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Numerical and Experimental Investigation of Reinforced Concrete Structures Subjected to Impact Loads

Final Report Summary - RCIMPACT (Numerical and Experimental Investigation of Reinforced Concrete Structures Subjected to Impact Loads)

Project objectives

Since the advent of reinforced concrete (RC) structures, the analysis and design of these structures against extreme loads, such as earthquakes, blasts and impacts, has been an objective of many researchers and designers. The demand for impact-resistant design has a wide spectrum, from protective barriers to rock sheds for bridging piers to industrial facilities. Moreover, as a result of recently elevated terror threat levels in the world, the impact-resistant design of buildings has become the focus of new attention. Numerous studies have been conducted so far on understanding and developing methodologies for predicting the behaviour of RC structures under impact loads. However, because of the major difficulties involved in modelling the behaviour of RC structures in high dynamic conditions, there currently exists no general, commonly accepted analytical analysis and design methodology for such structures against impact loads. One particular challenging aspect stems from the increased shear-dominant behaviour of reinforced concrete members under impact loads; modelling the shear behaviour of RC has long been problematic.

This project aims to expand our knowledge on the impact behaviour of RC structures and develop and verify a 3D nonlinear finite element analysis (NLFEA) software that can effectively be utilised in the impact analysis and design of RC structures. To achieve this objective, a NLFEA software, VecTor3, has been developed that employs high-level, state-of-the-art reinforced concrete modelling techniques. A well-instrumented experimental program was also conducted on RC slab specimens investigating the effective mechanisms involved in RC structures when subjected to impact loads. The study focused on impacts that do not induce significant local damage, allowing the dissipation of impact energy through global mechanisms.

Work performed

Development of the NLFEA software constituted the first phase of the project. The software development involved major modifications to an existing reinforced concrete analysis program, VecTor3, which is a high-level, 3-D NLFEA software, originally developed at the University of Toronto. The program was capable of analysing RC structures under static loads and, as a part of this project, it was modified and enhanced for the consideration of dynamic loads, including impacts. The program was tested through the modelling of impact tests conducted on RC beams available in the literature and a successful corroboration of the computed results with the experimental results was achieved.

The second phase of the project involved the impact testing of reinforced concrete slabs. Six RC slabs with the dimensions 2 150 x 2 015 x 150 mm were tested at the Structural Laboratory of the Izmir Institute of Technology. To facilitate a comparison between the static and impact behaviour of identical specimens, the slabs were cast in three identical pairs, so that one of the specimens would be tested under impact loads and its identical twin would be tested under static loads. Hence, three types of slabs with identical geometry but varying reinforcement details were tested, which included orthogonal meshes at the top and bottom of the slab manufactured.

To test the slabs under simply supported conditions, an innovative impact test set-up was designed and manufactured, supporting the specimens at 20 locations along the perimeter and holding the specimens in place during the impact-induced rebound. This set-up was also used for testing the specimens under monotonically increasing static loads at the midpoint. Impact loads were introduced on the specimens by a free falling drop-weight, impacting the specimens at the midpoint. The specimens were intensely instrumented with load cells, displacement transducers, accelerometers and strain gauges. The data was captured with the help of a high-speed data acquisition system. In addition to the sensors connected to the specimen, a high-speed camera was used to observe the deformations of the specimen during impact tests and so to measure the contact velocity of the drop-weight. The impact load was applied by means of a drop-weight free-falling from a 2.5 m height, resulting in 7 m/s contact velocity at the moment of impact. The drop-weight, 180 kg in weight, slid between rails with minimum contact and impacted the specimens at the mid-point. The drop-weight had a circular, flat, steel base of 200 mm in diameter, identical to the one used in static tests.

Main results

At the end of the static tests, the specimen with the highest reinforcement ratio carried the highest load but showed little ductility. Other specimens with lesser reinforcement ratios carried lesser amounts of load but they were able to sustain the load with increasing displacements. Nevertheless, all statically tested specimens failed in the end with punching, although they developed flexural cracks as well. At the loading surface, the circular loading plate suddenly punched, creating a punching cone that was visible from the other side as wide circular cracks around the centre. During impact tests, punching was also observed with little sign of flexural cracks. These tests proved that although the specimens showed a high degree of flexural damage in static tests due to the involvement of inertia forces, shear failure is the dominant failure mechanism in impact loads. This observation stresses the importance of the accuracy of shear-behaviour models used in modelling the impact behaviour. Thus, sophisticated shear-behaviour models of VecTor3 were justified in NLFEA modelling. Both static and impact test results were modelled using VecTor3 with significant success.

The behaviour of reinforced concrete members under impact loads was successfully investigated. A comprehensive analysis of test and NLFEA analysis results will further yield a solid understanding about the mechanisms involved, thus contributing to the development of a design methodology for RC structures to resist impact loads. Moreover, the literature includes extensive high-quality impact test data, which can be used as a benchmark by researchers working in this area.

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