Experimental and numerical study on retard-bonded-prestressed reinforced concrete frame with openings under vertical load

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To study the mechanical performance of the retard-bonded-prestressed reinforced concrete frame with openings (RRCFO) under vertical load, a scale model (1:4) was made. The effect of openings on crack patterns, failure modes, load-deformation curves, load-stress increment curves, load-maximum crack widths curves, load-strain curves, ductility, and energy absorption capacity was analyzed. The results show that mid-span and beam-end appeared the ideal plastic hinges. After the openings were strengthened, there was no sudden influence on the deformation of the RRCFO. The best effect was to configure diagonal bars on both sides of the openings. The transverse steel bars of the upper chord play a major shear effect. When the retarding agents were not hardened, the error of prestress increment calculated by BS8110–1 for retard-bonded prestressing tendons was −12.76 % and −22.47 %, with high accuracy. Furthermore, the influence of different sizes of the openings, concrete strength, loading points, prestress, loading modes, and opening eccentric on RRCFO was analyzed by the verified finite element model. With the increase of the opening length (>1.24 h)(h is the beam height), the bearing capacity gradually decreased, at the same time, the shear failure of the openings occurred. Compared with specimens without openings, the opening length of 0.74 h has no significant effect on bearing capacity of the beam, however, it influences the peak displacement. Considering prestress increment and section strain analysis, a new bearing capacity model of RRCFO under bending failure was established, and the average value (AV) and integral absolute error (IAE) were 0.939 and 0.061, respectively, which shows that it had a high degree of accuracy. In a word, the reasonable strengthened of the openings could not weaken the stiffness, and the scheme of RRCFO is feasible.