Modeling of welded angle connections in fire

Transcription

1 April 26-27, 211, Semnan University, Semnan, Iran Modeling of welded angle connections in fire Amir Saedi Daryan 1, Mahmoud Yahyai 2, 1- PhD. candidate of Structural Session, Civil Engineering Dept. K.N.Toosi University, Tehran, Iran 2- Associate Professor, Civil Engineering Dept. K.N.Toosi University, Tehran, Iran Abstract In this paper, the behavior of welded angle connections is studied at elevated temperatures using ABAQUS finite element software. In this study, steel members and connection components are considered to behave nonlinearly; the degradation of steel properties with increasing temperature is considered according to EC3 recommendations. The results of finite element and experimental tests conducted on welded angle connections in furnace fire conditions are compared, and the obtained failure modes and moment-rotation-temperature characteristics are in good agreement with those associated with experimental tests. Temperature loading similar to real fire condition is applied to each specimen to study the behavior of these connections in fire condition and the stiffness change with temperature increase under real fire condition is studied. The numerical results show that, in addition to material properties and connection geometry, the applied moments and temperature have significant effects on the stiffness of this kind of steel connection. Keywords: Welded angle connections, Finite element modeling; Fire modeling; Elevated temperature 1 INTRODUCTION The temperature sensitivity of steel is a weakness in steel structures. Since the mechanical properties of steel significantly deteriorate at high temperatures, the load capacity of steel structures under condition of a structural fire will decrease intensively. The failure of beam-column joints frequently induces the collapse of steel structures. However, the behavior of steel beam-column connections at elevated temperatures is very complicated and has not been fully studied. Different methods, including finite element analysis, are used to study the behavior of steel beam-column connections at elevated temperatures. The finite element method (FEM) provides an attractive means to investigate the beam-column joints in more detail than experimental tests would usually allow. A number of 3-D FE models have been developed (by various authors) that take into account any geometrical and material nonlinearities. Several studies have focused on the influence of elevated temperatures on end-plate connection behavior; a brief description of some of this research follows. Liu [1, 2] was the first to attempt to use FEM in modeling connection behavior at elevated temperatures. He developed a finite element model (FEAST) to predict the behavior of different types of joints at elevated temperatures. The beam, column, end plate and stiffeners were modeled using eight-nodded shell elements and considered the nonlinear behavior of the material with nonuniform thermal expansion across a section as well as large deformations in fire. The stress-strain-temperature characteristics were adopted based on recommended values from experimental tests. Close agreement was observed with experimental data for different types of joints. A 3-D FEM was developed by El-Houssieny et al. [3] to simulate the response of extended end plates at both ambient and elevated temperatures. Results from the developed model compared well with the experimental results and subsequent parametric studies were conducted to investigate the influence of connections on the behavior of sub frame elements in fire. Silva and Coelho [4] presented an equivalent elastic model to evaluate the response of steel joints under bending and axial force. Sarraj et al. [] also developed 3-D ABAQUS models of fin plate connections, which include the important contact interaction between the bolts and the fin plate and beam web. The models were validated against lap joint data at ambient temperature and a fire test conducted by Wald et al. 2 [6] at the Czech Technical University. Sarraj has used the FE modeling to develop a component spring model assembly. A finite element model was developed by Al-Jabri et al. [7] to study the behavior of flush end plate bare steel joints at elevated temperatures using the general purpose finite element software ABAQUS. The finite element model was used to establish the moment-rotation characteristics of the flush end plate bare steel joints with a concentrated force at elevated temperatures. The joint components were modeled using 3-D brick elements, while contact between the various components was modeled using Coulomb friction. Material nonlinearity was considered to model steel members and the joint components. Degradation of steel

2 April 26-27, 211, Semnan University, Semnan, Iran properties with increasing temperatures was taken in accordance with design code recommendations. The obtained FE-simulated failure modes and moment-rotation-temperature characteristics of the joints compared well with the experimental data in both the elastic and plastic regions. In the case of other connection details, Saedi Daryan et al. [8-1] modeled the bolted angle connections with and without web angles considering the effect of elevated temperatures and concentrated forces applied to the connections using ANSYS finite element software. In this research, stiffness and strength deterioration of steel and bolts as well as nonlinear behavior of materials were considered. The results were presented in the form of rotation-temperature curves and connection failure mechanisms are compared to those of experimental tests and good agreement is achieved, which shows the ability of finite element modeling to predict the behavior of these specific connection details at elevated temperatures. Saedi Daryan et al. carried out four experimental tests on Khorjini connections as semi rigid connections at elevated temperature. The connections were modeled using ABAQUS finite element program. Comparison between the result of numerical models and experimental test results showed good agreement in elastic and plastic ranges [11]. From the presented overview, it is clear that FE methods provide a reliable technique, which can be efficiently used in predicting the elevated-temperature behavior of joints to an acceptable degree of accuracy and enable a wider range of parameters to be considered than would be the case with a laboratory-based investigation. This study presents the full-scale fire tests of beam-column moment connections carried out by Saedi Daryan et al [12]. The general purpose finite element software ABAQUS is adopted to develop the 3-D finite element model. The numerical model is first verified by the experimental results. Then the fire response of steel semi-rigid beam-column moment connections is fully studied by the 3-D finite element analysis. 2 Experimental studies 2-1 Arrangement and test instrumentation The test carried out by Saedi Daryan et al. [12] is chosen to study the behavior of welded angle connections under temperature loading. In the tests, the temperature of the furnace was increased according to the ISO834 and ASTME119 curves. 2-2 Specimen details All specimens consisted of a symmetric cruciform arrangement with a single 8 mm high column made of IPE3 section connected to two 24 mm long cantilever beams made of IPE 22. The load was applied at a point 2 cm from the column flange. A series of eight experimental tests was performed on two types of welded angle connections. The connections were defined as follows: Connection group 1: Specimen without a web angle (SOW). Connection group 2: Specimen with a web angle (SWW). Connection group 1 (SOW) consisted of two angles. One of the angles was welded to the top flange of the beam and the other was welded to the bottom flange. This assembly was welded to the flange of the column, as shown in Figure 1. Figure 1. Specimen details for the SOW connection group 2

3 April 26-27, 211, Semnan University, Semnan, Iran For connection group 2 (SWW), in addition to the two SOW angles, two more angles were welded to the web of the beam and to the flange of the column. The web angles used in all specimens were 1*1*1 mm and are shown in Figure 2. Figure 2. Specimen details for the SWW connection group Considering the connection specimens tested in Refs. [12] Four connection specimens were selected. Two of these connections were selected from the first connection group and the other two were selected from the second connection group. The connection geometries are presented in Table 1. Table 1. Connection geometrical details Test NO Connection group Size of angle (mm) S2 ١ ١٥ *١٠٠* ١٥٠ S3 ٢ ١٥ *١٠٠* ١٥٠ S8 ١ ١٠ *١٠٠* ١٠٠ S9 ٢ ١٠ *١٠٠* ١٠٠ Specimen loading and material properties The values of applied moment to each specimen in the tests are presented in Table 2. As can be seen, the rotation capacity of connection is theoretically calculated first and then the applied moment is selected as a coefficient of connection rotation capacity and is applied to the specimens during the test. Table 2. Values of applied moment for each specimen in the tests conducted by Saedi Daryan et al. Specimen number Moment (M) level Applied M(kN m) S2.6*Mcc 8. S3.4*Mcc 8. S8.Mcc 4.2 S9.*Mcc 6. Material properties were measured by coupon test for all specimens, and dimensions were recorded before tests were started. The results of the mill test for this grade of steel are shown in Table 3. 3

4 April 26-27, 211, Semnan University, Semnan, Iran Table 3. Material properties Material Yield stress (N/mm2) Ultimate stress (N/mm2) Modulus of elasticity (N/mm2) Beam & Column & Angle *1 3 Numerical modeling The commercial finite element code ABAQUS is used to simulate the behavior of welded angle connections at elevated temperatures. Typical subparts of the 3-D finite element model are shown in Figure 3. Figure 3. Finite element model of the connection To obtain accurate results, we used a fine mesh in the vicinity of the connections where high stress and strain gradients were expected to take place, whereas a coarse mesh was used in areas far from the connection zone where low stress levels were expected. This modeling leads to accurate results around the connection, which is the region of prime interest. The structure is assumed to behave nonlinearly under high temperatures; elements are typically defined by the basic shape and chosen in such a way that they fulfill these requirements. Because of their reliability, eight-nodded reduced-integration brick elements were used. Contact between all connected parts was modeled using the surface interaction command in ABAQUS, allowing small relative sliding at the interface of the contact surfaces. Friction between the contact surfaces at the connection was modeled using the classical Coulomb model where the friction coefficient is taken as.1. The stress-strain characteristics of structural steel at elevated temperatures are defined in the design codes BS9, Part 8 and EC3: Part 1-2. The equations for degradation of welds are defined according to these codes. 4 Verifying the models To validate the results of the FE analysis, four elevated temperature tests were modeled. A comparison of the temperature-rotation response of the connections at different moments is shown in Figure 4. The temperature-rotation response curves of the connections agree well with tests at the elastic and plastic stages. Differences between the numerical simulations and the test results may have several causes, including numerical modeling simplification, test specimen defects, residual stress, contact surface interactions, frictional forces, or nonlinear constitutive models of materials at elevated temperatures. 4

5 April 26-27, 211, Semnan University, Semnan, Iran Temperature (c) S2 EXP S2 FE Rotation (Millirads) Temprature ( c) S3 EXP S3 FE Rotation (Millirads) Temprature (c) S8 EXP S8 FE Rotation (Millirads) Temprature(c) S9 EXP S9 FE Rotatio (Millirads) Figure 4. Comparison of FE and experimental results for four different tests One of the most important and applicable curves in connection design is the moment-rotation curve. This curve is shown in Figure for four types of connections at different temperatures using FEMs. For all tests, deterioration of connection characteristics resulting from temperature increases is predicted well by the models. Moment(kNm) S c 2 4 c c 1 c 1 6 c Rotation(Millirads) Moment(kNm) S3 2 c 4 c c 6 c 62 c Rotation(Millirads) Moment(kNm) 2 S c 1 4 c 8 c 6 c 6 c Rotation(Millirads) Figure. Moment-rotation-temperature curves Moment(kNm) 3 S c 4 c 1 c 1 c 6 c Rotation(Millirads) Figure shows that the moment-resistance of the connection is severely decreased. In general, these types of angle connections that have been made by usual constructional steel have no moment resistance at temperatures higher than 8 C.

6 April 26-27, 211, Semnan University, Semnan, Iran Results and discussion One of the most challenging subjects about the furnace fire experimental tests is the ability of these tests to represent the behavior of connections under real fire condition, since there are differences between real fire and furnace fire conditions. One of the basic differences is the uniform temperature in connection components in furnace fire test, while in the real fire condition the temperature is different in different parts of the frame. Thus, the stiffness change at different temperatures under real fire condition is calculated in the following, since the stiffness is one of the most important parameters for connection design. Then, the effect of the applied moment on the connection on stiffness changing in real fire condition is studied. It should be noted that the temperature magnitude applied to each connection components follows the temperatures measured in Cardington test [13]. First, the effect of increasing temperature on connection stiffness under a constant bending moment is studied and then the effect of the bending moment change applied to the beam on the connection stiffness is studied. Before the study of these parameters, an equation should be presented for calculation of connection stiffness. In this study, the stiffness of steel connection is defined as follows. [14] K = / Δθ Where M b r M b is the moment of the interface of beam and column and (1) Δ θr is the rotation of steel connection. Δ θ = θ θ = b c r r r Δ Δ Δ Δ h h t b t b 1 tan ( bf bf ) 1 tan ( cw cw ) bf bf (2) b where θr is the rotation of steel beam, θ c r is the rotation of steel column, Δ t bf is the horizontal displacement of an intersection point of middle surfaces of top flange and web of beam and the interface of beam and column, Δ is the horizontal displacement of an intersection point of middle surfaces of bottom b bf t flange and web of beam and the interface of beam and column, Δcw is the horizontal displacement of an intersection point of middle surfaces of top flange of beam and the centerline of column, Δ b cw is the horizontal displacement of an intersection point of middle surfaces of bottom flange of beam and the centerline of column, and hbf is the distance of middle surfaces of top and bottom flange of beam. Figure 6 shows the definition of these variables. Figure 6. Definition of joint deformation components 6

7 April 26-27, 211, Semnan University, Semnan, Iran The effect of increasing temperature on connection stiffness is studied in Figure 7. As can be seen, the stiffness of steel moment connections under a constant bending moment increases with increasing temperature before approximately 3 C and then a downturn occurs from the peak. In other words, the maximum stiffness of each connection under a specific bending moment is achieved at 3 C. 4 K(kN-M/Millirads) S3 S2 S9 S Average Connection Temperature (c) -1 Effect of the applied moment Figure 7. Stiffness temperature of the connections The effect of applied bending moment on connection stiffness at different temperatures is shown in Figure 8. The effect of applied bending moment is significant. The connection stiffness is increased by decreasing the applied moment value. K(kN-m/Millirads) S2, Vb=.6 1 Mb/Vb=3 Mb/Vb=4. Mb/Vb=6 Mb/Vb= Average Connection Temperature(c) K(kN-m/Millirads) S3, Vb=.6kN 1 Mb/Vb=3 1 Mb/Vb=4. Mb/Vb=6 Mb/Vb= Average Connection Temperature(c) K(kN-m/Millirads) S8, Vb=2.8 kn Mb/Vb=3 Mb/Vb=4. Mb/Vb=6 Mb/Vb= Average Connection Temperature (c) K(kN-m/Millirads) S9, Vb=4.3 kn Mb/Vb=3 Mb/Vb=4. Mb/Vb=6 Mb/Vb= Average Connection Termperature (c) Figure 8. Effect of applied moments on connection stiffness 6 Conclusion This study applied the general purpose finite element software ABAQUS to investigate the fire response of welded angle connections with and without web angle and built an effective numerical model to investigate the fire response of the steel connections. Results of the analyses were then presented as rotation-temperature 7

8 April 26-27, 211, Semnan University, Semnan, Iran curves and deformed shapes and the results were compared to those of experimental tests. The comparison confirms the ability of the finite element method in simulating the behavior of welded angle connections. Considering the importance of connection behavior under real fire condition, the effect of increasing temperature on connection stiffness under temperature loading similar to real fire condition was studied. The results show that the stiffness of steel moment connections increases with increasing temperature before approximately 3 C, then a downturn occurs from the peak. Finally, the effect of changes in the applied bending moment on connection stiffness at elevated temperature was studied and results indicated that the applied moments have a significant effect on steel connection behavior and the connection stiffness decreases with increasing applied moment. 7 References 1. Liu TCH, Morris LJ., (1994), Theoretical modeling of steel bolted connection under fire exposure, Proceedings of the international conference on computational mechanics, Hong Kong, pp Liu TCH., (1999), Moment rotation temperature characteristics of steel/composite connections, Journal of Structural Engineering, 12(1), pp El-Housseiny OM, Abdel Salam S, Attia GAM, Saad AM., (1998), Behavior of extended end plate connections at high temperature, Journal of Constructional Steel Research,46(3),pp da Silva LS, Coelho AMG., (21), An analytical evaluation of the response of steel joints under bending and axial force, Computers & Structures, 79(2), pp Sarraj M, Burgess IW, Davison JB, Plank RJ.,(29), Finite element modeling of fin-plate steel connections in fire, Proceedings of the fourth international workshop on structures in fire, Aveiro, Portugal, pp Wald F, Simo nes da Silva L, Moore DB, Lennon T, Chaldna M, Santiago M., (26), Experimental behavior of a steel structure under natural fire, Fire Safety Journal, 41(), pp Al-Jabri KS, Seibi A, Karrech A.,(26), Modeling of un-stiffened flush endplate Bolted connections in fire, Journal of Constructional Steel Research, 62(),pp Saedi Daryan A., (26), A Study on Behavior of Connections in Fire, MS thesis, Tehran, Iran K.N. Toosi University. 9. Saedi Daryan A, Yahyai M., (29), Behavior of bolted top-seat angle connections in fire, Journal of Constructional Steel Research, 6(4), pp Saedi Daryan A, Yahyai M., (29), Modeling of bolted angle connections in fire, Fire Safety Journal, 44 (6), pp Saedi Daryan A, Bahrampoor H., (21), The study of behavior of Khorjini Connections in Fire, Fire Safety Journal, 44(7), pp Saedi Daryan A, Yahyai M., (29), Behavior of welded top-seat angle connections exposed to fire, Fire Safety Journal, 44 (), pp F. Wald, L. Simoes da Silva, D.B. Moore, T. Lennon, M. Chladna, A. Santiago, M. Benes, L. Borges.,(26), Experimental behavior of a steel structure under natural fire, Fire Safety Journal, 41 (6), pp C.J. Mao, Y.J. Chiou, P.A. Hsiao, M.C. Ho., (29), Fire response of steel semi-rigid beam column moment connections, Journal of Constructional Steel Research, 6 (4), pp