This paper proposes a method to modify the strut effectiveness factor in the strut-and-tie model for CFRP-strengthened reinforced concrete deep beams. Two groups of deep beams comprising six ordinary reinforced concrete deep beams and six CFRP-strengthened reinforced concrete deep beams were experimentally tested under the four-point bending configuration. The shear span-to-effective depth ratio of the beams in each group was 0.75, 1.00, 1.25, 1.50, 1.75, and 2.00. The theoretical principal tensile strain in CFRP-strengthened struts was modified based on a proposed empirical relationship, based on two ratios: the experimental to the theoretical value of principal tensile strain and the shear span-to-effective depth of deep beams. KEYWORDS: RESUMEN:Modificación del factor de eficacia de las bielas en vigas de canto de hormigón reforzadas con laminados de polímero reforzado con fibras de carbono. En este trabajo se propone un método en el que se modifica el factor de eficacia que se aplica a las bielas en el modelo de bielas y tirantes para vigas de canto de hormigón reforzadas con laminados CFRP (polímero reforzado con fibras de carbono). Mediante el ensayo a cuatro puntos se determina la resistencia a flexotracción de doce vigas de canto divididas en dos grupos de seis, las del primer grupo de hormigón armado normal y las del segundo de hormigón reforzado con laminados de CFRP. En ambos grupos cada una de las seis vigas se caracteriza por una relación luz de cortante-canto útil distinta, con valores utilizados de: 0.75, 1.00, 1.25, 1.50, 1.75, y 2.00. El valor teórico de la deformación principal por tracción de la biela reforzada con CFRP se modifica de acuerdo con la relación empírica propuesta en este trabajo. Esta se establece a partir de otras dos: la relación entre los valores experimental y teórico de la deformación por tracción principal y la relación luz de cortante-canto útil de las vigas de canto.

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B-Regions are parts of the structure in which Bernoulli's principle of straight-line strain is used. D-Regions are parts of the structure with a complicated variation in strain. In essence, D-Regions contain the parts of structure which are near to the concentrated forces or steep changes in geometry which are so-called geometrical discontinuities or static discontinuities. Strut-and-Tie Model (STM) is one of the best models to analyse the D-regions. Nevertheless, according to the existing literature, there are still some challenges about STM which are addressed in this paper. STM and its details are investigated to show its common challenges presents some recommendations to overcome these challenges. According to this review, the major challenges in STM are related to the strut effectiveness factor, static uncertainties of STM, strain compatibility, and anchorage requirements in STM. The scope of this research is confined to the two dimensional STM.

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