Experimental Study and Finite Element Modelling of Reinforced Concrete Column Having An Opening

This study deals with the finite element modelling of reinforced concrete beam and column using ANSYS. The study covers a comprehensive introduction to ANSYS, such that any beginner can learn the useful and effective use of the ANSYS in order to model and analyze any type of structure like beam and column. ANSYS is a very handy and effective tool in modelling and analysis of various structural element based on finite element modelling. A detailed study of the work includes the modelling of beam and column with specific cross sections and reinforcement details. Further the column with a lateral hole at the center is evaluated in terms of cracking response around the hole. The experimental work is carried out in the concrete laboratory of civil engineering department UET Taxila .The results of that experimental work is then compared eventually with the results that’s were obtained by analysis through ANSYS. As a summary of the work the results were compared in terms of the stress, strain and cracking around the hole. It is concluded that the response of the column around the hole/opening in terms of cracking pattern in both elastic and post elastic stages was much more comparable with that of experimental work. Keywords: Finite element modelling, ANSYS, Column, Lateral hole, Stress, strain, Elastic, post elastic, cracking pattern. DOI : 10.7176/IEL/9-1-03


Introduction
For providing pipes for utility services, holes are required in structural members. Usually these holes are provided in beams and slabs as columns are critical members of a structural system and their failure results in partial or complete collapse of building. Limiting holes to only beams and slabs, results in complex network especially in multistory buildings. If holes (transverse) are provided in column by carefully taking in account the effect of stress distribution and reduction in load carrying capacity then it will result in more organized and economical network. Therefore this study is aimed at discussing alternate solutions using which hole in column can be provided without compromising on its original strength. The finite element method has thus become a powerful computational tool, which allows complex analyses of the nonlinear response of RC structures to be carried out in a routine fashion. With the help of this method the importance and interaction of different nonlinear effects on the response of RC structures can be studied analytically. A follow-up study will address the response of these structures under real time loading by Finite element modelling method using the most reliable tool i.e. ANSYS 17.0.
A brief review of previous studies on the application of the finite element method to the analysis of reinforced concrete structures is presented is this section.
In the earliest publication by Ngo and Scordelis (1967) on the application of the finite element method/modelling of RC structures, simple beams were analyzed by meshing it into triangular elements of constant strain and a special bond link element was used to connect the steel to the concrete. Also described the bond-slip effect while modelling RCC structure. A linear elastic analysis was performed on beams with predefined crack patterns to determine principal stresses in concrete, stresses in steel reinforcement and bond stresses.For the analysis of RC beams with material and geometric nonlinearities Rajagopal (1976) developed a layered rectangular plate element with axial and bending stiffness in which concrete was treated as an orthotropic material. RC beam and slab problems have also been treated by many other investigators (Lin and The behavior of RC columns with longitudinal hole was studied using three-dimensional non-linear finite element method and the finite element modeling of the columns predict much higher strength for the columns than experimental result ( Basravi 2010).Study conducted by (Qais Majeed 2012) in which experimental and nonlinear finite element analysis of creating square openings in existing RC beams and strengthening with CFRP laminate was carried. The beams were modeled using a FEM packaged (ANSYS 11).The results indicate that the strengthened beam recorded the highest failure load and its mode of failure was ductile. The numerical results seemed to be able to predict the behavior of the beams.The behavior of reinforced concrete columns with holes under axial load is researched to help designers and structural code officials (Ehab M. Lotfy 2013) and the experimental results were compared with finite element model using ANSYS.

A).Experimental method
In this Phase of the research work that is experimental program of the column as described has been discussed. Geometrical and material properties of columns with casting procedure along with test setup and testing procedure has been described in this phase.
In this section geometry and properties of columns which were tested have been summarized. A column specimen was square in shape of size 203 mm x 203 mm, having a length of 1 meter. 51 mm diameter transverse hole was left in column while concreting exc. 4 bars of #13 were used as longitudinal reinforcement.Column geometry and reinforcement detail are tabulated below.

Material Properties
Reinforcement bars and concrete used for casting of column specimens had the properties as described in given section.

Reinforcement Bars
Reinforcement bars used for column were deformed steel bars. For longitudinal bars #13 bars of Grade 420 steel were used. Whereas for transverse bars #10 bars of Grade 300 steel was used. Reinforcement bars conforming to ASTM A 615M were used. The Yield strengths of longitudinal and transverse reinforcement were approximately 420 MPa and 280 MPa respectively.

Concrete
For concrete, normal weight aggregates having maximum size of ½" were used. Cement used was Ordinary Portland Cement. Concrete mix design ratio of 1:2:4 was used for columns. Water to cement ratio was 0.5. Concrete strength was determined by casting concrete cylinders 6" diameter and 12" length and testing under compression testing machine as per ASTM C 39. Average value of compressive strength came out to be 29 MPa for column. The achieved concrete strengths of different batches of concrete as per tested concrete cylinders were quite close to desired value of strength. For desired strength of 28 MPa, 29 MPa was achieved.

Casting Procedure
Column specimen was casted in batch using wooden formwork. Concrete mixer was used for concrete making. Concrete mixture was poured in formwork and using electric vibrator its compaction was carried out. PVC pipe was used for hole formation and it was removed in some columns while in others it was let there even after concreting so that hole may retain its shape during testing and to avoid premature failure due to stress concentration and cracks formation near hole. A day later to casting; molds were removed and samples were cured for 28 days using wet hessian cloth.

Test Setup and Testing Procedure
After 28 days of curing of specimens, these were tested for axial compressive strength. This section discusses testing procedure of cast column, which is given in next paragraph. Both ends of columns were encased in 51 mm high steel collars to prevent localized failure of columns. This arrangement also prevents buckling of column and resultant premature failure. But it was seen that some of the columns still failed due to crushing of concrete at the ends of columns.
For measurement of axial deformation, the linear variable differential transducer (LVDT) having maximum axial deformation capacity of 100 mm was used with P3 micrometer. For fixing LVDT, a hole was drilled at a distance of 195 mm from lower end of column and LVDT was mounted there. Another hole from 195 mm distance from upper end of column was also drilled for fixing angled steel plate. This assembly resulted in a distance of 610 mm between two holes that is axial deformation was noted for this column portion. This distance was later used for conversion of axial deformation to axial strain. P3 micrometer was operated from computer which was set for 1 second interval. Values so recorded were later converted to strain. The testing assembly mentioned in below given figure.