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The present paper deals with composite steel-concrete floor with headed shear studs welded onto a flat steel plate. In bridges applications, which are the focus of the paper, very thick steel plates are often combined with a moderate layer of concrete. And, in some wide slab applications, the most performant lay-out for the shear studs could lead to them being welded on top of an existing butt weld between steel plates. Presently, we are ignorant about the shear resistance of the weld-to-weld interface and its possible detrimental effect on the overall resistance of the system. In addition, the ultimate behaviour of the system combines the flexural resistance of a composite cross-section followed by, when the concrete is cracked, the development of a compressed arch anchored in these headed shear studs, emphasising the importance of such connections. Both questions are addressed in the paper and, based on experiments, an in-depth analysis on the system’s behaviour at ultimate limit state is provided. Two types of tests are performed: pure shear tests complying to the push-out tests of Annex B of EN 1994-1-1 and beam tests with four-point loading where the studs in the shear spans are loaded with longitudinal shear. For each pair of tests, the reference specimen was made with a steel plate without a butt weld or comprising a butt weld directly under the line of shear studs. Typical floor dimensions from bridge applications are studied. No difference in shear capacity, stiffness or failure mode could be observed. However, it was noticed that the classic design rules provided in EN1994-1-1 cannot accurately predict the failure load of the chosen configuration. Based on assumptions made following the observations of the failure mechanisms, the authors propose a theoretical evaluation of the capacity of the system. ispartof: Engineering Structures vol:197 status: published