Boyan Mihaylov

Behavior of Large Deep Beams Subjected to Monotonic and Reversed Cyclic Shear

This paper describes an experimental study that was conducted on 8 large reinforced concrete deep beams, in which the variables were the quantity of stirrups (0.0 or 0.1%), shear span-depth ratio (a/d = 1.55 or 2.29), and type of loading (monotonic or fully reversed cyclic). Findings showed that all members failed in shear before yielding of the longitudinal reinforcement. The provision of only 0.1% of transverse reinforcement significantly increased the shear strength and significantly decreased crack widths at service load levels. For members with stirrups, the load-deformation response measured under monotonic loading provided a good envelope to the cyclic response and the shear strength under reversed cyclic loading was not significantly reduced. However, members without stirrups tested under reversed cyclic loading failed at significantly higher shear forces than their companion specimens that were tested under monotonic loads.

Three-Parameter Kinematic Theory for Shear-Dominated Reinforced Concrete Walls

This paper is aimed at addressing the need for physically accurate and computationally effective models for predicting the response of shear-dominated reinforced concrete walls. The presented theory is based on a three-degree-of-freedom kinematic model for the deformation patterns in walls with aspect ratios smaller than approximately 3. In the kinematic model, the wall is divided into two parts-a rigid block and a fan of struts-by a diagonal crack. The mechanisms of shear resistance across this crack are modeled with nonlinear springs to capture the prepeak and postpeak shear behavior of the member. The base section of the wall is also modeled to account for yielding of the reinforcement and crushing of the concrete. It is shown that this approach captures well the global and local deformations measured in a test specimen with detailed instrumentation. A more comprehensive validation of the theory is performed with 34 wall tests from the literature. The obtained peak load experimental-to-predicted ratios have an average of 1.03 with a coefficient of variation of 11.6%, while these values for the drift capacity are 0.99 and 16.4%. (c) 2016 American Society of Civil Engineers.

Shear Response of Prestressed Thin-Webbed Continuous Girders

While different design codes provide similar guidance for the flexural design of prestressed thin-webbed continuous girders, the shear design provisions differ greatly. This paper investigates these discrepancies with the help of 11 experiments and a number of analytical studies. Together, these provide the basis for recommendations for engineers conducting the shear design for new girders or evaluating the shear capacity of existing girders. It is shown that the traditional ACI approach of taking Vc as the smaller of Vci (flexural-shear cracking load) and Vcw (web-shear cracking load) can significantly overestimate the shear strength of such girders, particularly if they are highly prestressed and contain relatively small quantities of shear reinforcement. The other codes evaluated provided more conservative predictions. It is shown that the ACI predictions can be improved significantly by taking into account the effect of flexural stresses on web shear cracking. While the ACI Code uses different shear-strength equations for members subjected to external axial loads versus members subjected to internal prestressing forces, the studies summarized in this paper support the idea that unification of these ACI shear provisions is possible.

Behavior of Deep Beams with Large Headed Bars

While anchor heads are particularly useful in the case of large bars, which require significant development lengths, the ACI Code provides design guidance for bars No. 11 or smaller. This paper presents a test of a large deep beam reinforced with a single No. 18 headed bar. The behavior of the beam is evaluated in comparison to the behavior of a specimen with more conventional reinforcement of six No. 8 headed bars, and to the behavior of a specimen with lap-spliced anchor hooks. Despite the extreme detailing of the specimen with a single No. 18 bar, the beam had the same strength as the specimen with six No. 8 bars. The compressive stress in front of the anchor head reached approximately 1.5 times the compressive strength of the concrete, and the stress in the No. 18 bar reached approximately 414 MPa (60 ksi) before the beam failed in shear along a diagonal crack away from the anchorage zone. Shear strength calculations according to Appendix A of the ACI Code showed that the current strut-and-tie provisions can overestimate the shear strength of deep beams by as much as 23%.

Deformation Patterns and Behavior of Reinforced Concrete Walls with Low Aspect Ratios

The assessment of wall structures featuring low aspect ratios remains a challenging problem due to their complex deformation patterns and susceptibility to sudden shear failures under seismic actions. To address this issue, a three-parameter kinematic theory (3PKT) for shear-dominated walls has been recently proposed by Mihaylov et al. (2016). This rational approach uses only three degrees of freedom to predict the complete load-displacement response of a member, and captures shear failures occurring prior to or after the yielding of longitudinal reinforcement. Due to its relative simplicity and computational effectiveness, the 3PKT can be used for the performance-based evaluation of existing structures. To further validate and extend the kinematic approach, an experimental program was performed at the University of Liege, and is the focus of this paper. The test campaign consisted of testing to failure of three cantilever walls with an aspect ratio of 1.7, which featured uniformly distributed longitudinal and transverse reinforcement. The main test variable was the level of axial load which has a significant impact on the failure mechanism and lateral displacement capacity. In addition to more conventional measurements using displacement transducers, the experiments involved the measuring of the complete deformation patterns of the walls using an optical LED system. This paper presents the main test results in terms of load-displacement responses, crack patterns and failure modes. The walls developed major diagonal cracks but failed in flexure-dominated modes. The measured deformation patterns are compared to the patterns predicted by the kinematic model for shear-dominated walls. Even though the failure of the test specimens was governed by flexure, their deformations were predicted well by the model.

Effect of upgrading concrete strength class on fire performance of reinforced concrete columns

High strength concrete (HSC) provides several advantages over normal strength concrete (NSC) and is being used in multi-story buildings for reducing the dimensions of the columns sections and increasing the net marketable area. However, upgrading of concrete strength class in a building may affect the fire performance, due to higher rates of strength loss with temperature and higher susceptibility to spalling of HSC compared with NSC. Reduction of columns sections also leads to increased member slenderness and faster temperature increase in the section core. These detrimental effects are well known, but their impact on fire performance of structures has not been established in terms of comparative advantage between NSC and HSC. In other words, it is not clear whether the consideration of fire resistance limits the opportunities for use of HSC for reducing the dimensions of columns sections in multi-story buildings. This research aims to address this question by comparing the fire behaviour of reinforced concrete columns made of NSC and HSC using nonlinear finite element modelling. The evolution of load bearing capacity of the columns is established as a function of the fire exposure duration. A 15-story car park structure is adopted as a case study with alternative designs for the columns based on strength classes ranging from C30 to C90. Results show that, although the replacement of NSC by HSC accelerates the reduction rate of columns capacity under fire, the columns generally have significant reserves in resistance leading to sufficient fire resistance. This study gives an insight into the impact of replacing stocky sections in NSC by more slender sections in HSC on fire resistance rating for multi-story structures. creased member slenderness which may lead to increased second-order effects. For all these reasons, consideration of fire loading may partly neutralize the advantages of HSC. The objective of this research is to investigate to what extent the fire performance of structural members is affected when HSC is used instead of NSC. After discussing the various factors that can lead to a shorter fire resistance for a HSC design, compared with a NSC design, a case study is adopted to quantify these effects in a realistic design. The case study consists in a 15-story car park structure designed according to Eurocode. It focuses on the fire resistance of the car park columns. Alternative designs are adopted with concrete strength classes ranging from C30 to C90. The evolution of the load bearing capacity of the columns under standard fire is established by numerical modeling in order to assess the consequence of upgrading the concrete strength class in terms of fire resistance of the car park. 2 DESCRIPTION OF THE EFFECTS OF UPGRADING CONCRETE STRENGTH CLASS 2.1 Effect on the rate of strength loss with temperature The reduction of strength with temperature is more severe for HSC than for NSC, particularly in the range of 50-250°C (Phan and Carino, 1998; Cheng et al., 2004). In this study, the reduction of compressive strength at elevated temperature is considered according to the values given in Eurocode 1992-1-2 (EC2, 2004b), see Figure 1. For HSC, strength properties are given in three classes in Eurocode, which depend on the characteristic strength of concrete at ambient temperature. The higher the class, the more pronounced the relative reduction in strength is with temperature. As a result, HSC members lose a larger percentage of their capacity at a given temperature, compared to NSC members.

Macro-Kinematic Approach for Shear Behaviour of Short Coupling Beams with Conventional Reinforcement

Short coupling beams in wall structures work predominantly in shear and develop complex deformation patterns. For this reason they cannot be modelled based on the classical plane-sections-remain-plane hypothesis, and are typically designed with strut-and-tie models. However, because strut-and-tie models are inherently conservative, they can result in very large amounts of shear reinforcement (stirrups), and therefore significant construction difficulties. In addition, strut-and-tie models do not provide information about the deformation capacity of coupling beams, which is important for performance-based seismic design. To address these challenges, this paper discusses a three-parameter kinematic theory (3PKT) for the shear strength and deformation patterns of short coupling beams. The 3PKT approach is situated between simple and conservative strut-and-tie models and complex non-linear finite element (FE) models. While FE models use a large number of degrees of freedom (DOFs) to describe the deformation patterns in coupling beams, the 3PKT method is based on a kinematic model with only three DOFs. The paper presents the formulation of the model and its validation with tests.

Towards Mixed-Type Modelling of Structures with Slender and Deep Beam Elements

Concrete frame structures often include both slender and deep beams. Deep beams possess a large shear capacity, and thus are typically used as transfer girders to carry heavy loads over large spans. The overloading of such members due to extreme events such as earthquakes may result in the collapse of the entire structure. To evaluate the resilience of large frame structures under extreme loading, it is necessary to model the interaction between the deep girders and the rest of the structure in an accurate and computationally effective manner. To address this issue, this paper proposes a mixed-type modelling framework by formulating an innovative 1D macro deep-beam element and coupling it with 1D slender elements. The new macro element aims to combine the accuracy of 2D micro finite elements with the simplicity of 1D macro elements. The paper summarizes the formulations of this element, based on the three-parameter kinematic theory, and integrates it into an existing global nonlinear analysis procedure to create a mixed-type modeling framework. The verification study, including a frame structure with a deep transfer girder, has shown that this approach captures the response of the frame with an accuracy similar to that of 2D micro finite elements, while requiring approx. 20% of the analysis time.

Two-Parameter Kinematic Approach for Shear Strength of Deep Concrete Beams with Internal FRP Reinforcement

AbstractTests of deep concrete beams with internal fiber-reinforced polymer (FRP) reinforcement have shown that such members can exhibit lower shear strength than members with conventional steel reinforcement. To model this effect, the current paper proposes an approach based on a two-parameter kinematic theory (2PKT) for conventional deep beams. The 2PKT is built on a kinematic model with two degrees of freedom that describes the deformation patterns of cracked beams. Using this theory shows that large strains in FRP longitudinal reinforcement result in reduced shear resistance of the critical loading zones (CLZ) of deep beams. The original 2PKT is therefore modified by introducing a reduction factor for the shear carried by the CLZ. The extended 2PKT approach is then applied to a database of 39 tests of FRP-reinforced deep beams from the literature, resulting in an average shear strength experimental-to-predicted ratio of 1.06 and a coefficient of variation of 18.3%. The results show that the 2PKT adequ...