Fatigue and static behaviour of post-tensioned steel-concrete composite beams

Abstract

Strengthening steel and concrete composite (SCC) bridge superstructures using post-tensioned steel tendons offers many advantages. Since bridge structures are usually exposed to fatigue cycles, understanding the fatigue behavior of strengthened composite beams is vital. Also, bridges in practice are subject to various types of damages. Thus, when strengthening an in-service bridge component, the effect of pre-fatigue conditions on the effectiveness of post-tensioned steel tendons should be considered. In this research, the fatigue and static behavior of damaged and undamaged post-tensioned steel-concrete composite beams were evaluated experimentally and numerically. Experimental fatigue tests were conducted on five SCC samples under four-point fatigue loading. The effect of external post-tensioning (PT) as a retrofit on the cyclic crack patterns on the concrete flange, cyclic incremental deformations, and strains were investigated with various pre-fatigue conditions. The pre-fatigue conditions included exposure to outdoor environmental changes for 365 days, plastic deformations, and fatigue damages. The results of these experimental fatigue tests showed that external PT enhanced the performance of the individual components of the composite samples (steel beams, concrete flanges, and shear connectors), which enhanced the overall fatigue performance of the strengthened samples. However, the strengthened samples experienced longitudinal fatigue cracks in the concrete flanges because of PT while such cracks were not observed in the non-strengthened sample. The pre-damage conditions due to fatigue loading or environmental changes caused higher damages in concrete around the studs relative to the plastically pre-deformed strengthened sample and led to more incremental deformations and strains. Experimental static tests to failure were performed on the fatigued samples to investigate their residual capacities, deformations, and strains. The results of the static testing program showed that the crack patterns in the concrete deck were significantly affected by the type of pre-damage that was applied before the post-tensioning. The static overloading pre-damage reduced the number of cracks and their rate of propagation while the exposure to outdoor environment pre-damage induced more cracks, which negatively affected the crack patterns during fatigue loading. Subjecting the sample to the pre-damage of plastic deformation slightly improved its performance in terms of residual stiffness and ultimate load. The residual ultimate load was increased by 7 percent relative to the fatigued sample without pre-damage. However, the ductility was reduced by 40 percent due to the initial plastic deformation. This reduction in ductility was accompanied with a decrease in the interface slippage between the concrete deck and steel flange. Numerical investigations were performed to study the monotonic behavior of externally post-tensioned steel-concrete composite girders. A three-dimensional numerical model was developed, which was validated using the experimental test results presented in this research as well as experimental test results that are available in the literature. A parametric study using this validated numerical model was performed to investigate the effects of various parameters on the monotonic performance of composite girders strengthened with external post-tensioned tendons. The parameters investigated include variations in the degree of shear connection, layout and diameter of shear connectors, the initial post-tensioning force, the depth of the steel beam, the eccentricity of the tendons, the compressive strength of concrete, and the shear capacity of the studs. The numerical model provided a better understanding of the effect of these parameters on the behavior of the strengthened beams. The results of the parametric study show that the slippage between the concrete deck and steel beam increased as the degree of shear connection decreased. Also, as the shear connection degree decreased, its effect on the slippage behavior increased. The study also shows that, for the same degree of shear connection, beams with one row of shear studs had higher flexural capacity than beams with two rows of studs. The load-deflection and slippage behavior improved when smaller diameters of the studs were used. Also, The higher the post-tensioning force the higher the ultimate load capacity and the lower the tensile strains in the steel beam. The tensile strains at midspan were considerably reduced by increasing the depth of the steel beam. The lowest midspan tensile strains were obtained from the combination of increasing the depth of the steel beam and post-tensioning tendon eccentricity. Furthermore, numerical investigations were performed to study the behavior of damaged strengthened steel-concrete composite girders. Loads throughout the service life of bridges may cause failure in the form of fracture in the studs near the ends of the bridge girders. The effect of this stud failure on the residual static capacity and the residual fatigue life of composite girders is not well investigated in the literature. Therefore, this research presents a numerical investigation on the residual behavior of prestressed composite beams with fractured studs at the end regions of the beams. The objective of this study is to investigate the effects of the progressive failure of stud shear connectors on the residual static performance and the remaining fatigue life of strengthened steel-concrete composite beams. Also, the effect of stud fracture on the slippage, shear stress range and compressive and tensile strains were studied. The behavior of the composite girders in terms of the estimated fatigue life and residual capacity was affected by the number of removed studs. Until 15 percent of the rows were removed, the strengthened sample had a better response in terms of the tensile and compressive strains and residual ultimate load. The effect of stud fracture on the shear stress ranges experienced by the shear connectors mainly manifested at the beam ends where stud fracture occurred. Also, neglecting the steel-concrete interface slippages in the theoretical calculation of stud shear stress ranges resulted in a significant underestimation of the shear stress ranges experienced by the shear connectors. In summary, in this research, the effect of external PT and the effects of pre-existence of damage on the performance of the different components of the steel-concrete composite section during fatigue were experimentally investigated. The residual capacities of damaged and non-damaged fatigued steel-concrete composite beams strengthened with externally post-tensioned steel tendons were also experimentally tested. Furthermore, a three-dimensional finite element (FE) model that can simulate the behavior of composite steel-concrete girders strengthened with external post-tensioned steel tendons was developed and validated. Then, numerical parametric studies were performed using the validated FE model to investigate the effects of various important parameters on the flexural behavior of the strengthened composite girders. Also, the effect of progressive stud fracture on the behavior of strengthened SCC girders was investigated.