Deflection and Tensile Stress Experiments on Ground Floor Slabs Load - Research Paper Example

May 19, Review of Deflection and Tensile Stress Experiments on Ground Floor Slabs Load Case at the Edge between 2002 and 2011 Introduction According to Abu et al (2007) tensile membrane action can have a significant influence on the way composite floor structures perform. Various design tools have therefore been developed in order to account for this. Abu et al (2007) paper looks at tests on model scale slabs and other issues and indicates that experience from both fire tests and actual building fires has shown that it may not be important to apply fire protection to steel beams that are exposed because composite floor slabs consists tensile membrane which acts in such a way that provides support unprotected steel beams when their strength and stiffness is lost with rising temperatures. The consideration of large deflections was found to be much less clear because there was a lack of understanding of the conditions that result in failure specifically in relation to concrete floor slabs. Experiments According to Foster et al (2004) large scale fire tests carried out in the multi-storey steel for building at Cardington suggests that concrete slab is highly resistant to fire. These tests indicate that the floor system does not collapse even when deformations are extreme. This resistance is attributed to the action of membrane forces which act in such a way as to maintain the integrity of the structure. Abu et al (2007) found that the structure has the ability to provide enough bending strength at low temperatures and so no enhancement from membrane action is necessary. However, as the temperature increases the flexural strength of the floor declines and can fail if sufficient membrane action is not mobilised. They concluded that tensile membrane action is of extreme importance to the real structure fire resistance of buildings. However, for this action to be effective the slab edges needs to be supported vertically. It is felt that the horizontal restraint is not really essential but can have a positive impact n performance. According to Zhang et al (2009) ArceloMittal and CTICM launched a project FRACOF in which a composite floor slab with dimensions of 8.725 x 6.990m was tested under ISO Standard fire for 120 minutes. The failure of a welding line at the joint of the reinforcement bars resulted in the slab failing the test. The fracture could they indicated could have been avoided by construction measures. Zhang et al (2009) indicates that in 2008 Guo Qiang Li and Na-si Zhang developed a new method in order to estimate the load capacity of composite floor slab under fire condition while taking into consideration membrane action. Zhang et al (2007) carried out a total of four (4) full scale slab tests. These tests were performed at the Tongji University in China under the sponsorship of the National Science Foundation of China. The tests consisted of measurements of the temperature of the furnace; slabs and reinforcement; the strains of the reinforcement concrete; and the deflection of slabs. Four (4) pieces of composite floor slabs of dimensions 5.232m x 3.72m with unprotected steel decks were used in the test. They were contributed with profiled steel sheet that was commonly used in China. The thickness of the deck was 1mm with strengths greater than 270N/mm2. The total depths of the slabs were 146mm and the concrete on op of the decks had thickness of 70mm. The reinforcing mesh of the slabs consisted of smooth reinforcement bars with gird size of 150mm x 150mm. The diameter of the reinforcement bar was 8mm and the grade of the steel Q235. The protective layer of reinforcement was he same thickness for all except one of the floor slab. The slabs, primary and secondary beams were all designed in accordance with two specific Chinese code one of which is dated 2003. Zhang et al (2009) also pointed out that in Cardington test, it had been found that the slabs were fractured at the boundary and so in order to make the tests similar the reinforcement was not anchored at the boundary but exceeded the edge by 150mm. Everything was the same for all slabs in terms of the size, arrangement of the reinforcement, the thickness of the top of decks and the total depth except that one of them had a thickness of 21mm while the others were 30mm thick. The direction of the rid was 30mm for three of the slabs and 21 mm for one of them. The direction of the rib was along the long edge for three of the slabs and along the short edge for just one of them. The short edge along with one of the long edge had no secondary beam while the other two had the middle of the long edge unprotected. In order to stimulate uniform load the slabs were loaded at 24 points with a load ration of 0.60~0.65 above their room temperature load-bearing capacity. The temperature time curve of the furnace that was used was in accordance with ISO834 standard fire tests. Among the measurements included in the tests were the displacement of the slabs; the temperature at both the bottom and the top of the slabs; the temperature and strain of the reinforcements in the slabs as well as the strain of concrete. The load was applied to the slabs in a total of ten steps. The result of the tests after the loads were applied at ambient temperature was only a slight deflection in the middle of the slabs. There were no other occurrences such as cracks or other factors indicating failure. The slabs ultimate bearing capacity of design values of 30.64 and 29.51KN/m2 and the greatest test loads of 18.38 and 17.71 with a load ratio of 60% developed cracks after 75 and 90 minutes respectively beside secondary beam. This resulted from the decrease in both the stiffness and strength of the slabs. Major cracks were also found at running along the long edge of the slabs. This was said to be a result of the large deflection in the middle area of the slabs. Some cracks also developed along the short edge of the slabs and also extended to the side face of the slabs. The deflection of the slabs was major. The change in the formation of the slabs was an indication that membrane action was developed during the test in order to support the applied load. Even though the defection of the secondary beam was great there was no failure of the beam. A major finding was that the profiled steel deck did not melt down and this helped the decks to maintain stability of the slabs after approximately 90 minutes under what is described as standard fire. There were cracks in two of the slabs which were due to membrane action. There were no collapses in any of the tests which indicate that membrane action took place in order to keep the floor system stable and to carry the loads that were applied to the slabs. Another finding was that the distance between the reinforcement and the bottom of the slab impacted greatly on temperature of the reinforcement. Zhang et al (2009) further pointed out that the data from the tests reveal that the reinforcement was under pressure throughout most do the test, with the exception of the beginning and indicates that this phenomenon is proof of the occurrence of tensile membrane action in the test. The concrete in the middle of the boundary was also found to be under compression and therefore validate the existence of concrete compression rings which can provide a basis for the reinforcement. However, data taken at the corner by strain gauges reveals that chat compressive strains were found at angles between 30 and 60 degrees at the corner which is in line with the phenomenon of failure. It was also revealed that deflection can arrive at 1/25 of the short edge of slabs and so one can state that the load bearing mechanism of the slabs was changed from a mechanism that bends to membrane action under large deflections in the tests. According to Mensinger and Stadler (2009), slabs which have been subjected to fast heating of the standard fire curve will have a high curvature and a relatively small uniform extension. This curvature changes in both directions of the slab and at unrestrained slabs the edges are will not move together. Deformations are prevented due to the existence of compression rings around the slab’s perimeter. Mensinger and Stradler (2009) also provides a formula which can be used to calculate the stress in the rebar which appears at the first bend of the curve when the tensile strength of the concrete is reached and the first cracks appear. The formula is: ?s,cr = ?ct((1/?) + (Es/Ect)) However, no explanation was given by Mensinger and Stadler (2009) of the meaning of the variables in the formula. No attempts were made to use the formula in the paper. Conclusion These tests show that concrete slabs once made to the highest standard are highly resistance to load and fire as a result of tensile membrane action. This action helps to maintain the integrity of the structure of concrete slabs even though there may be high levels of deformity. References Abul, A.K.., Burgess, I.W. and Plank, R.J. (2007) Analysis Of Tensile Membrane Action In Composite Slabs In Fire. Paper. Presented at: Pacific Structural Steel Conference 2007: Steel Structures in Natural Hazards Wairakei, New Zealand 13-16 March 2007. Retrieved from: http://ir.canterbury.ac.nz/bitstream/10092/4156/1/12620002_AKA_PSSC_07_1.pdf Foster, S.J. Burgess, I.W. and Plank, R.J. (2004) High Temperature Experiments on Model Scale Concrete Slabs at High Displacement. Paper presented at The Third International Workshop: Structure in Fire, Ottawa May 2004 Mensinger,M. and Stradler, M (2009). Concrete and Composite Slabs in Fire: Discussion of the Load Bearing Characteristics. . Paper presented at Application of Structured Fire Engineering, 19-20th Feb 2009, Prague, Czech Republic. Retrieved: http://eurofiredesign.fsv.cvut.cz/Proceedings/6th_session.pdf Zhang, N., Li, G., Lou, G., Jiang, S. and Hao, K. (2009). Experimental Study on Full Scale Composite Floor Slabs Under Fire Condition. Paper presented at Application of Structured Fire Engineering, 19-20th Feb 2009, Prague, Czech Republic. Retrieved: http://eurofiredesign.fsv.cvut.cz/Proceedings/6th_session.pdf Read More