Analysis of the progressive collapse of a parking garage concrete structure due to punching shear

This work aims to clarify the causes leading to the structural collapse of the garage of a residential development located in Santander, Spain, in 2020. Visual inspections of the collapsed area were carried out to establish the first hypotheses about the collapse. Subsequently, the materials, geometry and acting loads were studied to compare them with the original project. The main conclusion was that the slab was subjected, in the area where the collapse began, to gravity loads of 34 kN/m 2 when it was designed to support 15 kN/m 2 . Then, structural verifications were performed in accordance with EHE-08 and Eurocode 2, in force during project design, to identify the failure mechanism(s) that caused the progressive collapse of the structure. Unacceptable safety factors were observed for punching shear, which was the main failure mechanism. Punching was aggravated by cavities detected in the concrete, which caused both a decrease in the slab strength (lower effective depth of the slab) and an increase in the stresses in the compression struts of about 20 % to 30 %, which considerably reduced the punching safety levels. This calculation was carried out by means of a finite element analysis with SAP2000. The assessment of this type of defect, very common during the execution phase, is not well developed either from a theoretical or regulatory point of view. This paper proposes, as a corrective measure, the inclusion both in the European and National Standards of a mandatory inspection procedure based on Ground Penetration Radar or ultrasounds, during the execution phase, to detect and correct this type of defect, not detectable by visual inspection. Finally, a bending and cracking analysis were also conducted, giving values of the corresponding safety factors of less than unity. Therefore, they also contributed to both the progressive loss of strength of the slabs and the final punching failure. These last two causes are also crucial to understand why the collapse occurred 15 years after its commissioning. • Two concomitant causes were established: excess load on slabs and concrete defects. • A FEM analysis was runed to determine stress increments due to concrete cavities. • Punching safety coefficients not admissible, even for non-factored loads. • The punching shear fracture-bending-slab cracking interaction was assessed. • A single column could cause the collapse of 475 dwellings and 500 parking spaces.