EXPERIMENTAL BEHAVIOUR OF END-PLATE BEAM-TO-COLUMN JOINTS UNDER BENDING AND AXIAL FORCE Database reporting and discussion of results

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1 DEPARTMENT OF CIVIL ENGINEERING UNIVERSITY OF COIMBRA ECCS TECHNICAL COMMITTEE 1 CONNECTIONS TWG 1.2 EXPERIMENTAL BEHAVIOUR OF END-PLATE BEAM-TO-COLUMN JOINTS UNDER BENDING AND AXIAL FORCE Database reporting and discussion of results by L. SIMÕES DA SILVA, L. LIMA, P. VELLASCO and S. ANDRADE Meeting in Ljubljana April 11-12, 21 UC/DEC/GCOM/22-8/RR/JOINT and ECCS TC1-2-WG2/4 (Version 1 9/4/22)

2 2 EXPERIMENTAL BEHAVIOUR OF ENDPLATE BEAM-TO- COLUMN JOINTS UNDER BENDING AND AXIAL FORCE Luís Simões da Silva 1, Luciano R. O. de Lima 2, Pedro C. G. da S. Vellasco 3 e Sebastião A. L. de Andrade 4 ABSTRACT The objective of the present paper is to present the results of an experimental research project on endplate beam-to-column bolted steel joints subjected to ending and axial force currently being currently carried out at the University of Coimbra. Because of the ongoing nature of this work, this paper is an updated version of a previous Document presented in Timisoara (Document TC1-1-WG2/11), 15 tests being now reported from flush and extended end-plate configurations. In addition, special emphasis is given to database reporting, to allow other research centres to use this experimental data for calibration of theoretical models. 1. INTRODUCTION A great part of the beam-to-column connections and beams-splices are subject to axial forces, bending and shear. The interaction of these efforts modifies the behaviour of the joint with respect to its initial stiffness, bending resistance and rotation capacity. In the Eurocode 3, [1], it is possible to evaluate the resistant capacity of a joint subject to bending moment and axial force whenever the latter is less than a maximum limit, given by the equation (1), N N pl.1 (1) where N is the beam axial force and N pl is the beam plastic resistance subjected to compression. Under these conditions, the axial force may be disregarded in the design of the joint. It is important to point out that there is no background to justify this empirical limit of 1%. For connections that exceed the limit range of this equation, Eurocode 3 does not make any specific recommendation. However, the general principles of the component method contemplate this case, since any component is characterized independently of the type of applied loading to the connection. It is up to the designer to define a coherent component 1 Department of Civil Engineering, University of Coimbra, Portugal 2 Department of Civil Engineering, PUC-Rio Rio de Janeiro Brazil (PhD Student - PDEE CAPES Brazil) 3 Department of Structures and Foundations, UERJ Rio de Janeiro Brazil 4 Department of Civil Engineering, PUC-Rio Rio de Janeiro Brazil and Department of Structures and Foundations, UERJ Rio de Janeiro Brazil UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

3 3 model that identifies the relevant interactions between the various components. Recent attempts to establish specific procedures for these types of connections were performed for Jaspart et al, [2], Cerfontaine, [3], Silva and Coelho, [4] and Silva et al, [5]. To provide as sound basis for further theoretical developments, a set of 15 tests was carried out at the University of Coimbra and are briefly described in the next section [6, 7] 2. DESCRIPTION OF THE EXPERIMENTAL TESTS A series of 8 (eight) experimental tests of beam-to-column steel connections with flush endplate, typically illustrated in Figure 1, were already carried out at the University of Coimbra, within a research program on behaviour of beam-to-column joints under bending and axial force. In the first test, FE1, only bending moment was applied through a hydraulic actuator, located a meter away from the face of the column flange, Figure 1(a). For the following tests - FE3, FE4, FE5, FE6, FE7, FE8 and FE9 - constants axial forces of, respectively, -4%, -8%, -2%, -27%, -2%, +1% and 2% of the beam plastic resistance were applied to the beam. A further series of 7 (seven) tests with extended endplate joints were also performed, Figure 1(b). As in the first test series, only bending moment was applied for test EE1. For the subsequent tests EE2, EE3, EE4, EE5, EE6 and EE7 - constant axial forces of, respectively, -1%, -2%, -27%, -15%, +1% and +2% of the beam plastic resistance were applied to the beam. In all tests, the columns were simply-supported at both ends and consist of a HEB24, the beams consist of an IPE24 and the endplate is 15 mm thick, all manufactured from a steel S275. The bolts are M2, class 1.9. The compressive force application system, Figure 2(a), was composed of a hydraulic jack that applies a tension force to four cables of prestressing with diameter φ = 15,2 mm. The transfer of this force for the connection was performed through a central load cell with capacity of 5kN (NOVATECH), Figure 2 (c). These cables pass by a deviator constituted by a profile HEM1 to guarantee that the axial force is always parallel to the beam axis. A load cell TML with capacity of 2 kn in each cable was also used, with the purpose of to obtain the force in each cable. The transmission of the applied axial force in the connection to the reaction wall was made through a reinforced concrete footing that was connected to the reaction wall by means of a profile HEB 2, and prestressed to the reaction slab through bars DYWIDAG. The tensile axial force application system is shown in Figure 3. Four hydraulic jacks placed in one of the extremities of a circular profile transmit the tension axial force. These circular profiles are simply supported in the other extremity for allow the rotation of these profiles and to guarantee that the axial force is applied always parallel to the beam axis. UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

4 4 (a) Figure 1 Endplate joints layout (b) desviator (b) (c) (a) Figure 2 (a) Frame of application loading, (b) hydraulic jack and (c) central load cell Figure 3 Experimental test layout for tests subjected to bending and tension axial force UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

5 All tests were instrumented as shown in Figure 4 and Figure 5, with single strain gauges (FLK 6-11-TML), rosettes to 45º (FRA 5-11-TML), bolts axial strain gauges (BTM 6- C-TML), and displacements

The registration of measurements was done with a data acquisition system TDS2-TMlL.

5 5 All tests were instrumented as shown in Figure 4 and Figure 5, with single strain gauges (FLK 6-11-TML), rosettes to 45º (FRA 5-11-TML), bolts axial strain gauges (BTM 6- C-TML), and displacements transducers (LVDT's), in order to evaluate the main characteristics of the connection, such as, bending moment resistance, initial stiffness and rotation capacity. The registration of measurements was done with a data acquisition system TDS2-TMlL. CH / CH B35 B36 25, 26 & 27 B37 B IPE24 2/ /3 31/32 23/ HEB IPE24 CH39 CH4 1 CH43 CH41 CH CH45 HEB24 CH47 CH48 CH Figure 4 Layout of Single strain gauges, rosettes to 45º, bolts axial strain gauges layout and displacements transducers (a) (b) Figure 5 (a) Displacements transducers and (b) data acquisition system For all tests, a constant axial force was applied first, maintained constant throughout the test, with subsequent application of a bending moment incremented to failure. Two unloadings were performed, the first for a bending moment of 25 knm (down to 5 KNm, to eliminate possible slack in the joint) and the second for a rotation of 2 mrad. Force control was used in the first part of each test, subsequently changed to displacement control. Tensile tests on coupons extracted from the beams were carried out, aiming at characterizing the actual properties of the material. Then, it was possible to calculate the beam plastic resistance and to determine which the true level of applied axial force to the beam for the other tests. These tensile tests were accomplished according to the following specifications, EN12 [6], EN12 [7] and EN125 [8], yielding the results of Table 1. UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

6 6 Table 1 Steel mechanical properties (beam of the first test - FE1) Specimen f y (MPa) f u (MPa) Flange3A Flange3B Flange average Web2B Web3B Wen3A Web average ANALYSIS OF THE EXPERIMENTAL RESULTS FOR THE FLUSH ENDPLATE JOINTS 3.1. Moment vs. Rotation Curves and Application of the Axial Force The moment vs. rotation curves of the seven tests are presented below where it may be observed that even for a level of equivalent axial force of 2% of the beam plastic resistance, the bending moment is still higher than the Eurocode 3. This is due to the fact that the components in tension for two bolt row are alleviated by the compression axial force and the components in compression, even increased, don't reach the strength, even in presence of the axial force, Figure 6. The results of test FE9 (applied axial force of + 2% of the beam plastic resistance) will be added to the results shortly FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Rotation (mrad) Figure 6 Moment vs. rotation curves With the purpose of calibrating the axial force application system, a initial test was performed (FE2) in elastic regime. It was verified that the applied axial load with the cables UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

7 7 was transmitted by the central load cell to the connection as shown in Figure 7. In this graph, it was verified that the axial force applied to the connection, measured through the strain gages located in the web and flange beam and the measured by the central load cell is similar adds of the individual cells (1, 2, 3 e 4) strain gagess central load cell (5) Axial Force (kn) Displacement (mm) Figure 7 Applied axial force verification For higher levels of bending moment, it was verified that the rotation of the beam provoked a reduction of the force in the lower cables and an increment of load of the top cables. Consequently, hydraulic jacks were placed in the lower cables to make the correction of the axial force as the test is being performed. In Figure 8, the variation of the axial force is exemplified in the four individual cables with the moment applied for each test, as well as a detail of the hydraulic jacks. The correction can be observed in item (c). The Table 2 presents the values obtained for the bending moment resistance and for the initial stiffness of the tests connections. The values of the initial stiffness were obtained from slope of the unloading part of the moment vs. rotation curves. The theoretical values calculated according to Eurocode 3 were respectively, 68.9 kn.m and kn.m/rad being disregarded the presence of the axial force. Table 2 - Experimental values of bending moment resistance and initial stiffness Test N (kn) M Rd (kn.m) S i (kn.m/rad) FE3 (- 4% N pl ) FE4 (- 8% N pl ) FE5 (- 2% N pl ) FE6 (- 27% N pl ) FE7 (- 2% N pl ) FE8 (+ 1% N pl ) FE9 (+ 2% N pl ) UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

8 2 4 8 1 2 4 8 1-1 (3) e (4) -2 (3) e (4) Axial Force (kn) -2-3 -4 (1) e (2) Axial Force (kn) -4 - -8-1 (1) e (2) (5) -5 - (5) y =.5x - 52.679-12 -14 y =.77x - 15.

8 (3) e (4) -2 (3) e (4) Axial Force (kn) (1) e (2) Axial Force (kn) (1) e (2) (5) -5 - (5) y =.5x y =.77x (a) Test FE3 (b) Test FE y =.79x (5) Axial Force (kn) (1), (2), (3) and (4) Bending Moment (kn) (c) Test FE6 Figure 8 Bending moment vs. axial force curves (d) Hydraulic jacks 3.2. Analysis of Individual Components The Table 3 presents the theoretical values of the bending moment resistance and initial stiffness for all components of the connection in study, calculated in agreement with Eurocode 3. Tension Compression Table 3 - Theoretical values of the resistance and stiffness of the components Component Resistance (kn) Stiffness (mm) Column web in tension (3) 458,7 7,3 Column flange in bending (4) 397,2 38,22 Endplate in bending(5) 321,7 13,35 Beam web in tension (8) 455,2 Bolts in tension (1) 441, 7,76 Column web in shear (1) 552,7 7,52 Column web in compression (2) 598,2 1,4 Beam flange in compression (7) 53,6 In Figure 9, the moment vs. rotation curves of the four tests can be observed with the experimental identification of the yielding sequence of the several components. It is clearly noticed that application of the compression axial force benefits the critical component of the tension zone (endplate in bending) and it decreases the capacity of the critical component of the compression zone (beam flange in compression). UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

9 FE1 5 5 Experimental - FE3 4 Experimental - FE1 4 Endplate in bending (5) Endplate in bending (5) 3 Beam flange in compression (7) 3 Beam flange in compression (7) Bolts in tension (1) Bolts in tension (1) 2 2 Column flange in bending (4) Column flange in bending (4) 1 Beam web in tension (8) 1 Beam web in tension (8) Column web in shear (1) Column web in shear (1) Rotation (mrad) Rotation (mrad) FE FE FE Experimental - FE4 3 Beam flange in compression (7) Endplate in bending (5) 2 Bolts in tension (1) Column flange in bending (4) 1 Beam web in tension (8) Column web in shear (1) Rotation (mrad) Experimental - FE5 Beam flange in compression (7) 3 Endplate in bending (5) 2 Column flange in bending (4) Beam web in tension (8) 1 Bolts in tension (1) Column web in shear (1) Rotation (mrad) Figure 9 Moment vs. rotation curves with yielding sequence of the components In the graph presented in the Figure 1 it can be observed that, for all the tests, the column flange in bending presented deformations according to mode 1, that is, complete yielding of the flange. The measured displacements for this component are presented in Figure 11 where it is noticed that the behaviour of the component is similar for all the tests, independently of the applied axial force FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Strain (µε µε) Figure 1 Moment vs. strain curves for column flange in bending component UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

10 FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Displacement (mm) Figure 11 Moment vs. displacements curves for column flange in bending component In agreement with the design rules of Eurocode 3, the resistance of the endplate in bending is equal to 321,7 kn. In the graph of the Figure 12, this component, in test FE1 reaches the yielding strain for a bending moment of, approximately, 45 kn.m. For this bending moment level, the tension load in the first bolt row, evaluated in agreement with the strain gages located in the beam flange, it is equal to 333,4 kn. However, for the test FE5, due to the contribution of the applied axial force, the endplate reached the yield strain for a higher level of applied moment. In Figure 13 the moment vs. displacements curves for this component are presented FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Strain (µε µε) Figure 12 - Moment vs. strain curves for endplate in bending component UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

11 12 1 FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) 8 4 2-25 -2-15 -1-5 5 Displacement (mm) Figure 13 -

11 FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Displacement (mm) Figure 13 - Moment vs. displacements curves for endplate in bending component Analysing the curves presented in Figure 14, it is clearly noticed that the beam flange also reaches yielding. According to Eurocode 3, the beam flange in compression resistance is 529,3 kn. For this bending moment level, the average of the measured strains in the inferior beam flange was 23µε, that it, is equal to a force of 57, kn, higher than the 529,3 kn presented above, fact that can be explained through the hardening of the steel. However, it is worthwhile to point out that for the first test, where the compression axial force was not applied to the beam, beam flange yielding occurred for a larger value of bending moment than in the other tests FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Strain (µε µε) Figure 14 - Moment vs. strain curves for beam flange in compression component UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

12 12 As it can be seen in Figure 15, the column web in compression component did not reach yielding for any of the tests FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Strain (µε µε) Figure 15 - Moment vs. displacements curves for column web in compression component The column web in shear component reached yielding for all tests, Figure 16. This graph was obtained through a rosette positioned in the centre of the column web panel where the used component was the ε 45º FE3 (N = - 4% Npl) FE4 (N = - 8% Npl) FE5 (N = - 2% Npl) FE6 (N = - 27% Npl) FE7 (N = - 2% Npl) 2 ε 45º FE8 (N = + 1% Npl) FE9 (N = + 2% Npl) Strain (µε µε) Figure 16 - Moment vs. strain curves for column web in shear component UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

13 4. ANALYSIS OF EXPERIMENTAL RESULTS FOR EXTENDED ENDPLATE JOINTS 4.1. Moment vs. Rotation Curves and Application of the Axial Force The moment vs.

13 13 4. ANALYSIS OF EXPERIMENTAL RESULTS FOR EXTENDED ENDPLATE JOINTS 4.1. Moment vs. Rotation Curves and Application of the Axial Force The moment vs. rotation curves of the seven tests are presented below where it may be observed that even for a level of equivalent axial force of -1% of the beam plastic resistance, the bending moment is still higher than the Eurocode 3. This is due to the fact that the components in tension for the two bolt rows are alleviated by the compression axial force and the components in compression, even increased, don't reach the strength, even in presence of the axial force, Figure 17. The results of test EE6 (applied axial force of + 1% of the beam plastic resistance) show that the bending moment was reduced by 1%. In Figure 18, some deformations that occurred in these tests may be observed EE1 (only M) EE2 (N = - 1% Npl) EE5 (N = - 15% Npl) 4 EE3 (N = - 2% Npl) EE4 (N = - 27 % Npl) 2 EE6 (N = +1% Npl) EE7 ( N = +2% Npl) Rotation (mrad) Figure 17 Moment vs. rotation curves Figure 18 Column flange, beam flange and endplate deformations UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

14 5. DATABASE REPORTING Currently, all tests

organised in EXCEL sheets with the general

723 y =.167x + 72.37 7 5 4 3 2 1. 1. 2. 3. 4. 5.. 7. 8.

http://www.dec.uc.pt/~luisss/eccs/eccspreview.

14 14 5. DATABASE REPORTING Currently, all tests performed at the University of Coimbra are organised in EXCEL sheets with the general organisation of Figure y = 4.763x y =.167x Rotation (mrad) Database on website: Figure 19 DATABASE information UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

15 15 The detailed organisation of the information is as follows: (i) Geometrical and Mechanical Properties.xls - This file described data for the complete experimental program (15 tests) in the following way: (i.1) FLUSH ENDPLATE TESTS - These worksheets include nominal values and all measured quantities (i.1.a) Geometrical Properties End-plate and beam (Figure 2) Column (i.1.b) Mechanical Properties* (i.1.c) Joint details (i.1.d) Test arrangement (i.1.e) Instrumentation - This worksheet identifies the relevant positions within the test arrangement (i.2) EXTENDED ENDPLATE TESTS * - These worksheets include nominal values and all measured quantities (i.2.a) Geometrical Properties End-plate and beam (Figure 2) Column (i.2.b) Mechanical Properties * (i.2.c) Joint details (i.2.d) Test arrangement (i.2.e) Instrumentation - This worksheet identifies the relevant positions within the test arrangement (ii) FE1.xls to FE9.xls and EE1.xls to EE7.xls** - These files, all sharing the same structure, contain all the results for each individual test, organised in the following way: (ii.1) (ii.2) DATALOGGER READINGS - This worksheet contains all readings from datalogger, for the various channels already identified in file (i)geometrical and Mechanical Properties.xls INDIVIDUAL GRAPHS (CE to CE ) - These worksheets contain graphical information for each channel, plotted agains load increments * Incomplete ** Not all available at this stage UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

16 16 Test d e f g h i l nominal FE FE FE FE FE FE FE FE Test m n o p q d1 d2 nominal FE FE FE FE FE FE FE FE Test d3 d4 bp hp tp tfb1 tfb2 nominal FE FE FE FE FE FE FE FE Test tw wbf dh nominal FE FE FE FE FE FE FE FE l dh p n wbf tfb1 tw tfb2 d1 d2 d e f g h i d3 d4 bp q o m hp tp Figure 2 Column flange, beam flange and endplate measured quantities This information is available for download from the following website: upon request (restricted use until the end of 22, being part of the doctoral thesis of Luciano Lima luciano@dec.uc.pt). 6. CONCLUDING REMARKS The experimental results obtained to the moment for beam-to-column connections with flush endplate allow the elaboration of a moment vs. axial force interaction curve corresponding to the elastic resistance of the connection, just as it illustrates the Figure 21. From the analysis of the Table 3 that illustrates the resistance and elastic stiffness of the all components of the connection in study, calculated in agreement with Eurocode 3, it is possible to conclude that, for this connection, the tension zone presents a resistance of the critical component that is equal at 63% of the component of the compression zone. Thus, the compression axial force results in an increase of the bending moment resistance of the connection, as it is verified. The results evidence the need to prepare specific clauses for the design of connections subjected to axial forces and bending moment. UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

17 % Npl Figure 21 M-N interaction diagram for flush endplate joints 7. REFERENCES [1] Eurocode 3 (1998), ENV :1992/A2, Annex J, Design of Steel Structures Joints in Building Frames. CEN, European Committee for Standardisation, Document CEN/TC 25/SC 3, Brussels. [2] Jaspart, J. P., Braham, M. and Cefontaine, F. (1999), Strength of Joints Subjected to Combined Action of Bending Moments and Axial Forces. In: Studnicka J, Wald F, Machacek J. Eurosteel 99, Proceedings of the 2 nd European Conference on Steel Structures, Czech Technical University in Prague, Prague, p [3] Cerfontaine, F. (2), Etude analytique de l interaction entre moment de flexion et effort normal dans les assemblages boulonnés (in french), Travail de fin d etudes du DEA en Sciences Appliquées, Département MSM, University of Liège, Belgium. [4] Silva, L.S. and Coelho, A.G. (21), A analytical evaluation of the response of steel joints under bending and axial force, Computers & Structures, vol. 79, [5] Luís Simões da Silva, Luciano R. O. de Lima, Pedro C. G. da S. Vellasco e Sebastião A. L. de Andrade, (21). Experimental and Numerical Assessment of Beam-to-Column Joints Under Bending and Axial Force. In Chang-Koon Choi and Hyo-Gyoung Kwak (eds), Techno Press, Proceedings of the First International Conference on Steel & Composite Structures, , Pusan, Korea. [6] EN 12 (199) Metallic Materials Tensile Tests. Part1: Method of Test (at ambient temperature). [7] EN 12 (1989) Steel Definition and Classification. [8] EN 125 (1994) Hot Rolled Products of Non-Alloy Structural Steels. UC/DEC/GCOM/22-8/RR/JOINT,and ECCS TC1-2-WG2/

DESIGN OF END PLATE JOINTS SUBJECT TO MOMENT AND NORMAL FORCE

DESIGN OF END PLATE JOINTS SUBJECT TO OENT AND NORAL FORCE Zdeněk Sokol 1, František Wald 1, Vincent Delabre 2, Jean-Pierre ueau 2, arek Švarc 1 ABSTRACT The presented work describes design model of end