Effect of asymmetric flexural reinforcement ratios on punching shear resistance

The impact of varying flexural reinforcement ratios in the orthogonal directions (⁠$ρx$ and $ρy$⁠) on the punching shear resistance (PSR) of flat slabs was experimentally investigated. Six slabs, measuring 1.5 m × 1.5 m × 0.15 m, were tested, with a constant average reinforcement ratio (⁠$ρav$⁠) of 0.3% and $ρx/ρy$ ratios ranging from 1.2 to 3.0. The results indicated an insignificant effect on punching resistance for $ρx/ρy$≤ 2.5. Increasing $ρx/ρy$ led to wider, more dispersed cracks and enhanced ductility. Comparisons of various codes (BS 8110-1:1997, ACI 318-19, the first and second generations of Eurocode 2 (EC2-2004 and EC2-2023, respectively) and fib Model Code 2010 (MC2010)) revealed that BS 8110-1:1997 provided conservative predictions except for $ρx/ρy$= 3.0, while ACI 318-19 consistently overestimated the punching resistance. MC2010 (level of approximation IV) accounted for the reduction in PSR at higher $ρx/ρy$ ratios. Tests from the literature with $ρx/ρy$ up to 4.4 exhibited further strength reduction. Comparisons of EC2-2004 and EC2-2023 showed that EC2-2004 gave conservative estimates, with diminishing accuracy as $ρx/ρy$ increased, while EC2-2023 accurately predicted the punching resistance for $ρx/ρy$ ≤ 2.5, but overestimated it for higher ratios. This study highlights the limitations of current codes in addressing reinforcement asymmetry effects, and recommends incorporating a reduction factor in EC2-2023 for $ρx/ρy$ > 2.5 to account for reduced PSR.