Mohamad Mansour

Stiffness, ductility, and energy dissipation of RC elements under cyclic shear

A new Cyclic Softened Membrane Model (CSMM) was recently developed to predict the stiffness, ductility, and energy dissipation of reinforced concrete (RC) elements subjected to reversed cyclic shear. Using the nonlinear finite element analysis, we can integrate these responses of elements to predict the behavior of a whole structure, such as a low-rise shear wall, subjected to earthquake action. This study of CSMM summarizes systematically the effects of the two primary variables: the steel bar angle with respect to the direction of the applied principal stresses and the steel percentage. The results clearly show that RC structures under cyclic shear stresses could be designed to be very ductile, have large stiffness, and possess high energy-dissipation capacities (just like flexural-dominated elements), if the steel bars are properly oriented in the directions of principal stresses and if the steel percentages are kept within certain limits.

Nonlinear Analysis of Shear-Critical Reinforced Concrete Beams Using Fixed Angle Theory

This paper proposes a solution methodology for the application of the fixed-angle theory to predict the shear response of reinforced concrete (RC) beams subjected to the combined actions of shear and flexure. The proposed solution, based on the fixed-angle theory, takes into account the effect of flexural moment on the shear strength of RC beams and calculates the concrete constitutive relationships by transforming the concrete stresses and strains from the principal direction of concrete stresses to that of the applied stresses. To verify the effectiveness of the proposed solution method, seven shear-critical RC beams were tested and their corresponding experimental shear stress-strain relationships were compared with the predicted ones by using the proposed methodology. Furthermore, the shear strengths of 150 RC test beams, reported in the literature with various shear span-to-depth ratios, steel reinforcing ratios, and support conditions, were compared with the predicted shear strengths obtained by the proposed method and other existing truss models. The results presented in this paper show that the proposed formulation can predict the shear response of RC beams with reasonable accuracy.

Pinching Effect in Hysteretic loops of R/C Shear Elements

This paper presents a non-linear analytical model capable of describing pinching behavior in hysteretic loops of reinforced concrete shear elements. The model is an extension of the fixed angle softened truss model (FA-STM) proposed for monotonic loading. The extension of FA-STM for application to reversed cyclic loading requires new constitutive models for concrete and steel in the unloading and reloading ranges. The three principles of the mechanics of materials: equilibrium, compatibility and constitutive relationships are satisfied by this rational theory. The paper illustrates the validity of this theory by comparing the behavior of three panels with three different steel bar angles.

Prediction of damage in R/C shear panels subjected to reversed cyclic loading

In this paper, the damage prediction of shear-dominated reinforced concrete (RC) elements subjected to reversed cyclic shear is presented using an existing damage model. The damage model is primarily based on the monotonic energy dissipating capacity of structural elements before and after the application of reversed cyclic loading. Therefore, it could be universally applicable to different types of structural members, includeing shear-dominated RC members. The applicability of the damage model to shear-dominated RC members is assessed using the results from reversed cyclic shear load tests conducted earlier on eleven RC panels. First, the monotonic energy dissipating capacities of the panels before and after the application of reversed cyclic loading are estimated and employed in the damage model. Next, a detailed comparison between the analytically predicted damage and the observed damage from the experimental tests of the panels is performed throughout the loading history. Subsequently, the effects of two important parameters, the orientation and the percentage of reinforcement, on the damage of such shear-dominated panels are studied. The research results demonstrated that the analytically predicted damage is in reasonably good agreement with the observed damage throughout the entire loading history. Furthermore, the orientation and percentage of reinforcement is found to have considerable effect on the extent of damage.

Cyclic Softened Membrane Model for Nonlinear Finite Element Analysis of Concrete Structures

This paper describes how a Cyclic Softened Membrane Model (CSMM) was developed to rationally predict the cyclic shear responses of reinforced concrete (RC) elements, including the pinching effect in the hysteretic loops, the shear stiffness, the shear ductility and the energy dissipation capacities. This CSMM model was verified by the tests of fifteen RC panels at the University of Houston. The test results confirmed that the orientation of the steel bars and the percentage of steel in a panel are the two most important variables that influence the cyclic response of RC panel elements. Using OpenSees as a framework, the concept of the CSMM was simplified from a 2-D model into a 1-D model and implemented into a finite fiber element program for the prediction of concrete frame structures subjected to cyclic or dynamic loading. The developed program is validated by the reversed cyclic load tests of a reinforced concrete column and by the shake table tests of a prestressed concrete frame. The CSMM has recently been implemented into an OpenSees-based finite element program for a 2-D RC element that will allow structural engineers to predict the monotonic, cyclic and dynamic responses of structures containing walls. This 2-D RC element is validated in this paper by the prediction of the monotonic responses of two RC panels subjected to shear stresses.

Nonlinear Analysis of R/C Low-Rise Shear Walls:

An analysis method for predicting the response of low-rise shear walls under both monotonic and cyclic loading is presented in this paper. The proposed analysis method is based on the softened truss model theory but utilizes newly proposed cyclic constitutive relationships for concrete and steel bars obtained from cyclic shear testing. The successfulness of the analysis method, when combined with new materials constitutive relationships, is checked against the test results of 33 low-rise shear walls reported in the technical literature. The theoretical predictions are compared with the reported test results and are found to be applicable throughout the loading history. The effects of loading types (monotonic versus cyclic) as well as the effects of the boundary elements on the predicted results are also addressed.

Axial Strain of Reinforced Concrete Columns

The reinforced concrete members are designed to fail in flexural member to behave ductilely. Also the failure doesn’t impose on columns but beams. But according to the plastic collapse mechanism, the plastic hinge potentially developed at the bottom of the RC column near the base of the structure after flexural yielding. These columns are generally dominated by shear which led to sudden failure in post yielding region because of its relatively short span-to-depth ratio, so special care is needed. The deformability of column with short span-to-depth ratio is small compared with larger span-to-depth ratio column under reversed cyclic loading. Therefore the design of these kinds of RC columns necessitates the prediction of both the shear strength after flexural yielding and corresponding ductility of such members. Ten RC columns with varying axial force ratio and shear reinforcement ratio were tested under monotonic and reversed cyclic loading. The most affectable factor to column behavior was the axial force. The result indicates that concrete contribution to shear resistance in the plastic hinge region and axial strain were decreased as axial force.