Four Discussions from the September 2019 ACI Structural Journal

Disc. 116-S113/From the September 2019 ACI Structural Journal, p. 201 Reinforcement Limits for Reinforced Concrete Elements with High-Strength Steel. Paper by A. W. Puranam and S. Pujol Disc. 116-S117/From the September 2019 ACI Structural Journal, p. 247 Novel Empirical Expression to Predict Shear Strength of Reinforced Concrete Walls Based on Particle Swarm. Paper by Hadi Baghi, Hani Baghi, and Sasan Siavashi Disc. 116-S141/From the November 2019 ACI Structural Journal, p. 251 Torsional Behavior of Reinforced Concrete Beams with High-Strength Steel Bars. Paper by C. Kim, S. Kim, K.-H. Kim, D. Shin, M. Haroon, and J.-Y. Lee Disc. 117-S04/From the January 2020 ACI Structural Journal, p. 43 Bond Behavior of 0.6 in. (15.2 mm) Prestressing Strand in Beltic Calcium Sulfoaluminate (BCSA) Cement Concrete. Paper by T. M. Bowser, C. D. Murray and R. W. Floy

Simplified Strut-and-Tie Model for Shear Strength Prediction of Reinforced Concrete Low-Rise Walls

In this study, an analytical model using the strut-and-tie concept was developed to predict reinforced concrete (RC) low-rise wall shear strengths. In the model, the failure mode considered was crushing of the diagonal compression strut. To accurately determine the strut area, a formula for calculating depth of compression zone at the bottom of wall was derived with the aid of nonlinear finite element analysis. A total of 100 RC low-rise wall specimens failing in shear obtained from available literature were used to verify the accuracy of wall strength predictions of the proposed strut-and-tie model. Furthermore, strength predictions from building codes and other analytical models were also included for comparison purposes. The analysis results show that the proposed strut-and-tie model is conservative and it has the lowest coefficient of variation as compared to other methods in predicting the shear strength of RC low-rise walls. In addition, the predictions of the proposed model are quite consistent and less scattered for wide ranges of wall height-length ratios and concrete compressive strengths

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Shear Strength of Normal to High Strength Concrete Walls

This paper presents an analytical study on the behavior of normal to high strength concrete walls. Experimental data of concrete walls were collected from available literatures and several building code provisions were evaluated by comparing nominal wall strengths calculated using code formulas with experimental wall strengths. Moreover, behavior of concrete walls as influenced by various parameters was investigated by plotting the normalized experimental wall strengths and average shear stresses against shear span ratio, axial load ratio, web reinforcement ratio, and concrete strength. The analysis results show that most code formulas underestimate wall shear strengths. It is shown that longitudinal web reinforcement also has contribution to the shear strength of concrete walls even though it is not accounted in code formulas. Furthermore, the accuracy of code formulas is also affected by variation in concrete strength. For example, the ACI code considerably underestimates the shear strength of high strength concrete walls due to its limitation on maximum wall shear stress which is quite conservative for high strength concrete walls. Thus, a modification of ACI code formula is proposed to enhance its accuracy. The results show that the modified formula yields better predictions of both normal and high strength concrete wall shear strengths

Cyclic Shear Behavior of High Strength Concrete Structural Walls

High-strength concrete (HSC) walls having compressive strength of approximately 100 MPa (14,500 psi) were tested under cyclic lateral loading to investigate their shear behavior. The parameters included were height-to-length ratio of the walls, vertical and horizontal web reinforcement ratios, and the effects of boundary elements in the form of flanges. The experimental results show that shorter walls exhibit greater shear strength than taller walls. Both vertical and horizontal web reinforcements contribute significantly to increasing the shear strength of the walls, with the horizontal web reinforcement being more effective for walls having height-to-length ratio from 1.0 to 2.0. With increase in height-to-length ratio of walls, the concrete contribution to the shear strength decreases while the web reinforcement contribution increases. The presence of flanges also significantly increases the shear strength of HSC walls. Experimental wall shear strengths from this study as well as from literature were compared with predictions from the ACI Code and Eurocode provisions. It can be seen that both ACI and Eurocode do not give consistent safety factors. The ACI method can be unsafe for low-strength concrete walls, while the Eurocode is overly conservative in almost all cases

Shear Analysis of Reinforced Concrete Slabs with Effective Moment of Inertia

The effective moment of inertia concept has been used to evaluate the bending stiffnesses of reinforced concrete (RC) members in design codes. Later, it was combined with finite element (FE) methods to calculate the deflection responses of RC slabs. However, the failure of the slabs cannot be predicted by the effective moment of inertia based method. In this work, an empirical failure criterion for RC slabs under bending and shear was adopted and modified to consider the stiffness degradation of shell elements in FE analysis. In order to predict the shear failure of the slabs, a softening curve for the bending and shear stiffnesses was proposed. The model parameters in the failure criterion were calibrated according to published test results. The comparison between the numerical and the experimental results shows that the proposed method can predict the deflection response and the shear strength of the analyzed slabs with acceptable accuracy

Truss Model for Shear Strength of Structural Concrete Walls

Numerous methods for calculating shear strengths of structural walls are available. However, due to the complexity of wall behaviors and possible loading combinations that they may be subjected to, it is quite challenging to derive a method that is reasonably simple but can accommodate various influencing parameters in order to acquire more accurate predictions of wall shear strengths. The authors had earlier tested a series of very-high-strength concrete wall specimens (fc′ = 100 MPa [14,500 psi]) to investigate the influence on shear strength of several parameters, such as: height-to-length ratios, shear (web) reinforcement ratios in the vertical and horizontal directions, as well as the presence of flanges (boundary elements). The conclusions of the authors’ experimental study in the light of other research results reported by other researchers will be summarized herein and will be used as a guide for deriving a proposed truss model. The proposed model is based on modern truss analogy principles (softened truss model, compression field theory) and it has been shown by comparing it with experimental results to be accurate and stable. The design and analysis procedure based on the proposed truss model will also represent an improvement over existing ACI and Eurocode design procedures

Analytical Study on High Strength Concrete Shear Walls

This paper presents an analytical study on the behavior of high strength concrete (HSC) shear walls. Several experiments on HSC shear walls with concrete strength above 60 MPa have been selected to be studied. Data from various experiments were collected and nominal wall strengths have been calculated using several building code formulas, such as those of the ACI (American), AIJ (Japanese), and EC (Eurocode). Subsequently, nominal wall strengths from the building code formulas were compared with actual wall strengths from experiments. Moreover, normalized actual wall strengths over nominal wall strengths and the average shear stresses were also plotted against some significant factors such as shear span ratio, axial load ratio, ratio of longitudinal and transverse reinforcements, etc., in order to observe the behavior of HSC shear walls as influenced by various parameters. The analysis results show that most of the building code formulas underestimate HSC wall strengths for low shear-span ratio (below 2.0) but they predict more accurately for high shear-span ratio (above 2.0). Furthermore, from the results, it seems that axial load up to 0.15 (f�c Ag) does not contribute much to the wall strengths. In addition, the comparative study shows that the contribution of longitudinal reinforcement to wall strengths is more significant than that of the transverse reinforcement. This phenomenon is not accounted for in most building code formulas. Thus, there is a need to develop an expression that can take into account this phenomenon and that can yield better predictions of the strength of HSC walls

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Punching Tests of Double-Hooked-End Fiber Reinforced Concrete Slabs

Ten (10) high-strength concrete slabs reinforced with new type of steel fibers, the double hooked-end steel fibers, were tested under punching shear loads. The strength of the concrete fc varied from 80 to 100 MPa. The fiber content Vf varied from zero to 1.2%. Two different values of flexural reinforcement ratios ρ (= As/bd) of 0.9% and 1.4% were chosen for this test program. The experimental results showed that the use of double hooked-end steel fibers in the concrete enhances slab performance significantly in many ways. As the fiber volume or fiber content Vf increased, the flexural stiffness of the slab throughout loading history also increased, while both the deflections and crack widths decreased considerably. At the ultimate load stage, the punching shear strength increased by up to 156% compared to non-fibrous concrete slabs. The increase in punching shear strength is significantly higher than the increase introduced by conventional single hooked-end steel fibers. The ductility of the slabs was also significantly improved. Comparisons between design methods with experimental results show that the design method by Concrete Societys TR-34 performs very well. Another method that was based on the yield line theory overestimates the strengths of the slabs. Model Code 2010 method also overestimates the punching shear strengths. Finally, some relevant design recommendations are given