The Study On Collapse Performance Of RC Flat Plate Frame Structures

Progressive collapse of structures caused by earthquakes, blasts, and other accidents results in catastrophic loss of life and money. Many practicing engineers and academic researchers have been engaged in the prevention of progressive collapse since the partial collapse of the22-storey Ronan Point apartment building in1968. Especially after the terror attack on the Alfred P. Murrah Federal building in1995and World Trade Center in2001, several changes to the philosophy and practice of design for important buildings have been made in the last decade. Resistance of building structures to progressive collapse has been an important task for the development of structural design codes. The RC flat plate construction systems are being widely used in residential and industrial buildings in many parts of the world. This system is highly advantageous because of the formwork costs reduction, fast execution and easy installation. The reduced storey height and increased architectural freedom in design result in lower overall construction and maintenance costs. However, because of brittle punching failure occurred at the slab-column connections, the flat plate construction is vulnerable under gravity or earthquake load.The study on the collapse-resistant experiment of RC flat slab-column structures has not been found in the literature. It is important to explore the alternative load path in a flat slab-column structure when the structure subject to a load-carrying member loss. As the vulnerable behavior of the flat slab-column structures after punching failure of the slab-column connections, it should be interesting to know the collapse-resistant performance after a column loss in the structure. However, the complete design concept is not explicit duo to the lack of relevant experimental investigation on the flat plate structures.1. This paper presents a collapse experimental research on two single-stories2×2-bay reinforced concrete flat slab-column structure in order to find the alternative load path in the flat slab-column structures when an interior/side/corner column was removed. The research described in this paper demonstrates the feasibility of using a static unloading approach to simulate column loss and to study the collapse-resistant behavior of flat slab-column structures. The data generated from this experiment will contribute to the much-needed database on collapse behavior of RC flat plate structures. The results of experiments and the analyses reported in this paper will contribute to the future development of collapse resistant design methods.2. The experiments utilize a quasi-static method to simulate the failure of a flat plate structure subjected to a sudden column loss, which reveal the plate load transmission mechanism change process and mechanical characteristics. During the collapse process of the flat plate structure, it has experienced elastic stage and plastic stage accompanied by membrane action. The experiment indicates that the compressive membrane action and tensile membrane action could be considered as an alternative load path, compressive membrane action or compressive-tensile composite membrane action is the main characteristics of new alternative load path when the flat plate structure subject to a column loss.3. Based on the experimental results, it can be judged that the conventional designed flat plate frame will not collapse after the interior column sudden removed, because the column-losing frame carried2times uniform design load characteristic value finally, the progressive collapse did not happen (dynamic impact factor<2),1.7time design load characteristic value with side column loss,1.17time design load characteristic value with corner column loss.4. The experimental results of flat plate structure show that the load-carried capacity is larger than the value predicted by yield-line theory. For flat plate, its plane stiffness is mainly provided by exterior cantilevered plate and surrounding column. Limited compressive membrane could play a certain action with limited plane stiffness, which makes the load-carrying capacity greater than plastic hinge line value.The compressive-tensile membrane action should have important effect on the load transfer mechanism when the flat plate structure with an interior column loss. However, the analytic value is hard to be obtained. A finite element model was established and verified by comparing the experimental results, the same loading procedures and constraints as in the testing were used. Finite element method was conducted to simulate the test and it shows a good agreement with the interior span test. The flat plate frame collapse-resistant assessment criterion obtained by the Finite element method is useful for the large deformation/strain analysis of the RC structure collapse, and it could be a preparation for the next parameter analysis.5. The collapse failure of the RC flat plate frame is ultimately controlled by the longitudinal reinforcement breaking. The reinforcements were almost all broken, and punching failure occurred when concentrated force113kN (uniform load unload to1time characteristic load stage) was applied downward on the interior column stub. It also shows good performance of deformation which conducive to collapse-resistant capacity of flat plate structure. The collapse-resistant capacity would be further enhanced by the reinforcement ratio which goes through the column.6. Damage identification and health monitoring is an important aspect in the evaluation of structural safety recently. This paper also presents a modal experiment and analysis for the RC flat plate frame structure. The displacement mode experiments were carried out by hammer-hitting excitation method in the age of frame concrete. The varieties of the modal parameters of the flat plate were obtained by dynamic testing along with the growth of elastic modulus of concrete and change of the environment temperature. It can be used to identify the location of the damage. Base on the missing of some specific structure modal shapes and reduction of the frame horizontal stiffness, it can be revealed that the interior slab-column connection cannot transfer the unbalance moment well. A3-D solid finite element models with the interior column fixed/hinged were constructed, they were validated to match the frame natural frequencies and mode shapes. The results demonstrate the effectiveness of modal parameters identification method in estimate the damage of column in RC flat plate frame.7. The design recommendations for the flat plate structure collapse-resistantant are proposed, the variety of the column axis compression ratio and the increment of the uneven punching shear force of the adjacent slab-column connection are analyzed, two. The structure system integrity, continuity, reducndancy, well-anchored structure and compressive-tensile membrane action would play important roles in the flat plate collapse-resistant performance.