Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges

Transcription

1 Fire Science and Technorogy Vol.23 No.3(2004) Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges Akio Kodaira 1, Hideo Fujinaka 2, Hirokazu Ohashi 3 and Toshihiko Nishimura 3,4 1 COE Engeneer, Center for Fire Science and Technology, Tokyo University of Science, Japan 2 Formerly Research and Development Institute, Takenaka Corporation, Japan 3 Research and Development Institute, Takenaka Corporation, Japan 4 Graduate School of Science and Technology, Tokyo University of Science, Japan ABSTRACT Fire resistance tests were performed for composite beams composed of rolled steel profile concreted between flanges. In the present paper, the fire resistant test results and numerical analysis results are described. As a result of the fire resistant tests, the relation between the fire resistance time and applied bending moment of the composite beam is clarified. Analytical results agreed with tests results and consequently, the analytical model can be used to predict the behavior of this composite beam in a fire. keywords : Fire resistance, Composite beam, Fire resistance test, Thermal conduction analysis, Thermal elasto-plastic analysis 1. INTRODUCTION Steel encased reinforced concrete and steel encased concrete structures are conventionally known as structural components combining steel and concrete. Compared to these structures, the composite beams of rolled steel profile concreted between flanges given in Figure 1 (a) (hereafter called SC composite beams) enable the omission of reinforcement and mould, and reduction in the weight. In addition, the improvement in stiffness is expected to prevent vibration or local and lateral buckling. SC composite beams are formed by filling concrete in the web of the rolled steel at the processing plant of rolled steels or the construction site, conducting assembly after the concrete hardens, and filled concrete in the junction with columns. They may be integrated with an reinforced concrete slab using stud bolts attached to the top flange as shear connectors. In these SC composite beams, since the bottom flange is exposed, the bending yield strength is decreased if heated during a fire. Therefore, composite beams with filled concrete reinforced with an axial reinforcement and strap (refer to Figure 1 (b) are adopted in Europe[1], and in Japan, too, the fire resistance of this type of composite beam has been considered[2]. However, this stiffening with reinforcements or others complicates the construction process. Load applied to the beams during a fire is gravity load. In Japan, where earthquakes

2 193 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA frequently occur, the cross section of the construction is usually determined considering the earthquake-resistant designs. Thus, the yield strength of the beam in a construction designed to be earthquake-resistant provides an margin more than sufficient against the stress caused by the gravity load[3]. Then, fire resistance may be ensured without reinforcing the filled concrete as shown in Figure 1 (b). Therefore, we studied the fire resistance by conducting a loaded fire resistance test mainly for SC composite beams (Figure 1 (a)) with simplified reinforcement of the filled concrete[4],[5],[6],[7]. (a) Example of the cross section of SC composite beam (b) Example of SC composite beam in Europe Figure 1 SC composite beam The SC composite beam has complicated distributions of temperature or stress in the cross section when heated by fire, and its behavior differs greatly between when positive bending (bending to cause a tensile stress on the bottom end of the beam) is applied to the middle of the beam and when negative bending is applied to the fixed end. This paper reports on the situation when positive bending is applied. 2. SPECIMEN 2.1 Test variable The size of the cross section of the specimen influences the distribution of temperature in the cross section to have an influence on the yield strength during a fire. It also largely influences the yield strength whether the Reinforced concrete slab is combined with the top end of the beam or not. Considering these points, we decided that 3 types of cross sections of the specimen from Series I through III were available as shown in Figure 2. Series I had 3 specimens that were provided with different reinforcing details under the same load in the fire resistance test. In Series II and III, the specimens were the same but under a different load in each series. Thus, the variables in this test were determined as the size, the presence of a junction with the floor slab, and the ratio of the applied bending moment against the bending strength of the specimens (hereafter called 'applied bending moment ratio').

3 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges Specimen In Series I, welded steel lattice (D6, #100) was attached at the position where the cover thickness of the concrete was 20 mm and fixed to the stud bolt (φ 9 mm) attached to the web of the rolled steel to prevent the concrete filled in the web from dropping. Series I has 3 specimens as shown in Figure 2 ; Specimen I-1 had no reinforcements, Specimen I-2 had 2 reinforcements (D25, SD345) at the bottom of the filled concrete, and Specimen I-3 had 2 pre-stressing strands (φ 17.8) at the similar position. In the filled concrete in Series II and III, welded steel lattice (D6, #100) was attached at the position where the cover thickness of the concrete was 40 mm to prevent dropping. For the bar arrangement in the slab, both top and bottom end bars were arranged in both directions at (SD295A), with a 30-mm thick cover of concrete. Series II had 2 specimens and III had 3 under different loads Figure 2 Cross section of specimen in each series. The result of the tensile test of the flange and web of rolled steel is given in Table 2. The IA test piece specified in JIS-Z-2001 was used for tensile test. The mechanical property of the reinforcement and pre-stressing strand used to reinforce the filled concrete in Series I are given in Table 3. The specifications of the welded steel lattice used to prevent the filled concrete from dropping, reinforcement and stud bolt for the slab are given in Table 4. The material characteristics of concrete for the fire resistance test are given in Table 5. Table 1 Size of specimen (unit: mm) H shape steel RC slab Section size Steel grade Thicknees Width H-450 * 200 * 9 * 14 SM490A No H-500 * 250 * 9 * 22 SM490A H-700 * 300 * 13 * 24 SM490A

4 195 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA Table 2 Mechanical characteristics of rolled steel Series Part Yield point (N/mm 2 ) Tensile strength (N/mm 2 ) Elongation (%) I II Flange Web Flange Web III* Flange *No test tensile strength of Web Table 3 Reinforcement and Pre-stressing strand used in Series I Constituents of material Material grade Yield point (N/mm 2 ) Tensile strength (N/mm 2 ) Elongation (%) Reinforecement (D25) SD Cable (φ 17.8) SWPR Table 4 Specifications of stud bolt and steel-frame Constituents of material Part Specification Stud For Web φ 9, L=80 (JIS B 1198) φ 16, L=80,100 (JIS B 1198) For slab φ 16, L=80,100 (JIS B 1198) Reinforcement For slab D10 (SD295A) (JIS G 3112) Weld wire net For Web φ 6#100 (JIS G 3551) D6#100 (JIS G 3551) Table 5 Material characteristics of concrete Series Part Age of material (days) Compression strength (N/mm 2 ) Percentage of water content (%) I For web II III For web For slab For web For slab

5 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges TEST METHOD 3.1 Loading and heating methods As shown in Figure 3, the specimen was set on the top of the fire-resistant test furnace for loading and heating, and heated with specified load applied from the top, following the standard heating curve specified in ISO-834 'Fire-resistance tests -- Elements of building construction.' Each specimen was simply supported and load was applied at the 2 points dividing the length into 3 equal parts. The length of the specimen was as described in Clause 2.2, and each distance between the supporting points was 3600 mm in Series I and 6600 mm in Series II and III. Loading was conducted with an oil hydraulic jack set on the top of the force-applying beam before starting heating, and constant load was maintained until the specimen reached its limit. The load value was set so that the ratio (M/ S M Y ) of the applied bending moment (M) of the middle of the beam against the yield bending moment ( S M Y ) of the rolled steel used in the specimen would be from 0.33 to The applied bending moment ratio of each specimen together with the test result is given in Table 6. Here, the yield bending moment was calculated based on the F value (=325 N/mm 2 ) of the steel materials. Figure 3 Heating and loading of specimen (example in Series III) 3.2 Measurement item and method The temperature measurement items were heating temperature, and temperatures of steel and inside concrete. In Series I, the temperature of the reinforcements and pre-stressing strands inserted into the concrete was measured. In Series II and III, temperatures inside the slab and on the unheated surface were measured. A 0.65 mmdiameter K thermocouple was used in the temperature measurement. Temperatures were measured at several locations along the length direction of the specimen. The measurement positions of temperature together with the result are given in Figures 4, 5 and 6.

6 197 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA The load was measured with a load cell set on the bottom of the oil hydraulic jack. In addition, the bending deformation and expansion deformations at loaded before heating and after starting heating were measured with an electric displacement gauge. 4. TEST RESULT 4.1 Result of temperature measurement The heating temperature in each specimen was in good agreement with the standard heating curve specified in ISO-834 'Fire-resistance tests -- Elements of building construction' within the range of the allowable tolerance. As typical examples of the temperature measurement result in each series, the temperature measurement results of the specimens, which were heated for the longest time in each series, are given in Figures 4, 5 and 6. Each figure provides the heating temperature, temperatures of the bottom flange, in the center of the web and of the top flange of the rolled steel, and in the middle of the filled concrete. In addition, the temperature of the reinforcement inserted into the inside of the concrete is given in Figure 4 (Series I-2), and the temperature on the unheated surface of the slab in Figure 5 (Series II-1) and 6 (Series III-1). These temperatures are the average value measured in 2 locations between the loading points including the center. In each series, the bottom flange temperature is sharply increased after starting heating. Though somewhat higher values are indicated during the early stages of heating in Series I with the thin plate thickness of the bottom flange compared to those in Series II and III, there is no significant difference among them and the temperature reaches C in 30 minutes after starting heating to exceed 700 C in 60 minutes. The temperature of the top flange in Series I indicates a remarkably higher value than that in Series II and III. This is because the top of the specimen is covered with an insulation material, which blocks the heat escape upward in Series I, while in Series II and III, the reinforced concrete slab is installed on the top of the specimens. Figure 4 Result of temperature measurement(series I-2)

7 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges 198 Figure 5 Result of temperature measurement(series II-1) Figure 6 Result of temperature measurement (Series III-1) The temperatures in the middle of the web and in the center of the filled concrete are lower than those of other parts, and there is a large difference between specimens. Those temperatures in Series I, II and III are given in Figure 7. The larger the beam width, the lower value the temperature indicates. This shows that the effect of the cross section size worked. 4.2 Result of deformation measurement Result of deformation measurement in Series I Figure 8 shows the secular change in the relative displacement between the middle and the supporting point of the specimen as a deformation of Series I. The deformation in every specimen was increased downward linearly until 20 minutes after starting heating, and from then, the deformation velocity was decreased, and again was increased as heating was continued. In Specimens I-2 and 3 in which filled concrete was reinforced with a reinforcement or pre-stressing strand, the deformation was barely increased until 20 to 60 minutes, so that the amount of deformation was smaller compared to Specimen I-1 in which filled concrete was not reinforced.

8 199 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA Figure 7 Comparison of result of temperature measurement ISO-834 'Fire-resistance tests -- Elements of building construction' defines the limit of the structural stability of the load bearing member to which bending moment is applied as the time when either the bending deflection (δa) or bending deflection velocity (δa/dt) reaches the following value: Where δa=l 2 /(400 d) [mm] (1) δa/dt=l 2 /(9000 d) [mm/min] (2) L : Distance between supporting point (mm) d : Depth of member to be bent (mm) Though the length of specimens to be heated is shorter than the distance between the supporting points as described previously, its effect is neglected here since the bending moment near the supporting points is so small that it is not affected by heating. Figure 8 shows the bending deflection equivalent of Equation (1) and the bending deflection velocity (the slope of time - deflection relation) equivalent of Equation (2). In every specimen, the bending deflection had reached the limit before the bending deflection velocity did. At this time, the specimen was able to support the specified load without being damaged. In addition, in Specimens I-2 and 3 with reinforced filled concrete, the deformation amount reached the limit 14 to 23 minutes later than in Specimen I-1 without the reinforcement in filled concrete; this indicates that the reinforcement is effective, however slight Result of deformation measurement in Series II and III The result of the deformation measurement of the specimens in Series II and III is given in Figure 9. In every specimen, the deformation was increased rapidly more than 30 minutes after starting heating, and the specimen under a small working load had the same behavior as in Series I whereby the deformation velocity was increased again as heating was continued after being decreased once. The deformation amount and deformation velocity each equivalent to Equation (1) and (2) is given in the figure. Though these values are different due to the difference in the beam depth in Series II and III, in every specimen, the deformation amount reached the limit in a state whereby the specimen supported the working load similarly to that in Series I.

9 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges Fire resistance time of each specimen As described above, in each specimen, the deformation amount reached the limit before damage was caused, which determined the fire resistance time. The applied bending moment ratio (M/ S M Y ) and the fire resistance time of each specimen are given in Figure 6 ; though the fire resistance time changes depending on the applied bending moment ratio, if the applied bending moment ratio is the same, it differs significantly depending on the size of the cross section of the specimen. Figure 8 Result of deformation measurement in Series I Figure 9 Result of deformation measurement in Series II and III Table 6 Applied bending moment ratio and fire resistance time Series No. of Specimen M/ S M Y Fire resistance time (min.) I I I I II II II III III III III

10 201 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA 5. ANALYTICAL STUDY Since Series I is a special case where there is no floor on the top of the beam, we studied the behavior during a fire by analyzing the temperature and the thermal deformation for Series II and III. 5.1 Temperature analysis Since the distribution of temperature in the cross section is consistent in the direction of the long axis, we studied its secular change by conducting a thermal conduction analysis using a two-dimensional differential method[8]. We divided the beam of the cross section of the specimen into 22 portions in the width direction and into 44 portions between the top and bottom of the beam as analytical models, and used a rectangular cell formed by dividing the slab into 52 portions in the width direction of the floor and into 10 portions in the thickness direction of the slab. The heating temperature was in accordance with the standard heating curve specified in ISO-834 'Fire-resistance tests -- Elements of building construction,' and we set the convective heat transfer coefficient at 25 (W/m 2 K) as a boundary condition, and the emissivity as follows: Flame: 1.0, Steel material:0.62, Concrete:0.7 In addition, it was assumed that free water contained in the concrete would remove evaporative latent heat when the temperature in each divided cell reached 100 C to maintain the cell temperature at 100 C until the water was lost from each cell. In principle, the values indicated in Eurocode 4 (1994)[9] were used as the thermal constants of the steel material and concrete used in the analysis, but for the heat conductivity of concrete, we used the value given by multiplying it by 0.85 since the Eurocode values tend to be somewhat higher than the test result. Analysis and test results in each series are given in Figure 10 and 11. The results of the tests in Specimen II-1 and III-1 in which the heating time is the longest in each series are also given. The figures indicate the top and bottom flange temperatures, the temperature in the middle of the web, and the temperature in the center of the concrete filled in the web. Heating was stopped after the specimen reached the limit in the actual test, but to conduct a thermal deformation analysis, it was assumed that heating should be continued along the standard heating curve in analysis. Thus, the test and the analysis results diverged at 150 minutes and beyond in Specimen II-1, and in 242 minutes and beyond in Specimen III-1. The analytical value of the temperature in the center of the concrete indicates a behavior that differs from the test value around 100 C. This is because the analytical technique does not deal with the detailed behaviors of free water or water vapor, but considers this will not have a large effect on the behavior of composite beams since it is within a low temperature range. In other portions, the results are in broad agreement though the analysis result tends to be somewhat higher than the test result

11 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges 202 Figure 10 Result of temperature analysis in Series II(Specimen II-1) Figure 11 Result of temperature analysis(specimen III-1) 5.2 ermal deformation analysis For the thermal deformation analysis, we used the technique of three-dimensional thermal elasto-plastic analysis[8] considering material nonlinear. For modeling, the specimen was divided into 6 portions in the axis direction to form a small member whose cross section was divided in the same way as in the thermal analysis. For the relation between the thermal expansion coefficient of steel materials and concrete and stress-strain, the relation equation given in Eurocode 4[9] was used. For the temperature progression of each cell in the cross section of the members, the temperature - time relation calculated in Clause 5.1 was used. Figure 12 indicates the results of the analysis and the test in Series II, and Figure 13 shows those of Specimen No.1 and 3 in Series III. Though the deformation amount in the analysis result tends to be larger than others in Series II as does the deformation amount of the test result in Series III, the results of the test and the analysis are in broad agreement. The mark δa is the limit value of the bending deflection given by Equation (1) and in the analysis as well, damage is caused after this value is exceeded. Figure 14 shows the distribution of temperature and the intensity of stress in the cross section of the middle of Specimen III-3. In Figure 14, the specific time from the point

12 203 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA when load is applied before starting heating (0 minute after starting heating) to 180 minutes after starting heating is covered; the upper column indicates the distribution of temperature on the web axis and the lower column shows the distribution of the intensity of stress on the web axis. The distribution of the intensity of stress under load before starting heating is linear and the neutral axis is caused at the position of 451 mm from the beam bottom end due to the effect of the filled concrete and the slab. The intensity of stress for the bottom flange is 92.5 N/mm 2, and that for the top flange is N/mm 2. The intensity of stress at the top end of the concrete slab is N/mm 2. In 15 minutes after starting heating, the temperature of the bottom flange of rolled steel was increased to reach 214 C. But the distribution of temperature in the web is nonlinear, suddenly decreasing upward from the position of the bottom flange. Thus, the thermal expansion of the bottom flange caused the curvature to increase suddenly and the nonlinear distribution of temperature caused a large tensile stress in the web, causing yield up to and around the center in the height direction. In 30 minutes after starting heating, the bottom flange temperature reached 435 C and the nonlinear distribution of temperature in the web got severer. Thus, the yielding area reached around 80% of the depth of the rolled steel and the curvature was around 2 times as large as that in 15 minutes. Since the bottom flange temperature approached 700 C and the strength of steel materials decreased significantly in 60 minutes after starting heating, the generated intensity of stress decreased suddenly. The increase in curvature from minutes after starting heating is smaller than that from minutes, with a curvature not exceeding 1.5 times that at 30 minutes. This change is shown as the change in deformation velocity in Figure 13. Figure 12 Result of deformation analysis in Series II

13 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges 204 Figure 13 Result of deformation analysis in Series III Figure 14 Distribution of temperature and stress on the web axis (center of specimen III-3) By 120 minutes after starting heating, the bottom flange temperature exceeds 900 C and the range of high temperature is spread in the web, as well. In addition, the temperature of and around the top flange is increased. As a result, the intensity of stress is decreased at the bottom of the web and a large compression intensity of stress is caused at the top flange. In Specimen III-3, the deformation amount reached the limit in 141 minutes but no damage was caused. In analysis, too, no damage was caused. In 180 minutes, the top flange temperature exceeded 470 C, the strength was decreased, and the generated intensity of stress was lower than that at 120 minutes. In addition, the range in which the stress was decreased at the bottom end of the web was spread. And then, at 240 minutes, the deformation was rapidly increased causing damage in the analysis. Intensity of the compression stress was caused in the slab, as well, though it is not shown in Figure 14. The intensity of stress under load was -7.6 N/mm 2 as described before, and it was increased after starting heating, leading to N/mm 2. After that, the intensity of stress was decreased to reach -8.9 N/mm 2 in 120 minutes, and then increased again to reach as high as N/mm 2. However, the compressive stress was 32.1 N/mm 2 as shown in Clause 2.2, and no damage was caused.

14 205 A.KODAIRA, H.FUJINAKA, H.OHASHI and T.NISHIMURA 6. SUMMARY We studied the behavior of composite beams composed of a rolled steel profile concreted between flanges during a fire by conducting a loaded fire resistance test with different cross sections and load ratios, temperature analysis, and thermal elasto-plastic analysis. The results were as follows:. 1) In SC composite beams which are simply supported and to which positive bending moment is applied, deformation is caused downward in the early period of fire, and then the deformation velocity is decreased once but increased again as heating is continued, leading to the limit of the fire resistance. The more remarkable this tendency, the smaller the applied bending moment ratio. 2) The temperature distribution in the cross section of composite beams during a fire is remarkably nonlinear, which has a large effect on the deformation property or the distribution of stress during a fire. 3) The temperature of the bottom flange of the rolled steel composed of SC composite beams is high in the early period of heating, but it is possible to ensure fire resistance performance without reinforcing the filled concrete if the applied bending moment ratio is equal to or lower than a certain value. 4) During a fire, the deformation ability of SC composite beams that are bent positively is large, and the fire resistance time is determined due to the limit value of the deformation amount defined in ISO-834 'Fire-resistance tests -- Elements of building construction.' In addition, damage is not caused at this point, but is caused as heating is continued. 5) SC composite beams with an normal-sized cross section to be used as cross beams have a fire resistance of 140 minutes when the ratio of yield bending moment in rolled steel applied bending moment in rolled steel against is 0.5, and 240 minutes when it is The fire resistance time is affected by the size of the cross section, whether SC composite beams are connected to the reinforced concrete floor or not, as well as the applied bending moment ratio. 6) The results of the fire resistance test, temperature analysis, and thermal deformation analysis are in good agreement to establish that the analysis is appropriate. Thus estimation of the fire resistance time of SC composite beams is possible by analysis.

15 Fire Resistance of Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges 206 REFERENCES 1. Fire Engineering Design for Steel Structures, International Iron and Steel Institute, Brussels, Haruhito OKAMOTO, Shigeki ITO, Kazuchika KONNO, Masatomo YOSHIDA, Hiromi SHIMOKAWA, Iwao KURAHASHI: A Research on Fire resistance of Steel and Concrete Composite Framework, Part 1: Architectural Institute of Japan, Summary of Lecture, Aug Takao YUTANI et al.: Stress of Structure under long- term Designed Load (Part 2 Beams), Architectural Institute of Japan, Summary of Lecture, Sep Hirokazu OHASHI, Akio KODAIRA, Hideo FUJINAKA, Koji IIZUKA : Fire Resistance Test for Composite Beams, Architectural Institute of Japan, Summary of Lecture, Oct Akio KODAIRA, Hideo FUJINAKA : Fire Resistance Test for Composite Beams, Architectural Institute of Japan, Summary of Lecture, Oct Toshihiko NISHIMURA, Akio KODAIRA, Hideo FUJINAKA, Hirokazu OHASHI: Fire resistance of the SC composite beams composed of rolled steel profile concreted between flanges, Architectural Institute of Japan, Summary of Lecture, Sep Hirokazu OHASHI, Akio KODAIRA, Hideo FUJINAKA, Yoshio TANNO, Nobuo NAKAYAMA: Fire Resistance of the Composite Beams Composed of Rolled Steel Profile Concreted Between Flanges - Middle-sized Cross Section -, Architectural Institute of Japan, Summary of Lecture, Aug Hirokazu. OHASHI, Akio. KODAIRA:Calculation of Load and Deformation Behavior of Structure Elements Taking into Account 3 - Dimensional Heat Flow, Fire Safety Science - Proceeding of the Fifth International Symposium, March EUROCODE 4, Design of Composite Steel and Concrete Structure, Part1.2 Structural Fire Design, CEN/TC250/SC4, 1994