Critical view on the level of maximum punching strength of flat slabs and column bases using database evaluations

Abstract

The level of maximum punching strength is of major importance to design highly stressed slab‐column connections and to choose the type of punching shear reinforcement. Since a couple of years, there is consensus that the maximum punching strength of shear‐reinforced concrete slabs can be defined as a multiple of the reference punching strength without shear reinforcement. The upper bound of this load‐increasing effect depends mainly on the anchorage and arrangement of shear reinforcement elements. Despite numerous research studies and database evaluations, there are ongoing discussions regarding the final effectiveness of stirrups and double headed studs installed in flat slabs and column bases. In this paper, a strictly filtered punching database is used to highlight the significant impact of underlying reference strength comparing the maximum load increase according to current Eurocode 2 and the draft for the next generation of Eurocode 2 (prEC2). Based on this unifying database, the origin of current discrepancies is discussed and a combination of constant lower bound coefficients with a refined formula as specified in prEC2 is proposed. Additionally, the effect of adapting current European Technical Approvals for double‐headed studs and punching shear optimized lattice girders to prEC2 is shortly analyzed. A further database evaluation also allows for setting the basis for optimized detailing provisions regarding the maximum permissible diameter of shear reinforcement.

Limits for the Punching Shear Capacity of the Flat Slabs Reinforced with Transverse Reinforcement

“…A 20 mm increase in the effective shear reinforcement depth could cause a 12% increase in the maximum punching capacity of a flat slab. Thus, the relation ( 2) is quite sensitive to dsys [8].…”

“…The maximum punching capacity of flat slabs was also investigated by the research team led by Prof. Muttoni (the reference [9]), where they suggested a modification of the more accurate relation (2), see Table 1. This topic has also been dealt by a team led by prof. Hegger (the reference [8]), statement that the parameters mentioned above have a negligible influence on the maximum possible increase in the punching shear resistance and recommended considering the constant value of ηsys, namely ηsys = 1.75 for shear studs and ηsys =1.4 for stirrups. If a more detailed calculation is needed, they recommend to a more accurate calculation (2) [8].…”

“…This topic has also been dealt by a team led by prof. Hegger (the reference [8]), statement that the parameters mentioned above have a negligible influence on the maximum possible increase in the punching shear resistance and recommended considering the constant value of ηsys, namely ηsys = 1.75 for shear studs and ηsys =1.4 for stirrups. If a more detailed calculation is needed, they recommend to a more accurate calculation (2) [8].…”

Doppelkopfanker als Querkraft‐ und Durchstanzbewehrung – Aktueller Stand und Ausblick

“…In Bild 9 werden grafisch die in [6] empfohlenen vereinfachten Koeffizienten η sys für Doppelkopfanker und Bügel in Abhängigkeit vom Verhältnis Stützenumfang b 0 zu statischer Nutzhöhe d v verglichen. Modellvergleiche und Hintergründe zur mechanischen und versuchsgestützten Herleitung des Koeffizienten η sys werden von Schmidt et al [22] und von Hernandez et al [23] (auf Basis der sog. „Critical Shear Crack Theory“ von Muttoni und Ruiz) gegeben.…”