Effect of Deformation History on Punching Resistance of Reinforced Concrete Slabs

Deformation-dependent punching shear resistance often constitutes the decisive design criterion for reinforced concrete slabs supported by columns. The increasing number of aging structures exhibiting insufficient punching resistance and detailing deficiencies, and undergoing changes of usage has resulted in a growing demand for strengthening against brittle punching failure. Hence, the research presented in this thesis aims at gaining a better understanding of the identified problems occurring especially in existing structures, such as pre-deformation. A further objective is the experimental and theoretical validation of the performance of a new strengthening concept. A detailed analysis is carried out of the structural behavior of centrically supported rotation-symmetric slabs subjected to punching. In the first part of this research project a literature review is conducted to characterize the available punching models for new slabs and discuss to what extent they are able to consider the problems linked with existing slabs. Benefits and limitations of current strengthening solutions are illustrated, suggesting local prestressing as a promising concept. In the second part such a concept is analyzed that has been developed to improve the system efficiency and punching resistance by immediately activating the post-installed shear reinforcement composed of carbon fiber-reinforced polymer (CFRP) composites, thus partially unloading the slab. An experimental campaign comprising sixteen slabs verifies that the installation of this strengthening concept leads to a more ductile system behavior and significant increase of punching resistance compared to a non-strengthened slab. In the third part the load-deformation behavior of slabs is analyzed and a modification of an analytical model applied to a slab sector is developed, considering the influence of shear on flexural behavior. An overestimation of the flexural capacity and consequently of the punching resistance is thus avoided. The agreement of the modified sector model results with experimental results is confirmed for a large number of experiments from literature. The model is also capable of predicting the load-rotation responses and punching resistances of slabs strengthened with prestressed CFRP straps. The fourth part of the research concerns the effect of pre-deformation, or load history, on the structural behavior of uniaxial members and biaxial slabs. Single unloading and reloading cycles, which reduce tension stiffening and result in additional deformations, leading to decreased concrete punching resistance, are investigated. In slabs prestressed CFRP straps can compensate the additional deformations caused by load history.

Effect of punching shear on load-deformation behavior of flat slabs

Thomas Keller, Robert Koppitz

Punching shear resistance usually constitutes the decisive design criterion for reinforced concrete flat slabs supported by columns. Modeling approaches based on Kinnunen and Nylander's rotation-symmetric sector model, such as the Quadrilinear Sector Model (QSM), allow the prediction of the slab's load rotation behavior. To take into account an orthogonal reinforcement layout, the steel Young's Modulus is usually reduced. A mechanically better justified flexural stiffness reduction factor is derived here which depends on the longitudinal reinforcement and modular ratio. The QSM also neglects shear deformations, which may reduce the flexural capacity. A Modified Sector Model (MSM) is derived which considers the influence of shear on the flexural behavior. A strength reduction factor for the shear crack-crossing longitudinal reinforcement is introduced which depends on the mechanical longitudinal reinforcement ratio. The factor takes into account that the longitudinal reinforcement also has to transmit forces resulting from shear. The MSM can also be applied to predict the load rotation responses and punching shear resistances of flat slabs strengthened with prestressed carbon fiber-reinforced polymer (CFRP) straps. (C) 2014 Elsevier Ltd. All rights reserved.

Elsevier Sci Ltd2014

Modelling shear–flexure interaction in equivalent frame models of slender reinforced concrete walls

Katrin Beyer, Panagiotis Mergos

Quasi-static cyclic tests on reinforced concrete (RC) walls have shown that shear deformations can constitute a significant ratio of the total deformations when the wall is loaded beyond the elastic regime. For slender RC walls that form a stable flexural mechanism the ratio of shear to flexural deformations remains approximately constant over the entire range of imposed displacement ductilities. This paper proposes a method for incorporating shear-flexure interaction effects in equivalent frame models of slender RC walls by coupling the shear force-shear strain relationship to the curvature and axial strain in the member. The suggested methodology is incorporated in a finite element consisting of two interacting spread inelasticity sub-elements representing flexural and shear response, respectively. The element is implemented in the general finite element code IDARC and validated against experimental results of RC cantilever walls. In a second step, it is applied in inelastic static and dynamic analyses of tall wall and wall-frame systems. It is shown that ignoring shear-flexure interaction may lead to erroneous predictions in particular of local ductility and storey drift demands.

2014

Structural Response of R-UHPFRC - RC Composite Members Subjected to Combined Bending and Shear

Talayeh Noshiravani

The addition of a thin overlay of Ultra-High Performance Fibre Reinforced Concrete (UHPFRC) to Reinforced Concrete (RC) members is an emerging technique to strengthen and protect existing structures and to design durable new structures. Combining UHPFRC with closely spaced, small-diameter steel rebars in Reinforced UHPFRC (R-UHPFRC) layers improves the UHPFRC's strain hardening behaviour. For reasons of practicality, R-UHPFRC layers are cast or glued (in the case of prefabricated elements) on top of RC members, thus changing the latter into R-UHPFRC - RC composite members. The high strength and deformation capacity of R-UHPFRC elements make them a suitable external flexural reinforcement for RC members over intermediate supports, e.g., bridge decks and slabs or beams in buildings. Over reinforcement of RC beams and slabs with tensile flexural reinforcement can result in their shear failure at either a lower resistance or deformation than the associated values for member failure in flexure. A comprehensive experimental program was conducted to study the flexure-shear behaviour of R-UHPFRC - RC composite beams. The program comprises two test series on cantilever beams and continuous beams. The test parameters include shear span-depth ratio (a/d), the amount of transverse reinforcement ( ρν), the amount of longitudinal reinforcement, and the strength and bond condition of the R-UHPFRC rebars. The experimental results reveal the different failure modes of R-UHPFRC - RC composite members and the contribution of the R-UHPFRC elements to the member resistance, ductility and capacity to redistribute the internal stress. It was shown that in R-UHPFRC - RC beams with ribbed rebars and a shear span to depth ratio greater than 2.5 the stresses are carried by beam action. Depending on the degree of longitudinal reinforcement, all but two of the beams with 3.0≤a/d≤3.4 and ρν≤0.17 had a flexure-shear failure; the rest failed in flexure. The flexure-shear failure of the composite beams was at an approximately equal rotation level as their RC reference beam but at a resistance 2.3 times that of the RC beam. This is due to (1) the debonding interface zone between the elements that allows the R-UHPFRC - RC beams to rotate more freely and (2) the out-of-plane resistance of the R-UHPFRC element that contributes to the shear resistance. The internal flow of forces and the structural response of composite members strongly depend on the bond condition between the R-UHPFRC and RC, the UHPFRC and its rebars, as well as the concrete and its rebars. Cracking of the concrete along the interface zone causes bond reduction, i.e., softening of the shear connection, between the two elements. In presence of high shear stresses and diagonal flexure-shear cracks, interface zone softening is observed between the elements prior to the maximum resistance, while UHPFRC is strain hardening. The cause of this softening behaviour is the prying action due to the relative rotational movement of the RC rigid bodies separated by the flexure-shear cracks. Static and kinematic solutions of the theory of plasticity for RC beams are extended to predict the collapse load of R-UHPFRC - RC composite beams at the ultimate limit state. A mechanical model for predicting the structural response of composite beams is proposed. In combination with truss models, the concept of an R-UHPFRC - RC plastic hinge is introduced to calculate the force-displacement response of composite beams. The failure criterion based on the collapse mechanisms (kinematic solutions) sets the limit of the force-displacement response. The model is corroborated by the experimental results. This model provides a tool for analysis of RC members reinforced with an added tensile R-UHPFRC element.

EPFL2012

Structural Response of R-UHPFRC - RC Composite Members Subjected to Combined Bending and Shear

Talayeh Noshiravani

EPFL2012