Rudolf Seracino

The Gradual Formation of Hinges Throughout Reinforced Concrete Beams

ABSTRACT The rotational capacity of reinforced concrete beams is of funda-mental importance to the collapse of reinforced concrete structures. Research that started in the eighties used numerical approaches to simulate the rotational behavior using discrete and independent blocks encompassed by adjacent cracks. In this paper, these numerical approaches are taken a stage further. A numerical procedure has been developed that: allows for the interaction between the blocks along the full length of concrete beams; can cope with any number of layers of reinforcing bars; automatically predicts the occurrence of flexural cracks; and automatically allows for slip of the reinforcing bars.

Partial-interaction flexural stresses in composite steel and concrete bridge beams

Even though mechanical shear connectors in composite steel and concrete beams require slip to transmit shear, most composite bridge beams are designed as full-interaction because of the complexities of partial-interaction analysis techniques. However, in the assessment of existing composite bridges this simplification may not be warranted as it is often necessary to extract the greatest capacity and endurance from the structure. This may only be achieved using partial-interaction theory which truly reflects the behaviour of the structure. This paper develops a new concept of the partial-interaction focal point and simplifies partial-interaction theory to derive a simple procedure for deriving the partial-interaction flexural stresses from standard and easily obtained full-interaction parameters. This will allow engineers to develop more accurate procedures for determining the strength and endurance of existing composite bridge beams. Use of the procedure is illustrated by way of an example.

Fatigue behaviour of composite steel and concrete beams with stud shear connections

The fatigue behaviour of composite steel and concrete beams with stud shear connectors is unusual because the mechanical properties of the connectors are continually changing under the application of cyclic loads. Tests have conclusively shown that stud shear connections reduce in strength and stiffness immediately after cyclic loads are applied; however, friction at the interface between the concrete slab and the steel beam reduces the forces on the stud shear connectors. These effects can both enhance or reduce the endurance, stiffness and strength of the composite beam, which makes the fatigue design and assessment a very interesting and unique problem. This review describes the changes in properties that occur due to cyclic loading and the mathematical models that have been developed both to simulate the fatigue behaviour of composite beams and to estimate their remaining strength and endurance.

Behaviour of Stud Shear Connectors Subjected to Bi-directional Cyclic Loading

Almost all stud shear connectors in composite steel-concrete bridge beams are subjected to bi-directional cyclic loading at some stage during the fatigue life of the structure. In the assessment of an existing structure, it is desirable to predict the extent of fatigue damage experienced by the stud shear connectors as accurately as possible as this component of the structure can not be visually inspected. However, very little research has been carried out to quantify the fatigue behaviour of stud shear connectors subjected to bi-directional cyclic loading. This paper presents a new push-pull specimen that was developed to provide a better understanding of the bidirectional cyclic fatigue behaviour of stud shear connectors. The tests confirmed that for a given range of load, the fatigue life of connectors subjected to bi-directional cyclic loading is longer than those subjected to unidirectional loads only, and that the strength and stiffness of the connectors reduce immediately upon cyclic loading and continues to do so throughout the fatigue life. It was also shown that the rate of increase in slip per cycle is constant over most of the fatigue life with a rapid increase near the end so that, if monitored, can be used to provide adequate warning of failure.

MOMENT REDISTRIBUTION IN ADHESIVELY PLATED RC BEAMS AND SLABS

ABSTRACT It is common practice these days to retrofit RC beams and slabs by adhesive bonding FRP or steel plates to their surfaces. However, research has shown that these external plates can debond prematurely at relatively low strains, due to intermediate crack (IC) debonding, and in a brittle fashion. Because of these low IC debonding strains, it can be shown, through the standard use of neutral axis depth factors at ultimate, that the ability of the plated section to redistribute moment to other sections is severely limited and to such an extent that guidelines often preclude moment redistribution. This limitation may have little effect on plating RC bridges but it can, at least theoretically, severely limit the use of plating in buildings where ductility is a requirement. Tests on steel and FRP plated continuous beams have shown that moment redistribution can occur and a design procedure has been developed to determine the amount of moment redistribution for any type of plated section, such as tension face plates and side plates, and for any type of plate material, such as steel or FRP. In this paper, an analysis approach to quantify the amount of moment redistribution is described. A parametric study is then used to illustrate how the plate material and geometric properties affect moment redistribution; in particular, the study looks at the effect of using carbon FRP plates and glass FRP plates, as well as steel plates that have been designed to either debond prior to yielding or yield prior to IC debonding. In summary, the paper will show that plated sections can redistribute moment and, hence, the present restriction can be removed which should extend the use of retrofitting by plating. Furthermore, the moment redistribution analysis procedure allows the engineer the freedom to choose the properties of the plate to design for moment redistribution.

COMPOSITE ACTION IN NON-COMPOSITE BEAMS

Local tests show that slab-on-girder bridges exhibit composite action. The results from experimental testing confirm load test results and show that there is a definite reduction in slip-strain and curvature due to interface friction. The paper notes that the material properties used in the fatigue assessment of an existing bridge should be determined carefully. In particular, the extent of the concrete cracking should be determined so that an appropriate flexural rigidity can be used.