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concrete as a material. He developed a most up-to-date finite element model of a reinforced concrete terrace unit with help from a parallel laboratory investigation. The author is confident that the general elasto-pastic, meso/macro scale level approach, combined with cracking and crushing modes of failure, has captured the non-linear behaviour of the unit to failure.
The author‟s decision to steer away of the currently unresolved issue of strain softening and introduce more realistic failure criteria for concrete proved to be successful. It is believed that if a similar approach is adopted for the bond failure between concrete and reinforcement, that is, if the process of debonding can be described in a more rigorous manner rather than as a sudden local failure, existing results should be substantially improved. In the current investigation, the problem of shear resistance of the main reinforcement was resolved by assigning the contribution of steel to the surrounding aggregate. This may not be entirely compatible with existing design models but in the absence of more realistic and accurate numerical representation it may be considered adequate and has produced acceptable results.
4.4.3 Benefits
The „sensitivity‟ of the non-linear model developed, depends on both cracking and crushing of concrete and the yielding of the reinforcement. This makes the model particularly attractive to practising engineers who deal with the design of „special‟
structures and are seeking advice and solutions beyond the conventional codes of practice.
The problem of shear transfer was resolved by assigning the amount of shear resistance normally provided by the re-bars, to the interlocking ability of the surrounding aggregate.
Moreover, when shear transfer (combined with tensile stresses) takes place the bond between steel and concrete starts to break down. Shear resistance is gradually passed to the interlocking ability of the coarse aggregate within the concrete itself until the re-bars can resist very little or no shear at all. Bond failure is a long process with the concrete material surrounding the reinforcement starting to slip long before the maximum bond strength is reached. Hence, although restricting bond stresses to a specific value would sound as a possible alternative, it would not however be representative. It is pointed out that the complicated bond mechanism of failure was outside the scope of a macro/meso-scale model like the present.
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There were more benefits associated with the representation of shear resistance. ANSYS recommends a dedicated 3D, eight node, solid, isoparametric element (SOLID65), with three translational DOFs per node, suitable to simulate the non-linear response of brittle materials like concrete. In addition, ANSYS recognises that concrete needs reinforcement and therefore allows an additional smeared stiffness distributed through the centroid of the element in a 3D Cartesian system orientation. Thus the analyst can define up to three different size re-bars in three different directions. These virtual rebars can resist tension and compression but unlike the real reinforcement, they cannot provide any shear
resistance. Neither the BEAM nor the LINK families of elements would solve the problem to its roots. The BEAM elements, although they would resist shear they are however purely linear elements, not suitable for non-linear analysis. The LINK elements on the other hand, like their smeared opposite numbers, would go beyond yielding but would not resist shear stresses. It seems therefore that there is no suitable element for RC representations.
Another drawback which is not perhaps highlighted enough in the paper is that, as the smeared stiffness is distributed through the centroid of the element in three orthogonal directions, its position and orientation is not always suitable to represent that of a typical system of steel bars (eg: tension, compression and shear reinforcement in a beam).
The author proposed a discrete representation of the reinforcement to address the problem of shear resistance by choosing LINK8 elements. This way the problems of position and orientation of the reinforcement were solved while the contribution of shear resistance was attributed to aggregate interlocking ability, as discussed earlier.
4.4.4 Originality
Paper No. IV demonstrates a number of original qualities. For example, it addresses the problem of shear transfer (other researchers have conveniently avoided it) in a simple and effective manner. The capturing of flexural cracks has opened new horizons in modelling failure conditions of reinforced concrete structures. It is now easier to predict more
complex modes of failure such as the combination of flexural and shear failure which is the most popular in reinforced concrete beams. The algorithm presented has been tested and validated with experimental data and therefore it is reliable and relatively easy to adapt and tailor to particular circumstances. Hence, the model can be used as a benchmark for further numerical analysis work into the area of reinforced concrete. The phenomenon of strain
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softening is by no means original. However, only a few researchers in the past have raised concerns about its suitability to represent concrete (Hughes, 1976), (Kotsovos, et al. 2005).
The author was able to report on both arguments and express his own reasoned opinion.
Finally, the initiative to deal with these types of problems at a macro/meso scale level has worked well and may be opening new horizons to practicing engineers and analysts and in particular those who are dealing with the simulation of large, civil engineering structures.
4.4.5 Value, Impact
It has been demonstrated by the author that the dedicated concrete element in ANSYS is not always the best option in modelling reinforced concrete and its performance under loading. However, the proposed combination of SOLID65 and LINK8 elements may be adequate to depict the full performance of RC terraces or, indeed other structures undergoing incremental loading. This is expected to be of value to researchers and practitioners who are interested on a relatively simple but rigorous solution of their problem. Also, the problem of transferring shear stresses from the main reinforcement to the concrete material was addressed by the author in an indirect and simplified manner and is expected to be utilised by others. It is relatively early to report any citations but both articles have enjoyed good comments from the Chairman of the editorial panel (Bothwick, 2009).