Ayman El-Zohairy

Parametric study for post-tensioned composite beams with external tendons

Strengthening of bridge superstructures composite beams with external post-tensioned tendons is a good technique for strengthening the existing structures. In this study, a numerical model is illustrated to study the nonlinear simulation of composite beams stiffened with externally post-tensioned tendons. The accuracy of the developed numerical model is validated using comparisons between the numerical and existing test data. The influence of various strengthening parameters is investigated, which include draped versus straight tendons, tendon length, the effect of post-tensioning on reinstating the flexural behavior of an overloaded beam, tendon eccentricity, and the degree of shear connection. A good agreement between the proposed model and the test data is obtained. The results demonstrate that at the same tendon eccentricity, the trapezoidal profile shows better behavior for the strengthened beams. However, more ductility is obtained when using the straight tendon profile. Applying post-tensioning through the beam of full length helps to reduce the creation of fatigue cracks, which always start at stress raisers, and subsequently increases the fatigue life of the composite beam. Also, the external post-tensioning effectively maintains the flexural behavior of the overloaded strengthened beam after unloading in comparison to the un-strengthened beam. It is observed that 80% degree of shear connection or higher is recommended to obtain the desired performance of the external post-tensioning force for strengthening composite beams.

Finite-Element Modeling of Externally Posttensioned Composite Beams

AbstractPosttensioning has been used successfully to improve the performance of existing bridge structures. High-strength tendons can be used to effectively increase the ultimate capacity of composite beams. The main purpose of this paper is to develop a reliable three-dimensional (3D) finite-element (FE) model to simulate the nonlinear flexural behavior of steel–concrete composite beams strengthened with externally posttensioned tendons. A 3D FE model was used, where the nonlinear material behavior and geometrical analysis based on incremental–iterative load methods were adopted. The effective posttensioning stress was applied as initial strain in the link element used to model the tendons. To verify the accuracy of the developed 3D FE model, comparison between the FE analysis results and previous experimental results is presented. In-depth study has been carried out on the overall behavior of the strengthened beam and the effect of external posttensioning on stiffness, induced stresses, slippage between...

Experimental Investigation of Segmental Post-tensioned Girders

The structural behavior of precast concrete segmental bridges largely depends on the behavior of the joints between segments. In this research, series of static tests were carried out to investigate the behavior of full-scale post-tensioned segmental girders with different types of joint configuration; multi-key joint, single key, and plain key joint. The reference specimen was monolithically cast girder and the other specimens consisted of five segments for each one joined together with epoxy paste. The testing measurements included deflection, elongation of strands, cracking load, ultimate strength and strains at different locations and loading stages. The general theme abstracted from the results of cracking loads tests reflects an approximate similarity in the behavior of the four types of girder with slight differences. In the segmental girders with different types of joints, it was noticed that the cracks at the joints occurred in the concrete adjacent to the epoxy mortar which can be attributed to the high tensile strength of the used epoxy in comparison to concrete.

A Case Study to Evaluate Live Load Distributions for Pre-stressed RC Bridge

The live load distribution factors (LLDFs) are typically used to calculate the stress resultants of the individual components from the total shear force and bending moment acting on the entire bridge cross-section. In this study, an evaluation of the LLDFs, suggested by the American Association of State Highway and Transportation Officials (AASHTO) specifications, for a pre-stressed reinforced concrete (RC) bridge was presented. A bridge consisting of five pre-cast pre-stressed RC girders was investigated experimentally and numerically using three cases of loading to obtain the maximum design live load and deflection values for all girders. In addition, a finite element (FE) analysis was implemented for the bridge by using ANSYS. Comparisons among the FE results and the available measured deflections showed a good agreement. The responses of the bridge, measured during the static loading test and the FE analysis, was used to evaluate the LLDFs presented by AASHTO. In addition, the FE results were used to evaluate the effect of Cross-Frame Diaphragms (CFD) on the LLDFs for pre-stressed RC bridges. Including CFD increased the LLDFs for exterior girders and decreased LLDFs for internal girders which confirmed the significance of the CFD to distribute live loads among the girders.

Experimental and Numerical Evaluations of Live Load Distributions of Steel-Concrete Composite Bridge

Live-Load Distribution Factors (LLDFs) are commonly used by the bridge engineers to represent the placement of design lanes to generate the extreme effect in a specific girder. Summing all of the distribution factors for all girders produces a number of design lanes greater than the bridge can physically carry. This research aims to assess the LLDFs suggested by the American Association of State Highway and Transportation Officials (AASHTO) specifications for steel-concrete composite girders bridge. A bridge consisting of four steel plate girders, 1650 mm depth; 33,950 mm span; and connected by shear connectors to a reinforced concrete deck, was investigated experimentally and numerically in this work. Eight trucks of 248 kN each were used with different arrangements to achieve the static test and to obtain the maximum design live load and deflection values for all girders. In addition, a finite element (FE) analysis was implemented for the bridge by using ANSYS. Comparisons among the numerical results and the available measured deflections showed a close agreement. The responses of the bridge, measured during the static test and the FE analysis, was used to assess the LLDFs presented by AASHTO. In addition, the proposed FE model was used to assess the LLDFs for shear and the effect of Cross-Frame Diaphragms (CFDs) on the LLDFs for composite steel girder bridges. The significance of the CFDs to distribute live loads among the girders was confirmed by increasing the LLDFs for exterior girders and decreasing the LLDFs for internal girders.

Experimental Study on Fatigue Performance of Steel-Concrete Composite Girders

Besides the static load, the fatigue load caused by vehicles always exists in the bridge structures. This type of loading may cause failure even when the nominal peak loads are less than the ultimate capacity of the structure. In this paper, an experimental work, consists of two specimens, was introduced to study the fatigue behavior of shear connectors and steel-concrete composite beams. The fatigue tests were conducted under a four-point bending test with two different stress ranges in a constant amplitude. The testing measurements during the fatigue test included deflection, strain in the shear connectors, and slippage between the concrete flange and the steel beam. After completion of the fatigue tests, it was obvious that providing top and bottom longitudinal reinforcement as well as enough transverse reinforcement in the concrete flange can provide adequate confinement to concrete to limit fatigue cracks in the concrete flange. In addition, a growth in the cyclic deflection limits was obtained by increasing the number of cycles due to the damage region that developed in the concrete flange by the shear studs and caused a loss of stiffness in the shear connection.

Continuous Composite Beams Stiffened with CFRP Sheet at the Hogging Moment Region

The improvement in the flexural response and cracking resistance of steel-concrete composite beams stiffened with Carbon Fiber Reinforced Polymers (CFRP) sheets at the hogging moment regions is investigated experimentally and numerically in this study. The experimental program consists of two composite beams tested in an inverted position to simulate a portion of a continuous composite beam at the hogging moment region. The concrete flanges of the beams are reinforced externally with corrugated steel deck which used also as a permanent formwork. The experimental findings are used to develop and validate a reliable finite element (FE) model which able to simulate the non-linear performance of the stiffened beams as well as plain beams. In addition, a FE analysis on full-scale continuous composite beams with reinforced concrete flange is carried out to confirm the influence of using CFRP strips as a strengthening technique at the hogging moment regions. The beam deformation, steel flange strain, concrete strain, and CFRP strain confirm good agreement between the FE analysis and the test data. The beam capacity is enhanced only by 11% due to a premature failure in the composite flange. Although the composite connection between the concrete flange and the steel beam is improved by enhancing the cracking resistance of the concrete, the ductility of the strengthened beams is reduced by 20% in comparison to the un-strengthened beam.

Behavior of Steel-Concrete Composite Beams Under Fatigue Loads

Besides the static load, the cyclic load caused by the vehicles always exists in the bridge structures. This kind of loading may cause failure even when the nominal maximum loads have not exceeded the ultimate resistance of the structure. So, the main objective of this paper is to evaluate the fatigue behavior of composite beams at the sagging moment regions. A numerical model is described to predict the fatigue response of each part of the composite section. The accuracy of the developed numerical model is validated using existing test data. Also, the transformed section method is used to evaluate the fatigue response of the composite beams in terms of deflection, strains in the concrete flange and the steel beam, plastic deformation, reduction in the static strength of the shear connectors, and residual capacity of the composite beam. The cumulative damage rule is analyzed through the S-N curve to assess the procedures presented in the design code. A rapid growth in the residual deformations of the composite beams is obtained at the starting and the end of the fatigue life with the number of cycles while a linear increase in the remaining part of the fatigue life occurs. In addition, a reduction in the static strength of the shear connectors is developed with the number of cycles and subsequently causes a drop in the monotonic beam capacity which can be calculated based on the new shear connection strength.

Finite element analysis and parametric study of continuous steel–concrete composite beams stiffened with post-tensioned tendons:

Externally post-tensioned tendons can cause an initial compressive stress in steel–concrete composite sections at the hogging moment region, and then a part of the tensile stress in the concrete flange can be relieved. This study presents a detailed finite element analysis of the nonlinear flexural response of continuous steel–concrete composite beams strengthened with externally post-tensioned tendons. The initial post-tensioning force is introduced as an initial strain in the truss element that used to simulate the external tendons. The accuracy of the finite element model is validated using existing experimental works. The effects of tendon eccentricity, longitudinal steel rebar ratio, and initial post-tensioning force on the beam behavior are explored. Furthermore, deterministic and stochastic shrinkage effects are carried out to obtain the long-term random responses of the strengthened beams as well as unstrengthened beams. However, the ultimate capacity of the strengthened beam increases only by 8%, the cracked moment redoubles, and an affirmative behavior over the unstrengthened beams is obtained. Also, a rapid decay in the long-term deformation of continuous steel–concrete composite beams is obtained at the early age while a linear decrease in the remaining part of the age occurs.

Behavior of Post-tensioned Steel-Concrete Composite Beams Subjected to Hogging Moments

This study presents a detailed finite element (FE) analysis of the nonlinear flexural response of continuous steel-concrete composite beams strengthened with externally post-tensioned tendons. The initial post-tensioning (PT) force is introduced as an initial strain in the truss element that used to simulate the external tendons. The accuracy of the FE model is validated using existing experimental works. The effects of tendon eccentricity, longitudinal steel rebars ratio, and initial PT force on the beam behavior are explored. Furthermore, deterministic and stochastic shrinkage effects are carried out to obtain the long-term random responses of the strengthened beams as well as plain beams. However, the ultimate capacity of the strengthened beam increases only by 8%, the cracked moment redoubles and an affirmative behavior over the un-strengthened beams is obtained. Also, using prestressed concrete flange deters the propagation of cracks and improves the composite action between the steel beam and the concrete flange. As the longitudinal steel rebars ratio increases, the contribution of the externally posttensioned tendons to enhance the beam capacity is diminished.