Encased girders

prof. Ing. Ján Bujňák, Csc.

Faculty of Civil Engineering ; University of Žilina

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.

2. Design procedures assuming

unbonded system

3. Analyses considering composite action3.1. 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 (MRD) : *Neutral axis in the web

1.Calculation of the position of the plastic neutral axis XG:

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.

2.The resulting ultimate moment in the sum of the moments of these forces

related to XG:

3.Ultimate limit states:

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.

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.

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.

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.

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.

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

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

5. Conclusions

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.

Composite construction in the

form of rolled beams encased in

concrete is an economical and practical

superstructure for bridges.

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