FLEXURAL STUDIES ON CONFINED STEEL COMPOSITE BEAMS SUBJECTED TO ELEVATED TEMPERATURES1JEEVAN REDDY, U., 2TENSING, D1School of Civil Engineering, Karunya University, Coimbatore, IndiaME, [email protected] of Civil Engineering, Karunya University, Coimbatore, IndiaProfessor, [email protected] experimental program was conducted to find the effect of temperature on thin-walled concrete-filled steel (TWCSCB) composite beams when subjected to four point bending load.
Moment curvature curves and load deformation graphs were plotted for 15 beam specimens and maximum load carrying capacity and deformation were studied and compared for specimens with different spacing of shear connectors at room temperature (270C), 2000C and 4000C respectively. The study showed that the use of shear connectors enhanced the load carrying capacity of TWCSCB beams. Closer spacing of shear connectors enhances the load carrying capacity of TWCSCB, but when specimen subject to higher temperature a significant loss of the strength properties are observed. It was observed that a specimen subjected to temperature in the order of 2000C, specimen was able to take up more load than the control specimen at room temperature, and this is largely due to evaporation of water content in the concrete. All specimens are air cooled before testing them.
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The study is mainly focused on experimental work. The study is made on post fire strength properties of the specimens. Keywords: Composite beam, Higher Temperatures, Flexural strength, Steel Confinement.1. IntroductionNow a days many industries are adopting the composite construction is an option to have greater stiffness of the member and smaller member size.
The industries are often prone to haphazard fire accidents. The fire load is not a frequent load in many cases however a careful adoption of the fire safety measures is paramount. Strength of the steel structures is very poor under higher temperatures, many steels may have different levels of fire load taking ability due to the manufacturing processes like quenching and tempering.
Every material will have a critical temperature, beyond that point the material cannot serve the intended purpose. Composite structures are mainly employs steel as the important material, steel need to be covered with some heat insulating material otherwise there will be some serious strength reduction. In case of composite beams steel members are stiffened by using concrete infill, if the outer skin of the load bearing member is steel then under elevated temperatures the steel member loses its strength greatly. To know how much amount of fire loading can be applied safely to the steel confined composite beams is required to be tested.
An attempt is being made in this experimental study to know the strength properties of confined composite beams at different temperatures. A survey of the available literature showed that very little research has been performed to investigate the behaviour of thin-walled concrete-filled steel (TWCSCB) composite beams when subjected to four point bending load. To ensure uniform heating all the specimen are heated in electric furnace then after specimens were air cooled and tested. 2. Research significanceA very little research has been focused on the behavior and flexural nature of the steel confined composite beams filled with concrete under higher temperatures.
A few studies are being done on the behaviour of concrete beams under flexural loading subjected to elevated temperatures and same has been done on structural steel but not on the steel confined concrete filed composite beams.Study was carried out by M. S. Khan 2. Fire safety measures to structural members are measured in terms of fire resistance which is the duration during which a structural member exhibits resistance with respect to structural integrity, stability, and temperature transmission.
Weight loss due to thermal loading has been studied since it has bearing on the strength. Loss in concrete weight takes place due to evaporation of the moisture from the body of concrete. Hence, higher the moisture, higher will be the loss in weight. From the graphs, the weight loss appears to linearly increase up to about 4 percent and becomes asymptotic thereafter. A maximum loss of about 5.95 percent has been observed in their study.Kristi L. Selden 1 conducted experiments on composite beam and development of a 2D fibre-based model that can be used to predict the moment–curvature–temperature (M–?–T) response of a composite beam cross-section.
The modelling tool can be used for a composite beam consisting of a steel beam with either a flat slab or profiled deck (oriented perpendicular or parallel to the beam). The model accounts for temperature dependent material properties and shear stud slip at the concrete-to-steel interface, and it can be used for full or partial composite beams. The developed approach was benchmarked with published experimental test data on composite beams with flat concrete slabs and profiled deckT.
Tamizhazhagan 4 researched on a simple computation procedure is developed to predict the general behaviour of composite beam with shear connector under bending. Different spacing of shear connector should be change cold form sheet considered. The experiments include four series of composite beams tested. The tests reported were used to ascertain the flexural strength of the beams and to validate the theoretical predictions. Companion specimens of concrete cylinders and cubes were tested for compressive strength and elastic modulus properties. The section was then exposed to bending, and the change in the behaviour was noticed.
The effect of a wide range of important parameters was studied on composite beams accompanied by bending.Metin Husem 3, conducted experiments on the compressive strength of High Performance Micro concrete cooled in air and water decreased up to 2000C. The compressive strength of HPMC was increased between 200 and 4000C. The compressive strength of Ordinary Micro Concrete was decreased continuously. The compressive strength gain was 13% for specimens cooling in air. For the specimens cooling in water the strength gain was 5%.
Venkatesh Kodur 5 conducted experiments on Thermal and mechanical properties of fibre reinforced high performance self-consolidating concrete at elevated temperatures. The research shows Steel fibres improve tensile strength of SCC up to 400 °C which can be beneficial to minimize fire induced spalling. Addition of polypropylene fibres reduces the tensile strength of SCC at higher temperature.
3. Experimental Programme3.1. Material properties3.1.1. Steel propertiesHot rolled steel sheet of thickness 1.
2 mm was used for the fabrication. Yield strength of steel was 250 N/mm2. Steel trough was fabricated by bending three sides into suitable dimension and welding the flat plate at the ends and middle portion of trough in case the beam is not providing with the shear connectors. If the beam is to provide with shear connectors we don’t need to provide any bracing plates at the top.3.1.
2. Shear connectorsIn TWCSCB beams, it is of great practical and economic interest to have mechanical shear connectors at the interface between the concrete core and the steel tube to achieve the composite action with the help of natural concrete bond. The strength of TWCSCB beams are significantly dependent on the behaviour of bond between concrete and steel trough.
6mm diameter bars with a width of 97.6 mm and height of 60mm is used as shear connectors. These shear connectors are welded to the steel sheets. As shown in figures. Fig.1 Shear Connector 3.
1.3. Concrete properties Concrete of M25 grade was used to fill the column. The mix design is done for M25 concrete as per IS: 456 – 2000 and IS 10262-2009. Three concrete cubes (150x150x150mm) and nine composite cubes were cast as companion specimens and tested to failure to determine the compressive strength.
Fig. 2 Composite cubes (a) without shear connector(b) Shear connector with 6D spacing(c) Shear connector with 9D spacing Fig.3 fabrication of beams Fig.
4 fresh concreted specimens3.1.4. Heating Specimens are placed in to the electric furnace which is available in our laboratory. Electric furnace is capable of producing 12000C with heating rate at 20C/min. the specimen are placed above the refractory brick to avoid contact with steel part of the test specimen to the electrical parts of the furnace. To reach 2000C the furnace will take about 100 minutes and to reach 4000C the furnace will take about 200 minutes. After heating the specimens are taken out and air cooled for 48 hours.
The cooled specimen are ready for the testing. Fig.5 heating of specimen at 4000C Fig.6 heating of specimen at 2000C Table 1 Description and Results of the tested cubes studieddescription numbers title Compressive strength Mean compressive strength temperature N/?mm?^2 N/?mm?^2 0CComposite cube without shear connectors 1 reference 46.18 46.72 27 2 reference 47.26 3 – 49.
78 50.86 200 4 – 51.96 5 – 35.56 37.34 400 6 – 39.11 Composite cube with shear connector at 6D spacing 7 reference 48.
89 48.91 27 8 reference 48.92 9 – 51.1 51.78 200 10 – 52.4 11 – 40.
89 41.78 400 12 – 42.67 Composite cube with shear connector at 9D spacing 13 reference 46.67 47.34 27 14 reference 48 15 – 47.
11 47.11 200 16 – 47.11 17 – 35.56 36.89 400 18 – 38.22 Fig.6 Cube compression strength comparison with Temperature Table 2 description and results of the tested cylinders studieddescription numbers Title Tensile strength temperature N/?mm?^2 0CComposite cylinders without shear connectors 1 Reference 9.3 27 2 – 7.
8 200 3 – 6.4 400Composite cylinders with shear connectors at 6D spacing 4 Reference 9.8 27 5 – 8.8 200 6 – 7.
6 400Composite cylinders with shear connectors at 6D spacing 7 Reference 9.5 27 8 – 8.1 200 9 – 6.
8 400 Fig.7 Split tensile strength comparison with TemperatureTable 3 weight loss of the beams after heatingBeam designation Spacing of shear connectors Description Temperature 0C weight in kg before heating weight in kg after heating % of weight loss1 No shear connectors Reference specimen 27 13.8 NA NA2 6D Reference specimen 27 14.2 NA NA3 9D Reference specimen 27 14 NA NA4 6D-1 200 14.2 13.1 7.
745 9D-1 200 14.1 12.7 9.926 No shear connectors-1 200 13.8 12.
6 8.697 6D-4 400 14.2 12.57 11.478 9D-4 400 14.1 12.2 13.479 No shear connectors-4 400 13.
8 12.1 12.3110 6D-2 200 14.2 13.
1 7.7411 9D-2 200 14.1 12.7 9.9212 No shear connectors-2 200 13.
8 12.6 8.6913 9D-3 400 14.1 12.2 13.4714 6D-3 400 14.2 12.
57 11.4715 No shear connectors-3 400 13.8 12.1 12.323.2.
Test specimenA series of 15 TWCSCB beam sections are filled with concrete were loaded to failure. Steel trough is made from hot rolled steel sheet of 1.2mm thickness. The cross section of the specimen is 500x100x100 mm. Concrete of M25 grade was used to fill the beam.
The parameters studied in this experimental investigation were specimens without shear connectors and specimens with varying spacing of shear connectors in four point bending loading under elevated temperatures. All other parameters such as beam size, beam height, shell thickness, connectors cross section, steel and concrete qualities were not changed. Beams were named as 1 through 15 in numbers the details of the same is depicted in Table 3. Beams 1, 6,9,12 and 15 were casted without providing shear connectors. Specimens 2, 4,7,10 and 14 and specimens 3, 5,8,11 and 13 were welded inside the steel section with shear connectors of 6 mm diameter bars having a height of 60 mm and width of 97.6 mm at a spacing of 6D and 9D respectively.
Table 3 designation and description of beamsBeam designation Spacing of shear connectors Description Temperature 0C Beam size in mm(lxbxd)1 No shear connectors Reference specimen 27 500x100x1002 6D Reference specimen 27 500x100x1003 9D Reference specimen 27 500x100x1004 6D-1 200 500x100x1005 9D-1 200 500x100x1006 No shear connectors-1 200 500x100x1007 6D-4 400 500x100x1008 9D-4 400 500x100x1009 No shear connectors-4 400 500x100x10010 6D-2 200 500x100x10011 9D-2 200 500x100x10012 No shear connectors-2 200 500x100x10013 9D-3 400 500x100x10014 6D-3 400 500x100x10015 No shear connectors-3 400 500x100x1003.3. Test setupAll the specimens were tested to failure using the 1000kN capacity UTM using four point load. Before the test was conducted, the specimen was centrally positioned in the rig to ensure that the applied compressive load was concentric and in line with the crossbeam as well. As shown in Fig.8. Two strain gauges were bonded at the middle portion of the beam specimens with one on top face and other is at bottom face of the composite beam to know compressive and tensile strains. An LVDT is employed to measure the central deflection of the composite beam.
All these were mated to the data acquisition system. Fig.8 specimen with strain gauge and LVDT attached3.4 Test procedureSpecimens were marked with support and centre lines before testing.
Specimens were tested under simply support conditions using LVDT for displacement measurement and strain gauges and instrumentation for strain measurement. Strain as well as central deflection of specimen under various loads was recorded during the test at a load increment of 5kN. The load was applied concentrically to the cross beam above test specimen which is in line with test specimen.
Specimens were tested to failure under flexural loading.4. Experimental results and discussion4.
1. Comparative study of load vs. deflection graph Fig 9. Load versus deflection plot for specimens without shear connectors The Fig9 has the details of load versus deformation of confined composite beams without shear connectors. Reference specimen not heated remaining specimens 6, 12 were heated to 2000C and 9, 15 were heated to 4000C. likewise in Fig 10 specimens 4,10 were heated to 2000C and 7,14 were heated to 4000C, in Fig 11 specimens 5,11 are heated to 2000C and 8, 13 were heated to 4000C.
The load carrying comparison of 6,12 with specimen 1 yielded 14.32% higher value than specimen 1 but when compared 9,15 with specimen 1 the load carrying capacity has been degraded very drastically to 27.49%. Load carrying comparison of 4, 10 with specimen 2 is yielded 15.54% higher value whereas 7, 14 specimens are given a degraded 40.9%. The load carrying comparison of 5, 11 yielded 13.2% higher value than specimen 3 whereas 8, 13 given a deprived value of 12.
56%. Fig 10. Load versus deflection plot for specimens with shear connectors spacing at 6D distance Fig 10.
Load versus deflection plot for specimens with shear connectors spacing at 9D distance 4.2. Comparative study of Moment vs. Curvature graph Fig 11.
Moment versus curvature plot for specimen without shear connectors Fig 12. Moment versus curvature plot for specimen with shear connectors spacing at 6D Fig 13. Moment versus curvature plot for specimen with shear connectors spacing at 9D4.3.
Comparative study of Flexural Rigidity vs. Moment graphThe flexural rigidity was computed as the ratio of applied bending moment to the curvature in the constant bending moment zone. Fig 14. Flexural rigidity versus moment plot for specimen without shear connectors Fig 15. Flexural rigidity versus moment plot for specimen with shear connectors spacing at 6DFlexural rigidity (EI) value was calculated as shown belowEI=Moment/CurvatureMoment(M)=Wl/6Where, W= applied load L= clear span of the beam Fig 16.
Flexural rigidity versus moment plot for specimen with shear connectors spacing at 9DThe load and deflection curves showing that the beams which were heated to 2000C are able to take-up more loads than the reference specimens. In the past many researchers noted that concrete specimens which are exposed to 1000C temperature are losing their weight significantly and by the time all those beams reached to 2000C they have been completely dehydrated and their strength properties were improved. This property is true in consultation with the results obtained. The effect of shear connector spacing also come in to play which has improved load taking ability of 6D spacing specimens and 9D spacing specimens in comparison with no shear connectors specimens. All the specimens are tested under unstressed condition such as while heating the specimen in side of the furnace we have not applied any pre load to the specimen. The moment versus curvature curves are showing that the specimens with 6D spacing are good enough to take-up higher moments before failing in comparison with specimens with 9D spacing and without shear connector cases. This property may be due to the closer spacing of shear connectors in 6D spacing beams. Among 6D spacing beams the beams which are heated to 2000C are able to take up higher moments.
Specimen-4 which is heated to 2000C is able to take up 7.33 KNm moment and produced 0.00007075rad/mm curvature and the same amount of moment and curvature is taken by specimen-10 which is also 6D spacing specimen and heated to 2000C. Specimen-4 and specimen-10 are reported to be highest moment taking specimens among all 15-specimens. Specimens 9, 15 reported to be beams without shear connectors are heated to 4000C produced moment carrying capacity of 2.53KNm and curvature of 0.
00006505rad/mm the values are reported to be lowest among 15-specimens. So it is evident that specimens which are provided with closer spaced shear connectors are able to take-up higher loads and higher moments even possess higher flexural rigidity. The flexural rigidity is a property being keeps on coming down when the specimen is loaded and heated. The flexural rigidity of all specimens are came down when we heated the specimen and applied loads. But the flexural rigidity of specimens 4, 10 are noted to be 2.
27×?10?^11 N-mm2. So it is evident that when a steel confined concrete filled composite beams were heated to 2000C produces higher flexural rigidity and higher moment carrying capacity, higher load carrying capacity. All the curves shows that the specimens heated to 4000C are produced lower values of flexural rigidity in comparison with reference specimens and also with specimens heated to 2000C. Many researchers were noted that the cooling of the specimens with air will produce good results in comparison with water cooling so we adopted air cooling for the entire test. Table 3 shows the weight loss of the steel confined concrete filled beams, it is being noted that specimens heated to 2000C are losing their weights in the order of 7% to 9% in comparison with specimens at room temperatures (270C) and specimens heated to 4000C were lost their weights in the order of 11% to 13% in comparison with specimens at room temperature. It is being noted that beam specimens 4, 10 were just lost 7% weight in comparison with specimen-2 whereas specimens 8, 13 were noted that 13% of weight loss in comparison with specimen-3.
Fig 6 shows the compressive strength of the steel confined concrete filled composite cubes of 6D spacing are heated to 2000C reported to be 51.78 N/?mm?^2 , composite cubes of 9D spacing are heated to 2000C reported to be 47.11 N/?mm?^2 and composite cubes of without shear connectors were heated to 2000C reported to be 50.86 N/?mm?^2 .among these cubes it is evident that cubes which are provided with shear connectors of spacing 6D is taking up higher strength. We have used M25 concrete to fill these composite cubes but the results obtained were double the strength of the M25 plain concrete cubes.
Composite cylinders are also taking up unusual split tensile strengths are reported in table 2 and fig 7. From the Fig 7 it is evident that split tensile strength of the composite cubes goes on decreased when the temperature is increased.5.
ConclusionIn the current experimental investigation 15 TWCSCB beams, 15 composite cubes and 9 composite cylinders were tested to failure. Beams are tested with four point flexure testing, cubes are tested under axial compression in compression testing machine and cylinders were tested for split tensile test in compression testing machine. The experimental results reveal that The research is being done on the mechanical properties of the specimens under elevated temperatures but the research has not been focussed on the time versus temperature properties of the tested specimens that many other researchers are reported in their research.
Instead of time versus temperature we were focussed on temperature effects on strength properties of the specimens. For transversely loaded beams under four point loading pure bending test on 15 TWCSCB beams with and without shear connectors and also under heating, no heating conditions the results reveal that specimens with 6D spacing of shear connectors are showing predominant properties in all tested parameters. The specimens heated to 2000C are reported to be showing higher strength properties in the case of composite beams, composite cubes but in case of composite cylinders the heating showed decrement in the tensile strength properties. In the cube compression test it is evident that the strength properties of composite cubes is almost doubled than the strength properties of M25 plain concrete cubes. Please note that all composite specimens are filled with M25 concrete and cured to 28 days and few of them were heated to 2000C and 4000C.
The steel confinement provided with and without shear connectors are improved the cube compression strength. Among the heated specimens the specimens heated to 2000C showed good compressive strength properties than specimens heated to 4000C. The presence of shear connectors enhanced the load carrying capacity and moment carrying capacity of composite beams under all temperatures.
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