JOINTS FOR MIXED BUILDING TECHNOLOGY WITH VIEW TO EXPERIMENTS OF COMPONENT STEEL PLATE IN BENDING AND CONCRETE IN COMPRESSION

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1 JOINTS FOR MIXED BUILDING TECHNOLOGY WITH VIEW TO EXPERIMENTS OF COMPONENT STEEL PLATE IN BENDING AND CONCRETE IN COMPRESSION Dalibor Gregor 1, František Wald 1, Martina Eliášová 1, Ivo Jírovský 2 ABSTRACT The paper describes the tests designed to develop and to evaluate a European analytical prediction model of behaviour of end plate joints for the mixed building technology. Tree sets of tests simulating the beam to-column joints under cycling loading were carried out in the laboratory of Czech Technical University. Three components were observed experimentally: the bolt in tension and the end plate in bending; the anchor bolt in tension and the end plate in bending; the concrete in compression and the end plate in bending. The last set of tests, the part of joint in compression, is listed in details in presented paper. Key Words: Steel structures, Mixed building technology, Structural joints, Concrete in compression and plate in bending, Experimental observations, Component method. 1. Introduction The applications of informatics in building process enable to utilize a freedom in architectural expression as well as asked level of safety economy for structural elements. Today s structures are mostly using mixed building technology [1], which enables to explore structural elements of different materials. The steel, concrete, composite steel and concrete, glass and timber elements (beams, columns and walls) are developed and shaped as products. The questions of safety level (probability of failure of different materials and technology) and tolerances are avoided by introduction of structural Eurocodes. To connect structural elements made by different technology is one of the major difficulties as well as challenge of today s engineers [2]. 1 Czech Technical University in Prague, CZ Praha 6, Czech Republic. 2 Institute of Theoretical and Applied Mechanics, AS CR, CZ 19 Praha 9, Czech Republic. 1

2 Component method is a traditional prediction tool modelling behaviour of joint based on its parts behaviour [3]. The application of component method requires three basic steps: listing of the components of the joint, evaluation of force-deflection diagram of each individual component, in terms of initial stiffness, strength and deformation capacity, and assembly of the components in view of the evaluation of characteristics of the whole joint. The modelling under cyclic loading describes well the reality in case of static as well as b) e) Force F c,t F j V j M j c) F c,c f) Force Deformation a) d) F d,bv Deformation Fig. 1 a) End plate joint of mixed building technology; b) anchor bolt in tension and plate in bending, c) concrete in compression and plate in bending, d) bolt in shear; e) one directional component behaviour (contact); f) two directional component behaviour (bending of plate) cyclic loading. The traditional approach for prediction of behaviour of joints is using sophisticating curve fitting procedure based on joint major parameters [4]. The model may be very accurate for joints inside the experimentally observed parameters ranges. But in case of moving outside the boundaries of even one parameter the application may be limited. The component method is based on analytical simulation of components, which allows more freedom in prediction but may limit the accuracy. To keep the asked prediction accuracy the stiffness, the resistance and the deformation capacity are assembled separately [5]. The application of component method for cyclic loading shows a good chance for sophisticated on one hand as well as simple design models on the other hand [6]. Two major types of components may be distinguished [4], an example of connection of steel beam to the RC column is at Fig. 1a). One-directional component represents the contact between two surfaces. The component resists in compression, but offers no resistance in tension. The other type is two directional component. It represents behaviour of components resisting in tension and compression, i.e. plate or bolt. a) b) Fig. 2 a) Test set-up of pullout of T-stub fixed to concrete block; b) failure mode of end plate 2

3 The knowledge developed for base plates [7], [8] and applied in European structural steel practice may be used for the connection between steel framing and the RC part of the structure, see Fig. 1 [4]. There are two major but dependent questions: the resistance of theconcrete block in compression under rigid plate [1] and the effective area of flexible plate [11]. The crushing of the concrete surface guides the deformation stiffness and represents the resistance of concrete in compression [12]. Factors influencing the resistance are the concrete strength, the plate area, the plate thickness, the grout quality and the thickness [13], the location of the plate on the concrete foundation, the size of the concrete foundation and the reinforcement [14]. a) b) Fig. 3 a) The steel plate on the grout; b) installed measurements At the CTU in Prague the research activities focus on behaviour of the materially nonhomogenous joints. Tree set of test with repeated load were carried out - the tests with the components the bolt in tension and the end plate in bending; the anchor bolt in tension and the end plate in bending; and the concrete in compression and the end plate in bending. These tests were focussed on knowledge concerning the behaviour of components subjected to the repeated load. The cast-in threaded rod without the anchor head, sufficiently embedded in the concrete block was pulled out applying the zero-to-tension low-cyclic load. The T-stub fixed to the concrete block by two cast-in threaded rods with nuts was subjected to the zeroto-tension low-cyclic loading, see Fig. 2. The concrete subjected to the local compressive repeated loading transferred through the steel plate was tested, see Fig. 3. The acting force was parallel and perpendicular to the direction of casting. Two types of grout layers were used. The grout quality was designed as 1 MPa and 5 MPa, see Fig. 4. a) b) Fig. 4 a) Deformed shape of the plate; b) damaged surface of the concrete, test 1C1-H- 2. CONCRETE IN COMPRESSION AND PLATE IN BENDING The general set-up is described in Fig. 5, for details see [9]. The cubes made of the plain concrete was cast in June 21. The test was performed on the upper horizontal surface of the cube, tests were labelled C1/1-H-, C2/1-H- and C3/1-H-. Next three experiment 3

4 sets were performed on the same cubes, which were overturned to be loaded by force perpendicular to casting. b) a) Fig. 5 a) Set-up of compression tests; b) c)position of measuring devices for c) The top surface of the block was cleaned, moistened and a thin (less than 1 mm) layer of a high strength grout was applied in its central part to smooth the surface. The steel plate with nominal dimensions 2 x 1 x 1 mm was placed on the fresh grout layer. The steel bar, nominally 1 x 1 x 22 mm, was cantered on the plate. The cube was positioned under the head of the hydraulic actuator. The cylindrical head was joined to the actuator by articulation. The layer of plaster was made under the block to ensure the horizontal position of the top surface and good transfer of force by the entire bottom surface to the laboratory floor. The actuator was fixed to the massive steel frame fastened to the laboratory floor. The set of experiments (C1/2-V-, C2/2-V-, C3/2-V-) were performed on the vertical surfaces of the cubes (force perpendicular to casting). The set of experiments (C1/3-V-1, C2/3-V-1, C3/3-V-1) were performed on another (not used) vertical surfaces of the blocks, which was cleaned, moistened and at this case a 15 mm thick layer of grout of 1 MPa was applied in its central part, see Fig. 5a. The plan view shape of the layer was rectangular and the grout over sized the steel plate by 5 mm at each side. The steel plate (No. 2 in the Fig. 5a) was placed on the fresh grout layer. Last set of experiments was performed in the same conditions with the only difference in the strength of grout 5 MPa (three specimens were labelled C1/4-V-5, C2/4-V-5, C3/4-V-5). During the tests the inductive transducers measured the vertical displacement of 11 points, see Fig. 5b) for the transducers position. I1, I11, I12, I13 were positioned at the corners of the top cube surface and they were placed on the little smooth cupreous plates glued on the concrete surface. These LVDTs measured displacements relatively to the 4

5 laboratory floor. I14, I15, I18, I19 were at the corners of the top steel plate surface. I16, I17 were connected to the steel bar 2 mm from its ends. I2 (not installed during tests C1/1 and C2/1) was placed on the little cantilever glued on the side of the steel cylindrical head, above I16. All I14 to I2 measured the displacement relatively to the concrete top surface edges. The measured values of displacement and the force value in the actuator were recorded by the UPM Hottinger Baldwin and a personal computer. The test procedure was controlled by the force. After setting of the required value of force, the values of displacements were recorded each 3 s until the differences between previous and following values of all displacements became less than 1% of their previous value. 3. MATERIAL CHARACTERISTICS Concrete blocks were made more than 12 days before the tests. The value 28,3 MPa was found as the average strength measured on 6 concrete cubes with the nominal edge length equal to 15 mm. The Young s modulus according to ISO 6784 was determined as 23 8 MPa. The strength tests on the surface by the Smidt rebound hammer were tabulated at the Tab. 1. The plate and the bar were made of S235 steel. The strength of the grout was determined by the normative tests on 3 beams of grout 4 x 4 x 15 mm for each concrete cube, results are summarized in Tab. 2. Test specimen Strength [MPa] C1/1- H- Tab. 1 - Strength of concrete by Smidt rebound hammer C3/1- C1/2- C2/2- C3/2- C1/3- C2/3- C3/3- H- V- V- V- V-1 V-1 V-1 C2/1- H- C1/4- V-5 C2/4- V-5 C3/4- V-5 24,3 29,5 27,7 29,1 31,33 25, 3, 29,5 33, 3,7 3, 32,7 Tab. 2 - Strength of grout Test Tensile strength in bending Compressive strength Density specimen [MPa] [MPa] [kg/m 3 ] C1/3-V-1 4,4 24, C2/3-V-1 2,8 1,3 215 C3/3-V1 4, 16,2 211 C1/4-V-5 5,9 49,5 223 C2/4-V-5 5,5 5,5 225 C3/4-V-5 5,5 49, LOADING PROCEDURE The loading procedure shown in Fig. 6 was applied for compression tests C1-H-. The measurements were recorded at the force levels marked by points in the diagram. At each level the measurement was repeated if necessary while the force value was hold. The actual loading procedure with repeated measurements is shown in Fig. 7 for the test C1/1-H-. Each point corresponds to one taken measurement of all installed transducers. The minimum value of the force was set as 1 kn. This value was chosen arbitrarily and it should ensure that neither transducers nor the steel bar and the steel plate would not move after the unloading. 5

6 6 4 2 Failure C2/1-H- C3/1-H- Failure C1/1-H- Time [s] Failure C2/2-V- C3/2-V- Failure C1/2-V- Time [s] Failure C1/3-V-1 Failure C3/3-V-1 C2/3-V-1 Time [s] Failure C1/4-V-5 Failure C2/4; C3/4-V-5 Time [s] Fig. 6 - Reached values of test failure Time [s] Fig. 7 - Actual loading procedure with measurement points for C1/1-H- test, [9] 5. TEST RESULTS Tests of 12 specimens C1-H-, C2-V-, C3-V-1 and C4-V-5 with this set-up were carried out. As the acting force was increased, all these tests failed because of the eccentricity of the cylindrical head positioned on the steel bar and the non-homogeneity of concrete (the head overturned). The tension failure in the concrete block due to the shear was not reached. (The limit state of crushing of the concrete is for design limited by the deformations.) The position in loading procedure, where each test failed, is marked in Fig. 6. The Fig. 8, 9, 1, and 11 show the deformation under the axes of symmetry of the tests set-up for all observed 6

7 positions. The Fig. 11 and 12 display deformations under the corner of the plate. The total deformations of the concrete block as well as full tests data see in [9]. 6 5 C1/1-H- C2/1-H- C2/1-H- 4 t = 1 mm w F P 1 x δ 1 -,5-1 -1,5-2 -2,5-3 -3,5-4 -4,5 Fig. 8 - Force-displacement relation of test C1/1-H-, C2/1-H- and C3/1-H- no grout, top surface, average of measure devices I16 and I17 at central position 6 5 C1/2-V- C2/2-V- C3/2-V- 4 3 t = 1 mm w F P 1 x2-1 2 δ 1 -,5-1 -1,5-2 -2,5-3 -3,5-4 -4,5 Fig. 9 - Force-displacement relation of test C1/2-V-, C2/2-V- and C3/2-V-, wall surface, average of measure devices I16 and I17 7

8 6 5 4 t w= 1 mm F t g = 15 mm P 1 x2-1 δ C1/3-V-1 C2/3-V-1 C3/3-V ,5-1 -1,5-2 -2,5-3 -3,5-4 -4,5 Fig. 1 - Force-displacement relation of test C1/3-V-1, C2/3-V-1 and C3/3-V1; grout 2 MPa, average of measure devices I16 and I C1/4-V-5 C2/4-V-5 C3/4-V t w = 1 mm t g = 15 mm F P 1 x δ 1 -,5-1 -1,5-2 -2,5-3 -3,5-4 -4,5 Fig Force-displacement relation of test C1/4-V-5, C2/4-V-5 and C3/4-V-5; grout 5 MPa, average of measure devices I16 and I17 8

9 6 5 I I15 t w = 1 mm t g = 15 mm + δ F P 1 x Fig Force-displacement relation of test C1/4-V-5 measure devices I14, I15 placed at the corner of steel plate 6 5 I18 I t w = 1 mm t g = 15 mm F P 1 x2-1 + δ Fig Force-displacement relation of test C1/4-V-5 measure devices I18, I19 placed at corner of steel plate 6. CONCLUSIONS The envelope of the results is comparable to the classical tests by Hawkins [1], who observed and established the concentration factor under a rigid plate round 6. The tests confirm the observations published by Steenhuis [12], which supports the today design practice [5]. No basic influences of the grout quality were found. The finite element and analytical prediction model for repeating loading is under developments. The shape of the unloading part of the force-deformation curve conforms to the linear assumption without weakening. 9

10 ACKNOWLEDGEMENTS The authors dedicate this work to Mr. Martin Steenhuis. This work has been supported by grant GAČR 13/1/78 of the Czech Grant Agency and by grant J1-98:214 of the Czech Ministry of Education. REFERENCES [1] Stark J., Hordijk D.A.: Where structural steel and concrete meet, in Connections between steel and concrete, University of Stuttgart, ed. Eligehausen R., Stuttgart 21, pp. 1-11, ISBN X. [2] Ando N., Nishimura I., Kamo K.: An experimental study on the connection joints between steel girder ans reinforced concrete columns with various types of embedded load transferring plates, in Connections between steel and concrete, University of Stuttgart, ed. Eligehausen R., Stuttgart 21, pp , ISBN X. [3] Zoetemeijer P.: Proposal for Standardisation of Extended End Plate Connection based on Test results and Analysis, Rep. No , Steven Laboratory, Delft 1983, p. 56. [4] Wald F., Mareš J., Sokol Z., Drdácký M.: Component Method for Historical Timber Joints, v The Paramount Role of Joints into the Reliable Response of Structures, NATO Science Series, Series II, Vol. 4, ed. Banitopoulos C.C., Wald, F., Kluver Academic Publishers, Dortrecht 2, ISBN , pp [5] PrEN : 2xx, (Eurocode 3 Part 1.8) Design of Steel Structures, Design of Joints, Fourth final draft, 16 Feb. 22. CEN, European Committee for Standardisation, Brussels 22. [6] Rassati G. A., Leon R.RT.: PR Composite Joints Under Cyclic and Dynamic Loading Conditions: A Component Modeling Approach, in Connections in Steel Structures IV, Steel Connections in the New Millenium, Roanoke, AISC, Chicago 2, in printing. [7] Alma J. G. J., Bijlaard, F. S. K.: Berekening van kolomvoetplaten, Rapport No. BI-8-46/ , IBBC-TNO, Delft 198. [8] Wald F., Steenhuis C. M., Jaspart J. P., Brown D.: Component method for base plate, Journal of Constructional Steel Research, in printing. [9] Gregor D.: Experiments with components of mix joints under cyclic loading, Research, CTU, Praha 22, p. 53. [1] Hawkins N. M.: The bearing strength of concrete loaded through rigid plates, Magazine of Concrete Research, Vol. 2, No. 63, March 1968, pp [11] TNO / SBR, Mortelvoegen in montagebouw, Rapport no. 34, Stichting Bouwresearch, Samson, Alphen aan de Rijn, [12] Steenhuis, C.M., Bijlaard F. S. K.: Tests on column bases in compression, Published in the Commemorative Publication for Prof. Dr. F. Tschemmernegg, ed. by G. Huber, Institute for Steel, Timber and Mixed Building Technology, Innsbruck 1999, pp [13] Sokol Z., Wald F. Experiments with T-stubs in Tension and Compression, Research Report PECO-AH-132, CTU, Praha 1997, p. 76. [14] Steenhuis C. M., Wald F., Stark J. W. B., Sokol Z., Taylor J. C.: Resistance and Stiffness of Concrete in Compression and Base Plate in Bending, JCSR in printing. 1