The introduction of strut-and-tie models in fib Model Code 1990 as a design basis for discontinuity regions constituted a significant step toward promoting consistent design methods for reinforced concrete structures. In fib Model Code 2010, the scope was broadened through the stress field method that was introduced as a complementary tool. The present article summarizes subsequent evolutions in both methods, which will be incorporated in the upcoming fib Model Code 2020. Besides emphasizing their suitability for the structural design and assessment, their adaptability to the “Levels-of-approximation” approach is also depicted. This article presents the theoretical ground of both methods and looks on their potential for computer-modeling implementation. With this respect, several strategies are presented by discussing their advantages and optimum field of application.

Donnell published in 1933 an elegant and rather simple differential equation for the stress analysis in circular cylindrical shells. The civil engineering structural mechanics group in Delft has a long and strong experience in the fifties and sixties of last century in working with this shell equation in close cooperation with the then building institute of TNO. Shallow roof shells, tanks and chimneys were examined. It is known that the equation has restricted reliability for shells in which wavelengths in circumferential direction are relatively large. In 1959 Morley replaced Donnell's equation by a slightly extended equation which retained the essential simplicity of the original but the accuracy does not decrease as the wave length of circumferential distortion increases. The impact on longitudinal bending stresses at the base of chimneys due to wind is not negligible as appears from here summarized research. Application of Morley's theory is easily extended to ring-stiffened and elastically supported chimneys. This note demonstrates how subsequent research in time of individual persons provides stepping stones in the process of discovering.

Because a rigorous bending theory for thin shells of revolution is complicated, attempts have been made for reliable approximations of the edge disturbance problem under axisymmetric loading. A well-known one was published by Geckeler [1, 2], who obtained his approximation by mathematical considerations. He started from kinematic, constitutive and equilibrium equations for the rotationally symmetric thin shell without approximations. Herein he introduced mathematical simplifications. Each time when derivatives of a function of different orders appeared, he just kept the highest order derivative and neglected all lower ones. This is permitted if the function varies rapidly, as is the case for edge disturbances. Here we will present Geckeler's result in an alternate way, which illustrates the physical background of his mathematical approximation. Said in another way, we offer a derivation in the language of structural engineers.

Economic advantages and environmental benefits encourage the use of recycled materials for road construction. Usually, however, these materials have lower stiffness and strength characteristics than typically used natural materials. Because of these inferior material properties, use of recycled aggregate materials for unbound road base construction may result in increased rutting, differential settlement, and reflective crack propagation. Placement of reinforcement between the underlying soil layer and the aggregate layer has been proposed to improve the load-carrying capacity of road bases constructed from recycled aggregate materials. To investigate the viability of such an approach, finite element analyses were performed of asphalt concrete pavements with base layers consisting of reinforced unbound recycled aggregate materials. The response of such pavements was compared with that of pavements consisting of unreinforced natural aggregates. The criteria chosen for comparison were the influence of the material characteristics of the recycled aggregate and the reinforcement on the development and speed of propagation of reflective cracking in the body of the pavement. Various combinations of reinforcement and aggregate material characteristics were simulated. It was concluded that the placing of reinforcement can reduce the speed of crack propagation into the top layer, improve load spreading in the unbound base layer, and prolong the economic life of the construction.