PSI - Issue 81

Serhii Filipchuk et al. / Procedia Structural Integrity 81 (2026) 401–405

402

structural solutions in the field of civil engineering. (Dovbenko et al. (2024); Drobyshynets et al. (2024); Sobczak-Piastka et al. (2023)).

Nomenclature ε c

deformation of concrete deformation of reinforcement

 s

load

F

modulus of concrete elasticity

E c

2. Methods of experimental research To investigate U-shaped reinforced concrete frames with artificial force regulation, reinforced concrete frame specimens were fabricated. The frames had the following centerline dimensions: span l = 2000 mm, height h = 1100 mm; the cross-sectional dimensions of the frame beam were 160 × 100 mm, while those of the frame column were 180 × 100 mm (Fig. 1). The frame columns were reinforced with spatial reinforcement cages consisting of four symmetrically arranged Ø14 A400C bars. The frame beam was reinforced with a spatial cage comprising b ottom reinforcement of 2 Ø14 A400C bars and top reinforcement of Ø18 A500C bars, which were bent and anchored into the column below the bottom face of the beam over a length of 450 mm. Transverse reinforcement in both the columns and the beam was provided in the form of closed welded stirrups made of Ø6 Bp -I bars, spaced at 60 mm in the beam and at 70 mm and 50 mm in the column within the support zone. Embedded steel plates with a thickness t = 5 mm were installed at the ends of the columns. The frame tie w as made of Ø18 A400 reinforcing bars, with an M18 thread provided at one end. Prestressing in the tie was applied using a torque wrench. Based on prism tests, the concrete strength at the age of 28 days was f c = 20.31 MPa; the ultimate strain was ε c1 = 143 .17 × 10 ⁻ ⁵ , and the initial modulus of elasticity was E c = 24.95 × 10³ MPa. According to the test results of reinforcing steel specimens with a diameter of 10 mm, the following properties were obtained: ultimate strength σ u = 638 MPa; yield strength σ y = 509.6 MPa; modulus of elasticity E s = 2.0 × 10 ⁵ MPa; and the maximum relative strain corresponding to the yield stress was ε s0 = 244.9 × 10 ⁻ ⁵ . The frame tests were carried out using a two-hinged system scheme in a specialized test setup, with the beam loaded by two concentrated forces applied at a distance of 750 mm from the column axes. The load was applied using a hydraulic jack, and the force was measured with a calibrated ring dynamometer (Fig. 1). Frames R1-P and R1-PR were subjected to ten cycles of repeated loading up to a level of F cyc ≈ 0.75 F u , and during the eleventh cycle they were loaded until failure.

Fig. 1. General view and structural scheme of the experimental frames

3. Research results Let us examine in more detail the behavior of frame R1-P. Repeated loading of the frames revealed specific features in the evolution of the stress – strain state of the cross-sections of their elements. During the first loading cycle, at a load of F = 10 kN, the crack widths were w = 0.08 mm at the frame joint and w = 0.04 mm in the span of the frame beam. As the load increased during the first cycle to F = 22.5 kN, the crack width increased to w = 0.18 mm in the beam span and to w = 0.28 mm at the frame joint.