assumed to direction to the annual growth ring of the wood some existing concrete data w characteristics wood constitutive model element model, as seen in Fig. 6.
-strain relationship Encased steel shows the constitutive model), which response in this analysis. Th condition, which : s is equal to 200000 MPa. of encased steel for flange and web MPa, respectively. In this yield criterion is applied.

Constitutive model for
Wood panel work, the properties of obtained from the material test. The ultimate compressive strength is modulus of 10500 MPa. The data from the experiment were used in the finite element is assumed to direction to the annual growth ring of the wood some existing concrete data w characteristics [10]. The slightly reduced of the real wood constitutive model about 5% is applied in the model, as seen in Fig. 6. , the properties of the wood panel were obtained from the material test. The ultimate is 36.5 MPa with modulus of 10500 MPa. The data from the experiment finite element analysis. is assumed to be applied in direction to the annual growth ring of the wood some existing concrete data were The slightly reduced of the real about 5% is applied in the model, as seen in Fig. 6 perpendicular to the bond connection between wood panel and concrete core, the ultimate coefficient Also, the fracture criterion of wood material characteristics.

2.3
There are two materials in the element steel and wood eight nodes in the nodal steel connection ultimate stress Considering the lower tensile strength in the direction perpendicular to the bond connection between wood panel and concrete core, the ultimate tensile strength coefficient of modified shear transfer ( Also, the model of William the fracture criterion of wood material characteristics.

.3 Element type
There are two materials in the composite element for concrete steel and wood, which is eight nodes. There are in the nodal x, y, and In this analysis steel -concrete interface concrete interface connection, which ultimate stress [12]

ype
There are two types of elements used to model composite column concrete and the , which is 3D hexahedral element . There are 3 translational , and z directions analysis, a perfect interface [11], wh interface was assumed , which is modeled [12]. the lower tensile strength in the direction grain and also there is no bond connection between wood panel and concrete core, tensile strength is taken as 5 MPa. modified shear transfer (β c ) Wranke [9] is adopted for the fracture criterion of wood material characteristics.
types of elements used to model olumn, that are the he SOLID185 3D hexahedral element translational DOF directions, as shown in Fig  a perfect bond was assumed for [11], whereas the was assumed as unbounded modeled by decreasing olid65 and Solid185 ANSYS elements [ the lower tensile strength in the direction grain and also there is no bond connection between wood panel and concrete core, as 5 MPa. The ) is 0.35 [8]. is adopted for the fracture criterion of wood material characteristics.
types of elements used to model the , that are the SOLID65 SOLID185 element for 3D hexahedral elements with DOF at each node as shown in Fig. 7 [4].
was assumed for the the wood panel as unbounded decreasing the wood olid65 and Solid185 ANSYS elements [4]. the lower tensile strength in the direction grain and also there is no bond connection between wood panel and concrete core, The [8]. is adopted for

Boundary
The boundary were modeled to The constant on EWECS column by movement, while the load was The bottom end of the simulate against all restrained against all as shown in Fig. 9 the upper end strength of column at the lower end [1 displacement are monitored

Loads
The combined constant axial load and were applied to EWECS column procedure for bidirectional cyclic loads are presented in this constant compressive applied on top of Boundary conditions boundary conditions were modeled to simulate constant axial and cyclic lateral EWECS column by fixing the bottom end against movement, while the load was he bottom end of the simulate against all DOF. Simultaneously, the upper end restrained against all DOF as shown in Fig. 9. The lateral cyclic the upper end using controlled column was measured using a reference point at the lower end [13]. In addition, t displacement are monitored

Parametric
From the above the FE model can provide an accurate prediction for its seismic behavior, which has been compared to experimental program. behavior of loading, a parametric the influence composite columns such as Concrete (FRC Indonesian wood friction element between wood and concrete a parameter in this study because of the importance of the material in structural resistance, and it can improve seismic behavior without significantly shows the different values of each parameter for the parametric stud model that has been validated with to the reference model [15].

Parametric study of EWECS
From the above numerical analysis of EWECS column, the FE model can provide an accurate prediction for its seismic behavior, which has been compared to experimental program. In order to of EWECS composite parametric stud influence of the possible material composite columns such as FRC) for the concrete core Indonesian wood. In addition, the effect of applying friction element between wood and concrete a parameter in this study. Th because of the importance of the material in structural and it can improve seismic behavior without significantly changing the shows the different values of each parameter for the study. In this parametric stud that has been validated with the reference model [15].

tudy of EWECS column
numerical analysis of EWECS column, the FE model can provide an accurate prediction for its seismic behavior, which has been compared to In order to know more composite columns study was carried out the possible material composite columns such as the use of ) for the concrete core addition, the effect of applying friction element between wood and concrete . These parameter because of the importance of the material in structural and it can improve seismic behavior without changing the column dimensions. shows the different values of each parameter for the parametric stud that has been validated with the the reference model [15].

Kapur wood Matoa wood
Conta and Targe Element ANSYS (FRC) is concrete c steel fibers and synthetic fibers, mechanical properties that are homogeneously dispersed distributed randomly in the concrete mix.
concrete will increase the tensile of the concrete. In addition, the fiber omparison of hysteresis loop of EWECS column olumn numerical analysis of EWECS column, the FE model can provide an accurate prediction for its seismic behavior, which has been compared to know more about the columns under seismic carried out to identify the possible material used in the the use of Fiber Reinforce ) for the concrete core and the use of addition, the effect of applying friction element between wood and concrete was used as parameters were chosen because of the importance of the material in structural and it can improve seismic behavior without n dimensions.   The features and elements in numerical constant and similar to The features of other structural elements in numerical simulations of parametric analysis remain constant. related data for parametric analysis are similar to model analysis. Fig. 12 and Table 2  In the experimental significant damage which was similar behavior obtained the parametric study. analysed, which was principal strain of 0.002 has been reached in the encased steel of Model VF1 at story drift 0.54 13. The elastic modulus of the steel is 200000 MPa, with the yield stress stress distribution crack in the concrete and propaga concrete elements of the column, in the middle of the

3.2
The panel wood is the outermost component of the EWECS composite column that interacts with the concrete. core to resist the addition, the wood panel contributes to prevent the column buckling types of effect typical wood commonly used the wood as properties of the wood (hysteresis loop) of EWECS column with having variation the differences between the stiffness, strength, and energy dissipation of each model, as

Fig. with varying
The model with Matoa wood higher model with Kapur wood displays a Model R. The Matoa wood panel have an increase at the flexural capacity around 3.3% than the Model R. The percentage differen columns by variating 16%. The results of of wood panel column.

The use of Indonesian
The panel wood is the outermost component of the EWECS composite column that interacts with the concrete. It contributes to give the core to resist the addition, the wood panel contributes to prevent the column buckling types of Indonesian effect of this parameter on the column behavior. The typical wood used in th commonly used the wood as Kapur and Matoa. properties of the wood Fig. 15 presents the shear force versus story drift (hysteresis loop) of EWECS column with having variation in the typ the differences between the stiffness, strength, and energy dissipation of each model, as

of Indonesian
The panel wood is the outermost component of the EWECS composite column that interacts with the contributes to give the core to resist the bending moment addition, the wood panel contributes to prevent the column buckling. In these parametric studies Indonesian wood are of this parameter on the column behavior. The used in this parametric commonly used the wood as a structural component Kapur and Matoa. Table 3 shows t properties of the wood.
presents the shear force versus story drift (hysteresis loop) of EWECS column with having type of wood panel. Th the differences between the stiffness, strength, and energy dissipation of each model, as

Properties
Value Matoa   Fig. 16 shows the the Matoa wood and Kapur wood models concentrated on the where sink (due to compression) and uplift (due to tensile) occur. that the FE accurately the especially the ultimate

The use of
In this study, the us investigate the effect of the applied fiction element in seismic performance the EWECS columns. of element, CONTA174 and TARGE170 the contact between Surfaces with whereas surfaces target surfaces. elements used in this analysis The standard sliding friction there is sliding behavior between pressure in the occurs between Fig. 18 shows the hysteresis loops of with and without friction element. As shown in the figure, the maximum lateral shear force obtained at last shows the 1 st principal the Matoa wood and Kapur wood models concentrated on the edge where sink (due to compression) and uplift (due to tensile) occur. These good analysis is the seismic performance y the ultimate strength of the composite column The 1 st principal normal stress on wood and Kapur wood models se of friction , the use of friction element was analy investigate the effect of the applied fiction element in seismic performance the EWECS columns.
CONTA174 and TARGE170 between two with finer mesh surfaces with coarser surfaces. Fig. 17 s used in this analysis standard unilateral friction behavior sliding with contact between connection's pressure in the FE analysis between the surfaces in contact. Fig. 18 shows the hysteresis loops of with and without friction element. As shown in the figure, the maximum lateral shear force last story drift riction element e of friction element was analy investigate the effect of the applied fiction element in seismic performance the EWECS columns. CONTA174 and TARGE170 difference and were used as coarser meshes 17 shows the s used in this analysis.
unilateral contact behavior is used in this analysis contact closing connection's surfaces analysis was set to the surfaces in contact. Fig. 18 shows the hysteresis loops of with and without friction element. As shown in the figure, the maximum lateral shear force story drift 5% for EWECS EWECS columns with types of Indonesian wood.