When employing the strut-and-tie modelling (STM) method in the conceptual design of reinforced concrete structures, a suitable strut-and-tie (ST) model indicating load transfer mechanisms first needs to be identified. Topology optimization (TO) methods have frequently been used for this purpose. However, although TO methods employing either a ground-structure or a continuum-based TO approach can be used, the performance and effectiveness of these two methods have not been systematically investigated and compared. To obtain a better understanding of the characteristics of both methods, a systematic comparison procedure is proposed to investigate the generation process and the resulting ST designs. Three aspects, relating to structural performance, economic issues, and method applicability are considered in the comparison, with six metrics formulated to quantify these aspects. Based on investigation of designs for three reinforced concrete elements incorporating typical discontinuity regions (two 2D cases and a 3D case), the performance of the two methods is assessed. It is found that both methods result in safe and efficient ST designs, with comparable structural performance, while some differences in terms of computation time and usability are observed.

Strut-and-Tie modelling (STM) has been widely applied to design D-regions of reinforced concrete structures. For economic and environmental reasons there is a need for optimized Strut-and-Tie models. How to optimize Strut-and-Tie models considering multiple load combinations has not been investigated extensively in the literature. In order to address this gap in this paper, we propose a method to generate multi-load optimization-based Strut-and-Tie (MOST) models to design D-regions under multiple load combinations. The proposed generation method involves the determination of basis vectors for the load combinations and generation of the corresponding optimization-based Strut-and-Tie models for each of the basis vectors by topology optimization and truss extraction. The generated model is then used to design D-regions under multiple load combinations. In order to check the effectiveness of the proposed method, three alternative approaches for multiple load combinations are investigated and discussed. These approaches comprise: (1) using manually created Strut-and-Tie models, (2) adopting multi-load topology optimization resulting in a single Strut-and-Tie model, (3) generating individual Strut-and-Tie models for each of the considered load combinations. In this paper, three 2D and one 3D D-regions are investigated to compare the effectiveness and applicability of the different methods. It is found that the proposed method results in more economical designs than the three alternative approaches.