High-strength concrete (HSC) has been widely used in the column construction of tall buildings because of its larger strength-to-weight ratio. The current design of HSC columns focuses on providing adequate flexural strength but not on flexural ductility. From structural safety point of view, however, it is important to provide a minimum level of ductility to all structures even they are not subjected to earthquake attack, Currently, this minimum level of ductility in concrete columns is provided by some deemed-to-satisfy rules limiting the minimum size and maximum spacing of the confinement. However, these rules are concrete strength independent, and therefore the ductility level provided is not consistent for normal- and high-strength concrete columns generally lower at higher concrete strength or higher axial load level. To overcome such shortcoming, an extensive parametric study based on nonlinear moment-curvature analysis that investigates the combined effects of concrete strength, axial load level, confining pressure and longitudinal steel ratio on the ductility of concrete columns is conducted in this paper. Based on the results, a design inequality and chart are developed, which ensures that a consistent level of ductility could be provided to all concrete columns by limiting the maximum axial load level and minimum confinement.

Due to its higher strength-to-weight ratio, high-strength concrete is increasingly adopted in the construction of tall buildings and long span bridges. However, apart from better utilising its strength potential, there was only little attention paid to the ductility design of HSC members. Currently, the ductility design of HSC beams rely on some deemed-to-satisfy rules to provide a nominal level of ductility. The main drawback of this method is that it would not provide the same level of ductility, and most importantly much lower ductility, to HSC beams. This is dangerous because the lower ductility in beams would disallow the moment redistribution to occur during earthquake, and eventually lead to brittle collapse. In this regard, a new method that would enable the design of HSC beams with a minimum ductility not less than that provided in the past for normal-strength concrete beams is advocated in this paper. With this fixed minimum ductility set as a nominal requirement in the beams design, a maximum limit of tension steel ratio or neutral axis depth is imposed. These limits are subsequently evaluated using a new method of nonlinear moment-curvature analysis taking into account the stress-path dependence of steel reinforcement. The associated flexural strength that can be designed while achieving the recommended minimum ductility are also evaluated and presented in the form of design charts for practical application.

The moment-curvature relationship of reinforced concrete beams made of normal- and high-strength concrete experiencing complex load history is studied using a numerical method that employs the actual stress-strain curves of the constitutive materials and takes into account the stress-path dependence of the concrete and steel reinforcement. The load history considered includes loading, unloading and reloading. From the results obtained, it is found that the complete moment-curvature relationship, which is also path-dependent, is similar to the material stress-strain relationship with stress-path dependence. However, the unloading part of the moment-curvature relationship of the beam section is elastic but not perfectly linear, although the unloading of both concrete and steel is assumed to be linearly elastic. It is also observed that when unloading happens, the variation of neutral axis depth has different trends for under- and over-reinforced sections. Moreover, even when the section is fully unloaded, there are still residual curvature and stress in the section in some circumstances. Various issues related to the post-peak behavior of reinforced concrete beams are also discussed.

In the flexural design of reinforced concrete beams, apart from the provision of adequate strength, it is also necessary to provide a certain minimum level of ductility. Traditionally, this has been done by limiting the tension steel ratio or the neutral axis depth to no more than certain fixed maximum values. However, this would result in a variable level of curvature ductility depending on the concrete grade and the steel yield strength. Of greater concern is that this would lead to a lower level of curvature ductility than has been provided in the past to beams made of conventional materials when high-strength concrete and/or high-strength steel are used. It is proposed herein that instead of limiting the tension steel ratio and the neutral axis depth, it is better to set a fixed minimum to the curvature ductility factor. The maximum values of tension steel ratio and neutral axis depth corresponding to the proposed minimum curvature ductility factor for various concrete grades and steel yield strengths have been evaluated. Based on these maximum values, simplified guidelines for providing minimum flexural ductility have been developed.

In the design of reinforced concrete beams, it is a standard practice to use the yield stress of the steel reinforcement for the evaluation of the flexural strength. However, because of strain hardening, the tensile strength of the steel reinforcement is often substantially higher than the yield stress. Thus, it is a common belief that the actual flexural strength should be higher than the theoretical flexural strength evaluated with strain hardening ignored. The possible increase in flexural strength due to strain hardening is a two-edge sword. In some cases, it may be treated as strength reserve contributing to extra safety. In other cases, it could lead to greater shear demand causing brittle shear failure of the beam or unexpected greater capacity of the beam causing violation of the strong column-weak beam design philosophy. Strain hardening may also have certain effect on the flexural ductility. In this paper, the effects of strain hardening on the post-peak flexural behaviour, particularly the flexural strength and ductility, of reinforced normal- and high-strength concrete beams are studied. The results reveal that the effects of strain hardening could be quite significant when the tension steel ratio is relatively small.

Yield line analysis is a useful method for design of reinforced concrete slabs, but has been limited to slabs of simple geometry because up to now there has been no generally applicable and fully automatic computational procedure for complex-shaped slabs. Herein, a new yield line method that can be applied to any convex polygonal-shaped slab is developed. In this method, the deflections of the slab regions divided by yield lines are measured in terms of the dip and strike angles of the slab surfaces, which can define the geometry of all kinematically admissible collapse mechanisms or yield line patterns. The external work done and the internal energy dissipation at yield lines are evaluated as functions of the dip and strike angles, and the principle of virtual work is used to determine the corresponding load factor The final solution is obtained by minimising the load factor with respect to the dip and strike angles. A computer program based on this method has been produced. Its correctness is verified by checking against results obtained by others for simple cases, and its versatility is demonstrated by applying it to complicated slabs subjected to point, line, patch and uniformly distributed loads.

With the advent of advanced mineral and chemical admixtures, the strength level of concrete has been raised dramatically and high-strength concrete (HSC) is becoming more and more commonly used. However, HSC is generally more brittle than normal strength concrete. In fact, it has been shown that the use of HSC, if not properly controlled, could significantly reduce the flexural ductility of reinforced concrete beams. Herein, it is proposed to compensate for the reduction in flexural ductility owing to the use of HSC by adding compression and confining reinforcements. A parametric study based on complete moment–curvature analysis of beam sections made of different grades of concrete and provided with different amounts of tension, compression and confining reinforcements has been carried out to evaluate the increases in flexural ductility achievable by adding compression and confining reinforcements. From the numerical results, the compression and/or confining reinforcements needed to maintain a consistent level of minimum flexural ductility at all concrete strength levels have been determined and correlated to the concrete strength for direct evaluation in the design of HSC beams.

The concept of the seismic slit shear wall was proposed in the early 1990's. A series of experimental and theoretic studies on the wall with reinforced concrete short connecting beams cast in the slit were carried out. In this paper another type of slit shear wall is studied. It is one with vertical slit purposely cast within the wall, and the rubber belt penetrated by a part of web shear reinforcement as seismic energy-dissipation device is filled in the slit. Firstly, an experiment under cyclic loading was carried out on two shear wall models, one slit and the other solid. The failure mechanism and energy-dissipation capacity are compared between the two different models, which testifies the seismic performance of the slit wall improved significantly. Secondly, for engineering practice purpose, a macroscopic analytical model is developed to predict the nonlinear behavior of the slit shear wall under cyclic loading. The mechanical properties of each constituent elements of this model are based on the actual behavior of the materials. Furthermore, the effects of both the axial force and bending moment on the shear behavior are taken into account with the aid of the modified compression-field theory. The numerical results are verified to be in close agreement with the experimental measurements.

A finite element method for analysing the nonlinear behaviour of reinforced concrete structures that accounts for cracking and compression-softening of the concrete and dowel action and confining effect of the reinforcing bars has been developed by the second and third authors. It was applied in this project to study the load–deflection behaviour and failure characteristics of deep reinforced concrete coupling beams by analysing the models previously tested by the first and second authors and using it to conduct a parametric study on the effects of varying the shear reinforcement and restraining the axial elongation of the coupling beams. The analysis showed good agreement between the theoretical and experimental results and revealed that deep coupling beams behave quite differently from ordinary beams; after cracking, a deep coupling beam behaves more like a truss with a diagonal concrete strut which rotates and causes axial elongation. On the other hand, the parametric study revealed that although adding more shear reinforcement could suppress shear-tension failure, it would cause shear-sliding failure at the beam–wall joints and that although restraining the axial elongation could increase the shear capacity, it would at the end lead to a more brittle failure.