Punching shear failure in three-pile caps: Influence of the shear span-depth ratio and secondary reinforcement

Abstract

The strut-and-tie model (STM) is currently established as the best approach for pile cap design. This model leads to efficient estimations of the main reinforcement placed in strips between piles. However, good practices and some international Concrete Design Standards recommend some secondary distributed reinforcement, and even vertical stirrups that are not considered by the STM. An experimental campaign with nine three-pile caps tested by a centered load is presented to show the influence of both secondary reinforcement and the shear spandepth ratio on pile cap strength. The experimental results show a potential redistribution of internal forces in pile caps after yielding of main reinforcement, finally collapsed due to punching. Secondary reinforcement proves efficient to enhance pile cap strength since it takes part in complementary resistance mechanisms. As expected, the failure load increases with shear span-depth ratio reduction. The STM neither captures the effect of this ratio nor considers punching failure. Checking this failure mode is also required for pile caps. The punching formulation of Eurocode 2 allows considering the influence of this ratio, but some interpretation is required whether one deals with pile caps, regarding the effective width of the shear enhancement factor and the definition of the basic control perimeter. A proper definition would prevent unsafe or very conservative results. Therefore, some recommendations for the verification of deep pile caps following the Eurocode 2 are presented. The contribution of vertical stirrups as punching reinforcement is also investigated. The proposed approach is applied to the existing experimental database of three-and four-pile caps to check formulation validity, and conservative predictions with low coefficient of variation are reached.

Environmental analysis of concrete deep foundations: Influence of prefabrication, concrete strength, and design codes

“…2). This approach is declared to be usually used (Miguel-Tortola et al, 2018) and suitable (EHE-08, 2011;Miguel et al, 2007;Souza et al, 2009) for designing thick pile caps, while the Eurocode (EC-2, 2004) states that flexural-based methods are also applicable. EN 1990EN :2002+A1:2005EN , 2002 (NA to BS EN 1990:2002+A1:2005) Bored piles tend to be reinforced only superior meters when there are no lateral forces or seismicity.…”

Strength of pile caps under eccentric loads: Experimental study and review of code provisions

“…All these proposals involve conservative hypotheses on the location of the upper node (z) and offer simplified criteria to verify the stresses in the concrete. According to Miguel-Tortola et al [15], for three-pile caps under centered load, the obtained STM provide safe designs since they are based on the Lower Bound Theorem of Plasticity, but they become excessively conservative and do not detect the increased resistance increase when slenderness decreases or when vertical secondary reinforcement is added.…”

“…al [10], Delalibera and Giongo [11], Gu et al [12]) experimentally analysed the influence of different design variables on pile cap behaviour. As some studies described brittle fracture due to punching shear, some authors (Adebar and Zhou [13] and (recently) Guo [14]) proposed methods based on checking the stresses in the bearing areas to limit the resistance of the pile caps due to this type of failure, while, Miguel-Tórtola et al [15] recommended limiting punching stresses to avoid this problem on the basis of their own experimental studies on pile caps with three piles and a centered load. The first STM-based calculation method for pile caps under eccentric loads was proposed by Souza [16] but due to the lack of experimental studies on eccentric loads, the proposed model was verified by a non-linear FE analysis.…”

Refined Three-Dimensional Strut-and-Tie Model for Analysis and Design of Four-Pile Caps

“…In the scientific literature for pile cap analysis and design, most analytical models proposed are based on spatial 3D truss models 3,4,5,6,15,16,17,18 ; some of these models are reviewed in this paper. Most of these references focus on the proposal of different formulae to estimate a concrete strength effectiveness factor in order to reproduce accurately the failure load, but little attention is paid on the truss geometry and a predefined truss geometry is generally assumed for simplicity.…”