290 T.F. Nip and J.O. Surtees Moment rotation curves for whole-connections FB2 and FB4 are presented in Figure 6. These display a rigid initial characteristic but are sufficiently ductile to develop 30 x 10 -3 rad rotation before failure. A summary of cruciform connection test results is shown in Table 3. A maximum applied moment of 1.2 Mp was recorded for both tests. The compressibility of the connections at compression flange level correlated well with corresponding values obtained from test CB5. Although FB4 is not fully equivalent to tests CB 1,CB2 or CB3, it was clear in the case of whole connections that sidesway buckling of the compression zone is totally inhibited by several factors. The carrying capacities obtained in the isolated compression tests understated the true capacities of whole connections TABLE 3 SUMMARY OF WHOLE-CONNECTION TEST RESULTS Test l[ Stiffening method FB2 8 (4 off-line) ~24HTS bars FB4 4 ~24 HTS bars Failure moment + Mp Failure mode 1.229 Beam flange and web local buckling 1.2715 Beam flange and web local buckling CONCLUSION Threaded bar compression stiffening has been shown to be an effective and viable alternative to traditional welded plate stiffening. Tests have confirmed that bearing strengths much in excess of those required for current typical end plate connections are possible. Use of threaded bar as an effective form of tension stiffening has been considered incidentally in this paper because of its application to the whole-connection tests. The case for tension stiffening is strong and its eventual practical acceptance will undoubtedly increase the appeal of threaded bar compression stiffening. ACKNOWLEDGEMENTS The research described herein was funded by the Engineering and Physical Sciences Research Council and British Steel. Further support and advice was provided by the Steel Construction Institute and British Constructional Steelwork Association Ltd. REFERENCES Grogan W. and Surtees J. O. (1995) Column flange reinforcement in end plate connections using bolted backing angles. Nordic Steel Construction Conference, Malm6, Sweden, pp 87-94. Grogan W. and Surtees J. O. (1999) Experimental behaviour of end plate connections reinforced with bolted backing angles. J. construct. Steel Research, vol. 50, pp71-96. Grundy P., Thomas I. R. and Bennetts I.D. (1980) Beam-to-column moment connections. J. Struct. Div., Am Soc. Civ. Engnrs, pp313-330. Murray T. M. and Kukreti A. R. (1988) Design of 8-bolt stiffened moment end plate. Engineering Journal, AISC, Vol.25, Pt. 2, pp45-53. Murray T. M. (1988) Recent developments for the design of moment end-plate connections. J. Construct. Steel Research, Vol. 10, pp 133-162. Surtees J.O. and Yeung K.W. (1996) A new form of high moment beam-to-column connection. EPSRC Final Report, Grant reference GR/J70758, University of Leeds. Experimental Study of Steel I-Beam to CFT Column Connections S.P. Chiew & C.W. Dai School of Civil and Structural Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. ABSTRACT This paper focused on the experimental study of the composite behavior of steel universal beam to concrete-filled tube (CFT) column connections. Eight specimens were designed and tested to failure, of which four specimens are simple beam-column connections and the rest are rigid connections with different type of stiffening details. For simple connections, the parameters investigated are the thickness and diameter of the steel tube and the beam size. For the rigid connections, the stiffening details investigated include cover plate, shear plate, extemal ring and re-bar respectively. Experimental results showed that the simple connections have weaker ultimate strength, ductility and stiffness, and their behaviors are influenced by the parameters investigated. All stiffening details improved the composite behavior, but to different extent. The specimen with the re-bar detail that is easy to fabricate and costs effective exhibited excellent behavior in terms of ultimate strength, stiffness and ductility. KEYWORDS: composite behavior, CFT column, re-bar stiffening detail. 1. INTRODUCTION In the building construction industry, composite construction is gaining widespread popularity in recent years. Its better structural performance and relatively lower costs compared to conventional reinforced concrete construction makes it especially attractive for high-rise building projects. In this connection, structural 291 292 S.P. Chiew and C.W. Dai engineers have been experimenting with different types of composite columns in a hope to produce the most aesthetically impressive and futuristic buildings. Undoubtedly, new breakthroughs with this form of construction will usher a new and exciting era into the building construction industry. Basically, there are two types of composite columns: concrete-encased structural steel section and concrete-filled tube (CFT) columns. CFT column has many advantages over other types of column. Architecturally, CFT columns have many attractive features; for example, the concrete filling has no visual effect on their external appearance. The advantages from a structural point of view are, firstly, the triaxial confinement of the concrete within the section, and secondly, the fire-resistance of the column which largely depends on the residual capacity of the concrete core. During construction, the steel tube will dispense with the need for formwork and prevents spilling of the concrete. Although the CFT column is an economical form of composite construction, their uses to date have been limited due to the lack of design information on the beam-to-column connections and to the limited construction experience. While extensive data is available on CFT column behavior under different loading conditions, relatively less work has been done on the connections to these columns. Experimental results on CFT column connections can vary significantly depending on the tube shape and other connection requirements. Broadly speaking, details can be generalized into two categories, i.e. connections with the beams attached to the face of the steel tube only and connections that use elements embedded into or passed through the concrete core. Connections to the face of the steel tube include welding the beam directly to the tube surface, using fin-plate [ 1 ] or cover plate to connect the beam to the tube and providing diaphragms or external tings [2,3] to stiffen the connections. Connections with embedded or passed elements include through bolting beam end plates and continuing structural steel shapes into and through the column [4,5]. This paper summarized an experimental investigation to study the connection details to circular CFT columns. The objectives are: a) to investigate the effect of different parameters on the composite behavior of the steel I-beam to CFT column ~J connections, and hence, an strength and stiffness prediction for this type of connection can be given; b) to compare the effect of different stiffening details on the composite Experimental Study of Steel I-Beam to CFT Column Connections 293 behavior, so that a relatively good stiffening detail can be recommended and c) to provide test results to verify the finite element model built for this kind of composite connection. Experimental results showed that the simple connections have weaker ultimate strength, ductility and stiffness, and their behaviors are influenced by the parameters investigated. All stiffening details improved the composite behavior, but to different extent. The specimen with the re-bar detail exhibited excellent behavior in terms of ultimate strength, stiffness and ductility. This detail which is easy to fabricate and cost effective proved to be the most promising of all. 2. EXPERMENTAL PROGRAM 2.1 Specimen Details A total of eight specimens were tested in this study. All specimens are modeled to 89 scale of the real size. Fig. 1 and Tables 1-2 show dimensions and details of the test specimens. The materials used in the specimens are equivalent to BS4360 grade 43A steel. Specimens consist of simple and rigid composite connections. For simple connections, the parameters investigated are tube thickness (6.3mm and 8.0mm), tube extemal diameter (219.1mm and 273mm) and beam size (203mm x 133mm x 31.3kg/m and 254mm x 146mm x 31.25kg/m). For the rigid connections, the stiffening details investigated include cover plate, shear plate, external ring and re-bar respectively. Table 1. Details of Test Specimens Specimen UCN'I Beam size (mm x mm x kg/m) . 203 x 133 x 31.3 Column size (mm x mm ) 219.1 x 6.3 Stiffener type No stiffener Simple ucN-2 203 x 133 x 31.3 d~ 219.1 x 8.0 No stiffener Connection UCN-3 203 x 133 x 31.3 d~ 273 x 6.3 No stiffener UCN-4 254 x 146 x 31.25 qb 219.1 x 6.3 No stiffener , , , UCN-5' "203 x 133 x 31.3 d~ 219.1 x 6.3 Cover plate Rigid UCN-6 203 x 133 x31.3 ~ 219.1 x 6.3 Shear plate Connection UCN-7 203 x 133 x 31.3 (~ 219.1 x 6.3 External ring I UCN-8 ' 203 x 133'x'31.3 d~ 219.1 x 613 Re-bar 294 S.P. Chiew and C. I41. Dai Fig. 1 Overall Dimensions of Test Specimens 2.2 Load Application and Instrumentation Monotonic static loads were applied as shown in Fig.2. Bonded strain gauges were installed to observe the stress distribution in the flange, web and stiffeners (if available). Also, 14 linear variable displacement transducers (LVDT) were used to measure the vertical displacement, lateral displacement and the rotation of the 1-beam as shown in Fig.3. In addition, two inclinometers were installed on the upper flange of the steel 1-beam (near column) to measure the rotation of the steel I-beam. Loading was terminated when the deformation was already excessive or when the composite connection lost its ultimate capacity altogether. Fig.2 Test Set-up Fig.3 Beam Rotation Measurement Experimental Study of Steel I-Beam to CFT Column Connections Table 2. Details of Rigid Connection 295 3. TEST RESULTS AND DISCUSIONS 3.1 Material Properties Material tests were performed to determine the mechanic properties of steel and concrete used in this experiment program. For concrete material, the 28 days cube 296 S.P. Chiew and C.W. Dai strength is 39.88 N/mm 2 and the cylinder strength is 34.4 N/mm 2. The properties of the structural steel are summarized in table 3. Coupon Thickness (mm) BF(203x133) 9.86 BW(203x133) 6.34 BF(254x146) 8.76 BW(254x146) 5.98 CL(219. lx6.3) 6.19 CL(219. lx8.0) 7.98 CL(273x6.3) 6.21 steel plate 10.47 Re-bar ~30 BF: beam flange Table 3 Steel Mechanical Properties Gy (N/mm 2) 355.9 385.0 351.2 407.3 357.4 ,, 409.5 347.7 283.0 479.4 BW: beam web 6y E o. o]o. Ob EIo. (xl0 "6) (N/mm 2) (N/mm 2) (%) (N/mm 2) (%) 1725 206357 498.1 71.5 371.0 25.9 1867 206201 507.2 75.9 401.2 26.4 1709 205560 483.8 72.6 362.2 27.8 2030 200620 503.5 80.9 404.6 21.6 1716 208290 443.8 80.5 321.07 26.4 1962 208764 499.3 82.0 377.0 24.4 1665 208850 455.87 76.3 357.6 24.9 1363 207566 429.9 65.8 336.13 37.0 2601 184338 596.8 80.3 430.6 25.1 CL: column Cy: yield stress Ou: ultimate stress Elo.: elongation 3.2 Load carrying capacity The moment-rotation relationships of all specimens are shown in Fig.4 and Fig.5. Table 4 shows the experimental and numerical analysis results. The yield load in table 4 was determined as the value at an intersection point between an initial tangential line from the origin point and a tangential line with a 1/3 slope of the initial tangential line [6] as shown in Fig.5. The yield loads of all specimens are compared with the numerical analysis results. In order to evaluate the load carrying capacity of the connection, the yield moment of the test result is also compared with the plastic moment capacity of the steel I-beam. For simple connections, except specimen UCN-2, all other specimens had a weak load carrying capacity. This was reflected on the coefficient a- the value of ot is just between 0.40-0.48. This means these connections can not even achieve half the beam' s capacity. The specimen UCN-2 had a higher load carrying capacity and this illustrated that the thickness of the steel tube is an important parameter on the load carrying capacity. It is also found that the load carry capacity will decrease when the outside Experimental Study of Steel I-Beam to CFT Column Connections E 297 Fig.4 Moment-Rotation relationship of Simple Connections Fig.5 Moment-Rotation relationship of Rigid Connections diameter of the steel tube increased by comparing specimens UCN-1 and UCN-3. Under the same cross-section area, the selection of the higher and wider, but thinner steel I- beam can improve the yield load of the connection about 42%, however, the value of the coefficient c~ is almost the same (0.45 and 0.48 respectively). This means that the load carrying capacity of the connection depends on the properties of the composite column, the steel I-beam has lesser effect on it. For rigid connections, all specimens have a higher load carrying capacity when they are compared with the standard one (specimen UCN-1). This means the different 298 S.P. Chiew and C.W. Dai Table 4 Comparison of Numerical and Experimental Results Specimen Test result M~ (kN.m) UCN-1 95.2 UCN-2 128.2 My, (~.m) FEA result M. (kN.m) My1/My2 1% 62.3 64.4 0.967 i37.95 87.8 0.968 137.95 90.7 55.3 UCN-3 96.1 54.8 0.991 137.95 UCN-4 140.4 88.5 91.7 ' 184.4 UCN-5 123.8 UCN-6 118.8 UCN-7 138.8 UCN-8 229.4 81.9 88.8 78.8 0.965 0.962 0.887 78.8 90.8 137.95 137.95 137.95 o~ = M,,,/~, 0.45 0.64 0.40 0.48 0.57 0.57 86.1 1.054 0.66 175.5 189.9 0.924 137.95 1.27 ,. Mu: ultimate moment Myl, My2: yield moment Mp: plastic moment capacity of steel I-beam Load 113 initial stiffness Initial sty" Yield Load Deflection Fig. 5 Determination of yield load stiffeners are useful on improving the connection' s load carrying capacity, but their effects are different. The yield load of specimen UCN-5 and UCN-6 are 1.26 times of that of the specimen UCN-1. The external ring stiffener can enhance 46% of the yield load and the degree can be higher if the external ring had a higher strength (the yield strength is only 283N/mm 2 in this experimental project). The re-bar stiffener is the most effective one in improving the load carrying capacity the yield load is 2.82 times of the specimen UCN-1. The numerical analysis was carried out with MARC version K7.0 software Experimental Study of Steel I-Beam to CFT Column Connections 299 package a general purpose finite element analysis program for nonlinear or linear stress analysis in the static and dynamic regimes [7]. Three kinds of element are used in the analysis: the 8-node doubly curved thick shell element is used to model the steel tube and beam, the 8-node isoparametric three dimensional elements is used for the concrete and the friction and gap link element is used to model the interaction between the steel tube and the in-filled concrete. The Von Mises yield criterion and Buyukozturk yield criterion are adopted for steel and concrete material respectively. The Newton-Raphson iterative procedure is used in the analysis. The yield load was obtained by inputting actual material properties into the program. From table 4, it can be found the numerical results agree well with the test results for all simple connections. For rigid connections, the prediction by numerical analysis is acceptable, except specimen UCN-7, where the prediction is not so good 11% lower than the test result. The good agreement between the numerical and test results shows that it is feasible to use finite element analysis to predict the load carrying capacity of this kind of composite connection. 3.3 Initial Stiffness and Ductility Initial stiffness and ductility of each specimen is represented in table 5. They are all compared with specimen UCN-1. For simple connections, from the table, it can be found that the investigated parameters affect the initial stiffness and ductility in different ways. When the thickness of the steel tube wall was increased, both the initial stiffness and ductility of the connection were improved. The increase in the diameter of the steel tube caused the increase of ductility, but it decreased the initial stiffness. The selection of higher and wider but thinner beam (having the same cross-section area with UCN-1) improved the initial stiffness of the connection, but it almost had no effect on ductility. For rigid connections, the use of cover plate stiffener (UCN-5) improved the initial stiffness of the connection, but it had no effect on ductility. Shear plate stiffener (UCN-6) increased the ductility greatly, also had some useful on improving the initial stiffness. The use of external ring stiffener (UCN-7) improved the ductility of the connection obviously, but the initial stiffness decreased. The reason caused the reduction in initial stiffness may be the use of lower strength steel plate (refer to table 3) in fabricating the . determined as the value at an intersection point between an initial tangential line from the origin point and a tangential line with a 1/3 slope of the initial tangential line [6] as shown in. of the steel tube include welding the beam directly to the tube surface, using fin-plate [ 1 ] or cover plate to connect the beam to the tube and providing diaphragms or external tings [2,3]. or passed elements include through bolting beam end plates and continuing structural steel shapes into and through the column [4,5]. This paper summarized an experimental investigation to study