The existing analysis models for torsional strength or torsional behavior of steel fiber reinforced concrete (SFRC) members typically adopted a simple approach, in which the tensile constitutive model of conventional concrete was modified appropriately for the SFRC considering that the steel fibers and concrete matrix behave in a fully composite manner. In such approaches, however, the pull-out behavior of the steel fibers at crack interfaces caused by the progressive loss of bond stress developed between the fiber and surrounding concrete is hard to be described in detail, and they also requires much experimental effort and cost to derive the constitutive models of SFRCs in tension for various types of SFRCs. In this study, steel fibers are modeled as separate direct tension force transfer elements and applied to a modified fixed-angle smeared-crack truss model for torsion so that the bond mechanism between the steel fibers and concrete matrix can be reflected in the tensile behavior of SFRC further in detail. The proposed fixed-angle model approach for torsional behavior of SFRC members take into account the difference between principal stress and cracking angle, and well reflects the unique characteristics of SFRC, such as fiber directionality and bond mechanism. The proposed approach provided a good agreement with the torsional behavior of 48 specimens obtained from previous studies.

The optimized-section precast slab (OPS) is a half precast concrete (PC) slab that highlights structural aesthetics while reducing the quantity of materials by means of an efficient cross-sectional configuration considering the distribution of a bending moment. However, since a tapered cross section where the locations of the top and bottom flanges change is formed at the end of the member, stress concentration occurs near the tapered cross section because of the shear force and thus the surrounding region of the tapered cross section may become unintentionally vulnerable. Therefore, in this study, experimental and numerical research was carried out to examine the shear behaviour characteristics and performance of the OPS with a tapered cross section. Shear tests were conducted on a total of eight OPS specimens, with the inclination angle of the tapered cross section, the presence of topping concrete and the amount of shear reinforcement as the main test variables and a reasonable shear-design method for the OPS members was proposed by means of a detailed analysis based on design code and finite-element analysis.