Effect of reinforced concrete between the flanges of the steel profile of partially encased composite beams

During the last 20 years composite structures have been built in increasing numbers. Composite steel concrete constructions are able to take full advantage of the respective properties and performance of both materials and lead to building with the following advantages: easy prefabrication, building time reduction, extremely reduced cross-sections, flexibility, fire resistance.... However in the last version of the Eurocode 4 (pr ENV 1994-1-1/April 1992)/§4.12(2)) it is not allowed to take into account reinforced concrete between the flanges of the steel beams with concrete-encased steel webs for the load bearing capacity and for the deflection calculation. The reinforcing bars and the concrete may be assumed to contribute only to resistance to local buckling or lateral-torsional buckling. From this point of view this version of the Eurocode 4 is too conservative. Indeed 13 tests, performed in 1991 and 1992 in the University of Bochum (Institut für konstruktiven Ingenieurbau), have indicated the importance of the reinforcing bars and the concrete between the flanges for the calculation of the ultimate bending moment, the ultimate shear force and the deflection. From these tests were deduced methods taking into account the increase in the bearing capacity and in ridigity due to the reinforced concrete. This research has already provoked changes in the Eurocode 4 and will be an important data source for writing the annex of EC4 dealing with partially encased composite beams.