International Journal of Civil and Environmental Research Volume 02, Issue 01, Pages 1-9, Structural Analysis of Steel Beam under Fire Loading

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1 International Journal of Civil and Environmental Research Volume 02, Issue 01, Pages 1-9, 2015 ISSN: Structural Analysis of Steel Beam under Fire Loading Hossein Rad *, Mahmood Yahyai Department of Civil Engineering, K.N.Toosi University of Technology, Tehran, Iran * Corresponding author. Tel.: ; address: hossain.rad2000@gmail.com A b s t r a c t Keywords: Steel structures, fire loading, temperature, connection. 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 be intensively decreased. Thus, researchers have special interest in studying of fire effect on steel structures and their components. Study of their behaviour has a high importance in one steel structure. The purpose of this paper is determination of the behaviour of beams and their connections under fire loading. Due to the high cost of fire test and also the limited number of parameter in each test as well as growth and development of powerful software, finite element method (FEM) is used to analyse beam and connections under fire loading. In accordance with reliance and ability of this method in modelling and analysis of steel beam and connections, behaviour of some steel beam samples with semi rigid connections with various characteristics under fire loading is studied. In addition. Sample duration time, failure mechanism and in general behaviour of them under fire loading studied. Accepted:22 February2015 Academic Research Online Publisher. All rights reserved. 1. Introduction According to the importance of dangers of fire a series of studies on the effects of fire on structures has been done.amongst these instigations are an analytical method of catenary action in steel beams at large deflections under fire loading reported by Yin et al. [1, 2], who validated their results in the form of a comparison between predictions using their proposed method and simulations using ABAQUS [3], owing to the scar-city of relevant experimental results. Bradford [4] presented a generic nonlinear analysis of a member in a frame sub-assembly in the elastic range of structural response using the principle of virtual displacements. It was shown that under thermal loading, certain thermal regimes exist (combinations of uniform temperature and temperature gradient) that can result in tensile forces developing in a beam, even in the elastic range of structural response. This study is a developing work of a research that has been done by Bradford et al [5], in which the effect of nonuniform degradation of the mechanical properties of the steel over the beam section was considered; this degradation of being caused by the nonuniform temperature distribution in the steel beam that result from a heat sink effect provided by the cooler concrete floor slab above, which may or may not induce composite action with the steel beam.

2 In 2007 Bradford et al. [6] conducted numerical studies of a steel beam exposed to fire inside a steel compartment. A member of the fire affected the temperature slenderness ratio of members, and the constraints created by the members and the adjacent cooler thermal regime. It also can be used to develop this component of the predicted response under fire by steel nodes were Simoes et al. [7] mentioned.the research component of the node to predict the behavior of steel under this fire was proposed and an analytical process for evaluating the behavior of steel nodes was presented. According to the behavior of complex structures at high temperatures and high cost of laboratory work, in recent years to examine the behavior of structures separately from the main column of July and the behavior and effects of the behavior of the elements of the structure has been heated. One of the important subjects in the effect of high temperatures and taking down connections in the BEAM is hardly fitting behavior. This can give a better view of the behavior of structures. Therefore, this study seeks to conduct real-beam fire study and joint stiffness at high temperatures is also considered. Choe et al. [8] presented the results of experimental investigations conducted to determine the fundamental behaviour of steel members under fire loading. A total of eleven fullscale steel members were tested under combined thermal and structural loading. The experimental investigations showed that the fundamental behaviour and strength of steel members is governed mostly by the steel surface temperature, and the strength and stiffness of steel columns decreases significantly with increasing temperatures, particularly in the range from C. 2. Modeling One of the important things in modeling the behavior of steel under temperature is to simulate the behavior of low properties of steel under high temperature. In this study, the simulation is done based on European regulation. Coefficient decreases with increasing temperature hardness and strength of steel as shown in Table 1. Note in Table 1, reduction of temperature resistance and hardness is Table1: Reduction factors for stress strain curves of steel Steel temperature,θ s (c) at elevated temperature Reduction factor for yield stress f y, and Young s modulus E s, at steel Temperature θ s k y,θ = f y,θ /f y k E,θ = E s,θ /E s Fire resistance tests in accordance with ISO standards such as ISO-834 on a single heating element and heat that can be done. The heating takes are based on certain graphs and standards. Several studies have been done to determine the actual temperature of the fire in the building. It may be because of safety factors to improve the quality of steel in fire damage in steel. Figure 1 shows the ISO-834 standard fire curve [9]. Fig 1: Standard fire curves 2 P a g e

3 3 Connection geometry To illustrate the behavior of steel connections under load at normal temperature and tested by fire two angles BOLT on the connections were previously by Yahyaei and saedi daryan []. 3.1.Specimen details All the specimens were configured as a symmetric cruciform. Arrangement consisting of a single 80 cm high column of IPE300 section connected to two 240 cm long cantilever beams of IPE 220.The load was applied on a point of each beam two meters from the end of the beam. All of the bolts in each specimen were tightened to 150 N m using a torque wrench to ensure consistency. The cruciform test arrangement was selected because: (a) it requires a less expensive test rig than the corresponding cantilever arrangement, (b) it provides an indication of the variability of the nominally identical connections on either side of the column, and (c) two connections (east-west) are tested in every specimen. Lateral bracing was provided to prevent beam torsion during the test. These braces were designed according to the maximum torsional moment of the beam. The braces and their orientation are shown in Fig. 1. In total, 12 experimental tests were conducted on two different connection types. beam by six M16 bolts and to the flange of column by two M16 bolts. The details of this type of connection are shown in Figure 2. Connection Type II (SWW) has two additional angles beyond What is used for Connection Type I. These angles are bolted to the web of the beam on one side and to the flange of the column on the other side. Web angles are connected to the web of the beam by twom16 bolts and to the flange of the column by twom16 bolts. Details about this group of connections are shown in Fig. 2. Details Of all specimens are given in Table 2. Connection Type I: (Specimen without Web Angle) (SOW) Connection Type II: (Specimen with Web Angle) (SWW) Connection Type I (SOW) consists of two angles, one connected to the top flange of the beam and the other connected to the bottom flange of the beam. The total system is then bolted to the flange of the column. Each angle is bolted to the flange of the Connection Type I 3 P a g e

4 Table 2: Detail of specimens Specimen number Group number Angle size (mm) Grade of bolt 3 1 0*0* *0* *0* *0* 8.8 S Connection Type II Moment(kNm) c 400 c 500 c 550 c 600 c Fig.2: connection detail Graphs moment - rotation - the temperature Rotation(Millirads) At the connection, since one of the most important application in the design connections, curves of the moment - rotation on them. The finite element model in Figure 3 charts moment - rotation to the four connection types at different temperatures are plotted. For all experiments, the binding properties decrease with increasing temperature as predicted by the model well. Moment(kNm) S Rotation(Millirads) 20 c 400 c 500 c 600 c 650 c 4 P a g e

5 Moment(kNm) S c c c c c Rotation(Millirads) S Moment(kNm) c c 500 c c 600 c Rotation(Millirads) r L = r R = r = β E 20I 2L k L = k R = k = β E 20A 2L (1) here E20 is the modulus elasticity of the steel at room temperature and taken as E20 = 200 3N/mm2; I and A are the cross-section second moment of area and area respectively; L is the member half length (Figure 4) and β is the dimensionless relative stiffness coefficient, which allows for the spring stiffness to be easily varied. Variations of the steel modulus of elasticity ET and yield stress fyt are taken from an empirical relationship given in the Australian Standard for steel structures AS 40 [] as Fig.3: moment-rotation-temperature Figure 3 shows that by increasing temperature the resistance of the moment connection decreases. Connection resistance at temperatures higher than 800 is also be ignored []. 4. Validations ANSYS software is used for verification of this model with beam sub-assembly, same as Bradford s. Bradford s numerical studies were conducted on an Australian 460UB82.1 section, subjected to thermal regimes defined by a temperature rise TC at the geometric centroid of the section and a linear thermal gradient ρ over the cross-section depth. The member is also subjected to a total downwards uniformly distributed load of q = 5N/mm, which was chosen to simulate the sustained service load that the member is carrying. End restraints are applied to the member via elastic springs, in which their stiffness can be written as Fig.4: generig flexural member (Bradford) When ζ = 0.25 C / min, uniform load q = 5N/mm length of member 2L = 6000mm, the relative hardness of the coefficient B = 1 to B = 0.01 will change. The high end restraint, despite an increase in the axial force member, the opening deformation is reduced. As can be seen in Table 5 with end restraint, the middle of the span is reduced significantly [6]. Table 5: Compares the results of Bradford with the results of ANSYS software β y max (Bradford study) (mm) y max (present study) (mm) P a g e

6 Table 5 compares the results of Bradford with the results of ANSYS software to fit well together. 5. Set up of the numerical problem In Figure 4, a member of length L in a steel frame sub-assembly is shown. The end displacements are taken to be v(z = ±L/2) = 0 and which is restrained at both ends viz elastically translational springs (kl, kr) and NON- elastically rotational springs (rl, rr), respectively. The members under uniform load q are the nonlinear spring behavior of m-θ diagram shown in Figure 4 is used. Also for linear elastic behavior of linear springs, lateral stiffness is considered in the frame. The model taken for the four samples bolt connection, under increasing temperature, until it exceeds the yield stress. And the tension created in the middle of beam and end restraint should be controlled. The middle deflection of the BEAM has been recorded at different temperatures. The aim of this work is to compare different conditions on the behavior of structures under fire. The modeling work of ANSYS software is used. provides significant constraints. Also the semi-rigid connections, steel beam rotational constraints are considered. Many researchers have indicated that the thermal expansions of warming are under compressive forces. In this study, a uniform load q = 17N/mm and length of beam L = 4000mm, the coefficient of relative stiffness of transition is intended to cover 4 floors. Figure 5 shows the temperature displacement of the beam. This graphs connection change. And other specificationsbeam are fixed. As can be seen with increasing levels of stiffness, displacement of the mid span the beam is low. According close to the stiffness of connection the axial force in beams under different types of connection does not change much. Of temperature higher than 420 degrees, the slope of displacement is steeply increased. And the displacement of mid span in beam by connecting harder (S5) lowers the displacement of the center span is in the other beams. Fig. 6: temperature displacement of the beam Fig.5: generig flexural member 6. Results and discussion 6.1. Effects of end restraint As can be seen in Table 4, in connection profile the heat can be withstood. Stiffness connection way is S3 <S13 <S9 <S5. According to that in a fire compartment, cooler columns, steel beam heating As shown in Figures 6 and 7. End restraint member is impressive on the critical temperature (the temperature structure loses its effectiveness) and the durability time of construction. As expected, with increasing stiffness of end restraint, the critical temperature structure and the durability time structural is higher. The critical temperature structure changes between degrees. Because in higher temperature, stiffness and strength of steel decreases. 6 P a g e

7 Fig.7: Critical temperature vs concentration type Fig.8: Durability time vs concentration type Fig. 9: temperature displacements on the beam Axial force and deformation heat generated in a long beam is more than one short member. And increasing the length member also increases axial force. In the early stages of the fire, caused by thermal expansion of the axial force in the affected members, But with increasing temperature, especially temperatures above 500 C due to a sharp reduction in steel mechanical properties (hardness and strength) of the member axial force decreases and displacement the mid span shows its greatest change Effects of slenderness ratio In this case, the uniform load q = 17 kg / cm, and the connection of S5 is considered. And length of the beams 3, 3.5, 4, 4.5.and the beam profile is ipe22.the slenderness ratio is defined the ratio of the length of the member on the cross-section radius of gyration about the strong axis. And the slenderness ratio value is 49.4, 43.9, 38.4, and As Figure 8 shows the slenderness ratio of the beam has a significant impact on beam displacement. And displacement mid span the beams increase with increasing slenderness ratio. According to Figure 8, at 500 displacement of the members have slenderness ratio 38.42, 43.9 and to shortest member of coefficient slenderness ratio values 32.93, 33% and 67%, and167% have increased respectively. Fig.: axial force on the beam According to Figure and 11, the critical temperature and the durability time of structures decreased with a reduced slenderness ratio. And the critical temperature is between 500 to 650 degrees Celsius. And structural performances are lost in this period. Due to the slow temperature rise in the standard fire temperature of 580 to transition from high temperature, the amount of durability time will greatly increase. As can be seen in Figure 11 7 P a g e

8 slenderness ratio 38/42 and 32/93 in the critical temperature of beam is 600 and 650. With increasing temperature of 580 degrees and Decreases SLOPE the standard fire curve of 580 degrees, increased durability time of the beam is high. times durability time). Which according to the low volume of connecting and high cost the shield can be used as a mechanism for rising the durability time. Fig.11: critical temperature Fig.13: The effect of temperature changes on the max deflection 7. Conclusion Fig.12: durability time 6.3. The effect of temperature changes in the connection In many studies, connection stiffness during the rise in temperature is considered constant. And temperature changes in the connection yet. And this is true if the connection is protected against fire. As can be seen in Figure, if the connection is protected against changes in temperature, critical temperature structure will increase dramatically. And as in Figure 1 and the standard fire was considered. Temperature of 580 degrees to the slope is gentler. In this example can be seen. If the protection connection, the durability time of the beam 289 seconds to 501 seconds increased (1.73 The purpose of this paper was determination of the behaviour of beams and their connections under fire loading. Despite in initial step of fire, thermal expansion effect on axial force is obvious on member deflection. Considering of this affection in design is useful, as can say with considering this affection can reach an ideal design, however one can say in most models failure in beam occurs before than failure in connection, but regard to beam-connection interaction cannot neglect effect of connection on beam s behaviour. References [1] Yin, Y.Z. and Wang, Y.C. A numerical study of large deflection behavior of restrained steel beams at elevated temperatures, Journal of Constructional Steel Research, 2004; 60: [2] Yin, Y.Z. and Wang, Y. C. Analysis of catenary action in steel beams using a simplified hand calculation method, Part 1: theory and validation for uniform temperature distribution, 8 P a g e

9 [3] ABAQUS, ABAQUS Theory Manual Version6.4, Hibbitt, Karlsson and Sorensen Inc, Paw-tucket, Rhode Island, USA, [4] Bradford, M.A. Elastic analysis of straight members at elevated temperature, Advances in Structural Engineering, 2006; 9 (5): [5] Bradford, M.A., Luu, K.T., Heidarpour, A. "Generic nonlinear modeling of a steel beam in a frame sub-assembly at elevated temperatures The International Colloquium on Stability and Ductility of Steel Structures (Edited by: D. Camotim, N. Silvestre, P.B. Dinis) IST Press, Lisbon, 2006; [6] M. A. Bradford., K. T. Luu. And A. Heidarpour., Numerical studies of a steel beam in a frame sub-assembly at elevated Temperatures. Construction and Maintenance of Structures - Hanoi, Vietnam, December [7] L. Simoes da Silva a, Aldina Santiago b, Paulo Vila Real A component model for the behavior of steel joints at elevated temperatures Journal of Constructional Steel Research 2001; 57: [8] Choe, L., Varma, A.H., Agarwal, A., Surovek, A. Fundamental behavior of steel beam-columns and columns under fire loading: Experimental evaluation, Journal of Structural Engineering 2011; 137(9): [9] European Committee for Standardization CEN. ENV , Euro code 3: Design of Steel Structures, Part 1.2: General Rules Structural fire design, Brussels, 1995 [] Saedi Daryan A, Yahyai M, Modeling of bolted angle connections in fire Fire Safety Journal, 2009; 44: P a g e