Hogging Moment Regions Research Articles

Experimental study and theoretical analysis of partially encased continuous composite beams

The mechanical behavior and the law of moment redistribution of the partially encased continuous composite beam (PECCB) are investigated in this paper. One common continuous composite beam and three PECCBs were tested. The main variable parameter of the three PECCB specimens is the amount of longitudinal reinforcement in the concrete slab of hogging moment region. Owing to the contribution of the web encasement, PECCBs have much slower crack propagation speed in the hogging moment region and higher ultimate capacity. The web encasement also contributes to postponing the yielding of the bottom flange, and therefore prevents local buckling in the steel girder. As the amount of reinforcement in the concrete slab of hogging moment region increases, the degree of moment redistribution of PECCBs decreases. On the basis of analyzing the moment redistribution requirement for plastic design and the ultimate rotation capacity of hogging moment section, the required and available moment redistribution factors of PECCBs are obtained. A parameter Rp is introduced to evaluate the relative relationship between the longitudinal reinforcement in the concrete slab and the rest part of the partially encased section, and the value of Rp should be restricted within 0.35 to enable full moment redistribution for plastic design of PECCBs.

Special Issue on Design, Analysis, and Construction of Segmental Bridges

Special Issue on Design, Analysis, and Construction of Segmental Bridges

Behavior of a Continuous Composite Box Girder with a Prefabricated Prestressed-Concrete Slab in Its Hogging-Moment Region

AbstractThe application of prestress is a very effective way to prevent the cracking of the concrete slab in the negative-moment region of continuous composite girders, and using a prefabricated prestressed-concrete slab in this region is an easier option than posttensioning. To elucidate the structural response of a continuous composite box girder with a prefabricated prestressed-concrete slab, two large, 18-m-long specimens were fabricated and tested under short-term loading, and the results are reported in this paper. One, a control specimen, was a conventional composite girder with its concrete slab cast in situ, whereas the other was a composite box girder with a prefabricated prestressed-concrete slab in the negative-moment region. The relative slip between the steel girder and the concrete slab; the load-deflection response; the distribution of strain in the steel girder, reinforcement, and concrete slab; as well as the crack widths in the concrete slab were measured during the tests. The paper sho...

Uplift-Restricted and Slip-Permitted T-Shape Connectors

Reliable connections between steel beam and concrete slab are important for composite beams. A new type of uplift-restricted and slip-permitted T-shape (URSP-T) connector with foamed plastic was proposed to improve mechanical performance of the hogging moment region in composite beams, i.e., to prevent concrete slab separation from the steel beam and to delay the cracking of concrete. To investigate the slip and uplift performance of URSP-T connectors, six pushout tests and three pullout tests were conducted. In pushout tests, obvious slip deformation of the URSP-T connector occurred and the foamed plastics around connectors significantly affected the slip performance. It was observed from load-deformation hysteresis curves that stiffness changed with loading history. Based on the Richard-Abbott curve, a new hysteresis model was conceptualized to simulate the slip mechanism of the connector. The predicted results correlated well with test data. In uplift tests, two types of failure modes, i.e., punching shear failure of concrete and failure of the connector’s webs, were observed. Based on test results, the relationship between uplift performance and dimensions of the connectors is discussed. Furthermore, a design formula for ultimate uplift capacity and construction details of this connector is proposed. Pushout tests and pullout tests indicated that URSP-T connectors have excellent performance. Meanwhile, finite-element analysis proved that URSP-T connectors permit larger slip deformation and reduce tensile stress inside concrete in the hogging moment region of composite beams. The investigation reported in this paper may provide a basis for design and application of URSP-T connectors.

ELASTIC RESTRAINED DISTORTIONAL BUCKLING OF STEEL-CONCRETE COMPOSITE BEAMS BASED ON ELASTICALLY SUPPORTED COLUMN METHOD

Restrained distortional buckling (RDB) is different from overall buckling and distortional buckling, and it usually occurs in the hogging moment region of composite beams. It may govern the member design for some cases, and should be taken into consideration. The present paper investigates the elastic RDB of composite beams based on elastically supported column method. First, Svensson's elastically supported column model is improved by including the participation of the web as part of the buckling column. The corresponding theoretical expressions for the improved elastically support column subjected to a varying axial force are derived by the potential energy expression and equilibrium differential equation, respectively. Then, the precision of several elastically supported column methods for the RDB problem is discussed, based on the finite element method (FEM). The results show that elastically supported columns subjected to a negative uniform moment or a triangular moment have a good agreement with FEM results, but they are not valid for the cases of nonlinear moment distributions. In order to consider the complicated load conditions of composite beams, the equivalent moment assumption is introduced and proved to be suitable for the RDB problem. However, there is an applicable range of the length of the hogging moment region. Thus, a critical length formula of equivalent moment assumption of RDB is put forward and verified by FEM. Furthermore, a three-step simplified method is presented which can solve the elastic RDB problem of continuous composite beams. The proposed method, in which values of elastically supported column methods can be tabulated previously, is simple and suitable for design purposes.

Flexural performance of fire damaged and rehabilitated two span reinforced concrete slabs and beams

Five two-span reinforced concrete (RC) slabs and seven two-span RC beams were tested under the ISO 834 standard fire with different durations. CFRP strengthening was then applied to some of the specimens after the damaged concrete was removed from the specimens and replaced with polymer mortar. All the specimens were loaded to failure to investigate the influence of fire-damage and the effectiveness of strengthening methods. Test results indicated that the flexural capacities of specimens decrease with the fire duration increases. Moreover, fire exposure had more significant effect on the flexural rigidity than on the bearing capacity of the specimens. After rehabilitation, the bearing capacities of specimens reached or even exceeded that of the reference RC specimen, and the strengthening methods seemed to have limited effect on flexural rigidity recovery. From the analysis of moment redistribution of tested beams, elevated temperature is found having different impacts on sagging moment region and hogging moment region. The damage of RC continuous member is definitely a comprehensive response of different regions.

MECHANICAL BEHAVIOUR OF COMPOSITE GIRDERS SUBJECTED TO HOGGING MOMENT: EXPERIMENTAL STUDY

In continuous composite steel-concrete structures, cracking of the concrete slab in the hogging moment region decreases the global stiffness of composite steel-concrete structures and reduces the effect of continuity, thus making the structural behaviour highly nonlinear even for low stress levels, so special consideration is necessary. An experimental investigation on the behaviour of steel-concrete composite beams in hogging moment region is presented in this paper. A total of four specimens were tested under point load in the mid-span. Two of the composite beams with headed studs as the shear connectors, while the other two specimens are using Perfo-Bond Strips (PBLs) as the connection devices between the steel girder and the concrete slab. Ultimate load bearing capacity of composite sections in negative moment region was calculated and compared with experimental values. Crack formation, crack widths development process and strain distribution of the composite section before and after cracking were observed and presented in this study. Interface slip distribution was also given. Research results indicate that the current specifications such as AASHTO, JSCE, and EUROCODE-4 can provide appropriate values for ultimate strength of a composite girder under negative bending moment.

MECHANICAL BEHAVIOUR OF COMPOSITE GIRDERS SUBJECTED TO HOGGING MOMENT: EXPERIMENTAL STUDY

In continuous composite steel-concrete structures, cracking of the concrete slab in the hogging moment region decreases the global stiffness of composite steel-concrete structures and reduces the effect of continuity, thus making the structural behaviour highly nonlinear even for low stress levels, so special consideration is necessary. An experimental investigation on the behaviour of steel-concrete composite beams in hogging moment region is presented in this paper. A total of four specimens were tested under point load in the mid-span. Two of the composite beams with headed studs as the shear connectors, while the other two specimens are using Perfo-Bond Strips (PBLs) as the connection devices between the steel girder and the concrete slab. Ultimate load bearing capacity of composite sections in negative moment region was calculated and compared with experimental values. Crack formation, crack widths development process and strain distribution of the composite section before and after cracking were observed and presented in this study. Interface slip distribution was also given. Research results indicate that the current specifications such as AASHTO, JSCE, and EUROCODE-4 can provide appropriate values for ultimate strength of a composite girder under negative bending moment.

Experimental and numerical analysis of composite beams strengthened by CFRP laminates in hogging moment region

An experimental and a non linear finite element investigation on the behavior of steel-concrete composite beams stiffened in hogging moment region with Carbon Fiber Reinforced Plastics (CFRP) sheets is presented in this paper. A total of five specimens were tested under two-point loads. Three of the composite beams included concrete slab while the other two beams had composite slabs. The stiffening was achieved by attaching CFRP sheets to the concrete surface at the position of negative bending moment. The suggested CFRP sheets arrangement enhanced the overall beam behavior and increased the composite beam capacity. Valuable parametric study was conducted using a three dimensional finite element model using ANSYS program. Both geometrical and material nonlinearity were included. The studied parameters included CFRP sheet arrangement, concrete strength and degree of shear connection.

Long-Term Behavior of Composite Beams under Positive and Negative Bending. I: Experimental Study

This paper presents an experimental program designed to examine the long-term behavior of composite steel-concrete beams under sustained loads. The test program consisted of two simply supported beams under positive bending moments and two cantilever beams under negative bending moments. Long-term deflections, strains, and the cracking behaviors of the specimen beams were monitored over a period of 3 years. The primary observations are: (1) the deflection of the simply supported beam under positive bending moment increased quite significantly over the time due to creep and shrinkage, with the midspan deflections after 3 years of loading reached 2.5 times of the initial deflections, and (2) the cracking of concrete occurred in hogging moment regions for cantilever beams due to the combined effects of concrete shrinkage and sustained negative bending moments. Current design codes were evaluated through a comparison between the calculations and testing results. It was found that the effective modulus method provides a satisfactory prediction on deflections and strains for beams under positive bending moments. But for beams under negative bending moment, the stiffness reduction due to cracking effect should be considered in calculation.

Effects of partial shear connection on the behavior of semi-continuous composite beams

In this paper, the effects of partial shear connection on the behavior of semi-continuous composite beams were studied numerically using two-dimensional finite element model with plane stress elements. The finite element model takes into account the nonlinearity of the different materials involved. For the shear connectors, a non-linear (shear-slip) relation, drawn from the push-out tests, was used. The accuracy of the proposed finite element model was verified against test results available in the literature. A simplified method to predict load capacity and deflection of the semi-continuous composite beams was also proposed. Based on the results obtained from the finite element analysis, the concept of partial shear connection in the hogging moment regions can be accepted provided that the shear connectors are sufficiently ductile.

A comparative study of continuous steel–concrete composite beams prestressed with external tendons: Experimental investigation

A comparative study of prestressed continuous steel–concrete composite beams is carried out based on an experimental investigation. Four full-scale continuous composite beams were tested, among which two were two-span beams and two were three-span beams. One of the two-span beams was a conventional non-prestressed composite beam, and the other was a composite beam prestressed with external tendons in both the positive and negative moment regions. One of the three-span composite beams was non-prestressed, and the other was prestressed only in the negative moment regions with external high-strength tendons. The cracking behaviors, distortional lateral and local buckling as well as the load carrying capacity of the beams were investigated experimentally. Full plasticity was developed at the mid-span section of each beam; however, the maximum moments attained at the internal supports were governed by local buckling, which is related to the slenderness of the composite section. It is found that in hogging moment regions, the ultimate moment resistance of a composite beam prestressed with external tendons is governed by either distortional lateral buckling or local buckling, or an interactive mode composed of distortional lateral and local buckling. The results show that exerting prestressing on a continuous composite beam with external tendons increases the degree of the internal force and moment redistribution in the beam. A design proposal based on moment redistribution to evaluate the load carrying capacity of continuous composite beams with external tendons is proposed.

Experimental study of moment redistribution and load carrying capacity of externally prestressed continuous composite beams

A comparative experimental study of prestressed continuous steel-concrete composite beams was carried out. Two continuous composite beams were tested, one of which was plain continuous steel-concrete composite beam, while the other was a composite beam prestressed with external tendons. Cracking behavior and the load carrying capacity of the beams were investigated experimentally. Full plasticity was developed in the mid-span section each beam, the maximum moments attained at the internal support sections however were governed by local buckling which was related to the slenderness of composite section. It was found that in hogging moment regions, the ultimate resistance of an externally prestressed composite beam would be governed by either distortional lateral buckling or local buckling, or interactive mode of these two buckling patterns. The results show that exerting prestressing on a continuous composite beam with external tendons will increase the extent of internal force and moment redistribution in the beam. The influences of local and distortional buckling on the behaviors of the composite continuous beams are discussed. The Moment redistribution and the load carrying capacity of the prestressed continuous composite beams are evaluated, and it is found that at the ultimate state, the moment redistribution in the prestrssed continuous composite beams is greater than that in non-prestressed composite beams.

Experimental and Numerical Investigation on Ductility of Composite Beams in the Hogging Moment Regions under Fire Conditions

This paper presents both the results of an experimental program and numerical simulations, which have been conducted to investigate the ductility of composite beams in the hogging moment regions under fire conditions. The main objective is to study the ductility issue related to the inelastic behavior of composite beams at an elevated temperature. A second objective is to investigate the feasibility of using a finite-element model for parametric studies. To fullfil the first objective, a total of four composite beams with decking slabs were tested to failure. They were designed to represent the internal joint of a continuous beam. The segment between the plastic hinge over support and adjacent point of inflection was represented by one-half of a simply supported beam subjected to a midspan point load. The specimens were heated to a certain temperature before they were subjected to a static point load up to failure. These four test results were then used to validate finite-element models constructed from MSC. Marc Mentat. It is demonstrated that the finite-element analysis gives reasonable accuracy compared to test results. Hence, it provides an efficient, economical, and yet accurate tool to study the inelastic behavior of composite beams in fire. Finally, a parametric study was conducted to investigate the influence of several parameters on the moment-rotation response and the rotational capacity at elevated temperatures.

Analysis of composite beams in the hogging moment regions using a mixed finite element formulation

Cracking of the concrete slab in the hogging moment region decreases the global stiffness of composite steel–concrete structures and also reduces the effect of continuity, thus making the structural behaviour highly nonlinear even for low stress levels. In this paper, the behaviour of continuous composite beams with discrete shear connection is investigated using a nonlinear mixed finite element model. The model includes appropriate nonlinear constitutive relationships for the concrete, the steel and tension stiffening effect. Furthermore, the discrete nature of the shear connection is embedded in the model and the tension stiffening effects are introduced in the analysis by using a concrete constitutive model proposed in the CEB-FIB Model Code 1990 which incorporates embedded steel. Special attention is paid to the hogging moment regions, where cracking occurs. Comparisons between the numerical analyses and experimental results in the current literature are undertaken to validate the accuracy of the model. Furthermore, a parametric study is carried out to study the influence of span length and degree of shear connection on the strength and ductility of continuous composite beams.

Loading capacity of composite slim frame beams

The estimation of the flexural stiffness and bending capacity of composite slim beams is rather complicated, because the influence of many factors should be taken into account. These factors include variable section dimensions, development of cracks and non-linear characteristics of concrete. This is especially true for the composite slim frame beam, in which the sagging moment region and the hogging moment region should both be considered. In this paper, experimental investigations have been conducted to investigate the flexural behavior of two specimens of composite slim frame beam with deep deck under monotonic loading. Based on the experimental results, formulas of calculating the bending capacity and flexural stiffness in the hogging moment region of the slim beam with deep deck has been proposed. Combining these formulas with existing formulas of sagging moment, formulas for the equivalent stiffness and the design method of the frame slim beam have been developed. The feasibility of the proposed formulas and design method has been verified by the test results.

Progressive collapse of multi-storey buildings due to sudden column loss—Part II: Application

The companion paper presents the principles of a new design-oriented methodology for progressive collapse assessment of multi-storey buildings. The proposed procedure, which can be implemented at various levels of structural idealisation, determines ductility demand and supply in assessing the potential for progressive collapse initiated by instantaneous loss of a vertical support member. This paper demonstrates the applicability of the proposed approach by means of a case study, which considers sudden removal of a ground floor column in a typical steel-framed composite building. In line with current progressive collapse guidelines for buildings with a relatively simple and repetitive layout, the two principal scenarios investigated include removal of a peripheral column and a corner column. The study shows that such structures can be prone to progressive collapse, especially due to failure of the internal secondary beam support joints to safely transfer the gravity loads to the surrounding undamaged members if a flexible fin-plate joint detail is employed. The provision of additional reinforcement in the slab over the hogging moment regions can generally have a beneficial effect on both the dynamic load carrying and deformation capacities. The response can be further improved if axial restraint provided by the adjacent structure can be relied upon. The study also highlights the inability of bare-steel beams to survive column removal despite satisfaction of the code prescribed structural integrity provisions. This demonstrates that tying force requirements alone cannot always guarantee structural robustness without explicit consideration of ductility demand/supply in the support joints of the affected members, as determined by their nonlinear dynamic response.

Elastic Bubble Augmented Spline Finite Strip Method in Analysis of Continuous Composite Beams

Local and overall instabilities of the steel beam occur in the hogging-moment region in a continuous composite beam and these forms of buckling have been recognised to be highly interactive. The overall mode of buckling in composite steel-concrete tee-beams in regions of negative bending is what is herein termed restrained-distortional buckling (RDB). In negative bending the slab restrains the tension region of the steel and the neutral axis is not located at the mid-height of the web. The neutral axis is shifted towards the top flange, and in negative bending the steel region is subjected to predominantly compressive loading. In addition, the web usually carries proportionally higher shear loads than in ordinary steel beams. The lateral-distortional buckling resistance of the steel portion in continuous composite beams is therefore dependent on the extent to which the web can provide a restraining action to the unstable compression flange. This form of buckling is usually prevented in bridge girders by the cross bracing and the common view amongst engineers is that such bracing is excessive and uneconomical. Although buckling of plain steel beams in both the elastic and inelastic ranges of structural response has been studied extensively and a great deal of research work has been devoted to the understanding of their buckling modes, and codes of practice for the design of structural steelwork contain relevant clauses that presently are considered to be quite accurate, buckling of the steel component in composite beams still represents a grey area in structural engineering research and is much less well documented. The bubble augmented spline finite strip method of analysis, developed by the authors, is thus employed herein to study extensively the issue of RDB in composite tee-beams in hogging bending regions. The numerical investigations demonstrate that RDB mode is a governing mode for steel I-sections with one flange fully restrained, variations of the web slenderness parameter have a most pronounced influence on the elastic buckling capacity of composite T-section beams and the deformations can be reasonably large in the proximity of the internal supports and near the concentrated forces.

Semi-rigid composite frames with perfobond and T-rib connectors Part 1: Full scale tests

This paper proposes an alternative construction system for composite portal frames. The proposed process uses a perfobond rib shear connector and a connection element, denominated “T-rib” for composite action. The “T-rib” connection element was developed with the aim of transferring the forces from the reinforcing bars, used in hogging moment regions, to edge and corner column flanges. Full-scale test results of a composite semi-rigid portal frame are presented in terms of load–deflection, load–deformation and load–rotation curves.

The effects of partial shear connection in composite flush end plate joints Part I — experimental study

The experimental results of partial shear connection composite beams subjected to hogging bending have been previously reported. An extension to this work which considers the presence of a semi-rigid, partial strength connection is described herein. Five sub-assemblages of cruciform composite joints that simulate the internal region of a semi-continuous frame were tested. The joints were constructed using a novel approach of blind bolting a flush end plate to concrete-filled square hollow columns. Apart from the level of shear connection, the other parameter varied was the reinforcement area. In addition, a bare steel joint was also tested to act as comparison specimen. Based on the experimental results, the intrinsic behaviour of the joints in terms of strength, stiffness and ductility was assessed. It was further affirmed that by lowering the levels of shear connection, the rotation capacity of the composite joints was enhanced with only a slight compromise if the stiffness and moment resistance. A notable effect on the connection behaviour was also observed by varying the reinforcement levels. The outcomes clearly affirmed the benefits of partial shear connection in contributing the additional flexibility to increase the joint ductility, which is extremely advantageous for plastic design purposes. In view of this, the current restriction of providing full shear connection design within hogging moment regions of continuous and semi-continuous structures could be relaxed.