FIRE DESIGN OF BOLTED STEEL BEAM-TO-COLUMN JOINTS Aldina Santiago; Luís Simões da Silva ISISE - Department of Civil Engineering, University of Coimbra, Portugal [email protected]; [email protected] Paulo Vila Real LABEST - Department of Civil Engineering, University of Aveiro, Portugal [email protected] ABSTRACT In this paper, the main approaches used for developing a practical consistent methodology to predict the behaviour of bolted steel beam-to-column joints under a natural fire are presented and discussed. This methodology incorporates the influence of the transient temperature variation on the time-varying forces that act on the joint and gives design guidance on how to avoid the failure of the joint throughout the fire event (heating and cooling phase). Validation of the proposed model is carried out by comparison against the available results obtained from an experimental programme of steel sub-frames under a natural fire undertaken at the University of Coimbra, Portugal (Santiago et al., 2008). 1. INTRODUCTION Under a natural fire conditions, the behaviour of steel joints within a structure highly depends on the redistribution of internal forces with time as a result of the global behaviour of the structure. In this situation, the actual behaviour clearly deviates from the results of isolated joint tests, being subjected to a full 3D stress state (N, My, Mz, Mt, Vz and Vy), resulting from local, distortional or global instability of the connected members that could lead to the failure of the tensile components (such as bolts or end-plates). This paper gives a brief description of a consistent methodology to predict the behaviour of bolted steel beam-to-column joints under a natural fire. This methodology incorporates the influence of the transient temperature variation on the timevarying forces that act on the joint and gives design guidance on how to avoid the failure of the joint throughout the fire event. Validation of the proposed model is carried out by comparison against experimental results (Santiago et al., 2008). 2. BEHAVIOUR OF JOINTS IN FIRE Based on the studies previously described, it is confirmed that it was in the last fifteen years that the subject of steel joints under fire conditions suffered its main developments. Several experimental tests were performed in different typologies of joints and under different boundary and loading conditions, and analytical and numerical models were developed, which tried to reproduce adequately the behaviour of such tested joints. However, some of these experimental tests were concentrated on predicting the behaviour of isolated joints at high temperatures under monotonic bending loading, while other tests used this known bending-rotational behaviour as HEA 300 HEA 300 300 1129 boundary conditions, in order to study the behaviour of the heated connected beams. Despite the evident importance of modelling the behaviour of beam-tocolumn joints under a natural fire, as part of a frame structure, low experimental studies concerned with this matter have yet been published in the open literature. In a research projected developed at the University of Coimbra (Santiago et al., 2008; Santiago, 2008), some fire tests on a sub-frame beam-to column were carried out (Figure 1). The structural definition consisted of two thermally insulated HEA300 cross-section columns (S355) and an unprotected IPE300 cross-section beam (S355) with 5.7 m free span, supporting a steel-concrete composite slab. The mechanical loading applied at room temperature corresponded to the self-weight and the concentrated loads equal to 20 kN at 700 mm from the mid-span crosssection; the thermal loading corresponded to a heating-cooling curve applied to the beam and joints.The parametric study is focused on the beam-to-column joint configuration (Table 1). IPE 300 1210 5700 Z Y X Figure 1. Structural model (mm). Table 1. Beam-to-column joint configuration. Test ID FJ01 FJ02 FJ03 EJ01 HJ01 WJ01 Joint typology Flush end-plate Extended end-plate Header plate Welded End-plate dimensions (mm) and steel grade (320×200×10); S275 (320×200×16); S275 (320×200×16); S275 (385×200×16); S275 (260×150×8); S275 ------------- Bolts / Weld 2 bolt row M20, 8.8 2 bolt row M20, 10.9 2 bolt row M20, 8.8 3 bolt row M20, 8.8 4 bolt row M20, 8.8 af = aw = 10 mm 3. COMPONENT METHOD IN FIRE 3.1 Overview Over the past three decades, a considerable effort was undertaken to give consistent predictions of the steel joints at room temperature using the component method (Jaspart, 2002); however due to the large number of parameters that need to be taken into account when modelling the joint's response in fire, very little research work has been conducted in fire situation; exception should be mentioned to the work developed by the University of Coimbra (Simões da Silva et al., 2001), the University of Sheffield (Block et al., 2007) and the Imperial College London (Ramli Sulong et al., 2007). From the available methods(Simões da Silva et al., 2005), the component-based approach is also chosen in this work to model the connection behaviour because of its computational efficiency and capacity to provide a reasonable representation of the full range of response starting from the actual geometrical and mechanical properties. 3.2 Proposed component method Considering the evidences reached from the experimental tests and the numerical simulations (Santiago et al., 2008 and Santiago, 2008), some important aspects were identified as relevant for the formulation of any component methodology to analyse steel joints under fire: i) components characterization; ii) material properties dependency with temperature; iii) variable combination of bending moment and axial force; iv); non-conservation of linear cross-sections; v) loading-unloadingreloading that characterise the changing temperatures; vi) effective length of the components. The spring model chosen in this work corresponds to the one developed by Cerfontaine (Cerfontaine, 2004) to analyse joints under bending moment and axial force at room temperature. Figure 2 depicts the proposed model to a flush end-plate joint; the number and location of each component depends on the joint typology. For a bolted joint, the compression components (beam flange in compression and column web in compression) are located at the level of the beam flanges axis and the tension components (column web in tension, column flange in bending, end-plate in bending, bolts in tension and beam web in tension) are located at the level of the bolt rows axis. Additionally, the shear column components are located independently of the tension-compression system, as suggested by Cerfontaine. However an important difference should be highlighted; in the Cerfontaine model, the axial force and bending moment is monotonic increased and proportional throughout the analysis; but under a natural fire, the axial force changes from compression to tension and the bending moment from hogging to sagging. tension components (1st bolt row) co lum n we b in sh ea r compression components (beam top flange) tension components (2nd bolt row) compression components (beam bottom flange) Figure 2. Proposed model to a flush end plate joint. For the performed experimental tests, the columns were maintained at low temperatures, and its deformability, compared with the global deformability of the joint, was much reduced (Santiago, 2008). So, on the application of the proposed model, the column web components could be disregarded: column web in shear, compression and tension. The application of the proposed model is feasible when the component response is introduced as a force – displacement curve. Due to the reduced number of studies on the component characterization at high temperatures, a bilinear law was assumed in this study: the plastic resistance and initial stiffness at room temperature were calculated according the EN 1993-1-8-2005; once the component was loaded beyond its yield capacity, post-limit stiffness defined on literature was adopted (San- tiago, 2008). For each step, the degradation of the strength and stiffness of each component material with temperature was considered using the reduction factors proposed by EN 1993-1-2-2005, and the component temperatures corresponded to the experimental measurements (Figure 3). The effective length of each component remains constant throughout the analysis and corresponds to the value calculated by the EN 1993-1-8-2005 at room temperature. 7 50 6 00 b o t t o m flan g e (jo in t ) w e b (jo in t ) 900 t o p flan g e (jo in t ) b o t t o m flan g e (b e am ) 750 w e b (b e am ) 600 t o p flan g e (b e am ) temp. (ºC) b e am b e am b e am b e am b e am b e am tem p . (º C) 9 00 bolts end-plate 450 4 50 300 3 00 150 1 50 t im e (m in ) 0 0 20 40 60 80 1 00 1 20 14 0 1 60 time (min) 0 0 20 40 60 80 100 120 140 160 180 18 0 Figure 3. Temperature applied to the FJ03 model. The variable combination of bending moment and axial force, derived from the finite element models, was introduced in the spring model as axial forces at the level of each component (Santiago, 2008). To respond to the changing loading-unloading-reloading characteristic, a modified Masing rule has also been implemented into the model. Masing rule assumes that a material like steel unloads with a stiffness equal to the initial stiffness of the loading curve, and then follows a hysteresis curve meeting the mirror image of the point at which unloading started in the opposite quadrant. However, if the temperature changes between loading and unloading, this process becomes more complicated because the components response is temperature dependent. In this case, the assumption that the plastic strain is not affected by the temperature distribution should be employed (Franssen, 1990). The main underline of this assumption it that each force-displacement curves at different temperature unloads to the same plastic deformation, p (Figure 4). One of the main problems of this approach originates from the fact that the tensile and compressive forces in the connection do not share the same line of action. So, it was assumed that the compression springs are plastically deformed and unload until the end-plate loses contact with the column flange, the compression springs are deactivated and the tension springs start taking load from this deformed position. However, if the tension springs are deformed plastically and unload to initial position, it is assumed that all subsequent compression forces in the tension spring is taken by the compression spring row adjacent to the unloading tension spring row. Another problem inherent to a fire situation is the large deformations developed on the beam. After large deformations, the well known Bernoulli’s hypothesis, according to which plane cross-sections remain plane in the deformed state of the beam, is not valid, as observed in the experimental tests (Santiago et al., 2008). The component method presented in the EN 1993-1-8 assumes that the cross-section remains always plane, even at large deformations. Here, the same simplifying assumption was adopted. 0 Force, F F Ed , 0 A A' F Rd , 0 F Rd , 1 F Ed , 1 1 1 > 0) B Deformation , O p 0 O' Figure 4. Force-displacement paths for loading with increasing temperature. 3.3 Application to a bolted end-plate beam-to-column joints The connection element has been validated against the experimental results (Santiago et al., 2008). The active components were chosen according the variation of the axial stresses integrated in a beam cross section near the connection and the axial stresses of the bolts; Figure 5 illustrates it for the flush end-plate joint FJ03: The active beam components were divided in five periods: t < 12 min - compression in the lower zone and tension in the upper zone; 12 ≤ t < 27 min - compression in the lower and upper zones; 27 ≤ t < 90 min - compression in the lower and tension in the upper zone; t ≥ 90 min - tension in the lower and upper zones. 400 160 200 t (min.) 0 -200 -400 -600 0 Fx (kN) 200 Fx (kN) 600 25 50 75 100 125 150 175 compression in the upper zone tension in the upper zone tension in the lower zone compression in the lower zone 200 tension in the 2nd bolt-row 120 tension in the 1st bolt-row 80 40 t (min.) 0 0 25 50 75 100 125 150 175 200 Figure 5. Forces introduced in the component model (joint FJ03). Applying the axial forces and the bilinear force-displacement response of the active components at each temperature (Figure 6), the main quantities relevant for the FJ03 joint, for some representative times, are set out in Tables 2 and 3. For t < 27 min; no active component reached its capacity. Between 27 ≤ t < 90 min. the beam bottom flange exhibits a decrease of the compressive force and the upper connection zone changes from compression to tension. This change of forces is shown in the active components during this period: the components reached their highest temperature leading to a relevant decrease of their resistances and to the yielding of the beam bottom flange in compression and beam web in tension (top). F (kN) 900 end-plate in bending bolts in tension beam flange in compression beam web in tension column flange in bending 750 600 450 300 150 dx (mm) 0 0.0 0.2 0.4 0.6 0.8 1.0 Figure 6. Force-displacement response of each component at room temperature. Table 2. Proposed model applied to the bolted joint FJ03 (27 ≤ t < 90 min). beam bottom flange in compression 1st bolt-row in tension end-plate in bending (top) column flange in bending (top) Beam web in tension (top) t (min) 27.0 42.0 89.0 27.0 42.0 89.0 27.0 42.0 89.0 27.0 42.0 89.0 27.0 42.0 89.0 temp. (ºC) 700.0 849.5 686.5 206.4 378.7 414.5 291.1 518.2 502.5 95.4 302.7 404.9 636.0 790.9 572.6 FRd,t (kN) 188.4 69.8 215.0 370.4 318.5 294.8 336.2 243.2 259.7 390.7 390.7 390.7 250.8 79.1 362.9 FEd,t (kN) t (mm) 189.0 0.396 51.1 0.396 27.2 0.396 41.2 0.027 110.8 0.090 77.8 0.067 33.0 0.015 87.1 0.057 113.9 0.069 41.2 0.008 110.8 0.028 77.8 0.023 33.0 0.000 87.1 0.423 113.9 1.609 note yield elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic yield elastic From Figure 5 it is observed that between 90 ≤ t < 165 min, only the tension components are active. According the proposed methodology, there was one component that reached its plastic resistance: end-plate in bending (bottom) at t = 131 min, FRd,t = 336.2 kN and FEd,t = 358.2 kN, with a corresponding displacement of t = 0.93 mm (Table 3). The last active component that reached its maximum capacity was the 2nd bolt-row in tension at t = 165 min, FRd,t = 376.7 kN and FSd,t = 377.1 kN, with a corresponding displacement of t = 13.6 mm. It should be referred that on the experimental test, this component failed at t = 190 min. For each bolted end-plate beam-to-column joints, Table 4 summarizes the components that reached their plastic capacity. 4. RECOMMENDATIONS FOR DESIGN RULES In the previous section, a methodology for the evaluation of the response of steel joints under fire loading based on the component method was developed and applied. It was able to reproduce with sufficient accuracy the transient response of the steel joints throughout the fire development and to identify the failure modes of the joint. This procedure provides an adequate basis for incorporation in advanced Table 3. Proposed model applied to the bolted joint FJ03 (90 ≤ t < 165 min). t (min) temp. (ºC) FRd,t (kN) FEd,t (kN) t (mm) note 1st bolt-row in tension end-plate in bending (top) column flange in bending (top) Beam web in tension (top) 2nd bolt-row in tension end-plate in bending (bottom) column flange in bending (bottom) Beam web in tension (bottom) 90.0 131.0 165.0 90.0 131.0 165.0 90.0 131.0 165.0 90.0 131.0 165.0 90.0 131.0 165.0 90.0 131.0 165.0 90.0 131.0 165.0 90.0 131.0 165.0 389.1 256.5 159.3 467.1 300.9 192.9 379.2 241.3 161.2 515.8 292.3 153.2 389.1 256.5 159.3 455.0 314.1 196.6 379.2 241.3 161.2 515.8 292.3 153.2 313.2 364.0 376.7 286.6 336.2 336.2 390.7 390.7 390.7 478.0 653.7 653.7 313.2 364.0 376.7 276.7 336.2 336.2 390.7 390.7 390.7 653.7 653.7 653.7 86.4 183.8 275.6 118.9 186.5 234.9 86.4 183.8 275.6 118.9 186.5 234.9 35.9 299.2 377.1 0.0 358.2 497.9 35.9 299.2 377.1 0.0 358.2 497.9 0.072 0.128 0.173 0.067 0.083 0.093 0.024 0.043 0.059 1.609 1.609 1.609 0.030 0.209 13.60 0.000 0.930 5.105 0.010 0.070 0.081 0.000 0.000 0.000 elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic elastic yield elastic yield plastic elastic elastic elastic elastic elastic elastic Table 4. Sequence of yield or failure (bolts) of the components to each bolted endplate beam-to-column joints. acting Top – T forces Bottom - C -----FJ01 FJ02 ------ FJ03 ------ EJ01 ------ Top – C Bottom - C Heating t = 22 (beam bottom flange in compression) t = 22 (beam bottom flange in compression) Top – T Bottom - C t = 32 (end plate in bending - top) Top – T Bottom - T Cooling t = 99 (end plate in bending - bottom) t = 44 (beam web t = 123 (column flange in bending in tension - top) bottom) t = 141 (column flange in bending top) t = 170 (failure of the 2nd bolt-row in tension is imminent: FEd,t = 0.99 FRd,t) t = 27 (beam bot- t = 42 (beam web t = 131 (end-plate in bending - bottom flange in in tension - top) tom). compression) t = 165 (2nd bolt-row in tension) t = 34 (beam t = 110 (end-plate in bending – 3rd web in tension bolt-row) bottom) -----t = 141 (beam web in tension - 3rd bolt-row) t = 190 (3rd bolt-row in tension) calculation methods through the development of specialized joint finite elements. However, for conceptual and pre-design, the proposal of simple design recommendations is a desirable goal. Although the number of tests carried out in this research work is clearly insufficient to validate wide-ranging simplified rules, it is nevertheless enough to propose a framework and a methodology for future simplified rules. 600 105 80 55 350 t (min.) 100 -150 0 -400 -650 -900 M (kNm) 850 Fx (kN) Focussing on bolted end-plate beam-to-column joints, simplified design rules should take into account two distinct design points (on top of the fulfilment of the cold-design criteria): (i) design period A that corresponds to the critical period during the heating phase; and (ii) design period B that corresponds to the critical cooling time. Based on the times when the active components yield or fail (bolts in tension): period A is in the range 20 ≤ t ≤ 40 min and period B in the range t ≥ 100 min. Naturally, the choice of these two design periods depends on the fire scenario that must be considered as a relevant parameter in the simplified design recommendations. To propose these periods, only the fire scenario adopted in this research work was considered. The second step in the proposed simplified procedure consists on the evaluation of approximate levels of bending moment and axial force corresponding at the two design periods A and B (Figure 7). 25 50 75 100 125 FJ02 - beam FJ03 - beam EJ01 - beam FJ01 - beam 150 175 30 t (min.) 5 200 -20 0 25 50 75 100 125 -45 FJ03 - beam -70 EJ01 - beam FJ02 - beam -95 -120 150 175 200 FJ01 - beam Figure 7. Numerical curves of the axial force and bending moments on the joints during the fire. Finally, the tensile capacity of the main brittle component, which could lead to the structural failure, should be compared with the active forces. In this case, special reference will be made to the bolts in tension during the cooling phase: Ften ,t ,Ed F ten ,t ,Rd 0.9f ub As k b ,θ (1) where Ften,t,Ed is the tensile force in the bolt; Ften,t,Rd is the design tension resistance of a single bolt in fire; fub is the ultimate stress of the bolts; As is the tensile stress area of the bolt and kb, is the reduction factor for bolt resistance at temperature This comparison is made in Figure 8. The tensile bolt forces are represented by thick lines, the bolt resistances are drawn using dashed lines and the failure of the bolts is represented by a circle. This approximation allows the identification of the bolt failure. Of course, the tensile forces in the bolts were obtained performing an exhaustive numerical model. The identification of the degree of lateral and rotational restraint of the beam and the evaluation of approximate levels of bending moment and axial force at the two design periods A and B could be an alternative to obtain these tensile forces. As example, expressions proposed by Yin and Wang could be used to approximate these values (Yin and Wang, 2005). Although appropriate calculation and benchmarking would be mandatory. 500 Fx (kN) 400 300 200 FJ02 - 2nd bolt-row FJ03 - 2nd bolt-row EJ01 - 3rd bolt-row FJ01 - 2nd bolt-row t (min.) 100 0 0 25 50 75 100 125 150 175 200 Figure 8. Bolts in tension. Moreover, to avoid failure of the joint throughout the fire development, it was shown that a crucial factor is the ability of the connection to redistribute the applied internal forces. In particular, the deformability of the end-plate vis a vis the forces in the bolts plays a most relevant role. In EN 1993-1-8-2005 it is stated that a bolted end plate joint may be assumed to have sufficient rotation capacity for plastic analysis, provided that both of the following conditions are satisfied: (i) the moment resistance of the joint is governed by the resistance of either the column flange in bending or the end plate in bending and (ii) the thickness t of either the column flange or the end plate (not necessarily the same basic component as in (i)) satisfies: t 0.36 f ub fy (2) where is the bolt diameter, fu.b is the tensile strength of the bolt and fy is the yield strength of the relevant basic component. The application of this expression to the tested joints, results in the following bolt requirements (Table 5). It is observed that only the joint FJ01 meets the ductility criteria. Table 5. Ductility criteria (EN 1993-1-8-2005). fub (MPa) fyp (MPa) (mm) tp (mm) joint failure mode FJ01 FJ02 FJ03 EJ01 810 1076 810 810 275 275 275 275 M20 M20 M20 M20 10 16 16 16 end-plate deformation stripping-off of the threads of the bolts bolt required M20 √ M24 χ M24 χ M27 χ 5. CONCLUSIONS AND GENERAL RECOMMENDATIONS In this paper, a component method and a design verification to analyse beam-to-column joints under a fire were proposed and compared with experimental tests. Based on the results and considerations achieved during this research work, some design suggestions were proposed: i) The application of a thin end-plate demonstrated to be a good option to reduce the large bolt strain and consequently the bolt failure (FJ01). However, even no bolt failure was observed, large deformations on the end-plate were developed and bearing failure around the bolts could be happen. ii) Special attention should me made when it is intended to increase the joint resistance. The application of a bolt class with higher resistance reduces the bolt deformations and reveals to be a good choice to increase the joint resistance. Howev- er, a joint typology with a higher resistance at room temperature only increases the resistance to the hogging moment, but not to sagging moment that controls the cooling phase. Additional bolt rows in the lower zone of the connections should be considered in order to increase the joint resistance during the cooling phase. ACKNOWLEDGMENTS Financial support from the Portuguese Ministry of Science and Higher Education (Ministério da Ciência e Ensino Superior) under contract grant POCI/ECM/55783/2004 is gratefully acknowledged REFERENCES Block, F., Burgess, I. Davison, B. and Plank, R.J. (2007), “The development of a component-based connection element for end-plate connections in fire”. Fire Safety Journal, vol. 42(6-7) (pp. 498-506). Cerfontaine, F. (2004), “Etude de l’interaction entre moment de flexion et effort normal dans les assemblages boulonnés” (in french), Thèse de Docteur en Sciences Appliquées, Faculté des Sciences Appliquées, University of Liège, Belgium. EN 1993-1-2-2005.European Committee for Standardization – CEN. Eurocode 3: Design of Steel Structures. Part 1.2: General Rules – Structural Fire Design, Brussels. EN 1993-1-8-2005. European Committee for Standardization – CEN. Eurocode 3: Design of steel structures. Part 1.8: Design of joints, Brussels. Franssen, J-M (1990) “The unloading of building materials submitted to fire”. Fire Safety Journal, vol.16, (pp. 213-227). Jaspart, J.P. (2002) “Design of structural joints in building frames” Progress in Structural Engineering and Materials, vol. 4 (pp. 18-34). Ramli Sulong, N.H., Elghazouli, A.Y. and Izzuddin, B.A. (2007), “Behaviour and design of beam-to-column connections under fire conditions”. Fire Safety Journal, 42 (6-7), (pp.437-451). Santiago, A., Simões da Silva L., Vila Real P., Gameiro Lopes A. and Vaz G. (2008), “Experimental evaluation of the influence of connection typology on the behaviour of steel structures under fire”, Engineering Journal, AISC (submitted). Santiago, A. (2008), “Behaviour of beam-to-column steel joints under natural fire”. Doctoral Thesis, Department of Civil Engineering, University of Coimbra. Simões da Silva L., Santiago A. and Vila Real P. (2001), “A component model for the behaviour of steel joint at high temperatures”. Journal Constructional Steel Research, vol. 57(11) (pp. 1169-1195). Simões da Silva, L., Santiago, A., Vila Real, P. and Moore, D. (2005), ‘Behaviour of steel joints and fire loading’. International Journal of Steel and Composite Structures vol. 5(6) (pp. 485-513). Yin, Y.Z. and Wang, Y.C. (2005), “Analysis of catenary action in steel beams using a simplified hand calculation method, Part 1: theory and validation for uniform temperature distribution”. Journal of Constructional Steel Research, vol. 61 (pp. 183-211).