The European standard [1] makes it possible to design much more effective structures with the combined advantages of steel structural sections and concrete structures. However many of structural problems are not included in this regulation. One of them is a composite frame or a Vierendeel connection made with a RHS chord filled with concrete and branches with a RHS steel profile.

From the structural point of view the optimal box chords section ought to have large dimensions and their wall thickness ought to be as small as possible. However for such structures two problems arise. Local instability of the section walls leads to degradation of the chord resistance and the very thin walls decrease the strength and stiffness of the joints. The strengthening of joints is possible using a steel plate welded to the face of a loaded chord. This however, does not strengthen the slender webs of the box section. The other possibility is a concrete filling of the hollow section. Such filling leads also to increasing the thermal capacity of structure and its fire resistance. The comparison of these two ways of strengthening is given in [2]. The aim of this paper is to derive a simple theoretical formula for calculating the strength and stiffness of such joints. Test results for twelve connections at full scale have been done and described here. The geometry and material properties of the joints tested and the theoretical predictions of their strength and stiffness are given. Comparisons between theoretical and experimental results and some conclusions are also presented. Finally, in conclusion, some general design recommendations are given.

It is shown in the paper that the new estimation predicts the strength well for the RHS joints of which chords are filled with concrete. As expected, it was noticed that the rotation capacity of filled joints is much smaller than the adequate steel joint. The stiffness of filled joints is about 30% greater than a steel joint. The test results show that such estimation could be accepted. However, more tests are required for confirmation of this data.

1

EN 1994-1-1:2004, "Eurocode 4: Design of composite steel and concrete structures - Part 1-1: General rules and rules for buildings", CEN, 2004.

2

J. Szlendak, "Improve the joints strength in steel frames with RHS Columns by concrete filling", in "Proceedings International Conference on Steel Structures of the 2000's", Istanbul, Turkey, 345-352, 2000.

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