Shear Friction and Strut-and-Tie Modeling Verification for Pier Caps Asala Asaad Dawood 1 and Khattab Saleem Abdul-Razzaq, M.ASCE 2 Abstract: This study reports on experimental test results of reinforced concrete pier caps with different shear spans to effective depth ratios (a/d ) of 0.5, 1, and 1.5. Each test specimen is then designed theoretically using both shear friction (SF) and strut-and-tie modeling (STM) approaches, according to Section 16.5 and Chapter 23 of ACI 318-14, respectively, and the results are compared with the pier caps experimental test results. The cracking load, failure load, deflection, crack pattern, crack width, steel reinforcement strains, concrete surface average strains, and failure modes are observed, recorded, and discussed. The experimental load capacities are compared with the theoretical load capacities of SF and STM. Experimental test results indicate that both STM and SF are conservative approaches and STM is more conservative than SF. The reason for this is because they do not take secondary reinforcement into direct consideration. That is why, a model is proposed, modifying STM, for estimating the ultimate capacity of pier caps based on calculating the strength of concrete and secondary reinforcement separately that gave more realistic results. DOI: 10.1061/(ASCE)BE.1943-5592.0001758. © 2021 American Society of Civil Engineers. Author keywords: Strut and Tie; Shear friction; Experimental verification; Proposed model; Reinforced concrete; Pier caps. Introduction Pier caps can be described as cantilever beams or corbels that sup- port the superstructure and transfer loads from the superstructure to the foundation, which cause geometric discontinuities. Due to the presence of these discontinuities, the nonlinear distribution of strains and the flow of stresses in the concrete are complex. Thus, the usual design assumptions are no longer valid because these assume that plane sections remain plane and that the shear stress can be assumed to be uniform over the nominal shear area (Abdul-Razzaq et al. 2017; Abdul-Razzaq and Jebur 2017; Jalil et al. 2018; Abdul-Razzaq and Jebur 2018; Abdul-Razzaq et al. 2019a; Al-Soufi 1990). The load capacity of the pier cap is essentially influenced by its span-depth (a/d ) ratio, where (a) is the distance between the load center and the pier vertical face and (d ) is the pier cap effective depth. Because the pier caps are under heavy concentrated loads, the required depth will be high when the a/d ratio is less than 2 or even sometimes less than 1. When the pier cap has an a/d ratio of less than 1, it can be treated like a corbel, and designers typically apply a semiempirical method based on the concept of shear friction developed between the cracks (which are assumed to be vertical). When a/d is greater than 1, the pier cap is sometimes designed con- ventionally as a cantilever beam using a section-by-section flexural approach. However, this design approach is not strictly correct when a/d is less than 2, as there will be nonlinear strain distribution in the disturbed zone. Also, the shear provisions in most codes are found to be overly conservative for these deep members because they are based on 45° truss models (Geevar and Menon 2019). A shear friction hypothesis is proposed by Birkeland and Birkeland (1966) and Mast (1968) for critical shear structures. In the shear-friction approach, it is assumed that a crack exists in the shear plane for some unspecified reason. The frictional resist- ance to sliding of one crack face over the other causes the assumed shear resistance. The applied normal force on the sliding surface equal to the crossing shear reinforcement yield strength. Strut-and-tie modeling (STM) has been developed as one of the most effective design approaches for critical shear structures with an a/d ratio of less than 2 despite its conservatism (Geevar et al. 2020; Dawood et al. 2018; Abdul-Razzaq et al. 2018a, c, 2019b). In STM, the RC member is converted into an equivalent truss, in which the tension and compression zones are transformed into equiv- alent ties and struts that are connected at nodes to form a truss that resists loadings. STM gives reliable results at angles between strut and tie (θ) of 25–62° (ACI 318-14, ACI 2014; Abdul-Razzaq and Jalil 2017; Yun et al. 2019; Abdul-Razzaq et al. 2018b; Dey and Karthik 2019; Abdul-Razzaq and Farhood 2019). Design procedures have typically adopted a simple construction of straight compression struts combined with a minimal amount of reinforcement that is uniformly distributed in horizontal and verti- cal directions. This uniformly distributed reinforcement serves in controlling cracking in disturbed regions. While uniformly distrib- uted vertical reinforcement reduces the demand to the main tension tie reinforcement close to the support region, it does not result in increased strength. This is due to the fact that the strength is con- trolled by the conditions at the midspan. The presence of horizontal reinforcement assists the main tension tie reinforcement and results in increased strength (McLeod 1997). There is a lack of experimental studies on RC pier caps with a critical range of a/d values (0.5–1.5) due to the fact that the pier cap oscillates between shear predominance and flexure predomi- nance. Less than 0.5 is decided shear, and more than 1.5 is decided flexure as well. This paper experimentally validates the applicabil- ity of both the strut-and-tie technique and the concept of shear 1 Instructor in the Dept. of Civil Engineering, Univ. of Diyala, Baqubah 32001, Iraq (corresponding author). Email: asalaasaad01@gmail.com 2 Professor, Dept. of Civil Engineering, Univ. of Diyala, Baqubah 32001, Iraq. ORCID: https://orcid.org/0000-0001-6843-9325. Email: dr .khattabsaleem@yahoo.com Note. This manuscript was submitted on November 4, 2020; approved on April 23, 2021; published online on June 23, 2021. Discussion period open until November 23, 2021; separate discussions must be submitted for individual papers. This paper is part of the Journal of Bridge Engineer- ing, © ASCE, ISSN 1084-0702. © ASCE 04021059-1 J. Bridge Eng. J. Bridge Eng., 2021, 26(9): 04021059 Downloaded from ascelibrary.org by Asala Dawood on 06/25/21. Copyright ASCE. For personal use only; all rights reserved.