under Buckling Behavior

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1 EXPERIMENTAL STUDY ON HOT ROLLED SQUARE HOLLOW SECTIONS UNDER BUCKLING Experimental Study on Hot Rolled Square Hollow Sections BEHAVIOUR Karthick raja.s a, Sundararooban.S.R b, Rangaraj.S c a,b,c - Final year student, Department of Civil Engineering, Thiagarajar College of Engineering, Madurai-15. under Buckling Behavior 1 S.Karthick Raja, 2 S.R.Sundararooban, 3 S.Rangaraj 1,2,3 UG Students, Department of Civil Engineering, Thiagarajar College of Engineering, Madurai, INDIA ABSTRACT 1. Introduction This paper investigates the performance of Hot rolled Square hollow sections (HSS) made of 210 F y steel. 24 Axial compression column tests on specimens with varying length and cross section were conducted. Initial geometric imperfection in the form of initial out of straightness were measured using Total station and verified to be within limits according to IS Experimental results in terms of load carrying capacity, buckling modes and buckling factor (χ).the experimental results were compared with the existing column design curves in IS 800:2007, EN , AISC 13 th edition and AS 4100:1998. Numerical models of SHS are created in FEM and Eigen value analysis were carried out which satisfactorily fits the equivalent Euler column curve analysis. The model are modified for Non-linear Buckling analysis which captured the Buckling modes of the column satisfactorily. The models are to be further studied and analyzed for ultimate carrying capacity. Experimental showed that the design resistance calculated based on the internal codes were conservative in case of SHS sections. In addition to improve the design efficiency the post buckling strength of the column must also be considered and incorporated in the design buckling curve. Keywords: Buckling curves, IS 800:2007, Hollow sections, Column compression, Design methods. 1. Introduction use of hollow sections as compression members. The initial out-of- straightness, Eccentricity of applied load, effect of residual stresses and Effect of strain hardening and the absence of well-defined yield point affects the compressive strength of the practical i.e., Intermediate columns. Since design strength of these columns are found to vary in higher degree than short columns and Long columns. This paper presents an experimental investigation which includes 24 hollow square columns of yield strength 210 MPa to clarify the flexural buckling behaviour. Parametric analysis are conducted for columns of varying cross section and length. The experimental results are compared with the European, Indian, Australian and American standards. The design method to be adopted for such hollow sections are proposed. The application of the hollow sections has been widely used in building structures worldwide. In open sections the outstanding flanges tend to buckle before the webs which are supported along all edges. On the other hand, in closed sections such as the hollow rectangular section, both flanges and webs behave as internal elements, as each element is connected to the adjacent element at the longitudinal edges. This offers more strength and stability to the Hollow sections. The hollow sections are known for their increased compressive strength because of excellent distribution of area around the axis. They possess full strength under bending moment due to enhanced torsional rigidity, since Hollow section only allow uniform torsion and which is distributed evenly through the entire area. The versatility of these sections allows for a wide range of applications as industrial sheds and exhibition stalls to space frames and sign supporting structures. Large span portal frames, amusement parks and playground equipment. From the literature survey we made, it was clear that there were several investigations done in the columns. Though the development of Multiple Column Curve has been providing us with satisfactory and safe results, there is a need for study on the strength developed by the Practical columns 2. Experimental approach 2.1 Test specimen The specimen were made from hot rolled raw material. The sheet of raw material were cold worked to form hollow section through the process of high induction wielding. The specimen were fabricated in top and bottom with steel plate of yield stress Fy 210 grade. Table 1 summarizes the geometric properties of the test specimen whilst Fig.1 shows the symbols of the c/s and length as listed in 24

2 Table 1. The cross section is classified as class 1 cross section in Euro code 3. The 24 specimen fall on the slenderness ratio (Kl/r ratio) of range 70 to 180 i.e. intermediate columns. The standard tension coupons were taken from the face of the specimen and the test results are tabulated in Table.2, the results will be discussed further. Table 1 Nominal and measured dimension of specimens. length Section Member spec Area Thickness Radius of gyration Slenderness X38 A X32 A X25 A X32 B X32 B X25 B X25 B X32 B X38 B X25 B X25 C X32 C X32 C X38 C X25 C X25 C X32 C X25 C X32 D X32 D X25 D X25 D X38 D Test Procedure: The tests were conducted by using a column tester of capacity 200 tonnes. The load is applied through hydraulic loading system. The test arrangement is shown in figure no Pinned end conditions were achieved by means of ball and sockets at the top and bottom. The lateral deformations are measured at the midpoint of the length of the member using linear variable differential transformer. The verticality of the whole arrangement is ensured using plumb bob. The loading is applied from the bottom through load cell. A data acquisition system is used to record the applied load and deformations at regular intervals. The specimens were loaded until the significant buckling is visible and also the column must reach its ultimate load. The ultimate load is the load at which the reading gets reversed. 25

3 2.3 Initial imperfections: Initial imperfection were measured with the help of total station. The imperfection were measured at an interval of 0.2 m in the specimen. The imperfection in the specimen are later validated with the help of south-well plot. The initial crookedness of members are within tolerance limits as specified in IS 4923:1997. Maximum allowable out of straightness as per IS 4923 is length/ Test results and Analysis 3.1 Buckling Deformation Majority of the columns fail by flexural buckling, while others failed by interactive local and global buckling. A typical load-displacement curve of specimen is shown in fig.2 in which the ordinate is the load applied and the abscissa is the displacement in the horizontal direction. The 24 specimen were within the limit of 70 to 180. The following modes of buckling were obtained under axial compression. Column with slenderness 70 to 90 exhibited a local bulging and insignificant global buckling Column with slenderness 90 to 140 exhibited inelastic behaviour, we could clearly see the bifurcation point of the two straight parts of the column with a hinge at the centre. Column with slenderness more than 140 exhibits overall flexural buckling of the specimen. We also could notice significant rotation of the ball and socket. Some specimen show inconspicuous instability. 3.2 Buckling capacity The maximum experimental test load for each column is summarized in table. The experimental buckling factor (i.e. non-dimensional buckling strength) was calculated by using the Equ (1) and the results are tabulated in the table. (1) Where F y is the yield strength and A is the measured cross sectional area. Column curve A is adopted for the hot formed SHS specimen according to the IS 800. There are 4 column curves in Indian code based on the cross section (α = 0.21). The SHS specimen falls under the curve A. In a similar manner Euro code has 5 different column curves. A single column curve is adopted in American code. The code follows bifurcated curve based on the tangent modulus theory. Table 2: Comparison between Test results and code estimate Member Spec Λ equi - Indian -AISI -AUS -Perry A A A B B B B B B B C C C C C

4 Buckling Factor International Journal of Mechanical Civil and Control Engineering C C C D D D D D Column buckling factor The slenderness ratio is the ratio between λ o is the ratio between the pin ended column lengths (L) and the radius of gyration R, and its non-dimensional slenderness ratio value λ n is equal to Stress reduction factor / buckling factor Where, is the imperfection factor, equal to 0.21 for hot formed tubes. A similar form is adopted in Eurocode with the same imperfection factor =0.21. A single column curve is currently adopted in American code and the equivalent form of AISC column curve is shown below, λ n 1.5, λ n 1.5 where λ n is the non-dimensional slenderness ratio. 3.4 Comparison of test results with analytical results Comparison of test results with IS 800: Comparison of test results with AISC LRFD 13 th EDITION Comparison of test results with AS 4100:

5 International Journal of Mechanical Civil and Control Engineering [3] Buckling of stainless steel square hollow section compression members Y. Liu, B. Young. [4] BS EN , Eurocode 3: design of steel structures: Part 1-1: general rules and rules for buildings. London: BSI; [5] ANSI/AISC , Specification for Structural Steel Buildings. Chicago: AISC; 2010 [6] INSDAG teaching material authored by Rangachari Narayanan,V. [7] IS 4923:1997 Specification for hollow section. [8] IS 800:2007 General construction in steel code of practice. [9] AS 4100:1998 Australian standard- Design of steel structures Comparison of test results with Perry Robertson curve Conclusion The comparison of test results with analytical results shows that the international standards are very conservative while calculating the strength of hollow square sections under axial compression. The experimental behaviour is due to the high torsional rigidity of SHS and the post buckling strength due to the low thickness. International codes must also consider these in to account in design of SHS. The entire area is considered to effective due to the low b/t ratio causing high strength behaviour. 5. Reference [1] Experimental study on column buckling of 420 MPa high strength steel welded circular tubes Gang Shi, Xue Jiang, Wenjing Zhou, Tak-Ming Chan, Yong Zhang [2] Buckling of elliptical hollow section members under combined compression and uniaxial bending K.H. Law, L. Gardner. 27