Experimental Studies on the Effect of Properties and Micro-Structure on the Creep of Concrete-Filled Steel Tubes

To study different lateral restraints, different constituents of expansion agents, the influence of different steel ratios, and concrete creep properties, we carried out experiments with lateral restraint and without lateral restraint conditions separately on 12 specimens with the expansion agent content accounting for 4%, 8%, and 12% respectively. In addition, the creep tests were performed on specimens with different steel ratios of 0.0%, 3.8%, 6.6%, and 9.2%. The test results show that the lateral restraint improves the strength of the system (concrete-filled steel tubes) which resists further load after the concrete ultimate strength is surpassed and reduces the creep. The creep degree of the concrete-filled steel tube with lateral restraint is about 0.09–0.30 times smaller than that of the tube without lateral restraints. The creep degree of the concrete-filled steel tube increases as the steel ratio decreases. Creep tests with different amounts of expansion agent indicate that the creep degree of the concrete structure increases as expansion agent content decreases. To study the internal mechanism of the creep of concrete-filled steel tubes with different lateral restraints and different expansion agent concentrations, a microscopic pore structure test on the steel core concrete was conducted using the RapidAir457 pore structure instrument. Microscopic studies show that the air content and the length of the bubble chord of the laterally restrained core concrete are lower than those without lateral restraint core concrete. The amount of air content and the length of the bubble chord of core concrete specimens increase as the expansion agent content in the core concrete specimens decreases from 12% to 4%. Under the same external loading conditions, as steel ratio increases, the lateral restraint causes a further reduction of creep. The results of this study suggest that the creep of concrete can be reduced by selecting appropriate lateral restraint conditions and an optimal amount of expansion agent in the mix design of concrete for concrete-filled steel tubes.

Experimental and analytical evaluation of disproportionate collapse flat-plate buildings

Reinforced concrete flat plate buildings without continuous integrity reinforcement may be vulnerable to disproportionate collapse if a supporting structural member was lost in an abnormal event. This research forces on the evaluation of potential of disproportionate collapse in older flat-plate structures subjected to the loss of a supporting column in extreme loading events. If a supporting column fails, then the load was carried by that column must be redistributed to the surrounding slab-column connections, which in turn may results in a disproportionate collapse over an entire building or a large portion of it. This progression can occur if the punching shear strength of the surrounding connections is not sufficient. In order to make the most accurate determination of the potential for disproportionate collapse of flat plate structures, this research seeks to accurately evaluation the punching shear capacity of slab-column connections using the conditions present in a potential collapses event. The in-plane lateral restraint provided by the floor slab can enhance the punching shear strength of surrounding slab-column connections and may be significant. In addition, the post-punching capacity of the original failed slab-column connection may reduce the amount of load to be redistributed to the surrounding connections. In order to investigate the effects of lateral restraint and post punching capacity, six restrained and unrestrained static tests was conducted at 1% and 0.64% reinforcement ratios. The static tests showed that the punching shear capacity can be increased 2-8% as lateral restraint stiffness varies from 17 to 75.6kN/mm but the increase is highly related to the in-plane lateral restraint stiffness. The tests also indicated that the slab without integrity reinforcement can develop 54% of maximum post-punching strength after punching. However, this capacity decreases dramatically as the deflection increases to a large amount after punching failure. Since isolated slab-column testing cannot fully represent behaviors of an actual building, multi-panel testing was done at a sub-structure system level. The specimens consisted of two 9 column portion of a flat plate building, one tested with an exterior column instantaneous removal and another tested with an interior column instantaneous removal. The tests further investigated the dynamic load redistribution, punching, and post-punching responses in a flat-plate structure. The multi-panel tests (with interior and exterior column removal) showed that flat-plate slabs are vulnerable to disproportionate collapse at load levels of approximately 50% of their design capacity. The recorded lateral movements on columns in the tests verified the existence of compression membrane forces in continuous slab panel. Compressive membrane forces form after a column removal and gradually transition to tension membrane forces at deflections approaching the slab depth. Punching failure did not happen in compressive membrane phase, but in the tension membrane phase and tests showed that pre-existing damage in flat-plate structures (from prior overloading or shrinkage cracking) may impede the formation of compressive membrane forces in the slab. Dynamic removal of a supporting column resulted in a dynamic load amplification factor (DLAF) of approximately 1.3. Therefore, surrounding connections need to be able to carry at least 30% more than the predicted redistributed static load in a collapse analysis."