In recent years the strength and stiffness upgrading of existing buildings with timber floors has drawn attention to an efficient type of floor system, the timber-concrete composite slab, which consists of timber members connected to a concrete slab using a connection system. Compared to timber floors the main advantages of this type of composite structure are increased strength and stiffness, improved sound insulation and fire resistance.The structural behaviour of timber-concrete composite members is mainly governed by the shear connection between timber and concrete. A rigid connection develops full composite action between the components and therefore conventional principles of structural analysis can be applied. Full composite action may be achieved by grooved connections. On the other side connections based on mechanical fasteners like screws, nails, glued-in dowels mostly show elastic behaviour and develop only partial composite action. Therefore the structural analysis requires in this case the consideration of the slip between the components. Eurocode 5, Annex B gives a simplified calculation model for mechanically jointed beams, based on the differential equation for the partial composite action. As the shear connection usually shows a non-linear load-slip relationship, two different slip moduli are considered for design purposes: Kser for the serviceability limit state (SLS) and Ku for the ultimate limit state (ULS). The slip modulus Kser is usually evaluated by push-out tests according to EN 26891 and corresponds to the secant value at 40% of the load-carrying capacity of the connection. For the slip modulus Ku it is suggested to use the secant value at 60% (see for example STEP 2). According to Eurocode 5 the slip modulus Ku may be taken as 2/3 of Kser. Depending on the structural behaviour of the connection used (rigid, semi-rigid, etc) the assumption for Ku according to Eurocode 5 may be not adequate, leading to conservative or not conservative results.As the shear connections markedly influence the efficiency of timber-concrete composite slabs, a large number of push-out tests on different connection systems as well as bending tests on timber-concrete composite members have been performed worldwide (Sweden, Italy, Switzerland, Portugal, Germany, USA, etc.). The test results in combination with additional FE numerical simulations permit to check whether the hypothesis of assuming the slip modulus for ULS as two-thirds of the slip modulus for SLS is adequate. The first part of the paper presents the slip moduli K40% and K60% as well as the ratios K60%/K40% evaluated by push-out tests for different connection types. Further the slip moduli K40% are compared to analytical calculations according to Eurocode 5. In the second part analytical calculations are compared to bending test results and numerical simulations for different timber-concrete composite members. For the analytical calculations based on Annex B of Eurocode 5 the slip modulus K60% as suggested in STEP 2 is used. The aim of the analysis is to demonstrate that the use of K40% and K60% leads to accurate results for SLS and ULS, respectively. Finally based on the ratios K60%/K40% the assumption according to Eurocode that Ku may be taken as 2/3 of Kser is verified and if necessary improved.