Advanced method for design of composite columns subjected to fire

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1 Advanced method for design of composite columns subjected to fire RAU ZAHARIA, DAN PINTEA Department of Steel Structures and Structural Mechanics The Politehnica University of Timisoara Ioan Curea, 1, Timisoara, ROMANIA Abstract: - One of the main problems of steel structures is their low resistance in fire situation. The advantage of composite steel-concrete solution, taking into account the increased strength in fire conditions, is that this solution may avoid, by a proper design, the use of expensive and lifetime-limited fire protection materials, thus leading to sustainable construction. The fire resistance of composite steel-concrete structures may be determined using simplified methods, based on analytical formulas or tables, provided in Eurocode4 Part1-2. For special situations or for complex structures it may be necessary to perform an advanced analysis, using special purpose programs for the analysis of structures under elevated temperature conditions. The paper presents the principles of fire design and gives examples of application of the advanced methods in fire design, for composite steel-concrete columns in multi-storey buildings. Key-Words: - advanced methods, fire design, composite steel-concrete 1 Introduction Since Sustainability is typical of large infrastructures and involves ecology, it is worth noting that on a world-wide scale, the binomial infrastructures + green energy and technologies accounts for little less than 50% of the head-lines cited by one of the major American Institutions in Civil Engineering (44% in December 2008) [1]. The materials of construction (steel, concrete) impact on the above mentioned field, as well as on most of the themes indicated by the Commission on Sustainable Development of the United Nations (2007) [1]. Steel and concrete may be combined to form very efficient structural solutions. Beside the structural performance of these solutions, it should be noted the simpler technology, as compared to the classical reinforced concrete structures. For example, in the case of composite concrete filled steel hollow sections, there is no need for shuttering, leading to very important savings in the cost of the structure. The use of composite steel-concrete building elements leads also to high levels of fire resistance without the need of fire protection, which is usually expensive and has a limited guaranteed lifetime. By avoiding the use of fire protection, important energy and cost-savings are made, and by a proper advanced fire design, sustainable unprotected composite steel-concrete structural solutions can be proposed. The basic principle in determining the fire resistance of a structural element is that the elevated temperatures produced by the fire reduce the materials strength and stiffness until possible collapse. When the temperatures on the crosssection of a structural element produce the reduction of the element resistance bellow the level of the effect of actions for fire design situation, it is considered that that element lost its load-bearing function under fire action. The fire resistance of steel or composite steelconcrete structures is calculated according to EN [2] or EN [3] respectively. Three methods are available in order to evaluate the fire resistance: - the tabulated data method, based on observations resulted from experimental study, easy to apply, but limited by the geometrical conditions imposed to the composite cross-section (only for composite structures); - the simple calculation method compute the ultimate load of the element by means of formulas or design charts, established on the basis of experimental data; - the advanced calculation method suppose an advanced numerical analysis of the elements or of the entire structure under fire, using specialized software for the mechanical analysis of structures under elevated temperatures. According to EN [2] and EN [3], the advanced calculation methods should include separate calculation models for the determination of: the development and distribution of the temperature within structural members (thermal response model) and of the mechanical behaviour of the structure or of any part of it (mechanical response model). ISSN: ISBN:

2 Advanced calculation models may be used in association with any heating curve, provided that the material properties are known for the relevant temperature range, and may be used with any type of cross-section. Advanced calculation methods for mechanical response should be based on the acknowledged principles and assumptions of the theory of structural mechanics, taking into account the changes of mechanical properties with temperature. The thermal response model for steelconcrete composite sections should consider: - relevant thermal actions from EN [4]; - variation of the thermal properties of the material with the temperature, from EN [2] and EN [3]. There are several fire models (thermal actions), accepted by the European Standard EN [4], which describe the thermal and mechanical actions to be considered for a structure under fire. The nominal standard temperature-time ISO model, considered for the design of the elements presented in this paper, is used for fire tests and it is generally accepted as being conservative for determining the fire resistance of the structural elements. The fire is considered an accidental situation, which requires, with some exceptions, only verifications against the ultimate limit state. The combinations of actions for accidental design situations are given in the European Standard for basis of structural design EN1990 [5] by one of the following formulas: G + P +Ψ Q + Ψ Q k k 1,1 k,1 2, i k, i i> 1 G + P + Ψ Q k k 2, i k, i i 1 (1-2) in which are the characteristic values of the permanent, variable and prestressing action. A structure, substructure or element in fire situation may be assessed in the time domain, where the failure time must be higher than the required fire resistance time. The failure time is the time for which the resistance of the structure (or substructure, or element, as considered) under elevated temperatures reach the effect of actions for the fire design situation, considering the combination of action in fire situation, as given in the corresponding Eurocode for the basis of structural design EN1990 [5]. A computer program using advanced calculation models according to the Eurocodes is the SAFIR program [6] which is a special purpose tool for the analysis of structures under ambient and elevated temperature conditions. The program, developed at the University of iege, accommodates various elements for different idealization, calculation procedures and various material models for incorporating stress - strain behavior under elevated temperatures and fulfils all conditions imposed by the Eurocodes for fire design to be considered an advanced calculation model, as mentioned above. The program was validated through comparison with experimental tests and by means of benchmarks, in comparison with other existing computer codes. The analysis of a structure exposed to fire consists of two steps. The first step involves predicting the temperature distribution inside the structural members, referred to as thermal analysis. The second part of the analysis, termed the structural analysis is carried out to determine the structural response due to static and thermal loading. 2 Advanced method for design of Concrete filled CHS columns in fire The first example presents the calculation of the fire resistance for the columns of a three-storey framed building structure for the INDAB-Romania Company Headquarters, in Bucharest. The office structure has three levels (ground floor, 1st floor, and an attic floor), two spans of 6m each, and 7 bays of 5m, with a total area of 1308 m 2. Taking into account the specific of INDAB Romania (systems of steel industrial buildings) the special architectural demand was that the resistant structure must be visible steel, made by circular columns. Because for this type of building, according to Romanian fire regulations, the columns must have 2 hours of fire resistance, the solution of reinforced concrete filled CHS columns was chosen, as shown in Fig. 1. Fig. 1 Concrete filled CHS cross-section The office building structural scheme is composed of interior moment-resisting frames and exterior eccentrically braced frames. The lateral force resisting system was considered to be composed of both moment-resisting and eccentrically braced frames, through the diaphragm effect of the r.c. slabs and roof braces. The steelwork is fabricated from European hot-rolled profiles, and partially from ISSN: ISBN:

3 built-up sections. The beams have I cross section, the eccentric braces are RHS sections and the horizontal braces are round bars. Reinforced concrete slabs are fabricated in classical solution, with secondary steel beams and corrugated sheet lost formwork. For this case, all three methods of design provided in the EN [3] were considered. As it will be shown in the following, the tabulated data method and the simple calculations models are not able to provide complete information about the resistance in fire capabilities of the concrete filled CHS columns, and therefore the advanced design method using the complex finite element program SAFIR was considered. 2.1 Tabulated data method For reinforced concrete filled CHS columns, Table 4.7 of EN [3] is suitable. The load level is For fire resistance class R120, corresponding to a load level of 0.3 (the lowest value provided in Table 4.7) the minimum cross-section diameter d, minimum axis distance of reinforcing bars us and minimum percentage of reinforcement p, are: d min = 260mm < d=355.5 mm us min = 50mm < us=62.5 mm p min = 6% > p=4.4% There is one condition that is not satisfied, the minimum percentage of reinforcement. Meanwhile, the other conditions are satisfied far away from limitations and the value of the load level represents less than 50% of the lowest one provided in the table. 2.2 Simple calculation model The simple calculation model was applied using the diagrams provided in the CIDECT Design Charts for fire resistance of concrete filled hollow section columns [7]. Using the design chart I15 from this reference, for fire resistance class R120, the ultimate load of a composite column, with the buckling length of 3.4m, made of CHS 355x5.6 profile and filled with C20 concrete with 4% percentage of reinforcement, is: N fi,θ, Rd daN > N equ = 61428daN Taking into account the superior characteristics of the column cross-section (8mm for CHS thickness, P=4.4%, C25/30 concrete) the fire resistance class R120 of the column is proved. 2.3 Advanced design method The verification according to the tabulated data method is not relevant for the case into consideration, because one condition is not fulfilled (percentage of reinforcement). On another hand, in the verification with the CIDECT Design Charts [7], even if the fire resistance R120 of the column is demonstrated, taking into account the superior characteristics of the studied column in comparison with the data available in the Design Charts, no information about the fire resistance of the column under the imposed load is obtained. Therefore, an advanced fire design method was considered Thermal analysis Fig. 2 presents the numerical model of the concrete filled CHS column cross-section. Due to obvious reasons of symmetry, only a quarter of the crosssection was represented. Fig. 3 shows the temperature distribution after 2 hours of standardized ISO fire. It may be observed that for the CHS profile, the temperatures are superior of 1000 o C, so the steel profile exhausted its loading capacity. In the same time, the temperatures of the reinforcing bars are around 500 o C and there is an important core of concrete with quite low temperatures. Fig. 2 Numerical model of the composite cross-section Fig. 3 Temperature distribution on cross-section after 2h of standardized ISO fire ISSN: ISBN:

4 2.3.2 Mechanical analysis under elevated temperatures The columns, considered as isolated elements, loaded with the axial force and the bending moments on both principal cross-section axes (efforts corresponding to the fire combination of actions), were modelled with 3D beam elements. The buckling length of the columns in fire situation may be different from the buckling length for the design at normal temperature. As shown in Fig. 4, if the column in a multi-storey braced frame is a continuous member that extends through several floors, then the buckling length of a column exposed to fire in an intermediate storey may be taken as l θ =0.5, and in the top storey as l θ =0.7 (where is the system length in the storey that is under fire). The reason for considering these reduced values, according to EN [3] is that the stiffness of the column in the fire compartment decrease as temperature increases, whereas the adjacent parts of the column that are located in the floors above or bellow remain at normal temperature and keep a constant stiffness. As a consequence, the adjacent parts become relatively stiffer and provide a significantly higher degree of restraint with respect to rotation. Two conditions must be fulfilled in order to consider the reduced buckling lengths in fire situation: each storey must form a separate fire compartment, and the fire resistance of the building elements which separate the storey under consideration must be at least equal to the fire resistance of the column. Taking into account that for the considered building the fire resistance of the floors is lower than the fire resistance of the columns, in the mechanical analysis under elevated temperatures the buckling length of the columns was considered conservatively as l θ =1.0, i.e. the system length. rigid core fire exposed column l θ Fig. 4 Buckling length of columns in multi-storey braced frames for fire design Equivalent imperfections according to EN [8] were imposed on both directions of the principal l θ cross-section axes. The horizontal displacements evolutions at the mid-height of the columns of ground floor, which have the highest load level, are plotted against time. As the characteristic timedisplacement demonstrate (Fig. 5) after 2 hours of ISO fire the column with the highest load level is still able to resist to the imposed static loads, due to the bearing capacity reserve provided by the concrete core and the reinforcing bars. The collapse of the column is produced after around 3h of ISO standardized fire. Displacement [m] Time [sec] Fig. 5 Time-displacement characteristic 3 Advanced method for design of partially concrete encased columns with crossed I section in fire The second example of application of the advanced method for fire design is the multi-storey Bucharest Tower Centre structure, the tallest building in Bucharest, Romania, at this moment. The building has 3 basements, one ground floor, 21 floors and 3 technical floors for a total height of 106.3m. The building is 25.5m by 41.5m in plan and has a total construction gross area of approximately 24135m 2. The columns are made by crossed I sections, made of hot rolled European profiles, partially encased in reinforced concrete, in order to increase strength, rigidity and fire resistance. According to Romanian fire regulations, considering the specific and particularities of the building, the columns must have 150 minutes of fire resistance. Figure 6 shows the cross sections types of the columns: octagonal sections with identical steel profiles 2HEB500, 2HEA800, 2HEB800, 2HE800x373 (a), octagonal sections with different steel profiles HEM800 HEM700, HEB800 HEB700, HEA800 HEA700 (b) double-symmetric rectangular sections HEB1000 HEB500 (c) and rectangular ISSN: ISBN:

5 sections with one axis of symmetry HEB1000 HEM500, HEB1000 HEB500 (d). The rebars have 25 mm diameter and the concrete is C30/37. For the purpose of this paper, only the 2HEB500, HEM800 HEM700, HEB1000 HEB500 and HEB1000 HEM500 cross-sections will be presented, as they have the lowest fire resistance under ISO fire from each set of cross section types, respectively. For this type of cross section, there are no specifications for calculating the fire resistance in the Eurocodes, excepting for the advanced method. a) 2HEB500 cross-section a) b) b) HEM800 HEM700 cross-section c) c) HEB1000 HEB500 cross-section d) Figure 6. Composite cross-sections 3.1 Thermal analysis Figure 7 a-d shows the temperature distribution on the cross sections of the considered columns, after 150 minutes of ISO fire. Due to symmetry, only a quarter or half of the crosssections was considered. The round reinforcing bars are represented by quadrilateral elements, with equivalent area. For all cross-sections, after 150 minutes of ISO fire, the steel profiles flanges exhausted practically their load capacity, having temperatures greater than 900 o C, while the profiles webs and the reinforcing bars have lower temperatures and there is an important core of concrete with quite low temperatures. Consequently, after 150 minutes of ISO fire, the sections have a reserve of load capacity. d) HEB1000 HEM500 cross-section Figure 7. Temperature distribution at 150 minutes of ISO fire. 3.2 Mechanical analysis under elevated temperatures For the reasons presented in section 2, in the numerical analysis under elevated temperatures, the buckling length of the columns was considered conservatively as l θ = 1.0, i.e. the system length. Equivalent imperfections according to EN [8] were imposed on both directions of the principal ISSN: ISBN:

6 cross-section axes. Similar analyses as for the concrete filled CHS isolated elements, presented in section 2, were performed. Excepting for the columns with rectangular cross-section with one axis of symmetry (d), all other columns of the ground floor does not resist to the 150 minutes of ISO fire under the imposed static loads. The fire resistance grows with each floor, as the load level in the columns decrease on the height of the building. Excepting the columns with octagonal cross section and identical profiles 2HEB500, all columns above the ground floor fulfil the 150 minutes requirement. For the 2HEB500 columns, the R150 fire resistance requirement is fulfilled only from the 11th floor forth. Table 1 gives the corresponding fire resistance times for all considered columns. Fire protection is needed all the columns on the ground floor, excepting for the columns with rectangular cross-section with one axis of symmetry, while the 2HEB500 columns need protection up to the 11th floor. These results are available for ISO standard fire. If a natural fire action is considered [9], taking into account all physical parameters (fire load density, surface of the fire compartment, openings, etc) all columns resist to the imposed fire resistance time. Table 1. Fire resistance times [min.] COUMN GROUND F OOR F OORS HEB HEM800HEM >150 HEB1000HEB >150 HEB1000HEM500 >150 >150 4 Conclusion The outstanding fire resistance obtained for the composite columns, together with their increased resistance and stiffness under usual conditions, as well as their cost-saving technology, confirms the efficiency of this structural solution. The advanced method for fire design using special purpose computer programs is a very efficient tool to calculate the fire resistance of the composite columns, considering their particular shape and the lack of information provided by other design methods. References: [1] Gambarova P. G., Migliacci A., Concrete structures, Studies and researches, Vol. 28, Politecnico di Milano, Starrylink Editrice, 2008 [2] EN : Design of steel structures - Part 1-2: General rules Structural fire design, European Committee for Standardization, Brussels, 2005 [3] EN : Design of composite steel and concrete structures - Part 1-2: General rules - Structural fire design, European Committee for Standardization, Brussels, 2005 [4] EN : Eurocode 1 - Actions on structures - Part 1-2: General actions - Actions on structures exposed to fire, 2005, European Committee for Standardization, Brussels [5] EN1990 Eurocode: Basis of Design, September 2004, European Comittee for Standardisation, Brussels [6] Franssen J. M., Safir A thermal/structural program modelling structures under fire, Engineering Journal, AISC, Vol. 42, No 3, , 2005 [7]. Twilt & all. Guide de dimensionnement Poteaux en profils creux soumis a l`incendie Edite par Comite Internaţional pour le Developpement et l`etude e la Construction Tubulaire CIDECT, Verlag TUV Rheinland GmbH, Koln, 1994 [8] EN : Eurocode 4 - Design of composite steel and concrete structures - Part 1-1: General rules and rules for buildings, European Committee for Standardization, Brussels, 2005 [9] Zaharia R., Pintea D., Dubina D., Fire analysis and design of a composite steel-concrete structure, Steel and composite structures, Taylor & Francis Group, ondon, ISBN , pp , 2007 ISSN: ISBN: