A comprehensive failure analysis method for large-scale concrete structures is presented, which includes snap-back failure. Energy observation is used to describe the macroscopic mechanism of snapback occurrences and other softening failure modes. The typical snapback failures of beams and columns by shear are numerically obtained using the arc length method with selected displacement control points. The fundamental understanding of the phenomena is extended to the failure mechanism of a column where shear capacity is affected by axial force.
A calculation procedure is described for estimating crack shear stresses and crack slip displacements from average strain measurements made on reinforced concrete panels. Several series of panels, previously tested, are examined and crack shear-slip data are extracted. These data are compared against the predictions of previously developed crack slip models, as well as against an alternative constitutive model proposed herein. Reasonable correlation is found between experimental and calculated values, particularly at near-ultimate load conditions. It is then shown that including crack shear slip behaviour in a computational model results in improved accuracy in terms of predicted load-deformation response and ultimate load capacity for reinforced concrete elements such as panels, beams and shear walls. Further, it is shown that rigorously accounting for crack slip displacements results in a better representation of various subtle aspects of behaviour, such as the failure mode and the capacity of elements to deform and redistribute load.
To investigate the rate effects on post-peak structural behavior accompanying the compression softening of structural concrete, experimental studies were carried out on over-reinforced concrete beams with and without confinement under varied rates of flexural loading. The effects of loading rate on the capacity and ductility of RC beams were found more pronounced in confined cases than unconfined cases. The generic time-dependent constitutive model of compression-softened concrete was applied to nonlinear collapse analysis and its applicability was verified by experiments. The strain rate in the compressive localized zone in structures rapidly increased after the member reached its peak capacity even though the rate of displacement was kept unchanged especially in the case of unconfined beams. In the case of confined RC beams, localization of weak strain occurred but with comparatively greater time-dependent plasticity and fracturing within the structure. These deformation characteristics were adequately simulated by nonlinear analysis using a time-dependent constitutive model for softened concrete in compression.
Finite element analysis was conducted for ten RC specimens, which were shear-strengthened with fiber-reinforced polymer (FRP) sheets. Four of the ten specimens were wrapped with unbonded sheets to observe influence of the bond to the strengthening effectiveness. The tests and analyses demonstrated that the unbonded sheets are not effective to increase the shear strength of a member without steel stirrups while the flexural ductility of a member can be improved by confinement with unbonded sheets. Parametric calculations were attempted with varied interfacial fracture energy Gf and maximum bond stress τy between the sheet and concrete. The calculations indicated that the Gf and the τy have limited influence on the shear strength while the local stresses and crack widths can be controlled to a certain extent.
The mechanism of diagonal tensile failure of RC beams without shear reinforcement, which is difficult to solve by means of experimental and analytical study, is investigated. The close relationship between the fracture modes and the transfer stress at shear cracks is clarified, and the experimental results are verified by the finite element method taking into consideration the influence of a splitting tensile crack and dowel action. In RC members without shear reinforcement, the width of a shear crack increases owing to the occurrence of a splitting tensile crack along the main bars. As a result, the transfer stress of a shear crack cannot be generated and the shear crack grows and propagates rapidly. This paper also puts forth that FE analysis, in which not only shear crack but also splitting tensile crack are modeled discretely, can predict the size effect on diagonal tensile failure strength of RC beams without shear reinforcement.
This study attempts to expand the application of the conventional 2D lattice model, which can express the shear resisting mechanism of RC structural members, to a 3D analytical model. Use of a 3D lattice model for the simulation of torsional and biaxial responses of RC structural members is presented. The results of static analysis are compared with several sets of experimental results for RC beams/columns subjected to pure torsion or cyclic combined loads of torsion and bending. In addition, the dynamic analysis of RC columns subjected to bilateral seismic loading is carried out. The applicability of 3D lattice model to the response prediction of RC structural members under 3D loading is examined by comparing analytical and experimental results.
Concrete is a heterogeneous material consisting of mortar and aggregate at the meso level. Evaluation of the fracture process at this level is useful to clarify the material characteristic of concrete. However, the analytical approach at this level has not yet been sufficiently investigated. In this study, two-dimensional analyses of mortar and concrete are carried out using the Rigid Body Spring Model (RBSM). For the simulation of concrete, constitutive model at the meso scale are developed. Analysis simulates well the failure behavior and the compressive and tensile strength relationship of mortar and concrete under uniaxial and biaxial stress conditions. Localized compressive failure of concrete is also simulated qualitatively.
In Japan, the seismic performance of existing RC buildings is evaluated by computing the seismic capacity index, Is, using the Standard for Seismic Evaluation of Existing RC Buildings, while the damage level of RC buildings that undergo earthquakes is assessed by the Standard for Post-Earthquake Inspection and Guidelines for Repair and Strengthening Technology. This paper reports the results of investigation of the relation between the Is value and the damage level for low-rise RC buildings designed according to the old code by conducting dynamic analysis on model buildings with a variety of Is values. The effects of the deformability type of columns and the number of stories on the relation between the Is value and the damage level were studied. Two levels of ground motions, the original level of past earthquake records and the design standard level, were considered. In the analysis, column hysteresis was derived from test results. Strength deterioration after shear failure and axial collapse that are commonly associated with hysteretic behavior of old columns, were considered. The method presented in this study enables assessment of the damage level of buildings and the damage condition of columns if the deformability type of columns, number of stories, Is value and ground motion are given. In addition, the assessed damage level of buildings are compared with the observed damage level from past earthquakes and the Is value required to prevent collapse of buildings is discussed.
Wall buildings with vertically irregular configurations have been severely damaged or even collapsed due to the formation of a story mechanism during severe earthquakes, particularly under near-fault earthquake excitation. This paper presents a criterion to prevent such failures. A story-safety factor is defined to represent the relative reserve strength against a story mechanism in the structure. The validity of this factor was examined by conducting dynamic response analyses of various analytical models of 7- and 11-story wall structures with discontinuous wall panels in the first story using 29 real earthquake records (mostly near-fault records) and their scaled motions with various intensity levels. The results show that the proposed story-safety factor controlled well the failure mechanism of the structures. When the story-safety factor was larger than the corresponding dynamic shear magnification factor (Paulay and Priestley 1992) minus unity, a story mechanism did not occur in the structures in all cases. A practical procedure for using the story-safety factor to prevent the formation of a story mechanism in irregular stories is also presented.
One of the important factors for compressive stress-strain curves of concrete is the localization of failure. The stress-strain curve of concrete strongly depends on the aspect ratio of the concrete specimen; therefore, a unique stress-strain curve is not adequate to express the softening behavior of concrete. To overcome the problem related to the localization of failure, a series of uniaxial compressive tests of concrete specimens was conducted. From the measured energy distribution, the failed specimen was assumed to be composed of 2 or 3 zones. Then, an equation for an envelope curve involving a characteristic of compressive strength of concrete was formulated so as to match the experimental curve of each zone. Combining 2 or 3 proposed equations considering the extent of each zone could express the experimental stress-strain curve of the specimen regardless of the aspect ratio.
Reactive Powder Concrete (RPC) is an ultra-high-strength, high ductility and low porosity cementitious material. RPC properties are improved by pressing fresh RPC samples, which can increase its specific weight as high as 3000 kg/m3. However, high specific weight may be a shortcoming, where weight saving is important. Following to this review, a set of tests on RPC samples with high silica fume contents were carried out in a laboratory. The results show that although high silica fume content increases the compressive strength, however it decreases the density. It is concluded that by means of silica fume it is possible to produce high strength RPC with a specific weight as low as 1900 kg/m3. This Light Weight Reactive Powder Concrete (LWRPC) could be used in areas where substantial weight savings can be realized and where some of the remarkable characteristics of the material can be fully utilized.
Peeling failure is one of the main drawbacks of reinforced concrete (RC) members strengthened by externally bonded plates. Two different techniques are suggested to prevent peeling failure at the ends of steel plates glued to the soffits of RC beams. In the first technique, concrete cover is replaced by grout to enhance the resistance of substrate to crack initiation and propagation. In the second technique, permanent compressive forces at the ends of bonded plate are applied using different plate end anchorage systems, i.e. end anchorage by bolts or by clamps. Furthermore, two different sizes of side plates, with and without end anchorage, are glued to the plated beam to delay or prevent the peeling failure. An analytical model is suggested to predict the load-deflection behavior of beams strengthened by bonded steel plates at the bottom and sides of the beams. The results of 4PB tests indicate that conventional plated beam (PB) experience reduction in ductility and limited enhancement in ultimate load, which increased only by 22% compared to unplated beams due to the occurrence of peeling failure. Peeling failure of plated beams can be prevented through the use of either a concrete cover replacement technique or bolted anchorage systems. Anchorage of a side plated beam increased the ultimate load by 264% compared to an unplated beam and 217% compared to PB.
The mechanical interaction of pre-induced damage and diagonal shear was investigated with regard to corrosion cracks in concrete and rupture of the web reinforcement along the main reinforcement. First, localized corrosion cracking was produced in concrete around the mid-span of beams in both compression and tension fibers, and a mechanical interaction was no longer identified. Next, cracking occurred near the beam supports inside a shear span and around the anchorage zones of longitudinal steel. A diagonal shear crack was observed to meet the pre-existing cracking around the anchorage zones of longitudinal steel. Crack propagation into the anchorage zones resulted in significant decline in the overall capacity. On the other hand, the overall capacity could be maintained or even increased by keeping anchorage failure from occurring. Finally, RC beams with artificial defects in both concrete and web reinforcement over the whole shear span were tested. Damage around anchorage zones was concluded to play a critical role in triggering shear sliding failure in pre-damaged reinforced concrete.