The strut-and-tie method (STM) has been acknowledged as one of the most reliable tools for designing discontinuity regions in structural concrete. It is capable of producing safe designs consistently since it was developed as an extension of the lower-bound theorem of plasticity theory. However, to aptly address a physical problem of a concrete element, STM often relies heavily on engineering experience and intuition. It is because the current STM has an inability to be transparent in informing the nonlinear consequences of choosing a certain ST model design, which make the method to be troublesome in more complicated design tasks.

In this thesis, a supplementary evaluation technique that employs nonlinear finite element analysis (NLFEA) is proposed as a solution. It is advantageous to incorporate NLFEA because it can provide nonlinear behavior of a structural concrete as objective insights for making a more informed decision in determining a suitable ST model. To incorporate NLFEA to STM, the concept of 'ties-as-extended-rebars' or TER model is introduced. A TER model is a numerical model used to assess the influence of a certain ST model design toward the nonlinear behavior of a concrete element. Through a TER model, an ST model is nonlinearly evaluated as a concrete element with embedded reinforcements. Additionally, to allow the TER model to generate a representative failure, the rebars in the TER model are extended with straight anchorage length. Then, the information generated by the model, i.e. failure mode and ultimate capacity, can be utilized as additional information to find a fitting ST model.

To assess its ability, the proposed technique was implemented on six ST models generated for two complex concrete beam elements. The implementation provided the TER model versions of the ST models. At the same time, the experimental results of the ST models were also validated using NLFEA. The validation attempt generated six numerical validation (NV) results, which were then compared with the TER model results. The result of the comparison indicated that five out of six TER models were able to suggest failure mode and ultimate capacity (RTER) that are comparable with the failure mode and ultimate capacity (RNV) from the NV results. In more detail, RNV to RTER ratio of models with similar failure mode has an average of 1.04 and a coefficient of variation of 11.1%, which suggests that the proposed technique can provide representative ultimate capacity value with relatively low variability.

In the past, many researches on the topic of size effect on concrete structures were mainly focused on the phenomenon of size effect in flexural cracking. The result of those studies can be found today in the concrete structure design specifications of well-known building codes, such as the Eurocode. Nevertheless, the inclusions of the results of those studies into the design specifications are still minimum and therefore, it is necessary to conduct more studies on size effect, especially on other types of cracking.
In this thesis, an investigation focused on the size effect in shear tension cracking at prestressed concrete beams was conducted. The model used for investigating the size effect is a prediction that a shear tension crack will occur when the principal tensile stress at a certain location on the web of a beam is equal to the concrete mean uniaxial tensile strength (σ1 = fctm). The investigation was conducted by studying premature shear tension cracking on a group of several I-profile prestressed reinforced concrete beams, called the trusted specimens, which were experimented by Hanson (1964), Choulli (2005), and Elzanaty (1986) under four-point bending tests. These tested beams were numerically investigated using linear elastic finite element analysis (LEFEA) with an aim to find the nearly realistic principal tensile stresses that caused the shear tension crack to initiate below the designated tensile strength of the beams.
To study the size effect, the obtained principal tensile stress distributions were analyzed using two new approaches proposed by the author, namely the σ1 area approach and the ratio-of-distances approach. The σ1 area method is a technique for detecting a structural size dependency of the uniaxial tensile strength by comparing rectangles which areas represents a group of σ1 values that have a higher likelihood in achieving the deviated values of fctm and initiate shear tension cracking on the web of the trusted specimens. In contrast, the ratio-of-distances approach investigates the size dependency of the uniaxial tensile strength by observing the locations of σ1max where a shear tension crack initiated in the web of each trusted specimen under an assumption that a shear tension crack is more likely to originate from near the beam neutral axis instead of near the web-flange junction due to the change of thickness at that interface.
In conclusion, the result of the investigation was presented. The σ1 area approach confirmed the presence of size effect in shear tension cracking at the trusted specimens by giving a relation that showed a tendency for the smaller specimens to have a higher resistance towards principal tensile stresses compared to the larger specimens. The ratio-of-distances result, on the other hand, implied that the approach has failed to detect the presence of size effect. In that result, the shear cracks from the smaller specimens and the shear cracks from the larger specimens had similar starting points locations, at which the σ1max was located.
In addition, several recommendations are provided for future studies on size effect in shear tension cracking. It was recommended to do research this topic on different physical problems and shear tension cracking with the presence of flexural cracks.