Influence of stirrup detailing on punching shear strength of flat slabs Alejandro Pérez Caldentey ⇑ , Patricio Padilla Lavaselli, Hugo Corres Peiretti, Freddy Ariñez Fernández Structural Concrete Research Group, School of Civil Engineering, Polytechnic University of Madrid, Spain article info Article history: Received 3 July 2012 Revised 13 December 2012 Accepted 14 December 2012 Available online 9 February 2013 Keywords: Punching shear Detailing Design codes Current construction practice abstract Most concrete design codes agree that it is important for punching shear reinforcement stirrups in slabs to engage the tensile longitudinal reinforcement bars. However, due to the practical difficulties that this anchorage detail entails, it has been common construction practice in some countries (including Spain) to place closed stirrups without encircling the main tensile reinforcement. The Structural Concrete Research Group at the Polytechnic University of Madrid tested eight slabs with four different shear reinforcement dispositions and the results show that slabs with the shear reinforcement disposition that matches Span- ish practice show punching shear strength that is quite similar to the one shown by slabs with the trans- verse reinforcement disposition specified in the codes. The results also show a significant reduction in punching shear strength when longitudinal reinforcement does not pass through the slab–column connection. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The majority of concrete design codes emphasize that punching shear reinforcement stirrups, to be acceptably effective, must be suitably anchored. Some codes, such as ACI 318-08 [1], specify that stirrups should engage the tensile longitudinal reinforcement of the slab and claim this disposition to be an essential aspect to ensure the stirrups’ anchorage. The reasoning behind this requirement is that when the structure’s reinforcement is analyzed as a Strut- and-Tie Model, it is necessary for the tensile longitudinal reinforce- ment to transmit the variation of stresses to the vertical branches of the stirrups. Nevertheless, ACI 318-08 as well as other international design codes [2] acknowledge that this anchorage requirement is particularly difficult to materialize, especially in thin slabs. Thus, it has been common practice to implement alternative transverse reinforcement dispositions that facilitate reinforcement installation as well as expedite construction. For instance, in the UK it is common to use single leg links, which do not fully encircle the tensile rebars, as punching shear reinforcement [3]. Additionally, Beutel and Hegger have performed tests that show that stirrups that do not enclose the bottom flexural reinforcement but use welded transverse bars as anchor elements are actually quite effective to in- crease the punching shear capacity of the structure [4]. In Spain, however, it is traditional practice for contractors to first place the longitudinal reinforcement in the slab’s formwork and then set punching shear stirrups without encircling the main longitudinal reinforcement bars but engaging them with construc- tion wire, or by other means not officially recognized by any code. This disposition does not meet the requirements set by the Strut- and-Tie Model (see Fig. 1). The purpose of this paper is to present the results and conclu- sions drawn from the punching shear tests that were carried out in the Structure’s Laboratory of the Civil Engineering School at the Polytechnic University of Madrid in order to assess the influ- ence that different transverse reinforcement dispositions have on the punching shear strength of reinforced concrete slabs and com- pare these results with theoretical punching shear strengths deter- mined with formulations of the Spanish Concrete Code (EHE-2008 [2]), ACI 318-08 [1], Eurocode 2 [5] and fib Model Code 2010 [6]. Although several authors have analyzed and compared sets of experimental results to theoretical values calculated with current concrete design codes for the case of RC slabs subjected to punch- ing shear failure [7–9], there are few studies available on the importance of stirrup detailing in ultimate punching shear strength and practically none considering the common construc- tion disposition seen in Fig. 2. 2. Experimental research A total of eight concrete slab samples were tested considering four different rebar disposition typologies. In this way, for each reinforcement bar disposition there were two identical specimens. 2.1. Definition of the test samples The test samples were concrete slabs with dimensions: 2.80  2.80  0.25 m 3 and a load introduction zone (column) with 0141-0296/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engstruct.2012.12.032 ⇑ Corresponding author. Address: School of Civil Engineering, UPM, C/Professor Aranguren, s/n, ‘‘Ciudad Universitaria’’, 28040 Madrid, Spain. Tel.: +34 91 336 6700x16; fax: +34 91 336 6702. E-mail address: apc@he-upm.com (A. Pérez Caldentey). Engineering Structures 49 (2013) 855–865 Contents lists available at SciVerse ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct