Truss-arch model for shear strength of shear-critical reinforced concrete columns

Experimental observations on 11 Reinforced Concrete (RC) columns tested at Nanyang Technological University (NTU), and existing experimental data of 79 shear-critical RC columns are presented. Significant arch action is found in columns of small shear span-to-depth ratio and high axial-load ratio under shear force. Utilizing the sectional method for shear strength of these types of columns, which does not consider arch action, would give a more conservative prediction. Based on the truss-arch model, an expression to predict the shear strength of shear-critical RC columns is presented, which considers both the contributions of concrete and transverse reinforcement to shear strength in the truss model as well as the contribution of arch action through compatibility of deformation. The proposed model is compared with other shear strength models using the available column test data consisting of 90 shear-critical RC columns, and the results show that the proposed model can improve the accuracy of shear strength predictions for shear-critical RC columnsAccepted versio

Evaluation of shear strength design methodologies for slender shear-critical RC beams

This paper seeks to examine the concrete contribution to shear strength, and determine the inclination of the compressive strut within the variable truss model for slender reinforced concrete (RC) shear-critical beams with stirrups. Utilizing the Modified Compression Field Theory (MCFT) in place of the conventional statistical regression of experimental data, the expression for the concrete contribution to shear strength was derived and the inclination of compressive struts determined. A simplified explicit expression for shear strength was then provided, with which shear strength can be calculated without extensive iterative computations. This method was then verified using the available experimental data of 209 RC rectangular beams with stirrups and compared to the current methods in ACI 318R-08 and CSA-04. The theoretical results are shown to be consistent with the experimentally observed behavior of shear-critical RC beams.Accepted versio

Dynamic Response Analysis of the Floor Structure under Random Crowd Excitation

The popularity of new structural systems and prestressing technology has led to the widespread use of the large-space floor structures in large buildings such as high-speed rail terminals, conference centers, and sports stadiums. The reduction of nonessential load-bearing elements and the increase in span of the structure result in a reduction in the natural frequency and damping ratio of the floor structure, while the floor is a crowded area with disorderly flow between people, which may lead to human-induced vibration problems. In order to assess the dynamic performance of the large-span floor structure under crowd load, the random crowd-floor vertical interaction equation is derived, and the correctness of the equation is verified by comparing it with the test. For the stochastic nature of walking crowds, a formulation modeling method for random crowd is proposed, including pedestrian-dynamics parameters, formulation model, and response parameters. The model is characterized by considering inter- and intrasubject variability and reflects the vertical interaction between pedestrians and the floor system. According to the random crowd-floor dynamic equation, the variation of modal parameters and acceleration response of the floor during random crowd walking are also analyzed. The research in this paper will help in analyzing the comfort of large-span floor structures under pedestrian excitation and better meet the needs of the development of lightweight large-span structures

Re-evaluation of CEB-FIP 90 prediction models for creep and shrinkage with experimental database

This paper aims to evaluate the CEB-FIP 90 model, which is commonly utilised to predict the creep and shrinkage effects of concrete structures, by comparing it with an extensive compiled database which combines the available data in literature and newly collected data from China. This database considers only concrete specimens with an average 28-day compressive strength between 30 MPa and 80 MPa, and restricts the relative humidity of the experimental environment to a maximum value of 95%. Three statistical methods are applied to evaluate the CEB-FIP 90 model: the residual method, the B3 coefficient of variation method, and the CEB coefficient of variation method. Based on the statistical regression analysis of the shrinkage and creep test data, the CEB-FIP 90 model is revised by modifying the influencing coefficients of the compressive strength of concrete and the time development functions of creep and shrinkage. The modified model is then subjected to evaluation and verification using the residual method, B3 coefficient of variation method and CEB coefficient of variation method. Based on verification with experimental data and corroboration with statistical analysis, the modified model performs better than CEB-FIP 90 model, especially with regards to high strength concrete.Accepted versio

Seismic behaviour of lightly reinforced concrete structural walls with openings

An experimental investigation was carried out to examine the seismic behaviour of three lightly reinforced concrete walls with openings subjected to reversed cyclic loading. The three specimens consisted of one solid wall as the control specimen while the other two walls were detailed with regular or irregular openings. The test results indicate that all three specimens eventually failed when the outermost reinforcing bars fractured while the concrete in the compression zone crushed and spalled severely near the base. The specimen with five openings had ultimate strength and stiffness degradation similar to the control specimen. The specimen with nine openings had a lower ultimate strength but exhibited higher ductility, slower stiffness degradation and a more significant shear contribution than the control specimen. Furthermore, strut-and-tie models were developed to predict the ultimate strength of walls with openings. The results obtained from the strut-and-tie models were found to be consistent with the experimentally observed results.Published versio

Statistical Evaluation of CEB-FIP 2010 Model for Concrete Creep and Shrinkage

An extensive experimental database consisting of 2838 shrinkage data points and 3598 creep data points is used to evaluate the accuracy of the newly proposed CEB-FIP 2010 model in predicting the creep and shrinkage of concrete structures. To study the applicability of the model for high-strength concrete in general environments, the database was developed by only retaining the test data of concrete components with the average compressive strength greater than 30 MPa and the relative humidity in the test environment less than 95%. On this basis, combined with the B3 and CEB variation coefficient methods, the paper mainly adopts the residual method to assess the accuracy of the CEB-FIP 2010 model and compare it with the previous model, CEB-FIP 1990. The influences of several properties, such as the compressive strength, the age of concrete, the relative humidity, and the component size on the prediction accuracy of these two models are further studied. The results show that for the CEB-FIP 2010 model within the time interval of 0–9000 days, 52% and 48% of the shrinkage strain residuals of the total specimens are located in the negative and positive regions, respectively, while the positive and negative regions of the CEB-FIP 1990 model account for 73% and 27%, demonstrating the CEB-FIP 2010 model has better performance in predicting shrinkage strain than the CEB-FIP 1990 model, whereas the two models have comparable accuracy in predicting creep compliance. The CEB-FIP 2010 model is more reliable for considering the effects of compressive strength, relative humidity, and age at loading on shrinkage and creep than for considering the effect of member size

Mechanical behavior of grouted sleeve connection for rebars in precast concrete structures and reliability analysis considering the ratio of anchorage length to rebar diameter

Grouted sleeve connection is the widely used method for the connection of stressed steel reinforcement bars in precast concrete structure. The reliability of the connection depends on the bond strength between the steel bars and the grouting materials. In this study, the uniaxial tensile test of 204 grouted sleeve connection specimens was carried out. The test parameters included steel bar diameter, steel bar anchorage length, grouting material strength, etc. Test results indicate that there are two main failure modes for the specimens, steel reinforcement tensile fracture and bond failure, which are mainly affected by the anchorage length of steel bars. When the anchorage length is greater than 5ds, the main failure mode is steel bar tension fracture. Furthermore, 236 sets of data in the relevant literatures were collected. Combined with the experimental data, a database of bond strength between steel bars and grouting materials containing 334 sets of data was established, and a calculation formula of ultimate bond strength of grouted sleeve with high applicability was proposed. Finally, the reliability of the grouted sleeve connection with different anchorage lengths and the failure probability under different anchorage lengths were analyzed. On this basis, the minimum anchorage length for different types of steel bars was proposed. The minimum anchorage length of HRB400 steel bars ranges from 5ds to 8ds, the minimum anchorage length of HRB500 steel bars ranges from 7ds to 10ds, and the minimum anchorage length of HRB600 steel bar ranges from 9ds to 12ds

Nonlinear Creep Amplification Factor Considering Damage Evolution of Concrete under Compression

Creep affects the long-term deformation of concrete structures. Nonlinear creep further overestimates the safety factor of structures and affects the safety service performance. The coupling of creep and a damage model considering the rate effect is conducive to accurate prediction of nonlinear creep, but the iterative process of strain makes the calculation method more complex. The purpose of this study is to propose a nonlinear creep explicit method that considers the damage evolution of concrete under compression. Two groups of axial compression members with compressive stresses of 0.2 fc and 0.4 fc were made. Considering the law of concrete damage evolution under uniaxial compression, coupled with elastic creep and damage incremental strain, the lower limit of the medium stress level that gives rise to nonlinear creep is analyzed. The concrete nonlinear creep amplification coefficient with a loading age of 28 days and loading duration of 360 days is studied with consideration for the uncertainty of relative humidity and the theoretical thickness of the component. On this basis, the explicit calculation formula of the nonlinear creep amplification coefficient related to the concrete axial compressive strength and stress level is given. The results indicate that the nonlinear creep amplification coefficient increases nonlinearly with an increase in the stress level, and, when the compressive stress level ratio is higher than 0.6, the nonlinear creep amplification coefficient increases significantly; when the stress level is determined, the creep amplification coefficient decreases gradually with an increase in the compressive strength of the concrete. It is suggested that a stress level range of 0.35~0.75 should be used for the study of a nonlinear creep amplification factor under the medium stress state

Theoretical and Experimental Study of Effective Shear Stiffness of Reinforced ECC Columns

Abstract Engineered cementitious composites (ECC) possesses characteristics that make it suitable in the zones of high shear and ductility demand of structural elements; however, there is a lack of an adequate model to predict its shear stiffness. A theoretical model for the effective shear stiffness of reinforced ECC (RECC) columns is proposed on the basis of the truss-arch model, with the consideration of the unique property of ECC material. A total of six column specimens subjected to cyclic reverse loading are conducted, and the main test variables include the shear span-to-depth ratio, the transverse reinforcement ratio and the axial load ratio. Results show that the shear contribution to the total deflection in the diagonally cracked RECC beam is significant, and the proposed theoretical model can predict the shear deformation with reasonable accuracy

Seismic Performance Analysis of RC Frames with ECC Short Columns Based on the IDA Method

Engineered cementitious composite (ECC) is a high-performance composite material with greater shear deformation and shear strength than normal concrete, which has been proposed for use as a shear component in structures. This study modeled three frames, a pure reinforced concrete (RC) frame, an RC frame with concrete short columns and an RC frame with ECC short columns, using the incremental dynamic analysis (IDA) method to evaluate the contribution of ECC to the structural performance. A modified IMK model was applied to model the entire history of the mechanical behaviors of the short columns. The IDA curves, interfloor displacement angle distribution and limit state of the vertex displacement of the frames were analyzed to investigate the seismic responses of the frames. The model analysis results showed that an RC frame with short columns would form a weak layer on the floor where the short columns were located, which greatly weakened the seismic performance of the structure. ECC was certified to be effective in improving the shear formation of the short columns in the frames. The frame with ECC short columns improved the seismic performance of the structure to a certain extent relative to the frame with RC short columns. The deformation capacity of the frame with ECC short columns was close to that of the pure RC frame at the collapse level