Han, Tan, and Song 2014

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This paper develops and tests steel reinforced concrete (SRC) columns under fire using a three-dimensional finite-element analysis (FEA), followed by tests performed on four SRC columns that follow the ISO-834 fire standards. The analytical study is used to design a model of a full-scale SRC column. A simplified calculation can then be used to predict the fire resistance of the SRC column. H-shaped and cross-shaped steel are used because the peripheral concrete protects the steel under fire due to concrete’s conduction coefficient, thus SRC structures have a much higher fire resistance to other structures.

Analytical Study

Using the ABAQUS software, a model including temperature field prediction as well as structural analysis is designed to aid in the study of fire resistance of SRC columns. For the temperature field prediction, 8-node brick elements were used for concrete, 4-node quadrilateral shell elements were used for shaped steel, and the 2-node link elements used for steel reinforcement. Spalling in the concrete can occur at the early, intermediate, or later fire stages, and with high strength concrete (higher than 60 MPa), explosive spalling can occur when the temperature of the concrete cover layer is greater than 350oC. In terms of structural analysis, mechanical properties for steel must be studied. The total strain for steel is separated into three parts: instantaneous mechanical strain caused by external stress, thermal strain caused by increase in temperature, and creep strain caused by dislocations. For concrete, there is one extra method that must be considered; transient strain due to differences in chemical composition. Slippage between steel and concrete must be acknowledged, and was simulated using two-node springs. The results from the FEA modeling was then compared to the ISO-834 standard fire test results.

Experimental Study, Results, and Discussion

H-shaped and cross-shaped steel was fabricated out of mild steel sheets, and thermocouples were mounted half way up the column to measure temperature of both the steel and concrete. For the steel section, tensile tests displayed average yield strength was 307 MPa, modulus of elasticity was 205,100 N/mm2, and for the steel bar, 383 MPa, and 201,700 N/mm2 respectively. The concrete was mixed with a ratio of cement, fine aggregate, and coarse aggregate if 1:2.2:2.48, and water cement ratio of .49. The tests were completed in Tianjin, China and a hydraulic jack with 5 MN capacity applied the vertical load to the column, while a furnace exposes the columns to fire. ISO-834-1 (1999) fire standards describe failure if the column contracts axially by .01L at a rate of .003L/min; with L being the length of the column in mm. In this test, all of the specimens failed because the concrete was crushed and flexural buckling occurred. The steel reinforcement temperature rose slowly compared to the furnace temperature, because of the layer of concrete protecting the steel, however the steel was heated to 740 oC, displaying significant strength loss. Structural responses were also recorded. Spalling of concrete occurred asymmetrically during the intermediate and later stages, thus speeding up the failure of SRC columns due to fire. Spalling occurred at the compressive zone where there is high compressive plastic strain, whereas diagonal and parallel cracks occurs in the tension zone, where there is high plastic tensile strain. The failure of the SRC columns can be described in three stages: ambient loading stage, softening stage, and accelerated failure stage. As the fire duration time increases, the bearing capacity of the column decreases, and when this decreased bearing capacity is equal to the applied load, the column has reached its ultimate limit state. The time to get to the ultimate limit is known as fire resistance (tR). Spalling of concrete significantly reduces the fire resistance.

References

Han, L., Tan, Q., and Song, T. (2014). “Fire Performance of Steel Reinforced Concrete Columns.” Journal of Structural Engineering, 141(4), July. doi:10.1061/(ASCE)ST.1943-541X.0001081