Degree Of Moment Redistribution Research Articles

Experimental and numerical investigations on moment redistribution in reinforced concrete frames subjected to vertical loads

In this study, experimental and numerical investigations of the moment redistribution behaviour in reinforced concrete (RC) frames subjected to vertical monotonic loads are presented. By testing 12 RC single-storey continuous frames over two equal spans of 3000 mm, the evolution of moment redistribution during the whole process of loading was observed, and two-stage moment redistribution that occurred at the serviceability limit state (SLS) and ultimate limit state (ULS) was proposed. The test results showed that with the increase in the yield strength of the reinforcement, the degree of moment redistribution at SLS was improved, which should not be neglected in the prediction of the permissible moment redistribution. Meanwhile, the strain penetration of the high-strength reinforcement was more obvious. This is beneficial to the moment redistribution in frame beams. To further investigate the influence of various parameters on the two-stage moment redistribution quantitatively, a nonlinear finite element (FE) model based on the damage plasticity constitutive law was developed and verified using the teste results. A parametric study was conducted based on 96 simulated specimens and the influential factors included the neutral axis depth factor, yielding strength of reinforcement, concrete strength, geometric dimensions of the frame columns and beams, M-V interaction, and loading patterns. The results indicated that M-V interaction had a detrimental effect on the plastic moment capacity, thus promoting the process of moment redistribution in RC structures. Based on the experimental and FE results, new practical calculation formulas that comprehensively reflected the influential factors were proposed to estimate degree of two-stage moment redistribution in RC frames.

New analytical method for computing moment redistribution in RC beams under concentrated load

ABSTRACT Computing the moments in RC structures after the yield by linear elastic analysis can lead to an inaccurate assessment of the behaviour due to the nonlinear behaviour. Therefore, it can become necessary to use more advanced methodologies to achieve a higher degree of performance optimisation of structures than those resulting from the simplified approaches adopted by existing design codes based on linear elastic analysis with redistribution of internal forces. The moment redistribution is supposed to start after occurring the cracks of concrete, but with small ratio. In this study, the moment redistribution before the yielding will be neglected, and the redistribution is focused after the yield. This paper suggests a mathematical model to investigate the moment redistribution in RC beams after yielding analytically. In the suggested mathematical model, the beam after forming the plastic hinges is converted into a virtual beam that can be analysed by structural linear analysis. The plastic hinges in the virtual beam will be represented as rotational springs having a linear rotational stiffness against the induced moment. The actual moments can be found through derived relationships in the mathematical model between it and the virtual moment. The mathematical model was verified and it gave values of moment matching experimental results. Also, a comparison for degree of moment redistribution among the suggested mathematical model and several design codes was performed. The analytical results indicate that the proposed mathematical model can be used for analysis of moment redistribution of RC beams.

Static behavior of a steel-ultra high performance concrete continuous composite box girder

To improve the crack-resisting behavior of normal-strength concrete (NC) bridge deck and to reduce the deadweight of conventional steel-concrete composite girders, a new steel-ultrahigh performance concrete (UHPC) continuous composite box girder was proposed. Aiming to reveal the static behavior of the proposed structure, including the load-deflection response, the strain distribution, the crack development, the moment redistribution and the failure mode, a comparative experimental study was conducted on two types of the composite girder, namely a steel-UHPC continuous composite girder (SUCG) and a prestressed steel-concrete continuous composite girder (SCCG). The test results indicated that the ultimate loading capacity of the SUCG was 20.4% higher than that of SCCG; the nominal cracking strength of the SUCG was 22.9 MPa, and its corresponding cracking moment was 2.1 times higher than the conventional one; the crack development mode of UHPC slab were significantly different from the SCCG, the cracks appeared in UHPC slab dominated by microcracks with smaller length were numerous and intensive. It is also shown that the SUCG has smaller degree of moment redistribution. Thus, the current study verified the SUCG could be one of effective solutions for long-span continuous composite girder bridges.

Flexural response of continuous unbonded post-tensioned beams strengthened with CFRP laminates

The sudden brittle rupture of carbon fiber-reinforced polymer (CFRP) materials is often confused with the redistribution capacity of statically indeterminate CFRP-strengthened structures. Few investigations have been carried out regarding the flexural behavior of continuous CFRP-strengthened unbonded post-tensioned (UPT) beams, despite such beams having been extensively applied in practice, owing to retrofitting alternatives. This study emphasizes the flexural response of continuous UPT beams strengthened with CFRP laminates subjected to monotonic loading to failure. Progressive fracture of the CFRP laminates appears in the hogging and sagging regions, accompanied by concrete crushing in the sagging regions. The degree of moment redistribution (MR) and ductility generally increase with decreasing compressive zone depth of the critical section over the inner support. When the ratio of the global reinforcement index of the inner support to that of the mid-span increases, the MR and ductility decrease.

Moment redistribution and ductility of CFRP strengthened and non-strengthened unbonded post-tensioned indeterminate I-beams composed of UHSSCC

Strengthening might be necessary for unbonded post-tensioned (UPT) concrete members to account for mistakes at the design stage or to solve construction errors. Various strengthening techniques have been developed that use carbon-fiber-reinforced polymer (CFRP) to increase the load carrying capacity and fulfill serviceability requirements. Although many in situ post-tensioned beams are continuous, there has been little research done on such beams with unbonded tendons. The brittle nature of UPT members, particularly with high strength concrete and CFRP, raises concerns about the ability to redistribute the moments of a continuous member. Knowledge about the degree of moment redistribution (MR) and ductility response of ultra-high strength self-compacting concrete (UHSSCC) in strengthened and non-strengthened UPT indeterminate members is crucial. The current study presents the experimental results of research to determine the flexural response of six non-strengthened or strengthened full-scale continuous UHSSCC unbonded post-tensioned I-beams with emphasis on ductility and moment redistribution. The results showed that, despite a decrease in MR, displacement, and energy ductility, the confinement provided satisfied the high MR demand in presence of strengthening, unbonded prestressing and use of UHSSCC. The stress measured in unbonded strands approached the point of yielding at the onset of failure for non-strengthened and strengthened beams.

FE modeling of inelastic behavior of reinforced high-strength concrete continuous beams

A finite element model for predicting the entire nonlinear behavior of reinforced high-strength concrete continuous beams is described. The model is based on the moment-curvature relations pre-generated through section analysis, and is formulated utilizing the Timoshenko beam theory. The validity of the model is verified with experimental results of a series of continuous high-strength concrete beam specimens. Some important aspects of behavior of the beams having different tensile reinforcement ratios are evaluated. In addition, a parametric study is carried out on continuous high-strength concrete beams with practical dimensions to examine the effect of tensile reinforcement on the degree of moment redistribution. The analysis shows that the tensile reinforcement in continuous high-strength concrete beams affects significantly the member behavior, namely, the flexural cracking stiffness, flexural ductility, neutral axis depth and redistribution of moments. It is also found that the relation between the tensile reinforcement ratios at critical negative and positive moment regions has great influence on the moment redistribution, while the importance of this factor is neglected in various codes.

Flexural Response of Continuous Concrete Beams Prestressed with External Tendons

This article presents the results of a numerical investigation of the flexural behavior of continuous externally prestressed concrete beams. Aspects of behavior studied include the increase in stress in external tendons, moment redistribution in the postelastic range, and secondary moments as a result of prestressing. A finite-element model for the full-range analysis of continuous externally prestressed concrete beams is introduced. The model predictions agree well with the experimental results. The analysis shows that the ultimate stress increase in external tendons of continuous beams is dependent on both the number and rotation of plastic hinges that can be developed at failure load. The degree of moment redistribution is significantly influenced by the nonprestressed tension steel and the pattern of loading. An approach based on the linear transformation concept is designed to examine the secondary moments over the entire loading up to the ultimate. The results indicate that the secondary moments increase linearly with the prestressing force and can be conveniently calculated by an elastic analysis.

Influence of the shear force and transverse reinforcement ratio on plastic rotation capacity

Ductility, particularly the plastic rotation capacity of critical regions, conditions the available degree of moment redistribution and the ability to exploit the additional resistance of hyperstatic structures. A theoretical model for calculating plastic rotation capacity, considering the influence of the main factors, is presented. Special attention has been paid to the influence of the tensile reinforcement ratio, the shear force and the confinement of compressed concrete on plastic rotation capacity. Theoretical results are compared with those obtained using an experimental programme designed to study the influence of these factors. Some extrapolations are made on the basis of the model, and conclusions are drawn.

Flexural Behavior of Two-Span Continuous Prestressed Concrete Girders with Highly Eccentric External Tendons

[Abstract]: It is generally known that the flexural strength of beams prestressed with external tendons is comparatively lower than that of members with internal bonded tendons. One possible method of enhancing the flexural strength of such beams is to place the tendons at high eccentricity. To obtain an insight into the flexural behavior of beams with highly eccentric tendons, an experimental investigation is conducted on single-span and two-span continuous beams. The test variables include external tendon profile, loading pattern on each span, casting method, and confinement reinforcements. It is found that continuous girders with linearly transformed tendon profiles exhibit the same flexural behavior irrespective of tendon layout. The presence of confinement reinforcement enhances the ductility behavior but does not increase the ultimate flexural strength. The degree of moment redistribution is affected by the tendon layout and the loading pattern on each span. The results of the experimental investigation are discussed in this paper.