PSI - Issue 64

980 6

Annalisa Napoli et al. / Procedia Structural Integrity 64 (2024) 975–982 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

Beams strengthened with EB plate: 57 FL

RC beams strengthened with EB plate

RC beams strengthened with MF plate

Beams strengthened with EB plate: 57

#1 (2%)

#4 (7%)

ID E-DB

#2 (3%)

FL+Bolt shearing

DB FL FL+ID n/a

#13 (23%)

#8 (50%)

#8 (50%)

#25 (44%)

#12 (21%)

(a)

(b)

Fig. 5. Distribution of RC beams based on the experienced failure mode: strengthening with EB plate (a); strengthening with MF plate (b).

The E-DB mode, observed in 13 out of 57 tested beams, was typical for  r /  s ratios greater than 1.0, in absence of end anchors, but also in cases with short lengths of plates which are arranged far from the end supports, even in presence of additional end anchors. However, few cases of E-DB failures were also observed for  r /  s ratios less than 1.00, either in presence of high mechanical percentages of internal longitudinal reinforcement or, again, in presence of short plate lengths. Conversely, the FL mode was typically observed for  r /  s ratios less than 1.00, with plates located at the beam intrados very close to the end support and, preferably, in presence of additional anchors; this mode was experienced only in one beam characterized by  r /  s much greater than 1 (  r /  s = 2.37) with the plate located very close to the support and in absence of end anchors. Also, in four beams the FL mode was combined with the ID mode even though, in these cases, the papers’ authors state that the strip debonding occurred when the beams’ ultimate capacity was already achieved and crushing of concrete in compression was observed. Conversely, the two beams which experienced a premature ID mode are representative of cases in which the use of a very small amount of internal reinforcement (  s =0.04) or even  s =0 has probably affected the beams’ flexural behaviour. In Figure 5b, the 16 beams are grouped in the following two typologies of failure mode: FL = flexural crisis characterized by crushing of concrete in compression and yielding of steel plate in tension; FL+Bolt shearing = flexural crisis associated to the shearing of one or more bolts nearest to the plate ends. The latter crisis was typical when using steel plates with thickness t p = 4 mm, corresponding to  r /  s = 1.15. Finally, Figure 6 shows the experimental relationship between the percentage increase in bending moment of the strengthened beam compared to the corresponding reference (as-built) member and the ratio  r /  s . The graph focuses solely on the strengthened members experiencing “FL” failure , for which the flexural response of the companion beams without external strengthening was available in the scientific papers; specifically, this includes 9 and 8 EB strengthened beams with rectangular and T-shape cross-section, respectively, and 16 MF-strengthened members. Except for a few experimental cases, Figure 6 clearly depicts an experimental trend wherein the flexural performance of the strengthened member closely depends on the parameter  r /  s : the higher the mechanical percentage of the external strengthening plate in relation to that of the tensile steel rebars, the greater the increase in bending moment compared to the unstrengthened member.