Issue 78

D. D. Joshi et alii, Fracture and Structural Integrity, 78 (2026) 315-345; DOI: 10.3221/IGF-ESIS.78-19

Zhang et al. [52] Load-displacement response, failure modes, stress contours for concrete as well as steel rebars

• strengthened specimens attained flexural action, compressive arch action and catenary action • beam longitudinal rebar rupture is main failure mode

• increased the peak load of the strengthened specimen by approximately 386 % compared to the control specimen • improvement in catenary action capacity of strengthened specimen • higher concrete strength, increased beam top and bottom reinforcement ratio, higher steel plate strength enhanced the progressive collapse resistance of specimens • strengthened specimen exhibited enhancement in peak load and energy dissipation by about 16.9 and 12.4 times, respectively, compared to control specimen • provision of end anchorage of bolted steel plates at the end of CFRP sheets enhanced the peak load resistance by about 10% due to the delay of the CFRP debonding • strengthening layers of the CFRP/ NSM retrofitting system should cover the plastic hinge region, by extending the strengthening layers to at least twice the depth of the beam beyond the column face

• assumption of idealized boundary condition • perfect bond between CFRP strips and concrete

Elsanadedy et al. [53] Load-displacement response, strain measurement, displacement ductility, energy dissipation, failure modes

• concrete crushing at beam ends for control specimen • CFRP rupture and fracture of NSM rebars near middle junction for strengthened specimen, crushing of concrete at the top side of beam near middle junction • interfacial debonding of CFRP end at later stage due to insufficient anchorage

• insufficient restraint provided by the end columns, • discontinuity of beams at the ends • discontinuity of columns due to the consideration of single story • absence of column axial load.

Table 5: Key observations of strengthening studies on precast specimens.

Summary of strengthening techniques This section briefly summarizes the main advantages, limitations and practical implications of different strengthening techniques used for RC and precast structures to enhance progressive collapse resistance. The available studies clearly demonstrate that strengthening can improve the performance of both RC and precast structural elements under progressive collapse scenario. The reviewed studies have reported improvements in load-carrying capacity, stiffness, displacement ductility and energy dissipation with the use of externally bonded FRP systems and hybrid strengthening techniques. Strengthening can also alter the failure mechanism by delaying the failure or preventing premature failure. In precast specimens, externally bonded strengthening systems prevented complete separation between structural elements and maintain the integrity of beam-column joints by providing additional load-transfer paths. Externally bonded FRP systems provide a flexible strengthening option for beams, columns, slabs and beam-column joint regions. Depending on the strengthening configuration and structural system, strengthened specimens can develop flexural action, compressive arch action and catenary action. However, interfacial debonding, fracture of FRP materials and inadequate anchorage remain major limitations, particularly under large deformations, which may reduce the effectiveness of FRP strengthening during the catenary action stage. Although externally bonded FRP systems offer lightweight and flexible strengthening with limited modification to existing members, their effective application requires adequate surface preparation, reliable anchorage and appropriate protection against debonding and environmental or fire exposure. External cable systems, bonded steel plates and steel tendons have also demonstrated effectiveness in improving the progressive collapse performance of beam–column assemblies. However, their practical implementation requires appropriate anchorage and connection detailing, which may affect constructability and compatibility with existing structures. Improper installation of such strengthening systems may adversely affect the behavior of beam-column joints. Implementation of such techniques also requires skilled manpower and specialized resources, including suitable anchorages and connectors. Anchorage failure under large deformation is therefore an important concern and requires appropriate consideration in the strengthening design. Carbon fiber ropes have also demonstrated potential for improving progressive collapse performance; however, their application requires careful control and monitoring during construction. The selection

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