Negative Bending Moment Research Articles

Experimental study on precast beam-column connections with continuity reinforcement for negative bending moments

Abstract In this paper, an experimental investigation is presented on semi-rigid interior beam-to-column connections constituted by precast concrete beams supported on precast concrete column corbels and bending continuity reinforcement bars for bending negative moments. The main purpose of this paper was to analyze the influence of vertical interface grout filling between the corbel and the beam and the position of the bending continuity reinforcement bars (crossing only the column or crossing only the slab) on the behavior of this type of connection. Tests on eight specimens were performed. It was noticed that the vertical interface grout filling contributed to increase both rotational flexural stiffness and flexural strength capacity of the connections when compared to the connections without grout filling. It was also noticed that in the specimens in which the continuity bars crossed only the column, the rotational flexural stiffness was higher. For these last ones, coefficients k and β for predicting the secant stiffness by simplified expression present in Brazilian Code NBR 9062 were evaluate from experimental results. These evaluated coefficients may be regarded as indicative values for structural designers and helpful for future researches.

Long-term behavior of continuous composite slabs made with 100% fine and coarse recycled aggregate

A composite slab is an efficient modern structural member. Recycled fine and coarse aggregates are alternative green materials for the depleting natural sand and natural gravels in the composite slabs. In recent years, the nonuniform shrinkage occurring in single-side-exposed concrete has been discovered to significantly affect the long-term behaviors of composite slabs, which have been included in newly published codes. Applying recycled aggregate concrete (RAC) would further increase the effect of nonuniform shrinkage on the long-term behaviors of composite slabs. However, most studies, with RAC or not, have only focused on simply- supported composite slabs. For continuous composite slabs (CCSs), in addition to increasing deflections, a nonuniform shrinkage would induce a larger negative bending moment, which would further increase the crack width at the interior support region. Hitherto, no experiment has been conducted regarding the time-dependent crack width of CCSs and CCSs with RAC. Therefore, the aim of this study is to investigate the effects of nonuniform shrinkage and RAC on the deflections, strains, and cracks of CCSs. Hence, a 500-d experiment was conducted on three full-scale specimens. Finally, a finite element (FE) model was established and benchmarked against the test results of this study and those reported in the literature. The results indicated that (1) nonuniform shrinkage significantly increased the crack width at the negative bending moment region; (2) the use of RAC resulted in a more significant effect of the nonuniform, e.g. mid-span deflections, soffit strains, and crack widths increased by 40%, 64%, and 38%, respectively; and (3) the FE model can predict the long-term behaviors of CCSs well.

Experimental and analytical research on the flexural behaviour of steel–ECC composite beams under negative bending moments

Due to the strain-hardening and multi-cracking properties, engineered cementitious composite (ECC) is a new solution to the cracking issue in the negative bending moment region of steel–concrete composite beams. This paper presents an experimental and analytical study on the flexural performance of composite steel–ECC beams subjected to negative bending moments as well as the tension stiffening behaviour of reinforced ECC (R/ECC) flange slabs. Three beams with different slab materials and reinforcement ratios were tested under a hogging moment. Experimental results demonstrated significant enhancement in stiffness and crack resistance for the steel–ECC composite beams. A practical four-parameter fibre-bridging model was established to describe the strain-hardening behaviour of different ECC materials. Then, a modified analytical tension-stiffening model for R/ECC was formulated considering the strain-hardening behaviour of ECC and rebar–ECC bond–slip interaction based on the conventional tension-stiffening model for reinforced concrete. This modified model was verified by several direct tensile tests of R/ECC members. In addition, by applying the model to the traditional fibre beam–column element model, the mechanical performance and crack opening of general R/ECC structures were derived. The simulation results of the steel–ECC composite beams demonstrated satisfactory accuracy compared with the test results. Finally, a parametric study based on the new model was conducted to identify the influence of several important material and structural parameters on the flexural performance of steel–ECC composite beams under negative moments.

On Flexural Performance of Girder-To-Girder Wet Joint for Lightweight Steel-UHPC Composite Bridge

Joints are always the focus of the precast structure for accelerated bridge construction. In this paper, a girder-to-girder joint suitable for steel-ultra-high-performance concrete (UHPC) lightweight composite bridge (LWCB) is proposed. Two flexural tests were conducted to verify the effectiveness of the proposed T-shaped girder-to-girder joint. The test results indicated that: (1) The T-shaped joint has a better cracking resistance than the traditional I-shaped joint; (2) The weak interfaces of the T-shaped joint are set in the areas with relatively lower negative bending moment, and thus the cracking risk could be decreased drastically; (3) The natural curing scheme for the joint is feasible, and the reinforcement has a very large inhibitory effect on the UHPC material shrinkage; The joint interface is the weak region of the LWCB, which requires careful consideration in future designs. Based on the experimental test results, the design and calculation methods for the deflection, crack width, and ultimate flexural capacity in the negative moment region of LWCB were presented.

Experimental investigation on mechanical behavior of segmental joint under combined loading of compression-bending-shear

The non-circular tunnels, for example the quasi-rectangular tunnel, may generate a considerable shear force at their shoulder joints owing to the geometry effect. To investigate the shear force effect on the mechanical behavior (e.g., capacity, deformation and duality) of the segment joint, a series of full-scale mechanical loading tests were conducted and reported in this paper. The loading scenarios include the combined axis force and moment scenario (N-M), and the combined axis force, moment and shear force (N-M-Q) scenario. Both the positive bending moment and negative bending moment are considered. DIC technique is also adopted to capture the detailed cracking process in the N-M-Q loading test. The test results reveal that the mechanical response of the segmental joint under both N-M and N-M-Q loading scenarios can be divided into three stages, which are separated by the initial joint opening and the initiation of cracks. The shear force would lead to a larger deformation (e.g., deflection, joint opening and rotational angle) before the failure but result in a brittle failure of the segment joint. Moreover, the shear force could reduce the moment bearing capacity under the positive moment loading condition while has little effect under the negative moment loading condition. These shear force effects are thought to be caused by the shear induced dislocation and inclined cracks.

A new protocol for testing RC beams in combined shear and tension loading conditions

The problem of the shear strength of reinforced concrete members which are simultaneously subjected to an axial tension occurs in a number of practical cases such as continuous beams rigidly connected to columns (where tension results from the restraint of the axial variation of length due to concrete shrinkage and temperature variation), columns or shear walls and foundation piles in beam-column or shear wall-slab frames subjected to high horizontal forces. An original protocol is proposed for testing in these loading conditions, where reaching shear failure represents the main difficulty for designing specimens. This protocol makes use of an original and reliable test setup, in which two horizontal forces are symmetrically applied apart the beam axis, making it possible to generate a constant axial force and a constant negative bending moment along span. The loading protocol is oriented towards a defined value of axial force, while shear at failure may be combined with a negative or positive moment. The loading path for bending moment and axial force is chosen as to remain inside the normal force-bending moment failure curve. The failure in shear occurs at constant axial force and increasing bending moment. The whole procedure was first implemented to perform three rather simple preliminary tests. Their results show the technical feasibility of this protocol. Some limitation of initial parameters was evidenced. So better parameters were chosen for carrying out the main experimental program (presented in another paper).

Shear behavior and shear capacity prediction of precast concrete-encased steel beams

A novel precast concrete-encased steel composite beam, which can be abbreviated as PCES beam, is introduced in this paper. In order to investigate the shear behavior of this PCES beam, a test of eight full-scale PCES beam specimens was carried out, in which the specimens were subjected to positive bending moment or negative bending moment, respectively. The factors which affected the shear behavior, such as the shear span-to-depth aspect ratio and the existence of concrete flange, were taken into account. During the test, the load-deflection curves of the test specimens were recorded, while the crack propagation patterns together with the failure patterns were observed as well. From the test results, it could be concluded that the tested PCES beams could all exhibit ductile shear behavior, and the innovative shear connectors between the precast concrete and cast-in-place concrete, namely the precast concrete transverse diaphragms, were verified to be effective. Then, based on the shear deformation compatibility, a theoretical model for predicting the shear capacity of the proposed PCES beams was put forward and verified to be valid with the good agreement of the shear capacities calculated using the proposed method and those from the experiments. Finally, in order to facilitate the preliminary design in practical applications, a simplified calculation method for predicting the shear capacity of the proposed PCES beams was also put forward and validated using available test results.

Investigation of Traffic Loading Effects for Different Codes on Medium‐ and Small‐Span Girder Bridges in China

With increasing traffic volume, the traffic load grade given by design codes has gradually increased. For new bridges, there is no problem, and the traffic load can be met through the requirements of the new code. However, for existing bridges, there is a lack of uniform standards on whether they can continue to be used. It is not clear whether these bridges will be judged according to the new code or the original design code. The traffic loading effects of different codes on medium‐ and small‐span girder bridges in China are investigated in this study. Three codes are introduced: JTJ 021‐89, JTG D60‐2004, and JTG D60‐2015. Simply supported girder bridges and continuous girder bridges are discussed. The traffic loading effects calculated based on JTG D60‐2015 are significantly larger than those calculated based on JTJ 021‐89. For simply supported girder bridges, most of the differences range from 20% to 40%, and the maximum value is almost larger than 60%. For continuous girder bridges, most of the differences in the positive bending moments are concentrated in the 20%∼40% range, while the differences in the negative bending moments range from 10% to 20%. Therefore, the differences in traffic loading effects calculated based on various codes cannot be ignored in actual bridge engineering. The conclusion in this study can provide a basis for bridge structure evaluation and life prediction.

Experimental Study on Fatigue Performance of Negative Bending Moment of Steel‐Concrete Continuous Composite Box Girder

The experimental work presents results on the fatigue performance of composite beams in the negative moment region and the changes of stiffness and deformation of composite beams under repeated loads; fatigue tests were carried out on two double‐layer composite beams. The fatigue performance of composite beams with different reinforcement ratios under complete shear connection and the variation of deflection, strain of the reinforcement, strain of steel beam, and crack growth under fatigue load were obtained. The results showed that the fatigue resistance performance of concrete slab with low reinforcement ratio was much lower than that of concrete slab with high reinforcement ratio whereas, under the fatigue load, the stress of the welding nail in the negative moment region was small and the slip was almost negligible. The degradation of stiffness and the development of cracks were mainly due to the degradation of bond‐slip between the concrete and reinforcement. The fatigue failure mode was the fracture of the upper reinforcement in negative moment region. The results obtained in this study are helpful in the design of composite beam.

Measurement of deformation of the concrete sleepers under different support conditions using non-contact laser speckle imaging sensor

Prestressed concrete sleepers play a significant role in distributing wheel loads from the rails to the ballast layer. Cyclic wheel loads may cause local deterioration of the ballast layer which would affect the support conditions beneath the sleepers. Therefore, understanding the influence of different support conditions on the deformation of sleepers is necessary for maintaining safe railroad operations. In this study, a non-contact and non-destructive laser speckle imaging sensor (LSIS) was developed and successfully applied to investigate the flexural behaviour of concrete sleepers under various support conditions, including “full support”, “partial support” and “centre binding”. The results show that LSIS was in agreement with foil strain gauges of achieving high-resolution strain measurements with a maximum mean absolute error of 5.41±3.12με (equivalent to a mean absolute error of 7.15%) as the detection threshold (20 με) is exceeded. More than three times change in the negative bending moment was observed at the mid-span of the sleeper as the support condition was changed from “full support” to “centre binding”, which indicates the potential risks of voided ballast support and importance of ballast tamping in railway maintenance.

Flexural behavior of an innovative dovetail UHPC joint in composite bridges under negative bending moment

The 5th Nanjing Yangtze River Bridge is a three-tower cable-stayed bridge with a main span of 600 m and a composite cross section consisting of a steel box girder- and precast ultra-high performance concrete (UHPC) deck slab. The precast UHPC slabs are connected using cast-in-place UHPC, and joints between the UHPC slabs are prone to cracking under negative bending moment. This paper investigates the flexural behavior of an innovative dovetail UHPC joint in seven UHPC slabs under negative bending moment. An innovative method using steel wire mesh is presented to enhance the interface between precast and cast-in-place UHPC at the joints. Test parameters of the UHPC slabs included interface treatment method, joint material, reinforcing bar overlapping form, and prestressing level. The steel wire mesh generated fibers that bridge the interface between the precast and cast-in-place UHPC, thus significantly enhancing the mechanical performance of the jointed UHPC slabs: (1) the nominal cracking strength was increased by 2.4 MPa; (2) the post-cracking stiffness was retained at about 80% of the initial stiffness; (3) the ultimate stiffness was retained at about 35% of the initial stiffness; and (4) closely-spaced multiple cracks occurred at the joints. A new ductility index defined as the ratio of the ultimate deflection and the flexural cracking deflection is proposed to characterize the post-cracking ductility. The research findings are useful in understanding and improving the flexural behaviors of UHPC bridge decks subjected to negative bending moment.

Exploring the progressive failure characteristics of a large special-shaped shield tunnel lining based on ‘standing’ prototype loading tests

A series of prototype structural loading tests were performed in this paper to explore the mechanical behavior of a large special-shaped shield tunnel lining towards failure. To replicate the real working conditions, the testing specimen was fabricated and tested in a ‘standing’ state. It was shown that, similar to circular tunnels, the internal forces and overall deformation of the special-shaped lining structure increased with increasing burial depth. However, the maximum negative bending moments of the special-shaped lining structure were present at the shoulder areas rather than at the waist areas. The two key design parameters, effective transverse rigidity ratio and rotational stiffness, were shown to evolve during loading. A unique relationship between the joint rotational stiffness and eccentricity was obtained, which could be divided into an axial force-dominating phase and a bending moment-dominating phase. Different cracks appeared at different loading stages and the crack width also evolved as loading proceeded. Empirical correlations between various structural properties and cracking characteristics were established, based on which the progressive failure process was elucidated. It was found that due to the transferring of bending moment, cracks were mostly concentrated on segmental bodies in the ring(s) adjacent to the longitudinal joints.

Three-dimensional analysis of effects of ground loss on static and seismic response of shafts

During the construction and use of underground structures, it is difficult to avoid ground loss. Previous studies suggested a correlation between ground loss and damage of underground structures. But the current studies are mainly focused on the two-dimensional analysis of tunnels. In the two-dimensional analysis, the simulation of ground loss is flawed, and the results cannot be applied to the shaft structures. Based on the Suzhou River deep underground drainage and storage system, this paper establishes a three-dimensional finite element model of the shaft. Voids of different depth, angle and location are set behind the shaft wall, and the effects of ground loss on static and seismic response of the shaft are analysed. It is found that the existence of ground loss leads to a stress concentration which changes the static deformation and internal force of the shaft. And large positive and negative bending moments occur in the circumference direction which may cause damage to the shafts. During the earthquake, the ground loss will increase the bending moments and reduce the axial pressure. But it has no significant effect on the response in the cross sections. This paper indicates that ground loss can change the static and seismic response of the shaft and it is a potential unsafe factor. Ground loss should be avoided as much as possible during shaft construction, and the impact of ground loss should be considered as appropriate in the design.

Transverse bending behavior of the steel-UHPC lightweight composite deck: Orthogonal test and analysis

The lightweight composite deck (LWCD) is a new countermeasure to address issues of fatigue cracking in orthotropic steel decks (OSDs) and premature damage in asphalt overlays, which is composed of an OSD and a thin ultra-high performance concrete (UHPC) layer. The objective of this paper is to reveal the transversal bending behavior of the LWCD under negative bending moments. An orthogonal experimental test was accomplished on 40 steel-UHPC composite specimens. The specimens covered four parameters, i.e., the stud spacing, the UHPC thickness, the cover thickness, and the number of steel reinforcement in UHPC. The responses of the specimens were analyzed and discussed from different aspects, including the failure mode, the load-deflection curve, the strain distribution, the cracking status, and the interfacial slip. To facilitate the design of the LWCD, the influence of the four parameters on the average crack spacing, the cracking stress in UHPC and the ultimate load was discussed. In addition, equations for calculating the ultimate load of the composite specimens were proposed and were validated through the test results.

Experimental study of steel-timber composite beam-to-column joints with extended end plates

This study presents the results of laboratory push-down tests conducted on one pure steel and four steel-timber composite (STC) cruciform subassemblies to assess the failure characteristics, stiffness, flexural resistance and ductility of the extended end plate STC beam-to-column connections subjected to negative (hogging) bending moment. In the proposed composite system, the cross -laminated timber (CLT) panels were connected to the top flange of steel girders using coach screws and the steel beams were connected to the steel columns by bolted extended end plates. Moreover, the two juxtaposed CLT slabs (subject to tension) were connected by the mechanically anchored threaded rod and/or surface spline joints with steel plates. The experimental results showed that the extended end plate STC connection have enough rotation capacity to provide for plastic analysis/design of the STC beams. Furthermore, it was shown that the composite action in conjunction with continuity of timber slab can increase the bending moment capacity of the connection more than 50% of that for a pure steel connection.

Dynamic Response and Shear Demand of Reinforced Concrete Beams Subjected to Impact Loading

Reinforced concrete (RC) beams under the impact loading are typically prone to suffer shear failure in the local response phase. In order to enhance the understanding of the mechanical behavior of the RC beams, their dynamic response and shear demand are numerically investigated in this paper. A 3D finite-element model is developed and validated against the experimental data available in the literature. Taking advantage of the above calibrated numerical model, an intensive parametric study is performed to identify the effect of different factors including the impact velocity, impact mass and beam span-to-depth ratio on the impact response of the RC beams. It is found that, due to the inertial effect, a linear relationship exists between the maximum reverse support force and the peak impact force, while negative bending moments also appear in the shear span. In addition, the local response of the RC beams can be divided into a first impact stage and a separation stage. A shear plug is likely to be formed near the impact point at the first impact stage and a shear failure may be triggered near the support by large support forces. Based on the simulation results, simplified methods are proposed for predicting the shear demand for the two failure modes, whereas physical models are also established to illustrate the resistance mechanism of the RC beams at the peak impact force. By comparing with the results of the parametric study, it is concluded that the shear demand of the RC beams under the impact loading can be predicted by the proposed empirical formulas with reasonable accuracy.

EXPERIMENTAL STUDY ON INTERNAL FORCE VARIATION OF STEEL-CONCRETE COMPOSITE BEAM UNDER FIRE

In practical engineering, most steel-concrete composite beams have axial constraints, which cause the composite beam to produce catenary action to continue bearing more external loads at large deformations. To study the mechanism of catenary action of steel-concrete composite beam under fire, two full-scale tests of steel-concrete composite beams were carried out to obtain the internal force variations with the time. The test designation, loading scheme and measurement contents are presented in this paper. The experimental phenomena and failure characteristics are also described. The test results show that the temperature gradient distribution of the composite beam section changes greatly under fire, and so it is easy to generate additional bending moment, which has an adverse effect on the ultimate bearing capacity of the composite beam. The failure mode of the composite beam is mainly the formation of obvious plastic hinge at the negative bending moment area at the beam ends, and the overall lateral instability occurs on the tensioned lower flange of the steel beam at the bottom of the beam, which is contrary to the common phenomenon that overall unstability of a steel beam is usually attributed to the compression flange buckling of a steel beam. The load ratio is one of the important parameters affecting the fire resistance of the composite beam. Through the actual measurement, the changing process of the internal force of the composite beam in fire is obtained. It is found that a larger load value leads to more obvious catenary action of the composite beam at the same time under the same conditions.

Slab corner effect on torsional behaviour of perimeter beams under missing column scenario

Perimeter beams in reinforced-concrete buildings are subjected to considerable torsion from slab negative bending moments. In the event of progressive collapse caused by column removals, torsion effects on such beams become critical, owing to redistribution of loads. The aim of this study is to explore the effects of slab corners on the torsional behaviour of perimeter beams in reinforced concrete beam–slab buildings, particularly in the case of column loss. A series of torsional tests on peripheral beams was conducted, comparing cases with and without slabs. Thereafter, physics-based numerical models were developed, based on the test results, to investigate the effect of varying the slab's parameters, such as thickness and reinforcement ratio, on torsional capacity. The models showed that the presence of reinforced concrete slabs could increase the torsional strength of perimeter beams by as much as 97%. The enhancement came first from the slab corner, which acted as a set of compressive struts. This effect increased with increasing slab thickness. It was also observed that the slab corners tend to shift torsional failure (which occurred at the twisted beam near the support) to about one beam depth away. The contribution of negative bending moment from slab reinforcement along the edges could also enhance overall structural capacity.

Research on Application of Uplift-Restricted Slip-Permitted (URSP) Connectors in Steel-Concrete Composite Frames

Tensile stresses and cracks in concrete slabs induced by a hogging moment have always been a disadvantage of steel-concrete composite structures and key issue of concern in the design of such structures. To reduce the tensile stress and control the crack width of the reinforced concrete (RC) slab, a new type of connector, called the uplift-restricted and slip-permitted (URSP) connector has been proposed and successfully applied in the area subjected to a negative bending moment in steel-concrete composite bridges. The feasibility of the URSP connector in steel-concrete composite frame buildings is investigated in this study based on a comprehensive parametric analysis. The effects of URSP connectors on the cracking behavior, as well as the stiffness and strength of composite frames, are systematically analyzed using an elaborate finite element model, which resembles a typical composite beam-column joint subjected to both lateral loads and vertical loads. In addition, an optimized arrangement length of URSP connectors is proposed for practical design. The research findings indicate that the application of URSP connectors greatly improves the crack resistance of RC slabs without an obvious reduction of the ultimate capacity and lateral stiffness of the composite frame. It is recommended that the distribution length of URSP connectors at each beam end should be 20–25% of the frame beam length.

Structural behaviour of steel-timber composite (STC) beam-to-column connections with double angle web cleats subjected to hogging bending moment

This paper deals with an experimental study of steel-timber composite (STC) beam-to-column connections with double web angles, with the aim of developing an efficient composite flooring system that exhibits superior structural performance yet is simple and cost-effective from a construction perspective. The experimental program involved the testing of six cruciform subassemblies representing internal beam-to-column connections subjected to negative (hogging) bending moments. In four specimens, STC beams were connected to the flanges of a universal column by double web angles. Additionally, a specimen with bare steel beams and another specimen with steel-concrete composite (SCC) beams were fabricated and tested to provide benchmarks for comparing the structural behaviour of the STC with bare steel and SCC beam-to-column connections. It is shown that the STC connections exhibit semi-rigid behaviour with negative bending moment and rotation capacities larger than that of SCC connections. Lastly, an analytical model which is an extension of the existing component-based method is proposed for estimating the negative bending moment capacity of the STC connections with double web angles.