Encased girders. prof. Ing. Ján Bujňák, Csc. Faculty of Civil Engineering ; University of Žilina

Transcription

1 Encased girders prof. Ing. Ján Bujňák, Csc. Faculty of Civil Engineering ; University of Žilina

2 1. Introduction Originally developed only for railway bridges, decks with encased beams have also been widely and effectively used for road bridges. The filler beam deck consists of the concrete slab with stiff longitudinal reinforcement made of rolled beams and transverse reinforcement of steel bars. Closely spaced steel beams and concrete act compositely without specific mechanical shear connection means. The span covered by filler beam decks range up to 40 meters for road bridges and up to 30 meters for railway bridges. For continuous structural system the maximum spans are about 10 meters larger. Very shallow depths of the bridge deck, quick and easy erection without temporary supports and falsework are the maine advantageous features of this construction system.

3 2. Design procedures assuming unbonded system

4 3. Analyses considering composite action 3.1. Load clasification and internal forces calculation (Strength design at overloads) Calculation of the design value of the resisting bending moment of a filler beam cross-section (M RD ) : *Neutral axis in the web 1.Calculation of the position of the plastic neutral axis X G : For the equilibrium of the cross-section the force resulting from the tension stresses in the beam must be equal to the sum of the forces resulting from the compression stresses in the steel and concrete.

5 2.The resulting ultimate moment in the sum of the moments of these forces related to X G : 3.Ultimate limit states:

6 3.2. Elastic analysis for serviceability loads The behaviour of encased beams should be firstly verified by linear elastic analysis for working or service loads (without partial safety factors) to control serviceability limit states such as deflections and crack widths and the limit state of fatigue. The stress limits in recommended grades of steels, compressive concrete strength as well as the limit stresses of reinforcing steel are given in appropriate standards.

7 4. More realistic analysis supported by experimental study Using the Cosmos finite element program, a more generalized discredisation scheme that treats the bridge structure with encased beams as a three-dimensional system can provide acceptable numerical results. In the adopted computer model of the encased beam structure, the concrete with tensile stresses greater than its strength is not assumed to assist in resisting the moment. The steel carries all the tension in this area. The neutral axis position is determined from the corresponding interior forces distribution. In the next step, the stresses relevant to the actual location of the neutral axis can be recalculated. Ultimate load is limited by weakness in the tension steel or weakness in the compression concrete.

8 To test the validity of the above computer model, the experimental investigation was executed with the objectives to comprehend the behaviour of slab bridge with encased beams at both the service and ultimate load levels. Three models of composite beam structure were fabricated. The 1,5 m long composite slab had a span 1,4 m between the end supports, and consisted of two rolled-steel section I 120 and a concrete encasement. The concrete was 160 mm thick with a 28-day concrete strength of 30 MPa. A mesh of smooth welded wire reinforced the concrete. The models were instrumented for the purpose of measuring deformation, strains across the depth of the encased steel beams, applied load and slip between steel and concrete.

9 The presented method applied to a model, whose cross-section is shown in previous page gives concentrated mid-span collapse load 95,4 kn, greater than value 91,3 kn, provided by actual code analysis considering composite action and the very underestimated load 61,2 kn declared by the classic unbonded calculation. The limit load corresponding to the first yield in experimental test was 150 kn. It was moreover found that the encased beam models continued to carry loads long after the formation of the first yield. Comparison of test deflections with calculated ones showed similar behaviour. The calculated values are slightly greater.

10 Ansys finite element program Using the Ansys finite element program, a more generalized discredisation scheme that treats the bridge structure with encased beams as a three-dimensional system can provide acceptable numerical results. The applied elements distribution is illustrated at the next figure.

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12 The typical outputs of numerical analyses based on finite elements method for the ultimate experimental force 150 kn are represented. Particularly, values of vertical deflection of a half structure at the following figure. Shapes of vertical deflection of a composite beam and a steel beam alone

13 The corresponding longitudinal stresses distribution in a filler beam as well as in a steel beam for the vertical load represented by the extreme ultimate force load 150 kn are illustrated for a half of the span at the next figure. Distribution of longitudinal stresses in a composite beam and in a steel beam alone

14 5. Conclusions Composite construction in the form of rolled beams encased in concrete is an economical and practical superstructure for bridges. The presented design procedure considering the bond in the compression portion at the steel-concrete interface and in this way the concrete participation is sure to be more realistic. The load carrying reserve can be discovered by application of more sophisticated numerical model given in the paper and tested by experimental study.

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